Advances in Heat Transfer Enhancement
Abstract
This Brief addresses the phenomena of heat transfer enhancement. A companion edition in the SpringerBrief Subseries on Thermal Engineering and Applied Science to three other monographs including “Critical Heat Flux in Flow Boiling in Microchannels,” this volume is idea for professionals, researchers, and graduate students concerned with electronic cooling.
Chapters (9)
The advances in heat transfer enhancement techniques in all its aspects have been dealt with.
The advances in heat transfer enhancement techniques in all its aspects have been dealt with.
The advances in heat transfer enhancement techniques in all its aspects have been dealt with.
The advances in heat transfer enhancement techniques in all its aspects have been dealt with.
The advances in heat transfer enhancement techniques in all its aspects have been dealt with.
Passive techniques are commonly used for heat transfer enhancement as no external energy is needed. Many passive enhancement techniques were given in details by Webb and Kim (2005). However, recently developed enhancement techniques such as nanoscale structures and composite micro/nanostructures are not mentioned. In general, this part will mainly cover surface coatings (microscale and nanoscale), roughened or finned surfaces, inserts, curved geometries, surface tension devices and additives (e.g., nanoparticles, surfactants).
Active techniques require external power, such as electric or acoustic fields, surface/fluid vibrations, suction and jet impingement. In general, active techniques are not as common as passive techniques in industry as active techniques need external energy. The majority of commercially interesting techniques are mainly limited to passive techniques. However, active techniques such as electrohydrodynamic (EHD) enhancement of boiling and condensation indicate significant potential (Akira and Hiroshi 1988; Seyed-Yagoobi and Bryan 1999). EHD enhancement of heat transfer in two-phase flow features several advantages: (a) able to control the heat transfer coefficient by changing the applied voltage, (b) significant enhancement, (c) no moving parts, and (d) the electric power input is usually negligible. The heat transfer enhancement is highly dependent on quality, flow regime, heat flux, mass flux, and the strength of the radial EHD forces relative to the flow axial momentum. The EHD phenomena involve the interaction of electric fields and flow fields in a dielectric fluid medium. This interaction, under certain conditions, results in electrically induced fluid motion and/or interfacial instabilities, which are caused by an electric body force. When this force is enhancing heat transfer it is thinning and/or destabilizing the liquid layer, depending on the mass flux. However, this force can thin the liquid layer to a point of removing it and can drastically reduce the heat transfer, especially at low mass fluxes and high heat fluxes (Bryan and Seyed-Yagoobi 2001). The electric body force density acting on the molecules of a fluid in the presence of an electric field consists of three terms, as shown below (Melcher 1981):$$ \bf{f}_e={\rho}_eE-\frac{1}{2}{E}^2\nabla \varepsilon +\frac{1}{2}\nabla \left[{E}^2\rho {\left(\frac{2\varepsilon }{\partial \rho}\right)}_T\right] $$The three terms in Eq. (1) stand for the electrophoretic, dielectrophoretic, and electrostrictive components of the electric force. For two-phase flows, the dielectrophoretic force dominates because the gradient in the dielectric permittivity, ∇ε, is very high at the vapor-liquid interface, resulting in a large EHD force acting on the interface. This force can cause interfacial instabilities that force the liquid with higher permittivity to move to the regions of higher electric field. This phenomenon is usually referred to as the liquid extraction phenomenon and is believed to be the primary mechanism responsible for flow regime transitions which cause heat transfer enhancement. Sadek et al. (2006) studied in-tube condensation of R134a in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. As shown in Fig. 10.1, the heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor of 1.5 at the same conditions of the maximum heat transfer enhancement. The EHD force extracts sufficient liquid from the liquid stratum at the bottom region to cause flow regime transition from stratified flow to annular flow. The decrease in the liquid layer thickness and introduction of droplets into the vapor core (Cotton et al. 2001) result in a large improvement in heat transfer coefficient and a relatively moderate increase in pressure drop.
Flow direction directly affects the two-phase flow patterns, while flow pattern is closely related to heat transfer and pressure drop. Stratified flow is common in horizontal and inclined conventional channels under earth gravity conditions but not in vertical channels. Stratified flow is not an efficient heat transfer flow pattern. The thick liquid film in the bottom part of the horizontal channel presents a large thermal resistance, while for flow boiling, intermittent dryout might occur in the upper part of the channel. Various techniques can be used to redistribute the liquid film in stratified flow and thus enhance the heat transfer. For example, the EHD force, when appropriately applied on the two-phase flow, can carry liquid at the bottom part to wet the upper part of the heated surface. Microfin and herringbone tubes, can also push part of the liquid at the bottom surface to the upper and side surfaces, leading to earlier stratified-intermittent and intermittent-annular flow transitions, with increment in heat transfer.
Increase in heat duty and demanding requirements on clean and sustainable technology are constantly moving the developments of heat transfer enhancement technology. This chapter presents an up-to-date overview of heat transfer enhancement techniques for two-phase flow (e.g., boiling and condensation) and mainly emphasizes those either commercially used enhancement techniques or the most recent enhancement methods. A special focus is on the enhancement technologies with a relatively low pressure drop penalty. Passive enhancement techniques such as surface coating, roughened and finned surfaces, insert devices, curved geometries and additives are highlighted. Several recent enhancement techniques, e.g., nanoscale surface coatings, microfin tubes and nanoparticle additives, are outlined for their promising potential in enhancing phase-change heat transfer, especially nanoscale surface coatings. Microchannels which can be considered as one of the promising passive enhancement techniques, are not detailed in this chapter as many aspects of microchannels have been covered in the recent literature. Among active enhancement techniques, electrohydrodynamic (EHD) phenomenon and jet impingement are briefly described.
... The interaction of an electric field with a magnetic field in a dielectric fluid medium is used to improve electro hydrodynamic heat transfer. Electrically generated fluid motion and/or interfacial instabilities are the result of this interaction, which is caused by an electric body force under particular conditions [9]. In this procedure, the fluid 040005-2 is exposed to a high voltage and current. ...
... Where,E is the electric field strength( v/m), Fe is the electro hydrodynamic force ( N/m2 ), qc is free electric charge density( C/m3 ), the fluid permittivity(F/m), q the fluid density( kg/m3 ), and T the temperature in (K) [10]. The electrophoretic, dielectrophoretic, and electrostrictive components of the electric force are represented by the three terms in equation [9].This force can produce interfacial instabilities, forcing greater permittivity liquids to travel to areas with higher electric fields. This process is commonly referred to as the liquid extraction phenomenon, and it is thought to be the major mechanism responsible for flow regime shifts that result in increased heat transfer [9]. ...
... The electrophoretic, dielectrophoretic, and electrostrictive components of the electric force are represented by the three terms in equation [9].This force can produce interfacial instabilities, forcing greater permittivity liquids to travel to areas with higher electric fields. This process is commonly referred to as the liquid extraction phenomenon, and it is thought to be the major mechanism responsible for flow regime shifts that result in increased heat transfer [9]. ...
Energy conservation requires a reduction in the consumption of primary resources, environment protection, cost savings on materials and production inputs. Monetary considerations (material and energy savings) motivate initiatives to improve heat exchange efficiency. In heat transfer operations such as energy storage and recovery, the heat exchanger plays a crucial function. Heat transfer augmentation technologies are employed in a variety of industrial uses to improve the performance of heat exchangers. Heat transmission technologies are commonly utilized in HVAC systems, new thermal power plants, automobiles, and aircraft. Transmission of heat refers to methods for increasing the thermal and hydrodynamic performance of heat exchange equipment. One of the most significant study disciplines in the thermal engineering arena is heat transfer augmentation, where various approaches to improve heat transfer are investigated, tested, and analyzed. These techniques are divided into three groups-1. Active techniques 2. Passive techniques 3. Compound techniques. In this paper, an exhaustive literature evaluation of several heat transfer improvement techniques has been conducted.
... At a fixed Re, they demonstrated that the value of Nu becomes maximum in case of trapezoidal-triangular baffles. Similar types of work are available in the literature [14][15][16][17][18][19][20][21][22][23]. ...
... Hydro-thermal characteristics of turbulent air flow are governed by the following continuity (equation (1)), momentum (equation (2)) and energy (equation (3)) equations [10][11][12][13][14][15][16][17][18][19][20][21][22][23]: For analyzing the hydro-thermal phenomena, near the walls and far away from the walls, the AA" <model is suitable due to the presence of L D term, blending function. To predict the accurate results, this model is used to transform the inner region of boundary layer to the <model at a higher . ...
... For 3 × 10 6 ≤ ≤ 5 × 10ˆ , # & has been taken as per Petukhov [18]: ...
... where U 0 is the velocity of the main flow. Furthermore, according to the Blasius friction law [14], C f of the internal flow with 1 × 10 4 < Re < 5 × 10 6 can be calculated as ...
... As mediated by the second law of thermodynamics, the irreversible loss generated in domain V can be calculated as S is obtained by combining Equations (5) and (6), and T 0 is the environment temperature. For the numerical simulation, the integral computation can be solved by a summation of the value in every mesh cell, as shown by the leftmost part of Equation (14). i represents an arbitrary mesh cell and n is the total number of mesh cell. ...
... The curves in Figure 8b represent the distribution of the integral wake-flow loss along the blade spanwise direction of the wake-flow region. The integral wake-flow loss is calculated using Equation (14). A schematic of the control volumes in the wake-flow region is shown in Figure 8a. ...
A local loss model and an integral loss model are proposed to study the irreversible flow loss mechanism in a linear compressor cascade. The detached eddy simulation model based on the Menter shear stress transport turbulence model (SSTDES) was used to perform the high-fidelity simulations. The flow losses in the cascade with an incidence angle of 2°, 4° and 7° were analyzed. The contours of local loss coefficient can be explained well by the three-dimensional flow structures. The trend of flow loss varying with incidence angle predicted by integral loss is the same as that calculated by total pressure loss coefficient. The integral loss model was used to evaluate the irreversible loss generated in different regions and its varying trend with the flow condition. It as found that the boundary layer shear losses generated near the endwall, the pressure surface and the suction surface are almost identical for the three incidence angles. The secondary flow loss in the wake-flow and blade-passage regions changes dramatically with the flow condition due to the occurrence of corner stall. For this cascade, the secondary flow loss accounts for 26.1%, 48.3% and 64.3% of the total loss for the flow when the incidence angles are 2°, 4° and 7°, respectively. Lastly, the underlying reason for the variation of the secondary flow loss with the incidence angle is explained using the Lc iso-surface method.
... Various techniques in active and passive analysed for thermal rates augmentation within pipes, the varied inserts types were used, mainly when the flow turbulent was considered. Many approaches have been applied to rise thermal heat performance devices for example, rough surfaces, coiled tubes, treated surfaces, swirling flow devices, twisted tape, dimples and corrugated surfaces [3][4]. The twisted tapes devices can classify as swirl device and hence improvement of heat performance in heat exchangers is ascribed to several influences. ...
... In order to calculate heat transfer coefficient (h) researchers were used the below expression [3]: ...
Numerous inserts types are employed in different heat transfer improvement application devices. In this review study is forced on various types of twisted tape inserts in heat exchanger pipe. Geometrical configurations of twisted tape for example twist direction; length, width, space, twist ratio etc. were highly effect on flow pattern, hydrodynamic flow and heat transfer performance. In this review study observed that using different types of twisted tapes can improve thermal performance and hydrodynamic as compared to smooth pipe (without twisted tape). The review investigations found that improvement of thermal performance happens owing to decrease in pipe cross area, leads to rise in mixing flow, turbulence flow intensity flow and rise in swirl flow established through different kinds of twisted tapes. This article dealt with investigations published in corrugated pipes with varying field applications to provide good information for engineers and designers whom dealing and concerning with improvement of heat performance in heat exchanger corrugated pipes. K ey w ord s: Heat exchangers, Twisted tape inserts, Pressure drop, Thermal heat performance, Enhancement of heat transfer
... The traditional approaches of heat transfer enhancement were scrutinized and tested thoroughly over decades and merely nothing left to investigate [1,2]. Hence, it is imperative to follow a new approach and embrace innovative techniques to override the current challenges and vouched for the upcoming challenges and demands regarding heat transfer [3][4][5][6]. The emergence of nanotechnology helps in overcome of many issues especially in the heat transfer field. ...
The present study considered an impingement jet using hybrid nanofluid CuO–Cu/water. A single rounded nozzle was used to impinge a turbulent coolant (water) on the hot circular plate at Reynold’s number range of (5,000–15,000). CuO–Cu nanoparticles were physically synthesized at 50 nm size and dispersed by one-step preparation method. The experimentations were conducted with nanoparticle concentrations range of (0.2–1%) by volume. The results showed that the presence of hybrid nanoparticles exhibits a significant improvement in the overall thermal performance of the working fluid. Where the gained heat interpreted by the Nusselt number was found to be 2.8% (in comparing with deionized water) at ϕ = 1% and Re = 15,000, while the minimum gain in the heat was found to be 0.93% at ϕ = 0.2% and Re = 5,000. Furthermore, it was noted that the excessive increase in CuO–Cu nanoparticle concentration causes more pumping power consumption. Moreover, the CuO–Cu nanoparticles residual layer was found to be formed at a high CuO–Cu concentration, which acts as an insulation layer that hinders the heat exchange. It was also found that the threshold of nozzle-to-plate spacing is H = 4, before which, the heat gain is positive, and negative plummet after.
... The relation introduced by Petukhov et al. [36] was used to measure the friction coefficient ( f ) used in the Gnielinski relation [35]: ...
The rapid growth of the electronics industry and the increase in processor power levels requires new techniques to improve the heat transfer rate in their cooling systems. In this study, ultrasonic vibration technology was introduced as an active method to enhance the thermal performance of water-cooling systems. The effects of ultrasonic vibrations at power levels of 30, 60, and 120 watts for different cooling airflow rates were investigated experimentally. The results were validated with available empirical correlations to ensure the accuracy of the measurement systems. The findings indicated that the ultrasonic vibrations enhanced the heat transfer in the liquid-cooling heat exchangers. In addition, the thermal performance of the ultrasonic vibrations was improved by reducing the airflow rate and increasing the ultrasonic power. In addition to the feature of heat transfer improvement, ultrasonic waves are widely used for the cleaning of different types of heat exchangers. Regarding the anti-fouling and anti-accumulation effects of the ultrasonic vibrations, the introduced technology could provide a practical way for developing high-performance nanofluids-based computer cooling systems.
... Comparison of numerical and empirical coefficient of friction in the turbulatorless model. As shown in Fig. 5, the numerical results are compatible with the values calculated by the Moddy diagram and the Petukhov equation.In order to verify the data of numerical results, the empirical equations of Petukhov, Gnielinski, Dittus-Boelter and Sieder-Tate, which can be found in the literature, were applied for the fully developed flow in the tube[48][49][50][51]. Comparison of numerical and empirical Nusselt numbers in a turbulatorless tube. ...
... What is more, the Nu number, which is dimensionless CHTC, does not include the effect of increased pumping costs due to the dispersion of nanoparticles in the base fluid. Therefore, the following are three thermo-hydrodynamic performance criteria (TEF-thermal enhancement factor [110,[134][135][136], PEC-performance evaluation criterion [137,138], and EEC-efficiency evaluation criterion [134]) that can be used to evaluate the experimental thermal and hydrodynamic performance of nanofluids together and rationally: ...
This study aimed to experimentally investigate the effects of Al2O3-water nanofluids with six different volume concentrations (0.02%, 0.1%, 0.2%, 0.4%, 0.6%, and 0.8%) instead of water as working fluid on heat transfer and pressure drop on the tube side of a mini-channel shell and tube heat exchanger (MC-STHE). The shell-side hot fluid flow rate was kept constant at 180 L/h, while the flow rate of the tube-side cold fluids ranged from 125 to 600 L/h. The convective heat transfer coefficient (CHTC) of the nanofluids increased in the transition and turbulent regions. The nanofluids also extended the transition region. The nanofluids had higher friction factors than water. The nanofluids had an optimal volumetric concentration and a flow rate of 0.4% and 375 L/h, respectively, at which the CHTC enhanced by 49%, while the friction factor increased by 21% compared to water. Compared to macrotube heat exchangers, the CHTC enhanced 3 to 7 times with the combined effect of the mini-channels and nanofluids in the shell-and-tube heat exchanger (STHE). Consistent with earlier studies (maximum TEF of 1.8), the thermal enhancement factor (TEF) for the optimal volumetric concentration Al2O3-water nanofluid in the MC-STHE was 1.6 at most under the same hydrodynamic conditions. The performance evaluation criterion (PEC) and the efficiency evaluation criterion (EEC) showed that there was no point (not enhancement) in using nanofluids instead of the base fluid (water) in the MC-STHE.
... Two-phase flow is a condition that includes two of the basic phases encountered in technical applications such as combustion, boiling and condensation. Two-phase flow situations during phase change such as boiling and condensation are frequently observed in engineering applications where high heat fluxes occur [1]. Boiling is the physical phenomenon associated with the transition of a fluid from the liquid-phase to the gas-phase at the interface. ...
In this study, a brief review of the studies in the literature conducted for boiling in double-pipe heat exchangers at horizontal, vertical and different angles of inclination is presented. There are lots of studies in the literature for optimum heat transfer performance in double-pipe heat exchangers. Although there are many theoretical and experimental studies on double-pipe heat exchangers that involves with single and/or two-phase flow studies, almost all of them investigated the horizontal or vertical positions of the testing section. In addition, although there are studies conducted with enhanced (corrugated inner surface, micro-finned, insert, curved, etc.) tubes in the literature, the effect of inclination angle on heat transfer and pressure drop was not taken into account in most of these studies. Therefore, within the scope of this study, a summary has been done on the basis of boiling in inclined double-pipe heat exchangers with smooth and corrugated inner surface using different refrigerants.
... As detailed in section 3.1.1, heat transfer enhancement is an innovative field of research [205] using many techniques whether active (requiring an external energy input) such as the use of oscillating walls, or passive such as the addition of mixing promoters in the flow channel [63] [58] or the search for optimized transfer fluids. Convective mass transfer enhancement is also an area of particular interest for chemical processes [44] and biotechnology [129,222] that take advantage of passive [5,3] or active methods [207,73]. ...
This thesis work is part of research aimed at improving the performance of concentrated solar power plant receivers with large temperature gradients and asymmetric thermal boundary conditions. It is necessary to analyze the power lost due to thermal and viscous irreversibility: this is achieved by studying the entropy generation rate within the flow and by adopting three different axis of analysis that provide complementary insights: (1) the detailed study of the entropy generation rate in a laminar boundary layer by examining in particular the effect of the thermal boundary condition type (imposed temperature vs. fixed heat flux density) (2) the use of the calculus of variations to determine which velocity fields optimize an objective functional related to the entropy generation rate in a flat plate channel flow, one-third of one of the walls being at imposed heat flux density (3) the study of the entropy generation rate in a flat plate channel flow, turbulent, quasi-compressible and for a fluid which thermo-physical properties depend on temperature.
... The heat-exchanger design was a very challenging part of the system design. To achieve the design basis and sizing of a heat exchanger, the thermal energy transfer considerations as published in handbooks by Lienhard and Lienhard (2011), Saha et al. (2016) and Schreyer (2011) were used. The system was designed to be used with an existing commercially available electric geyser, namely, a standard 150-litre household geyser with an inlet port with an inside flange diameter of 120 mm. ...
A low-cost heat-exchanger system that can be used in high-pressure/low-pressure isolated solar water-heating systems in South Africa was developed for household applications. The combination of a copper coil and electrical heater allowed for isolation of the high-pressure and low-pressure sections of the system and enabled the utilisation of large low-cost solar heat-absorber platforms that operated at low pressure with a low risk of fouling and leaking. The design comprised a copper coil heat exchanger to be installed inside a conventional geyser, to replace the normal heating element and thermostat system in a conventional commercially available household geyser. The electric heating element still supplements the system in low solar energy conditions. The circulation in the system is created by a small separate photovoltaic panel and a circulation pump. An integrated switch allows the system to alternate between conventional electrical heating and solar water-heating according to prevailing weather conditions. Current tests show that the system of 15 m2 area can be installed at a cost of approximately ZAR 10 000–12 000. The system can provide hot water at approximately 12 cents per kWh, with a total heat storage capacity of up to 10 kWh per day. This implies a saving to the customer of up to ZAR 600 per month. The accumulated saving to a household over the ten-year lifetime of the product is estimated at ZAR 200 000. As the thermal energy storage capacity of current systems as available on the local market is approximately 1 kWhr per day for a 2 m2 collector. A typical increase in thermal energy collection capacity of tenfold more than the capability of conventional systems on the market is hence achieved. The system offers implementation possibilities for South Africa’s low-cost housing schemes and can provide for creating numerous new business and job opportunities on the African continent with its abundant solar irradiation resources.
... The limit pressure drop in heat exchangers is set as 10 kPa, which is the convergence condition for design process. For the cases where no phase change occurs (thermal oil in gas-oil heat exchanger, single-phase R245fa in evaporator and water in condenser), the convective heat transfer coefficient of tube-side is calculated by the Petukhov and Popov correlation [33]: ...
The Organic Rankine Cycle (ORC) has been proved a promising technique to exploit waste heat from Internal Combustion Engines (ICEs). Waste heat recovery systems have usually been designed based on engine rated working conditions, while engines often operate under part load conditions. Hence, it is quite important to analyze the off-design performance of ORC systems under different engine loads. This paper presents an off-design Medium Cycle/Organic Rankine Cycle (MC/ORC) system model by interconnecting the component models, which allows the prediction of system off-design behavior. The sliding pressure control method is applied to balance the variation of system parameters and evaporating pressure is chosen as the operational variable. The effect of operational variable and engine load on system performance is analyzed from the aspects of energy and exergy. The results show that with the drop of engine load, the MC/ORC system can always effectively recover waste heat, whereas the maximum net power output, thermal efficiency and exergy efficiency decrease linearly. Considering the contributions of components to total exergy destruction, the proportions of the gas-oil exchanger and turbine increase, while the proportions of the evaporator and condenser decrease with the drop of engine load.
... In compound techniques, two or three of the above-mentioned methods can be used together for further heat transfer enhancement. A review on these combinations has been done by Saha et al. [218]. Promvonge et al. [207] combined helical-ribbed tube and a twisted-tape inserts in a heat exchanger. ...
Gas-phase solar receivers use atmospheric or pressurised gas as their heat transfer fluid (HTF). The ideal gas-phase receiver would provide high thermal efficiency and high HTF outlet temperature with low pressure drop and low capital cost. In practice, these four objectives are hard to achieve simultaneously since it is difficult to (cost-effectively) overcome the intrinsically poor heat transfer performance between the solid absorber and the gaseous HTF. Thus, this review provides an in-depth look at the recent progress towards solving this challenge of pressurised gas-phase receivers to identify the remaining knowledge/research gaps. In general, gas-phase receivers can be classified as direct or indirect solar absorbers and by the type of heat transfer enhancement (HTE) employed to address the poor absorber-to-HTF heat transfer rate. The present review suggests the receiver designs that show the most promise for further improvement from each of the active, passive and compound HTE methods. This study also finds that there is a need for more proof-of-concept tests for these receiver designs, since the number of studies which include real prototypes operating under real weather conditions is limited. Based on a review of successful prototyping research, this review suggests proper prototyping procedures for high-temperature receivers. Overall, the authors believe the present study presents an up-to-date, comprehensive review of the progress on gas-phase receivers, along with some meaningful, specific guidance on the necessary next steps in their development. This is significant because gas-phase receivers represent the best near-term solution for pushing solar systems to higher temperatures, enabling integration with advanced/combined cycles and solar thermochemical reactors with endothermic chemical reactions at high temperature (e.g. mineral processes and solar fuels).
... Therefore, the value of η for a case above unity signifies that the modification of the VG improves the rate of energy used to achieve the desired heat transfer. The mathematical expression of thermal performance factor is provided in Eq. (10) [13]. ...
For many industrial applications, cooling of a hot surface is of greater importance for an improved performance of the system. The wetted surface area plays a vital role in heat transfer process. Increase of the wetted surface may improve the heat transfer rate from the hot surface to the flowing fluid. Further, the optimisation of the shape of the extended surface may promote superior thermal interaction between the surface and the fluid. These extended surfaces are responsible for creating an abrupt pressure difference across it. This results in formation of longitudinal vortices downstream of the surfaces. The induced longitudinal vortex may interact with the boundary layer, which in turn enhances the convective rate of heat transfer. In the current work, the vortex generators with surface textures are studied numerically, which is an extension of previous work performed by same group of authors Kashyap et al. (2018). The CFD results show that multiple textures on the leading and the trailing faces of the vortex generator enhances the strength of the primary vortex, downstream of the vortex generator. As a result, the heat transfer from the heated surface increases to about 14.4%. The phenomenon is realised by increment in the surface average skin friction coefficient along with the surface average Nusselt number of the plate. The surface temperature distribution of the plate downstream of the vortex generator show a downfall due to the stretching of the primary vortex with a minimal pressure drop.
... where, U 0 is the velocity of main flow. Meanwhile, according to the Blasius friction law [18], the C f of internal flow with 10 4 < Re< 5*10 6 can be calculated as: (10) where, Re is the Reynolds number at the inlet. According to the law of the wall [17], the velocity gradient is a constant inside the viscosity sub-layer. ...
The loss-generating mechanism of a linear compressor cascade at the corner stall condition was numerically studied in this paper. The hybrid RANS/LES method was used to perform the high–fidelity simulations. By comparing the results captured by SSTDES, DDES, SAS models with the experimental data, the SSTDES model is proven to be more accurate in capturing the detailed flow structure of the corner stall than the other two models. Taking the turbulence dissipation term of SSTDES model into account, the volumetric entropy generation rate and a new dimensionless local loss coefficient are proposed and used to analyze the loss-generating mechanism in this work. It was found that the main flow loss generated in this cascade could be sorted as the wake flow loss, the profile loss, the secondary flow loss and the endwall loss according to their amounts. The corner separation significantly affects the secondary flow loss, wake flow loss and profile loss in the cascade passage. The mixing between the separated boundary layer flow and the main flow, the shear between a tornado vortex and the main flow are the main sources of the secondary flow loss. The wake flow loss is the largest loss source of the cascade, accounting for 41.8% of the total loss. There are two peaks of the wake flow loss along the spanwise direction near the corner stall region. This phenomenon is related to the appearance of large velocity gradient flows when the main flows and the corner separation flows mix together. The profile loss takes up 40.06 % of the total loss. The profile loss intensity in the corner region is lower than the mid blade span due to the interaction of the boundary layer on the suction side with the corner separation.
... where Here, f o is the friction factor for fully developed turbulent flow in a smooth channel which was obtained from a Petukhov empirical correlation [26] , Dp is the pressure drop over the whole computational domain, r is the density of air, U in is the average velocity at the inlet, and pi is the pitch between two adjacent ribs. ...
The present paper deals with a flow of a viscous incompressible fluid along a heated vertical cone, with due allowance for variations of viscosity and thermal diffusivity with temperature. The fluid viscosity is assumed to be an exponential function of temperature, and the thermal diffusivity is assumed to be a linear function of temperature. The governing equations for laminar free convection of the fluid are transformed into dimensionless partial differential equations, which are solved by a finite difference method with the Crank–Nicolson implicit scheme. Dependences of the flow parameters on the fluid viscosity and thermal conductivity are obtained.
... where Here, f o is the friction factor for fully developed turbulent flow in a smooth channel which was obtained from a Petukhov empirical correlation [26] , Dp is the pressure drop over the whole computational domain, r is the density of air, U in is the average velocity at the inlet, and pi is the pitch between two adjacent ribs. ...
... In a developed turbulent flow, Nu and f are obtained from relations (16), [23] and (17), [24], respectively. ...
In this study, thermo-physical and geometrical parameters affecting entropy generation of nanofluid turbulent flow such as the volume fraction, Reynolds number and diameter of the channel and micro-channel with circular cross section under constant flux are examined analytically. Water is used as a base fluid of nanofluid with nanoparticles of Ag, Cu, CuO and TiO 2. The study is conducted for Reynolds numbers of 20000, 40000 and 100000, volume fractions of 0, 0.01, 0.02, 0.03 and 0.04, channel diameters of 2, 4, 6 and 8 cm and micro-channel diameters of 20, 40, 60 and 80 micrometers. Based on the results, the most of the generated entropy in channel is due to heat transfer, and also, with increasing the diameter of the channel, Bejan number increases. The contribution of entropy generation due to heat transfer in the micro-channel is very poor and the major contribution of entropy generation is due to friction. The maximum amount of entropy generation in channel belongs to nanofluids with Ag, Cu, CuO and TiO 2 nanoparticles, respectively, while in the micro-channel this behavior is reversed; and the minimum entropy generation happens in nanofluids with Ag, Cu, CuO and TiO 2 nanoparticles, respectively. In channel and micro-channel, for all nanofluids except for the water-TiO 2 , with increasing volume fraction of nanoparticles, the entropy generation decreases. In channel and micro-channel, the total entropy generation increases as Reynolds number augments.
... The optimization was carried out to maximize the thermal performance of a boot shaped rib (F TP ), which is defined as: where is the area averaged value of the normalized local Nusselt number, is the Nusselt number obtained from the Dittus-Boelter correlation [9] for a fully developed turbulent flow in a smooth pipe, is the wall heat flux, is the thermal conductivity of working fluid, is the wall temperature, is bulk temperature that is calculated by interpolating temperatures of the inlet and the outlet, Re is the Reynolds number based on the hydraulic diameter of the channel( ), and Pr is the Prandtl number. is a friction factor of the channel, ∆ is the pressure drop of total channel, is the mean velocity of the inlet and is a friction factor for a fully developed flow in a smooth pipe obtained from Petukhov empirical correlation [10]. The averaged area A is area of the ribbed surfaces (without ribs).Table 1 lists the design variables and their ranges used for the optimization of the boot shaped ribs.Table 1 Design variables and the design space ...
Shape optimization of a boot-shaped rib installed in a rectangular cooling channel has been performed with three-dimensional Reynolds-averaged Navier-Stokes analysis, surrogate modeling and Particle swarm optimization. The low-Re k-omega turbulence model was used as a turbulence closure. The numerical results for the heat transfer performance were validated by comparison with experimental data in a Reynolds number range of 8,000-20,000. In order to maximize thermal performance of a boot-shaped rib in the channel, ratios of tip width to rib width , rib width to rib height, rib height to channel height and tip height to rib height were selected as design variables for the optimization. The design points were generated using Latin hypercube sampling. The surrogate model for the objective function was constructed using the radial basis neural network model. As a result of the optimization, the thermal performance of the optimal design was found to increase by 11.5%, in comparison to the reference boot-shaped rib.
... where f ∆ U f 2 2.236 ln Re 4.639 where f 0 is a friction factor for a fully developed flow in a smooth pipe. It is obtained from the Petukhov empirical correlation [16], which is modified from the Karman-Nikuradse correlation for the best fit in the range 10 4 Re 10 6 . ...
A stepped circular pin-fin array is formulated numerically and optimized with Kriging metamodeling technique to enhance heat transfer performance. The problem is defined by two non-dimensional geometric design variables composed of height of the channel, height of smaller diameter part of the pin-fins, and smaller diameter of the pin-fins, to maximize heat transfer rate compromising with friction loss. Ten designs generated by Latin hypercube sampling were evaluated by three-dimensional Reynolds-averaged Navier-Stokes solver and the evaluated objectives were used to construct the surrogate model. The predictions of objective function by Kriging model at optimum point show reasonable accuracy in comparison with the values calculated by RANS analysis. Optimum shape of pin-fins strongly depends on the weighting factor which measures importance of the friction loss term in the objective function. The thermal performances are much higher than that of the straight pin-fin at sampling optimum points with different weighting factors.
... is a friction factor for fully developed flow in a smooth pipe, and is obtained from Petukhov empirical correlation [20] which is modified from the Karman-Nikuradse correlation for the best fit in the range, 10 4 < Re < 10 6 . ...
A numerical optimization procedure for the shape of three-dimensional channel with angled ribs mounted on one of the walls to enhance turbulent heat transfer is presented. The response surface based global optimization with Reynolds-averaged Navier-Stokes analysis of fluid flow and heat transfer is used. Shear stress transport (SST) turbulence model is used as a turbulence closure. Computational results for local heat transfer rate show a reasonable agreement with the experimental data. The pitch-to-height ratio of the rib and rib height-to-channel height ratio are set to be 9.0 and 0.1, respectively, and width-to-rib height ratio and attack angle of the rib are chosen as design variables. The objective function is defined as a linear combination of heat-transfer and friction-loss related terms with the weighting factor. Full-factorial experimental design method is used to determine the data points. Optimum shapes of the channel have been obtained in the range from 0.0 to 0.1 of the weighting factor.
... where f 0 is a friction factor for fully developed flow in a smooth pipe, and is obtained from Petukhov empirical correlation [16]. ...
The present study investigates on design optimization of rib-roughened two-dimensional channel to enhance turbulent heat transfer. Response surface method with Reynolds-averaged Navier-Stokes analysis is used as an optimization technique. Standard k-¥ å model with wall functions is adopted as a turbulence closure. The objective function is defined as a linear combination of heat transfer and friction drag coefficients with weighting factor. Computational results for overall heat transfer rate show good agreements with experimental data. Four design variables are optimized for weighting factor of 0.02.
... Here, f 0 is the friction factor for fully developed turbulent flow in a smooth channel, and was obtained from the Petukhoz empirical correlation (Petukhov, 1970), which was modified from the Karman-Nikuradse correlation (Schlichting, 1968) for the best fit in the range, Volume 20, Number 2, 2013 10 4 < Re < 10 6 . ∆p, ρ, U in , and S x are the pressure drop, fluid density, average axial velocity at the inlet, and distance between the pin-fins in the x-direction, respectively. ...
The performance of pin-fin arrays installed in the trailing edge internal cooling channel of a turbine blade was investigated using three-dimensional Reynolds-averaged Navier-Stokes equations. The turbulence was modeled using the low-Reynolds shear stress transport turbulence model. The Reynolds number based on the hydraulic diameter was 20,000. The measured and calculated velocity distributions in the trailing edge internal cooling channel were compared to verify the accuracy of numerical analysis. To examine the effects of the configuration of the pin-fin arrays on heat transfer, pressure drop, and flow rate uniformity at the exit of the channel, the number of pin-fin rows and spaces between the pin-fins were selected as the geometric parameters to be tested. As the number of pin-fins increased, the heat transfer and exit flow uniformity were enhanced. In addition, the flow uniformity was also improved by adjusting the spaces between the pin-fins.
In this paper, the simultaneous impacts of using nanofluid and ultrasonic vibrations in a double-pipe heat exchanger are experimentally investigated. The vibrating heat exchanger is designed so that the ultrasonic waves with the power of 60 watts and frequency of 40 kHz are applied to its body at equal length distances in a uniform and effective manner. Water-based Al2O3 nanofluid is used in this research. The available empirical correlation has been used to confirm the accuracy of the measurements and validate the results. The effective thermal parameters have been tested in three cases using water, nanofluids, and ultrasonic-excited nanofluids as the working flow of the double-pipe heat exchanger. These tests have been performed in a relatively wide range of flow rate (113–257 lh−1), Reynolds number (3230–7431), inlet hot fluid temperature (40–60 °C), and nanoparticle volume fraction (0.4–0.8%). The results indicate the positive effect of adding nanoparticles and applying ultrasonic vibrations, especially at higher inlet hot fluid temperatures and higher nanofluids concentrations. The nanoparticles are more effective at high-flow rates, whereas the ultrasonic vibration is highlighted at low-flow rates. Also, the effectiveness-NTU analysis carried out for the current heat exchanger shows that using nanofluid and ultrasonic-excited nanofluid instead of water can increase the efficiency of the thermal system up to 18.3% and 42.3%, respectively.
Technical volume on predicting heat transfer for nuclear thermal hydraulics. Technical background, a detailed discussion of advanced analysis methods, and a view on future developments. Download at: https://www.imeche.org/industry-sectors/power-energy/digital-reactor-design-nuclear-thermal-hydraulics
The mean problem in using the falling film technology is the total liquid film wettability. The current work aims to propose a new configuration of the falling film technology in which the whole heat transfer area is achieved. A theoretical modeling of an evaporator proposed as flooded-liquid film technology is developed, whose geometry consists of a spiral where the refrigerant flows inside the tube and the secondary flow passes over the contact area reproducing a cross-flow heat exchanger. The two-phase heat transfer was analyzed based on the homogenous model in which Shah's correlation and the NTU method were taken into account to study the evaporator performance. The influence of some operational parameters on the vaporization processes such as the liquid film thickness, the mass flow rate of both stream flows and the coolant water temperature were studied. Numerical results showed thermal effectiveness is affected by mass flow rate of external fluid and the liquid film thickness, mainly. Vapor quality changed in a linear fashion as a function of the length of the tube.
South Africa is, due its specific latitude location in the southern hemisphere, exposed to high solar irradiation levels. Black thermal absorbers have a high absorbance for solar incident radiation, while commercial photovoltaic technology only converts about 10% of energy available in the solar spectrum. In this article, low-cost Peltier conversion cells, that are normally used for cooling purposes, and that are freely available in supply stores in South Africa, were identified as suitable conversion cells for converting thermal energy into electricity. Two prototypes of thermal-to-electricity energy conversion systems were subsequently designed and developed. Particularly, advanced pulse mode DC- to- DC conversion technology, a special electronic control system, was developed, that could extract high amounts of electrical energy from the cells and could store the energy in standard storage batteries. A 3 W and a 30 W output continuous conversion capacity system were developed. A power conversion of up to 2 W capacity per individual cell was achieved. The systems used no movable parts, and the lifespan of the systems is projected to be at least twenty years. Cost and viability analyses of the systems were performed and the results were compared to existing solar photovoltaic energy conversion systems. Combining the 30 W capacity system with a black body and reflector plate absorber system revealed a cost structure of only ZAR 0.8 per kWh, as compared with a derived ZAR 3 per kWh for a combined photovoltaic and solar geyser combination, as calculated for a ten-year term. The technology as developed is suitable to be incorporated in South African households and rural Africa applications.
Air is a potential heat transfer fluid that can be used in Brayton cycles, however its poor heat transfer characteristics makes it quite undesirable. Therefore an improvement needs to be made in the enhancement of heat transfer is solar receiver tubes operating. This study involves the design process of multiple twisted tapes and helically twisted tapes as potential inserts in solar receiver tubes to enhance heat transfer. A modified form of the helically twisted tape called the half pitch helically twisted tape is proposed. Optimization was performed to maximize Nusselt number while minimizing friction factor using the Spherical Quadratic Steepest Descent algorithm. The final optimized design yielded a good Nusselt number and low friction factor, and a final thermal enhancement factor of 0.991.
The organic Rankine cycle (ORC) system can effectively recover waste heat from engines of heavy‐duty trucks, and is a promising method to improve the efficiency of on‐board engines. However, engine operating conditions fluctuate greatly while driving, the waste heat recovery system must often work under off‐design conditions, which significantly affects system performance. Further, different component structures can also affect the off‐design performance of the system. Thus, a novel design method of preheating organic Rankine cycle (P‐ORC) system harvesting waste heat of heavy‐duty trucks based on off‐design performance is proposed in this study. The design method includes selection of the optimal types of components and design point to optimize the comprehensive performance of the waste heat recovery system in all road conditions. In this study, different heat exchanger combinations are applied to the P‐ORC system to obtain six different design systems. According to the principle of uniform coverage, the scatter diagram of exhaust temperature and mass flow rate of the engine under real road conditions are discretized into 19 alternative design points. Each system is designed with 19 discretized design points, and a total of 114 design systems are obtained. The optimal heat exchanger combination and design point are selected based on the off‐design performance. It is concluded that a P‐ORC system using a combination of plate preheater, finned tube air cooler, and shell‐tube evaporator is the optimal system. The optimal design point number is 10, and the corresponding engine speed at DP10 is 1471 rpm, the engine torque is 474 Nm, the occurrence probability is 14.48%, the exhaust temperature is 350°C, the exhaust mass flow rate is 0.11 kg/s, and the maximum combined net power output is 4.26 kW. The results reveal that the optimal design point of the system can be selected at the design point with medium engine load and high occurrence probability. It guides the system design toward a more practical direction, so as to obtain an optimal system that could operate efficiently and recover more waste heat under the full working conditions of the engines. This novel design method can be extended for other cycle configurations.
In this study, mixed convection heat transfer of silver-water nanofluid was experimentally investigated under laminar flow regime inside a horizontal rectangular channel filled with an open-cell copper foam. Thermophysical properties of silver-water nanofluid at 1,000, 2,000, and 4,000 ppm concentrations were also measured. The morphological properties of copper foam were further measured by image processing. At the highest Reynolds number, metal-foam in the channel could increase the heat transfer coefficients of the top and bottom walls 1.6–3.7 times, respectively. Compared to deionized water, 15%, 20%, and 26% enhancement in heat transfer coefficients were achieved at nanofluid concentrations of 1,000, 2,000, and 4,000 ppm, respectively. The heat transfer coefficients of the top and bottom plates were increased by 4.7 and 2.7 time, respectively, at the highest Reynolds number and nanofluid concentration. Finally, two performance coefficients were proposed for the overall evaluation of the simultaneous application of porous-medium and nanofluid.
One way of achieving higher efficiency in electro-mechanical is by inducing vortices over the heated surface with the help of vortex generator (VG). The strength of these vortices is proportionate to the amount of heat transported. In this paper, the evolution and propagation of the produced primary vortex behind a VG with the attached secondary surface (SS) are studied experimentally and numerically. The addition of SS is found to augment heat transfer significantly with an additional drag. The obtained experimental results complement the numerical predictions for the modified VG. Linear regression analysis is performed to optimise the geometry of SS for higher heat extraction rate and lower drag. The SS placed at an optimum location increases Nusselt number on the heated plate by 8.9%, with a decrement in the drag by 3.2%, compared to the reference case. The addition of SS produces a vortex of higher strength and propagates downstream at a slower rate. Moreover, it exposes the vortex to greater shear in the flow, which in turn enhances the heat transfer rate.
The nanofluids have been able to occupy an important place in engineering, in spite of being a young science. While nanoparticles are very effective in increasing heat transfer of base fluids, they cause a significant pressure drop in the flow. In this paper, the effect of different concentrations, 0.1 to 0.4 wt.%, of carbon nanofluid in water have been investigated on the pressure drop of fluid flow over the Reynolds range from 14,000 to 28,000. The variation of pumping power was measured and the corresponding results illustrated increasing in the friction factor of the nanofluid at concentration 0.4 up to 70%, leading to a 68% increase in the pumping power.
A computational solution of an enhanced tube equipped with a backward louvered strip insert with various pitches was evaluated in this work. A k-ε renormalized group turbulence model has been applied for the turbulent model. Different pitches, S, of 40, 50, and 60 mm were investigated for the Reynolds number with a range of 10,000–17,500 using water as a working fluid. The extra louvered strip caused fluid flow disturbance, so that the flow pattern formed more turbulence. The turbulent flow was characterized by the flow pattern on the back of the inserts that form a vortex. The vortices formed caused a better heat transfer. The results of the computational analysis showed that the enhanced tube had a louvered strip with a pitch distance S = 40, 50, and 60 mm could increase the Nusselt numbers to 1.81, 1.75, and 1.72, and the friction factor to 7.59, 6.51, and 5.77 times greater than the plain tube, respectively.
Many of the industrial and engineering problems need to know the value of the convective heat transfer coefficient of fluid flows within various tools, systems, or tubes. In the present study, a predictive formula has been obtained for the Nusselt number for compressible laminar flows passing the velocity developed and thermal developing zone of a hot tube with different angles with the horizon. A fractional factorial experimental design method and an optimization method have been utilized for obtaining a width range and accurate formula. Finally, the ability of this formula has been examined using a set of testing data.
In this study, a numerical simulation has been conducted in order to evaluate the thermal hydraulic performance of a turbulent single-phase flow inside an enhanced tube equipped with a square-cut twisted tape (STT) insert. The classical twisted tape (CTT) insert was also investigated for comparison. The k-ε renormalized group turbulence model has been utilized as the turbulent model. Various twist ratios (y/W) of 2.7, 4.5, and 6.5 were investigated for the Reynolds number range of 8000-18,000, with water as the working fluid. The numerical results indicated that, in comparison with the plain tube (PT), the tube equipped with the STT with the twist ratios of 2.7, 4.5, and 6.5 led to an increase in the values of the Nusselt number and friction factor in the inner tube by 45.4-80.7% and 2.0-3.3 times, respectively; in addition, the highest thermal performance of 1.23 has been obtained. The results further indicated that the tube equipped with the CTT of the same twist ratios improved the Nusselt number and friction factor in the inner tube by 40.3-74.4% and 1.7-3.0 times, respectively, in comparison with the PT; further, the maximum thermal performance of 1.18 was achieved.
A numerical study was performed to investigate the thermal performance characteristics of an enhanced tube heat exchanger fitted with punched delta-winglet vortex generators. Computational fluid dynamics modeling was applied using the k-ε renormalized group turbulence model. Benchmarking was performed using the results of the experimental study for a similar geometry. Attack angles of 30 • , 50 • , and 70 • were used to investigate the heat transfer and pressure drop characteristics of the enhanced tube. Flow conditions were considered in the turbulent region in the Reynolds number range of 9100 to 17,400. A smooth tube was employed for evaluating the increment in the Nusselt number and the friction factor characteristics of the enhanced tube. The results show that the Nusselt number, friction factor, and thermal performance factor have a similar tendency. The presence of this insert offers a higher thermal performance factor as compared to that obtained with a plain tube. Vortex development in the flow structure aids in generating a vortex flow, which increases convective heat transfer. In addition, as the angle is varied, it is observed that the largest attack angle provides the highest thermal performance factor. The greatest increase in the Nusselt number and friction factor, respectively, was found to be approximately 3.7 and 10 times greater than those of a smooth tube. Through numerical simulations with the present simulation condition, it is revealed that the thermal performance factor approaches the value of 1.1. Moreover, the numerical and experimental values agree well although they tend to be different at high Reynolds number conditions. The numerical and experimental values both show similar trends in the Nusselt number, friction factor, and thermal performance factor.
In this paper, the effects of dual twisted tape inserts with different pitches on turbulent heat transfer and pressure drop are numerically investigated. A nanofluid is flowed inside a circular tube, which is under a constant heat flux condition. The Reynolds number varies from 5000 to 20 000 at a fixed Prandtl number of 7. Nine different cases are considered in the current study; three cases consist of a single twisted tape insert, three cases are related to twin twisted tapes with identical pitches, and the remaining cases consist of dual twisted tapes with different pitches for each insert. The predicted results indicate that inserting a dual twisted tape effectively increases the heat transfer 1.5 times more than that of the single insert with the penalty of high pressure drop. Also, the relative Nusselt number decreases with increase in Reynolds number for all the investigated cases. The heat transfer rates induced by dual inserts with different pitch ratios are higher than those with identical pitch ratios. Moreover, the maximum and minimum thermal performances belong to cases with Tr 1 = 2, Tr 2 = 3 and Tr 1 = 2, Tr 2 = 2, respectively. And finally, it is stated here that adding nanoparticles improves the thermal performance of all cases in all the investigated Reynolds numbers.
The impact of double-sided delta-winglet tape (DWTs) inserts on convective heat transfer and friction behaviors in a tube was experimentally investigated. Three DWTs with ratios of winglet-height (b) to inner tube diameter (di) called blockage ratio (Rb) of 0.28, 0.35 and 0.42 were tested and their performance was compared to that of a longitudinal strip and plain tube under similar test flow conditions. Experiments were conducted over a wide range of flow rates, 3.35 × 10−5–8.33 × 10−5 m3/s, which correspond to 5500 ≤ Reynolds number (Re) ≤ 14,500 in the 14.3 mm i.d. tube. The results revealed that using DWTs dramatically increased the Nusselt number (Nu) by as much as 364.3% and the friction factor (f) by 15.5 times compared with those of a plain tube. Thermal performance (η) increased with a corresponding increase in Rb. The highest thermal performance (η) obtained was 1.4. Showing a notable improvement on the thermal performance of the system, DWTs are proposed as a favorable insert device.
The enhancement of convective heat transfer in single-phase heat transfer through the use of helicoidally corrugated tubes has been studied numerically. By comparing the large eddy simulation (LES) results with detailed Stereo-PIV and Liquid Crystal Thermography measurements obtained at the von Karman Institute for Fluid Dynamics (VKI), a validated numerical framework was obtained. Heat transfer enhancements of 83-119% were seen, at the cost of pressure losses that were approximately 5.6 to 6.7 times higher than for a bare tube. In order to extrapolate the results to industrial Reynolds numbers at which experimental data is scarce, the simulation data was used to develop an improved near-wall Reynolds stress transport model (RSTM) that more accurately describes the heat flux vector. Comparison of both global and local flow characteristics at different Reynolds numbers confirms that the approach allows more accurate predictions over a wider range of design and operating parameters than using two-equation turbulence models, while the computational cost is still significantly lower than LES. This article is protected by copyright. All rights reserved.
Large thermal-hydraulic systems computer codes are most often applied to investigate safety issues in existing nuclear facilities. One such code is applied to aid the design process for a proposed state-of-the-art research reactor. The RELAP5 computer code is used to simulate system response to hypothetical loss-of coolant accidents (LOCAs) in an early design of the Advanced Neutron Source (ANS). Among accident scenarios, a LOCA event is expected to be one of the most challenging to the ANS reactor core; similar analyses for other accident types are in progress. This is the first detailed study of ANS transient system response during accidents, and the outcome of the analysis is used to benefit the design process. The ANS model used is based on an early (preconceptual) cool ing system design layout. This early design has since been superseded by an improved design that is partly based on the results of these studies. The calculated responses of the early design to representative LOCA events are described; the simulations indicate that fuel melting and damage would be experienced for medium and large breaks. The effectiveness of employing a gascharged accumulator on the primary coolant system for preventing fuel damage following medium- and large-break LOCAs is evaluated. As a result of this evaluation, the new ANS design incorporates such accumulators. Analysis uncertainties are addressed, and the findings from this study that were used for the next phase of ANS design are highlighted.
Convective heat transfer in the thermally developing region in a circular channel of the first wall and limiter/divertor plates of a fusion reactor has been analyzed numerically. The surface heat flux on a coolant channel in these plasma facing components varies circumferentially. The flow is assumed non-MHD fully-developed laminar and turbulent in a circular tube. The nonuniformity of surface heat flux greatly affect the Nusselt number and thermal entry length. For the cosine distribution of surface heat flux, the steady-state Nusselt number can be reduced at the point of maximum heat flux by as much as 38%, 62% and 37% for fully-developed laminar Poiseuille, laminar slug and turbulent flows, respectively. Thermal entry length can be increased by up to 2.4 times for laminar flow and 3.5 times for turbulent flow due to the nonuniformity of surface heat flux. If this reduction of Nusselt number due to the nonuniformity of surface heat flux is disregarded, the film temperature drop in the coolant channels of plasma facing components of a fusion reactor will be underestimated by 37% to 62%. This will result in an underestimation of the maximum structure temperature. The increase in entry length is not likely to affect the thermal-hydraulic design of a conventional divertor plate.
Currently, technical advances in the area of power electronics have enabled the more frequent use of frequency inverters for compressor speed control in air conditioning refrigerating systems. This reduction in compressor speed, aiming at controlling the cooling capacity, reduces the compression power, increases the evaporating temperature and decreases the condensing temperature, resulting in a higher coe�fficient of performance
(COP) and a smaller fluctuation of the internal ambient temperature.
At the same time, in response to environmental restrictions concerning refrigerants with high GWP (Global Warming Potential), applications using propane (R-290) are becoming more frequent, since R-290 has low
environmental impact and thermodynamic advantages when compared to the most used fluids R-410A and R-22.
The purpose of this study is to develop a computational model that allows the analysis of variable capacity air conditioning systems via the SEER performance factor. Sub-models were created for each component of the air conditioning system, including the connecting lines. The study also contributed with the evaluation of several aspects that permit the charge reduction of R-290 in air-conditioners. With focus on this latter objective, peripheral �ns heat exchangers were explored and applied in the model.
Also, an analysis of the irreversibilities of the system from the second law of themodynamics point of view, considering several geometrical characteristics of heat exchangers and of the compression mechanism was
carried out. In order to make the analysis more realistic, the mixture R-290/POE ISO 22 was considered as working fluid. The thermodynamic properties were obtained from a departure-function approach using the Peng-Robinson cubic equation of state. The oil eff�ects on the heat transfer coeffi�cients and on the pressure drop were also taken into account.
In order to validate the proposed model, a air-conditioner calorimeter was constructed and tested under several ambient temperatures and compressor speed. This experimental facility is proposed as an alternative
test procedure for standardized air-conditioning tests.
To complete the experimental activities, the oil mass flow rate was measured in a compressor evaluation facility adapted for this purpose. These measurements were applied to characterize the compressor pumping oil sub-model. The tests were performed with a high capacity calorimeter using a scroll compressor with same geometrical characteristics simulated in the model, under di�fferent operating conditions.
Good agreement was observed between the computational model and the experimental results. The increase of oil concentration resulted in a decrease of the cooling capacity and an increase of the system power input. Moreover, it has been seen that the use of peripheral fi�ns allow a reduction of the mixture mass presented in the system, despite the decrease of the SEER factor.
This paper presents the results of an experimental investigation conducted for high heat flux subcooled boiling heat transfer and pressure drop in a tubular channel under both smooth- and swirl-flow of high velocity water. High heat flux flow boiling is of interest to Fusion reactor first wall cooling. Test conditions covered a mass flux range from 5 to 10 Mg/m s, inlet temperatures from 100 to 175°C and system pressures from 2.0 to 5.0 MPa. The maximum heat flux tested was 12 MW/m2. The test section diameter used in this study was 5.30 mm (I.D.) with an axial heated length of 356 mm. To ensure accurate results, a significant number of heat balance tests were performed with a minimum and maximum heat balance error of 1.5%. Swirl-flow tests were performed using twisted tape inserts with thickness 0.8 mm with twist ratios between 2 and 4. To measure heat transfer performance, 15 miniature thermocouples were used to measure the tube outside wall temperature at various axial and circumferential positions. Differential pressure transducers were used to measure the axial pressure drop at several locations along the test section under single-and two-phase conditions.
In recent years efforts have been spent in the development of innovative reactors capable of operating with flexible Conversion Ratio (CR). Fast Reactors (FR) are natural candidates since they allow to achieve high CR, as well as an efficient TRU (TRansUranics) burning through a low CR and the closure of the fuel cycle. Among the fast-spectrum systems, a peculiar role is played by the Molten Salt Fast Reactor. This reactor lacks the sound technological basis available for the solid-fuelled liquid-metal-cooled FRs, but it shows fuel cycle potential benefits: it uses Th, which features vast natural resources and mitigates waste management issues due to a low generation of TRUs; it can naturally operate with flexible CR without design modifications thanks to the online reprocessing system; it can achieve high CR, with doubling times of the order of 40 years or lower; it can achieve good TRU-burning rates and very high burning rates of minor actinides. However, such fuel cycle flexibility implies a wide variety of fuel salt compositions. Along with the variation of the fuel salt properties, concerns arise for the varying safety features of the core, especially when using the MSFR as TRU-burner. This work first summarizes results regarding the fuel cycle performances of the MSFR when used as breeder, isobreeder or burner reactor. Subsequently, safety parameters are computed for each fuel cycle option and a simple approach based on reactivity and energy balances is employed to predict the reactor steady-state after major accidental transient initiators, thus giving indications of its inherent safety features for different fuel cycle strategies.
Numerical optimization of a wire-wrapped fuel assembly in liquid metal reactor combining three-dimensional Reynolds-averaged Navier-Stokes analysis with Kriging method as an optimization technique is presented in this work. Two geometric design variables are selected for the optimization, and design space is sampled using Latin Hypercube Sampling. Optimization problem has been defined as a maximization of the objective function, which is as a linear combination of heat transfer and friction loss related terms with a weighing factor. The optimal values of the design variables are obtained by varying the weighting factor.
A hybrid multiobjective evolutionary approach to the design optimization of a seven-pin wire-wrapped fuel assembly is applied to achieve an acceptable compromise between two conflicting objectives: enhancement of heat transfer and reduction of pressure drop. Two non-dimensional variables, the ratio of wire-spacer diameter to fuel rod diameter and the ratio of wire-wrap pitch to fuel rod diameter, are chosen as design variables. The Latin hypercube sampling method is used to determine the training points. The response surface method is used to approximate the Pareto-optimal front with Reynolds-averaged Navier-Stokes analysis of the flow and heat transfer. The shear stress transport turbulence model is used as turbulence closure. The optimization results are processed by the Pareto-optimal method. The Pareto-optimal solutions are obtained using a combination of the evolutionary algorithm NSGA-II and a local search method. The Pareto-optimal front for the wire-wrapped fuel assembly has been obtained. Six optimal values of the design variables have been obtained using clustering. With the increase in the wire-spacer diameter, both heat transfer and pressure drop in the assembly increase. Increasing the wire-wrap pitch reduces the pressure drop in the assembly at the cost of heat transfer.
The steady flow of nanofluids has been analyzed for water and ethylene glycol as base fluids and alumina oxide nanoparticles. Also they were studied in turbulent flow inside a pipe by using fluent and Gambit softwares. The numerical results show a good convergence with previous existing relations. The friction coefficient, pressure drop and viscous drag force increase with increasing the volume fraction of nanoparticles. This increase of course is not more considerable than the base fluid in the low volume fractions but with increasing volume fraction, this increase is significant. Between three alumina oxide nanoparticles AF, AR and AK, the nanofluid containing alumina oxide nanoparticles AF have the highest friction coefficient, viscous drag force and pressure drop and the nanofluid containing alumina oxide nanoparticle AR has the lowest. Because of this reason can be due to more viscous nanofluid containing alumina oxide nanoparticle AF. The increasing the Reynolds number reduces the friction coefficient and increases the pipe wall viscous drag force and the pressure drop. Finally, the use of nanofluids has no a significant impact on the developed velocity field. Introduction The commonly used fluids in heat transfer have low thermal conductivity. Solid particles due to the high conductivity with distribution in the base fluid, increase the thermal conductivity of the fluids, Cooling systems is one of the main concerns in industries such as electronics. With the advancement of technology in industries such as electronics, rapid and large operations with very high speed (multi-GHz) take place, the use of engines with high efficiency and high thermal load will be most important. Therefore, the use of advanced and optimized cooling systems is inevitable. Optimization of existing heat transfer systems, in most cases takes place by increasing their surfaces, which always increases the size of the systems. To overcome this problem, a new and efficient cooling method is required. Conventional heat transfer fluids such as water or ethylene glycol, used in cooling or heating applications are characterized by poor thermal properties. In the past years, many different techniques were utilized to improve the heat transfer rate in order to reach a satisfactory level of thermal efficiency. The heat transfer rate can passively be enhanced by changing flow geometry or by improving thermophysical properties for example, increasing fluid thermal conductivity. One way to enhance fluid thermal conductivity is to add small solid particles in the fluid. Maxwell (1881) was the first to show the possibility of increasing thermal conductivity of a solid-liquid mixture by more volume fraction of solid particles. He used particle of micrometer or millimetre dimensions. Those particles were the cause of numerous problems, such as abrasion, clogging, high pressure drop and poor suspension stability. Therefore, a new class of fluid for improving thermal conductivity and avoiding adverse effects due to the presence of particles is required. To meet these important requirements, a new class of fluids, called nanofluids, has been developed by Choi (1995). Wen and Ding studied the convective heat transfer in the entrance region under laminar regime using aluminium oxide nanofluid in a circular tube with constant heat flux.
Nanofluids, solid-liquid suspensions with solid particles of size of the order of few nanometers, have created interest in many researchers because of their enhancement in thermal conductivity and convective heat transfer characteristics. Many studies have been done on the pool boiling characteristics of nanofluids, most of which have been with nanofluids containing oxide nanoparticles owing to the ease in their preparation. Deterioration in boiling heat transfer was observed in some studies. Metallic nanofluids having metal nanoparticles, which are known for their good heat transfer characteristics in bulk regime, reported drastic enhancement in thermal conductivity. The present paper investigates into the pool boiling characteristics of metallic nanofluids, in particular of Cu-H2O nanofluids, on flat copper heater surface. The results indicate that at comparatively low heat fluxes, there is deterioration in boiling heat transfer with very low particle volume fraction of 0.01%, and it increases with volume fraction and shows enhancement with 0.1%. However, the behavior is the other way around at high heat fluxes. The enhancement at low heat fluxes is due to the fact that the effect of formation of thin sorption layer of nanoparticles on heater surface, which causes deterioration by trapping the nucleation sites, is overshadowed by the increase in microlayer evaporation, which is due to enhancement in thermal conductivity. Same trend has been observed with variation in the surface roughness of the heater as well. [DOI: 10.1115/1.4002597]
This paper describes recent work at Imperial College on the development of an electrohydrodynamically (EHD) enhanced shell/tube condenser for use with fluorocarbon heat transfer fluids. Results are presented for EHD enhanced condensation of Freon 114 and Freon 12 on a single horizontal tube (either smooth or integrally finned) using several electrode geometries. The investigation follows on from fundamental research carried out in the late 1960s and early 1970s which showed that application of electric fields to condensing dielectric vapours could increase heat transfer coefficients, under favourable conditions, by up to ten times. EHD condensation enhancement arises from electric forces acting on a condensate film to which an electric field is applied. Refrigerant-side heat transfer coefficients are shown to be EHD enhanced by a factor of up to 2. 5 on a smooth tube.
An exploratory experimental program was conducted to determine the improvements in heat transfer which could be achieved in the condensation of vapor by the application of electrostatic fields. Freon-113 was utilized as the working fluid in the tests, and condensation of the Freon vapor was produced on a cooled vertical copper plate. A series of electrodes was utilized to vary field strength and geometric configuration of the field, and the particular electrostatic action being studied. Results of the tests indicated that very large increases in heat transfer can be obtained with the use of screen electrodes placed parallel to the cooled copper plate. Increases of 150 percent were achieved which were controllable and readily reproducible.
The condensation of Freon-114 in the presence of a nonuniform, alternating, 60-cycle, electric field was examined experimentally. The condensing surface was a grounded, cooled flat plate, and the electric field was produced by applying a voltage to a second plate placed above the first. Voltages up to 60 kv were imposed, and nonuniformities in the field were created by varying the angle between the plates. Analytical predictions were made of the expected heat-transfer rate, and reasonable agreement with the experimental data was obtained for voltages less than 40 kv. Above 40 kv the results were unpredictable, but increases in the heat-transfer coefficient as high as ten times that for no field were obtained.
With fast growing power consumption and device miniaturization, micro/minichannels are superior to macrochannels or conventional channels for high heat-flux dissipation due to their large surface area to volume ratios and high heat transfer coefficients. However, the associated large pressure drop penalty and flow boiling instability of micro/minichannels hinder their advancement in many practical applications. Therefore, enhancement techniques are required to stabilize the flow and further augment the heat transfer performance in micro/minichannels. This work first presents the classification of micro/minichannels for single-phase flow and flow boiling and gives a general statement of heat transfer enhancement. Then a state-of-the-art overview of the most recent enhancement techniques is specifically provided for further sing-phase flow and flow boiling enhancement in micro/minichannels. Two promising enhancement techniques, i.e., interrupted microfins and engineered fluids with additives are discussed for single-phase flow. For flow boiling, the focus is given on several selected enhancement approaches which can effectively mitigate flow boiling instability and another hot research topic, i.e., nanoscale surface modification. Besides, effects of wettability on bubble dynamics are presented, and a concept of flow-pattern based heat transfer enhancement is proposed. For both single-phase flow and flow boiling enhancement, a special emphasis is on those enhancement techniques with high thermal performance and relatively low pressure drop penalty.
The objective of this study is to find out an optimum surface geometry of vertical condenser tubes where condensation takes place on the outer surfaces. The guiding principle of optimum performance is to make the thickness of condensate liquid on the surface as thin as possible. The vertical tube provided with longitudinally parallel tiny fins is preferable because condensate is made thinner over the widest possible region. It is found by an analysis that four controlling factors for the optimum fin are the sharp leading edge, gradually changing curvature of fin surface from its tip to the root, the wide groove between the fins to collect condensate and the horizontal disc set to the tube to remove condensate. The analytical result is checked by experiments using R-113. The optimum fin shape, fin pitch and spacing of discs are found by numerical calculations for R-113 and water.
An experimental investigation was performed for single-phase flow and condensation characteristics inside five micro-fin tubes with the same outer diameter 5mm and helix angle 18°. Data are for mass fluxes ranging from about 200 to 650kg/m2s. The nominal saturation temperature is 320K, with inlet and outlet qualities of 0.8 and 0.1, respectively. The results suggest that Tube 4 has the highest condensation heat transfer coefficient and also the highest condensation pressure drop penalty, while Tube 5 has the highest enhancement ratio due to its lowest pressure drop penalty and intermediate heat transfer coefficient. Condensation heat transfer coefficient flattens out gradually as G decreases when G
Existing database in literature on the adiabatic two-phase frictional pressure drop in evaporative micro/mini-channels were reviewed. The collected database contains 769 data points, covering 12 fluids, for a wide range of operational conditions and channel dimensions. The whole database was analyzed using five existing correlations to verify their respective accuracies. The importance of the Bond number, which relates the nominal bubble dimension or capillary parameter with the channel size, was revealed. A particular trend was observed with the Bond number that distinguished the entire database into three ranges. Using the Bond number, improved correlations of adiabatic two-phase pressure drop were established for small Bond number regions. The newly proposed correlations can predict the database well for the region where BoRel0.5⩽200.
An experimental investigation was performed for convective vaporization of R22 and R410A inside one smooth tube and five micro-fin tubes with the same outer diameter of 5 mm. Data are for mass fluxes ranging from 100 to 620 kg/m2 s at 279 K saturation temperature. The results suggest that the tube with fin height of 0.15 mm, apex angle of 25° and 38° starts has the best thermal performance for convective vaporization when mass velocity is less than 400 kg/m2 s, while the tube with fin height of 0.12 mm, apex angle of 25° and 58° starts has the best heat transfer performance at larger mass velocities, which is probably due to the relative size between fin height and liquid film thickness. Considering the effects of micro-fin on flow boiling, a new general semi-empirical model has been developed based on the present data and recent data from literature. The new model is applicable for intermittent and annular flow patterns, covering different fluids, nominal diameters from 2.1 to 14.8 mm, mass fluxes from 100 to 650 kg/m2 s, heat fluxes based on the total inner surface area from 0 to 30 kW/m2, and reduced pressure from 0.08 to 0.69. The model predicts the parametric trends correctly and the average and local heat transfer coefficients accurately. The heat transfer mechanism can also be observed clearly by the new model.
The enhancement of condensation heat transfer is of practical
importance in many industries such as HVAC, power, and aerospace. The
present work is concerned with the electrohydrodynamic (EHD) enhancement
of external condensation on single commercial enhanced tubes. Future
phases of this work will include applying the technique to practical
heat exchangers and developing a prototype EHD condenser. Single-tube
experiments were performed on two types of enhanced tubes. The
refrigerant was R-134a, with the EHD voltage in the range 0 to 25 kV,
saturation temperature 10 to 40°C, and heat flux 10 to 40
kW/(m<sup>2</sup>K). The results indicated that the external heat
transfer coefficient significantly increased under the effect of
electrical field. The optimum heat transfer enhancement was
approximately 3-fold for either tube, with respective EHD power
consumption lower than 1% of the test section heat transfer rate