No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Parameter optimization of the composite honeycomb tip in a turbine cascade
Abstract
The geometric parameters of the composite honeycomb tip have been extracted and optimized using Kriging model and genetic algorithm numerically. A total of 36 sets of input-response combination are included from full factor design. The Kriging model for the tip leakage mass flow rate is established and assessed by leave-one-out cross validation. The computation of the optimal composite honeycomb tip case has been completed to affirm the mathematical precision. Moreover, the tests of flat tip, normal honeycomb tip and optimal composite honeycomb tip were carried out in a low-speed wind tunnel. The results show that the combination of Kriging model and genetic algorithm is suitable for the present geometry optimization of the composite honeycomb tip. The optimal composite honeycomb tip reduces the tip leakage mass flow rate by up to 16.81%. In detail, the effects of these geometric parameters are discussed using the surrogate database. The pressure field on the casing is employed to analyze the blocking effect of these cavities on the tip leakage flow. The pressure distributions around the blade surface are also captured. Besides, the downstream secondary velocity streamlines and loss contours are compared. The simulated averaged loss is reduced by 5.49% in the optimal case.
... The largescale vortices were supported by the injection in deep cavities, the vortices were limited to one side of the cavity in shallow cavities, and the least leakage was generated by the injection with the downstream side and the small injection angle. Wang et al. 32 optimized the composite honeycomb tip. The optimization variables were the cavity depth, height ratio, and length ratio of the honeycomb prism, and the simulated averaged loss was reduced by 5.49% in the optimal case. ...
... The basic tip honeycomb was taken from the optimized composite honeycomb structure of Wang. 32 The upper hexagonal prism and bottom hexagonal length ratio b bot /b tip , the frustum height δ 2 and cavity depth δ hc ratio δ 2 /δ hc , and the honeycomb cavity depth δ hc are all optimized and verified. The tip honeycomb parameters and blade profile are amplified in the same ratio. ...
... The advantage of the k-ω turbulence model is the near-wall treatment at a low Reynolds number, which is more accurate for leakage flow prediction. 32,36 The simulations were performed on a computer with an AMD 3900X CPU and 64 GB of 2400-MHz memory, and the total time for solving a single case is approximately 5 h. ...
To reduce the secondary flow loss of the high-pressure turbine caused by the tip leakage flow, the design of the composite honeycomb tip has been experimentally and numerically examined in this study. The arrangement angle of the composite honeycomb is defined and optimized using the radial basis function and genetic algorithm. The tip flow field of the high-pressure turbine blade with a 2.128 mm clearance is simulated by CFX 18.0, and the arrangement angle of the composite honeycomb is optimized to obtain a lower loss. Moreover, the flat tip, basic honeycomb tip, and optimized honeycomb tip are tested in a low-speed cascade test facility. The results show that the vortices and radial velocity induced by the honeycomb structure cavity increase the kinetic energy loss of the leakage flow and reduce the leakage flow rate. Compared with the basic honeycomb, the optimized honeycomb has a supplemental blocking effect on the tip leakage flow by reducing the crosswise pressure gradient in the blade tip. The optimized honeycomb also changes the outlet angle of the leakage flow in the tip clearance, reduces the leakage vortex, and the total pressure loss is further reduced by 2.5%.
... Efficient and accurate aerodynamic prediction is a prerequisite for turbomachinery design and optimization [1][2][3]. Generally, the flow field over a turbomachine is obtained by Computational Fluid Dynamics (CFD) techniques. ...
... However, it is time-consuming and computationally expensive to perform CFD simulations in aerodynamic design and optimization, where there are requirements of quickly evaluating the performance of multiple designs based on detailed flow fields 3 and making adjustments accordingly. Therefore, there is a need for developing a cost-effective and accurate method, which can obtain a detailed flow field more efficiently than the conventional CFD simulation without much compromising the accuracy. ...
The main challenge of establishing a model to predict the flow fields of turbomachinery was insufficient data. This study aimed to establish a generalizable and accurate model on a small-scale dataset to cost-effectively predict the surface pressure distribution of a turbine rotor cascade with widely varying geometries and boundary conditions. To meet this purpose, a novel concept of transfer learning was introduced, which was defined as transferring knowledge from a large-scale but low-fidelity dataset to a small-scale but high-fidelity dataset. A Conditional Generative Adversarial Neural Network was designed as the pre-trained network for the transfer learning to regress the surface pressure distributions. Two models transferred from datasets with different fidelity and an independent model were established and compared in detail. The results showed that the proposed method successfully reduced the modeling cost with a low error in predicting the surface pressure distributions. The model transferred from the higher-fidelity dataset had better generalization performance, which reduced the root mean square error and modeling cost by 40.2% and 9 times, respectively. The presented method could serve as a base framework for modeling surface pressure distribution of complex objects using a small-scale dataset.
... Passive flow control methods can be classified into blade treatment techniques, which directly modify the blade profile, and casing treatment techniques, which modify the casing profile, based on the installation location of the control equipment. Blade treatment techniques, such as shrouded tips, 15,16 winglet tips, 17 squealer tips, [18][19][20][21][22] and honeycomb tips, [23][24][25][26] have been widely researched and have shown promise in enhancing the aero-thermal performance. However, directly changing the blade profile can have a significant impact on the safety of structure and take the risk of damaging the expensive blade. ...
In order to investigate the impact of design parameters (depth, width, and location) of the single-circumferential groove (SCG) on tip leakage flow (TLF) reduction in a transonic turbine stage, a systematic investigation was conducted by high-precision numerical simulation method. The numerical results show that the introduction of SCG could improve the stage efficiency (η) of turbine significantly. The optimal design parameters of depth coefficient (ζ), width (W), and location (L) were ζ = 3, W = W3, and L = 20%Cax (where Cax denotes the axial chord of the blade), which could reduce tip clearance leakage rate (m*) by 28.9% and improve the η by 0.56%. The implementation of flow field display methods reveals that the introduction of SCG influences the formation and development of the vortex system at the rotor tip region. The circumferential groove vortex (CGV), induced by SCG, divided the tip leakage vortex (TLV) into two segments TLV_1 and TLV_2 due to its blocking effect. TLV_1 interacted with the passage vortex (PV) and depress the strength of PV. Meantime, TLV_2 would regenerate from the trailing edge of SCG, due to its limited development range and insufficient growth, the strength and scale of TLV_2 were weakened. Given that the TLF passing through SCG flows in an opposite direction to the original TLF, the SCG at appropriate location could lead to a fact that the net flow rate of the leakage flow reached the minimum value, thereby considerably reducing the m* and flow loss caused by TLF.
... It is a very powerful tool to deal with the multi-objective optimization problems, and has been widely applied to solve practical optimization issues in different industrial fields. For example, Wang et al. [26] successfully optimized the geometric parameters of the composite honeycomb tip with the help of GA. By combining the response surface methodology (RSM) with the GA, Udayakumar et al. [27] developed a multi-objective optimization method to optimize the process parameters in friction welding process. ...
In this study, artificial neural network (ANN) was adopted to predict the quality of SPR joints. Three ANN models were developed respectively for the key joint quality indicators: the interlock, the remaining bottom sheet thickness at the joint center (Tcen) and under the rivet tip (Ttip). Experimental SPR tests were performed and the results verified the high prediction accuracy of the ANN models. The mean absolute errors (MAE) between the experimental and prediction results for the interlock, Tcen and Ttip reached 0.058mm, 0.075mm and 0.059mm respectively, and the corresponding mean absolute percentage errors (MAPE) were 14.2 %, 22.4 % and 10.9 %. Moreover, two innovative approaches were proposed to simplify the selection of rivet and die for new joint designs. One was realized by combining the genetic algorithm (GA) with the ANN models, and can generate optimal rivet and die combinations for different joint quality standards. The second was achieved by plotting application range maps of different rivet and die combinations with the help of ANN models, and can quickly select the suitable and accessible rivet and die. Furthermore, interaction effects between different joining parameters on the joint quality were also discussed by analyzing the contour graphs plotted with the ANN models.
Experimental studies are very important in designing of a new product. In the aerodynamic designs, wind tunnel tests are commonly used in experimental studies. In the experimental studies accuracy is the most critical point to verify the results. Laminar flow is preferred at test section to obtain accurate results. Turbulence intensity is the main drawback of the wind tunnel tests. Hence in this study, turbulence intensity of the hexagonal, square and circular sectioned honeycomb is investigated. In the numerical studies, a circular sectioned subsonic wind tunnel is used. The computational fluid dynamic (CFD) analysis of three-dimensional (3D) flow in a circular sectioned desktop size wind tunnel is used for comparison.
Two novel tip modeling methods are proposed in this paper. One novel tip is established by the mathematical method and the other is generated by the B-spline surface. And the genetic algorithm, the orthogonal design and the Kriging model are applied to optimize two kinds of novel tips numerically. The optimal mathematical tip and the optimal B-spline tip are acquired respectively. The aerodynamic tests were investigated at the wind tunnel test section, including the flat tip, the squealer tip, the optimal mathematical tip and the optimal B-spline tip. The static pressure distributions on the casing and the blade surface were measured by the pressure taps. Meanwhile, the secondary flow and the loss were obtained by the five-hole probe at the exit section. A comparison of the simulation analysis results with the experimental results shows good agreement. Compared with the flat tip, the measured loss on the squealer tip, the optimal mathematical tip and the optimal B-spline tip is decreased by 7.2%, 10.5% and 12.6% respectively.
Parametric optimization of sandwich composite footbridge was presented in the paper. Composite footbridge has 14,5 meters long and has U-shaped cross-section with inner dimensions 2,6 x 1,3 meters. The aim of analysis was to minimize the mass of the new footbridge that can lead to minimize the cost of structure. After optimization was conducted, the new structure was compare with the realized one. The results show that while mass can be decreased the state variables of structure like maximum displacement, strain or natural frequency do not change significantly. Moreover, results show that, although the mass is decreased, stiffness of the new structure is even higher.
Optimization and sensitivity analysis, the divisions of theory of designing, provide the designer a significant support. Despite presented optimization, also sensitivity analysis can additionally be effectively used in issues related to strengthening or modernizing structures as well as in the process of identifying quantities describing the computational model.
Buildings account for a substantial proportion of global energy consumption and global greenhouse gas emissions. Given the growth in smart devices and sensors there is an opportunity to develop a new generation of smarter, more context aware, building controllers. Therefore, in this work, surrogate, zone-level artificial neural networks that take weather, occupancy and indoor temperature as inputs, have been created. These are used as an evaluation engine by a genetic algorithm with the aim of minimising energy consumption. Bespoke 24-h, heating set point schedules are generated for each zone in a small office building in Cardiff, UK. The optimisation strategy can be deployed in two modes, day ahead optimisation or as model predictive control which re-optimises every hour. Over a February test week, the optimisation is shown to reduce energy consumption by around 25% compared to a baseline heating strategy. When a time of use tariff is introduced, the optimisation is altered to minimise cost rather than energy consumption. The optimisation strategy successfully shifts load to cheaper price periods and reduces energy cost by around 27% compared to the baseline strategy.
Tip leakage loss reduction is important for improving the turbine aerodynamic performance. In this paper, the flow field of a transonic high pressure turbine stage with a squealer tip is numerically investigated. The physical mechanism of flow structures inside the cavity that control leakage loss is presented, which is obtained by analyzing the evolution of the flow structures and its influence on the leakage flow rate and momentum at the gap outlet. The impacts of the aerodynamic conditions and geometric parameters, such as blade loading distributions in the tip region, squealer heights, and gap heights, on leakage loss reduction are also discussed. The results show that the scraping vortex generated inside the cavity is the dominant flow structure affecting turbine aerodynamic performance. An aero-labyrinth liked sealing effect is formed by the scraping vortex, which increases the energy dissipation of the leakage flow inside the gap and reduces the equivalent flow area at the gap outlet. The discharge coefficient of the squealer tip is therefore decreased, and the tip leakage loss is reduced accordingly. Variations in the blade loading distribution in the tip region and the squealer geometry change the scraping vortex characteristics, such as the size, intensity, and its position inside the cavity, resulting in a different controlling effect on leakage loss. By reasonable blade tip loading distribution and squealer tip geometry for organizing scraping vortex characteristics, the squealer tip can improve the turbine aerodynamic performance effectively.
The blade-tips of high-pressure turbine blades in a gas turbine engine are subjected to strong convective heat transfer and continued to present a significant design challenge to manufacturers. This paper is concerned with developing an understanding of the unsteady flow physics that influences the blade-tip heat transfer. Experimental investigations of bladetip heat transfer and aerodynamics have been conducted in a transonic turbine stage test facility. The data reveal the effect of vane-rotor interactions on the unsteady heat transfer along the blade-tip mean camber line. In particular, the vane shock and potential field interaction establish characteristic unsteady heat transfer signatures at different axial positions along the blade-tip. The fluctuations in heat transfer are discussed in terms of vane-periodic changes in both relative total temperature and aerodynamic conditions.
The effect of the cooling injection orientation and position on the tip leakage flow has been studied numerically in a honeycomb-tip turbine cascade. The injected fluid enters each honeycomb cavity through the bottom pipe at a fixed incident angle. The effect of the relative casing motion with six casing speeds is considered. The Reynolds-averaged Navier-Stokes (RANS) method and k-ω turbulence model were used in the three dimensional computations. Then the leakage mass flow rate and the averaged total pressure loss are evaluated. The static pressure and limiting streamlines on the casing are discussed and the flow direction around the gap are extracted to precisely analyze the leakage feature. Besides, the thermal conditions on the blade tip and suction surface are described by the Nusselt number and adiabatic film cooling effectiveness. Finally, the dimensionless temperature contours and velocity vectors inside the gap are plotted to explore the effect of the injection position. The results indicates that the injection feature mainly depends on the injection orientation and finally determines the aerodynamic and thermodynamic performance.
Aerodynamic loss measurements and flow visualizations have been conducted for a squealer tip with different winglets. The present results for seven winglets indicate that winglet coverage along the pressure side has a negative effect in the loss reduction, whereas winglet coverage along the suction side has a positive effect. Winglet coverage along both sides performs better than that along the pressure side but worse than that along the suction side. The upstream half of the suction-side winglet plays a crucial role in the loss reduction. Thus, this portion needs to be included in the winglet for better aerodynamic performance. The pressure-side winglet brings additional flow disturbances similar to the ones existing over the plane tip back to the tip gap flow over the squealer tip. For the suction-side winglet, however, the tip gap flow is separated from the casing in a wide range between the leading edge and mid-chord.
The influence of different arbitrary blade tip shapes on restraining the tip leakage flow in a highly loaded turbine cascade has been numerically studied. A combined method of establishing and optimizing the arbitrary blade tip shape is proposed by using B-spline surface modeling, Kriging model and genetic optimization algorithm. The results show that the Kriging model established by the B-spline surface modeling method can accurately fit the relationship between the arbitrary blade tip shape and the relevant aerodynamic parameters. The optimal leakage mass flow tip and the optimal total pressure loss tip obtained by genetic algorithm both have strong inhibitory effects on tip leakage flow. Compare to the flat tip at 1%H gap height, the tip leakage mass flow of the optimal leakage mass flow case and the optimal total pressure loss case decrease by 11.14% and 10.23%, respectively, the area-average total pressure loss at exit section is reduced by 8.08% and 7.41%, respectively.
Two innovative composite tip structures that combine a honeycomb with a prismoid or spherical bottom have been proposed to actively control the tip leakage flow in a turbine cascade. The mechanisms of inhibiting leakage flow are mainly analyzed from the aspects of detailed flow field, vortex structures, and turbulent kinetic energy dissipation. The influence of geometric parameters like depth and dihedral angle of the prismoid and depth of spherical bottom are discussed. The results show that both composite structures can enhance the vortices generated in cavities and intensify the mixing between honeycomb jets and upper clearance flow, thus achieving better control effect. For a honeycomb-prismoid tip, a 45° dihedral angle performs best, and the control effect is obviously weakened when the depth is greater than 1.5 times that of the honeycomb. A honeycomb-spherical bottom tip reduces the vortex development resistance, which benefited from the smooth transition of the spherical bottom. Its control effect is parabolic with the depth of spherical bottom, and the optimum depth is one-third of the diameter of its corresponding inscribed ball. Compared with a flat tip, a honeycomb tip, honeycomb-prismoid tip, and honeycomb-spherical bottom tip reduce the tip leakage flow by 11.11%, 13.77%, and 15% as well as the loss by 8.17%, 9.4%, and 10.42%.
The effects of the cooling injection with various injection positions and angles on the tip leakage flow have been numerically studied in an axial turbine cascade with honeycomb tip. The coolant is injected through the bottom hole into each honeycomb cavity and coupled with the internal vortices to obstruct the leakage flow. Tip leakage mass flow rate and total pressure loss coefficient at the cascade exit are used to evaluate the aerodynamic performance of the tip design. Nusselt number and adiabatic film cooling effectiveness are chosen for the thermodynamic evaluation. Numerical results show that the injection position and angle play an important role in affecting aerodynamic and thermodynamic performance, especially for the injection positioned near downstream sidewall or with small injection angle in the center. Compared with the flat tip, tip leakage mass flow rate drops by up to 26.7%, and total pressure loss coefficient is reduced by about 4.6%. Furthermore, the tip leakage vortex approaches the suction surface, resulting in reduced affected area and weakened loss strength. Meanwhile, the heat transfer performance of the honeycomb tip is improved, especially for the inclined injection with small injection angle in the bottom center.
In this paper, the effect of a novel honeycomb tip on suppressing tip leakage flow in turbine cascade has been experimentally and numerically studied. Compared to the flat tip cascade with 1%H blade height, the relative leakage flow in honeycomb tip cascade reduces from 3.05% to 2.73%, and the loss also decreases by 8.24%. For honeycomb tip, a number of small vortices are rolled up in the regular hexagonal honeycomb cavities to dissipate the kinetic energy of the clearance flow, and the fluid flowing into and out the cavities create aerodynamic interceptions to the upper clearance flow. As a result, the flow resistance in the clearance increased and the velocity of leakage flow reduced. As the gap height increases, the tip leakage flow and loss changes proportionally, but the growth rate in the honeycomb tip cascade is smaller. Considering its wear resistance of the honeycomb seal, a smaller gap height is allowed in the cascade with honeycomb tip, and that means honeycomb tip has better effect on suppressing leakage flow. Part honeycomb tip structure also retains the effect of suppressing leakage flow. It shows that locally convex honeycomb tip has better suppressing leakage flow effect than the whole honeycomb tip, but locally concave honeycomb tip is slightly less effective.
In this paper, the effect of a novel honeycomb tip on suppressing tip leakage flow in a highly-loaded turbine cascade has been experimentally and numerically studied. The research focuses on the mechanisms of honeycomb tip on suppressing tip leakage flow and affecting the secondary flow in the cascade, as well as the influences of different clearance heights on leakage flow characteristics. In addition, two kinds of local honeycomb tip structures are pro-posed to explore the positive effect on suppressing leakage flow in simpler tip honeycomb structures.
Based on the experimental and numerical results, the physical processes of tip leakage flow and its interaction with main flow are analyzed, the following conclusions can be obtained. Honeycomb tip rolls up a number of small vortices and radial jets in regular hexagonal honeycomb cavities, increasing the flow resistance in the clearance and reducing the velocity of leakage flow. As a result, the structure of honeycomb tip not only suppresses the leakage flow effectively, but also has positive effect on reducing the associated losses in cascade by reducing the strength of leakage vortex. Compare to the flat tip cascade at 1%H gap height, the relative leakage flow in honeycomb tip cascade reduces from 3.05% to 2.73%, and the loss at exit section is also decreased by 10.63%. With the increase of the gap height, the tip leakage flow and loss have variations of direct proportion with it, but their growth rates in the honeycomb tip cascade are smaller. Consider the abradable property of the honeycomb seal, a smaller gap height is allowed in the cascade with honeycomb tip, and that means honeycomb tip has better effect on suppressing leakage flow. Two various local honeycomb tip structures has also been discussed. It shows that local raised honeycomb tip has better suppressing leakage flow effect than honeycomb tip, while local concave honeycomb tip has no more effect than honeycomb tip. Compare to flat tip cascade, the leakage flow in honeycomb tip cascade, local concave tip cascade and local raised honeycomb tip cascade decrease by nearly 17.33%, 15.51% and 30.86% respectively, the losses at exit section is reduced by 13.38%, 12% and 28.17% respectively.
This paper presents a continued study on a previously investigated winglet-shroud (WS) geometry for a linear turbine cascade. A plain tip and a full shroud tip are experimentally and numerically examined as the datum cases. Various width (w) of double-side winglets (DSW) involving 3%, 5%, 7% and 9% of the blade pitch (p) is numerically investigated. It is observed that the DSW cases do not alter the flow fields including the separation bubble and reattachment flow within the tip gap region, even for the case with the broadest width (w/p = 9%). Meanwhile, the horse-shoe vortex near the casing is not generated even for the case with the smallest width (w/p=3%). Larger width of the DSW geometry is indeed able to improve the aerodynamic performance, but in a slight degree. With the w/p increasing from 3% to 9%, the mass-averaged total pressure loss coefficient over an exit plane is just reduced by 2.61%. Therefore, a favorable width of w/p=5% is chosen to design the WS structure. Based on this, three locations of the partial shroud (linkage segment) are devised, which are located near the leading edge, the middle and close to the trailing edge respectively. Results illustrate that all three WS cases have advantages in lessening the aerodynamic loss over the DSW arrangement, but with the linkage segment located in the middle having optimal effect. This conclusion verifies the feasibility of the previously studied WS configuration.
Numerical investigations have been performed to investigate the heat transfer characteristic on the squealer–winglet tip, which is also compared with the conventional squealer tip and squealer tip without pressure-side rim. The validations and verifications of the numerical methods are carried out based on the existing experimental data. At a range of winglet leading edge angles and winglet coverage percentages, the total pressure losses in the cascade, as well as the flow structures in the tip gap, are analyzed in detail. The numerical results show that, by removing the pressure side rim from the conventional squealer tip, the high heat transfer area on the tip cavity floor near leading edge has disappeared, and the total pressure loss in the cascade has been reduced. If the conventional squealer tip is modified into a full-coverage of pressure side winglet configuration, the area-averaged total pressure loss in the cascade can be reduced by 12.2%, and the area-averaged heat transfer coefficient on the tip can be reduced by about 10% (PR = 1.236). For the squealer tip configured with pressure-side winglet, the heat transfer distributions as well as the total pressure loss in the blade are not sensitive to the winglet leading edge angle. By choosing proper winglet leading edge angle and winglet coverage percentage, the area-averaged heat transfer coefficient on the tip can be reduced by 15.8%, and the area-averaged total pressure loss in the blade can be reduced by 13.8%, in contrast to the conventional squealer tip.
Superior rotor tip geometries possess the potential to simultaneously mitigate aerodynamic losses and severe thermal loads onto the rotor overtip region. However, classical design strategies are usually constrained to a specific type of geometry, narrowing the spread of shape topologies considered during the design phase. The current paper presents two novel multi-objective optimization methodologies that enable the exploration of a broad range of distinct tip configurations for unshrouded rotor blades. The first methodology is a shape optimization process that creates a fully carved blade tip shape defined through a Bezier surface controlled by 40 parameters. Combined with a differential evolution (DE) optimization strategy, this approach is applied to a rotor blade for two tip gap sizes: 0.85% (tight) and 1.38% (design) of the blade span. The second methodology is based on a topology optimization process that targets the creation of arbitrary tip shapes comprising one or multiple rims with a fixed height. The tip section of the blade has been divided into more than 200 separate zones, where each zone can be either part of an upstanding rim or part of the cavity floor. This methodology was tested with a level-set approach in combination with a DE optimizer and coupled to an optimization routine based on genetic algorithms (GAs). The current study was carried out on a modern high-pressure turbine operating at engine-like Reynolds and high subsonic outlet Mach numbers. A fully hexahedral unstructured mesh was used to discretize the fluid domain. The aerothermal performance of each tip profile was evaluated accurately through Reynolds-averaged Navier-Stokes (RANS) simulations adopting the shear-stress transport (SST) turbulence model. Multi-objective optimizations were set for both design strategies that target higher aerodynamic rotor efficiencies and simultaneous minimization of the heat load. This paper illustrates a wide variety of profiles obtained throughout the optimization and compares the performance of the different strategies. The research shows the potential of such novel methodologies to reach new unexplored types of blade tip designs with enhanced aerothermal performances.
The effects of hst/s (squealer rim height-to-span ratio) on heat/mass transfer rate on the floor of the cavity squealer tip have been investigated in a high turning turbine blade cascade by employing the naphthalene sublimation technique along with oil film flow visualizations. Tested squealer rim heights are hst/s = 0.00% (plane tip), 0.94%, 1.88%, 3.75%, and 5.63% for a tip gap height-to-span ratio of h/s = 1.02%. For comparison purpose, the tip gap height is changed from h/s = 0.34% to 1.70% for hst/s = 3.75%. Heat/mass transfer rate on the cavity floor upstream of the mid-chord is affected by the leading edge tip gap vortices as well as by the reattachment of the incoming tip leakage flow to the cavity floor for lower hst/s, as in the plane tip case, whereas it is influenced mainly by the impingement of the incoming tip leakage flow onto the cavity floor near the leading edge for higher hst/s. On the other hand, heat/mass transfer rate downstream of the mid-chord is determined by the downwash flow which is entrapped by the suction-side squealer rim. With increasing hst/s, average heat/mass transfer rate on the cavity floor decreases steeply at first and then decreases mildly in the same manner as over-tip leakage loss. Average heat/mass transfer rate on the cavity floor is more sensitive to the squealer rim height than to the tip gap height.
This article reviews the design and analysis of simulation experiments. It focusses on analysis via two types of metamodel (surrogate. emulator); namely, low-order polynomial regression, and Kriging (or Gaussian process). The metamodel type determines the design of the simulation experiment, which determines the input combinations of the simulation model. For example, a first-order polynomial regression metamodel should use a ”resolution-III” design, whereas Kriging may use ”Latin hypercube sampling”. More generally, polynomials of first or second order may use resolution III, IV, V, or ”central composite” designs. Before applying either regression or Kriging metamodeling, the many inputs of a realistic simulation model can be screened via ”sequential bifurcation”. Optimization of the simulated system may use either a sequence of low-order polynomials—known as ”response surface methodology” (RSM)—or Kriging models fitted through sequential designs—including ”efficient global optimization” (EGO). Finally, ”robust” optimization accounts for uncertainty in some simulation inputs.
A comparative experimental and numerical analysis is carried out to assess the aerodynamic performance of a novel partial shroud in a straight turbine cascade. This partial shroud is designed as a combination of winglet and shroud. A plain tip is employed as a baseline case. A pure winglet tip is also studied for comparison. Both experiments and predictions demonstrate that this novel partial shroud configuration has aerodynamic advantages over the pure winglet arrangement. Predicted results show that, relative to the baseline blade with a plain tip, using the partial shroud can lead to a reduction of 20.89% in the mass-averaged total pressure coefficient on the upper half-span of a plane downstream of the cascade trailing edge and 16.53% in the tip leakage mass flow rate, whereas the pure winglet only decreases these two performance parameters by 11.36% and 1.32%, respectively. The flow physics is explored in detail to explain these results via topological analyses. The use of this new partial shroud significantly affects the topological structures and total pressure loss coefficients on various axial cross sections, particularly at the rear part of the blade passage. The partial shroud not only weakens the tip leakage vortex (TLV) but also reduces the strength of passage vortex near the casing (PVC) endwall. Furthermore, three partial shrouds with width-to-pitch ratios of 3%, 5%, and 7% are considered. With an increase in the width of the winglet part, improvements in aerodynamics and the tip leakage mass flow rate are limited.
Tip leakage flows in unshrouded high speed turbines cause large aerodynamic penalties, induce significant thermal loads and give rise to intense thermal stresses onto the blade tip and casing endwalls. In the pursuit of superior engine reliability and efficiency, the turbine blade tip design is of paramount importance and still poses an exceptional challenge to turbine designers. The ever-increasing rotational speeds and pressure loadings tend to accelerate the tip flow velocities beyond the transonic regime. Overtip supersonic flows are characterized by complex flow patterns, which determine the heat transfer signature. Hence, the physics of the overtip flow structures and the influence of the geometrical parameters require further understanding to develop innovative tip designs. Conventional blade tip shapes are not adequate for such high speed flows and hence, potential for enhanced performances lays in appropriate tip shaping. The present research aims to quantify the prospective gain offered by a fully contoured blade tip shape against conventional geometries such as a flat and squealer tip. A detailed numerical study was conducted on a modern rotor blade (Reynolds number of 5.5 x 10(5) and a relative exit Mach number of 0.9) by means of three-dimensional (3D) Reynolds-averaged Navier-Stokes (RANS) calculations. Two novel contoured tip geometries were designed based on a two-dimensional (2D) tip shape optimization in which only the upper 2% of the blade span was modified. This study yields a deeper insight into the application of blade tip carving in high speed turbines and provides guidelines for future tip designs with enhanced aerothermal performances.
Superior rotor tip geometries possess the potential to simultaneously mitigate aerodynamic losses and severe thermal loads onto the rotor overtip region. However, classical design strategies are usually constrained to a specific type of geometry, narrowing the spread of shape topologies considered during the design phase. The current paper presents two novel multi-objective optimization methodologies that enable the exploration of a broad range of distinct tip configurations for unshrouded rotor blades.
The first methodology is a shape optimization process that creates a fully carved blade tip shape defined through a Bezier surface controlled by 40 parameters. Combined with a differential evolution optimization strategy, this approach is applied to a rotor blade for two tip gap sizes: 0.85% (tight) and 1.38% (design) of the blade span. The second methodology is based on a topology optimization process that targets the creation of arbitrary tip shapes comprising one or multiple rims with a fixed height. The tip section of the blade has been divided into more than 200 separate zones, where each zone can be either part of an upstanding rim or part of the cavity floor. This methodology was tested with a level-set approach in combination with a differential evolution optimizer, and coupled to an optimization routine based on genetic algorithms.
The current study was carried out on a modern high-pressure turbine operating at engine-like Reynolds and high subsonic outlet Mach numbers. A fully hexahedral unstructured mesh was used to discretize the fluid domain. The aerothermal performance of each tip profile was evaluated accurately through RANS simulations adopting the SST turbulence model. Multi-objective optimizations were set for both design strategies that target higher aerodynamic rotor efficiencies and simultaneous minimization of the heat load.
This paper illustrates a wide variety of profiles obtained throughout the optimization and compares the performance of the different strategies. The research shows the potential of such novel methodologies to reach new unexplored types of blade tip designs with enhanced aerothermal performances.
In this paper, the effect of cooling injection on the aerodynamics of tip flows in transonic turbines is investigated. Experiments are performed using an idealized model of a transonic tip flow. Schlieren photography, probe, and surface pressure measurements are used to determine the transonic tip flow structure and to validate the computational method. Computational simulations are performed to investigate the effects of cooling injection in a transonic blade environment. The results show that cooling injection has the potential to reduce overtip leakage loss.
This paper investigates the design of winglet tips for unshrouded high pressure turbine rotors, considering aerodynamic and thermal performance simultaneously. A novel parameterization method has been developed to alter the tip geometry of a rotor blade. A design survey of un-cooled, flat-tipped winglets is performed using RANS calculations for a single rotor at engine representative operating conditions.
Compared to a plain tip, large efficiency gains can be realized by employing an overhang around the full perimeter of the blade, but the overall heat load rises significantly. By employing an overhang on only the early suction surface, significant efficiency improvements can be obtained without increasing the overall heat transfer to the blade.
The flow physics are explored in detail to explain the results. For a plain tip, the leakage and passage vortices interact to create a three-dimensional impingement onto the blade suction surface, causing high heat transfer. The addition of an overhang on the early suction surface displaces the tip leakage vortex away from the blade, weakening the impingement effect and reducing the heat transfer on the blade. The winglets reduce the aerodynamic losses by unloading the tip section, reducing the leakage flow rate, turning the leakage flow in a more streamwise direction and reducing the interaction between the leakage fluid and endwall flows. Generally these effects are most effective close to the leading edge of the tip, where the leakage flow is subsonic.
Experimental and numerical methods were used to investigate the aerodynamic performance of a winglet tip in a linear cascade. A flat tip and a cavity tip are studied as baseline cases. The flow patterns over the three tips are studied. The flow separates over the pressure side edge. For the cavity tip and the winglet tip, vortices appear in the cavity. These vortices reduce the discharge coefficient of the tip.
The purpose of using a winglet tip is to reduce the driving pressure difference. The pressure side winglet of the winglet geometry studied in this paper has little effect in reducing the driving pressure difference. It is found that the suction side winglet reduces the driving pressure difference of the tip leakage flow near the leading edge, but increases the driving pressure difference from midchord to the trailing edge. This is also used to explain the findings and discrepancies in other studies. Compared with the flat tip, the cavity tip and the winglet tip achieve a reduction of the loss to the size of the tip gap.
The effects of the rounding of the pressure side edge of the tips were studied to simulate the effects of deterioration. As the size of the pressure side edge radius increase, the tip leakage mass flow rate and the loss increase. The improvement of the aerodynamic performance by using a winglet remains similar when comparing with a flat tip or a cavity tip with the same pressure side radius.
In a previous study, vane-rotor shock interactions and heat transfer on the rotor blade of a highly loaded transonic turbine stage were simulated. The geometry consists of a high pressure turbine vane and downstream rotor blade. This study focuses on the physics of flow and heat transfer in the rotor tip, casing and hub regions. The simulation was performed using the URANS (Unsteady Reynolds-Averaged Navier-Stokes) code MSU-TURBO. A low Reynolds number k-ε model was utilized to model turbulence. The rotor blade in question has a tip gap height of 2.1% of the blade height. The Reynolds number of the flow is approximately 3×106 per meter. Unsteadiness was observed at the tip surface that results in intermittent ‘hot spots’. It is demonstrated that unsteadiness in the tip gap is governed by inviscid effects due to high speed flow and is not strongly dependent on pressure ratio across the tip gap contrary to published observations that have primarily dealt with subsonic tip flows. The high relative Mach numbers in the tip gap lead to a choking of the leakage flow that translates to a relative attenuation of losses at higher loading. The efficacy of new tip geometry is discussed to minimize heat flux at the tip while maintaining choked conditions. In addition, an explanation is provided that shows the mechanism behind the rise in stagnation temperature on the casing to values above the absolute total temperature at the inlet. It is concluded that even in steady mode, work transfer to the near tip fluid occurs due to relative shearing by the casing. This is believed to be the first such explanation of the work transfer phenomenon in the open literature. The difference in pattern between steady and time-averaged heat flux at the hub is also explained.
In high-speed, unshrouded turbines, tip leakage flows generate large aerodynamic losses and intense unsteady thermal loads over the rotor blade tip and casing. The stageloading and rotational speeds are steadily increased to achieve higher turbine efficiency, and hence, the overtip leakage flow may exceed the transonic regime. However, conventional blade tip geometries are not designed to cope with supersonic tip flow velocities. A great potential lies in the modification and optimization of the blade tip shape as a means to control the tip leakage flow aerodynamics, limit the entropy production in the overtip gap, manage the heat-load distribution over the blade tip, and improve the turbine efficiency at high-stage loading coefficients. The present paper develops an optimization strategy to produce a set of blade tip profiles with enhanced aerothermal performance for a number of tip gap flow conditions. The tip clearance flow was numerically simulated through two-dimensional compressible Reynolds-averaged Navier-Stokes (RANS) calculations that reproduce an idealized overtip flow along streamlines. A multiobjective optimization tool, based on differential evolution combined with surrogate models (artificial neural networks), was used to obtain optimized 2D tip profiles with reduced aerodynamic losses and minimum heat transfer variations and mean levels over the blade tip and casing. Optimized tip shapes were obtained for relevant tip gap flow conditions in terms of blade thickness to tip gap height ratios (between 5 and 25) and blade pressure loads (from subsonic to supersonic tip leakage flow regimes), imposing fixed inlet conditions. We demonstrated that tip geometries that perform superior in subsonic conditions are not optimal for supersonic tip gap flows. Prime tip profiles exist, depending on the tip flow conditions. The numerical study yielded a deeper insight on the physics of tip leakage flows of unshrouded rotors with arbitrary tip shapes, providing the necessary knowledge to guide the design and optimization strategy of a full blade tip surface in a real 3D turbine environment.
A numerical study has been performed to investigate the effect of casing motion on the tip leakage flow and heat transfer characteristics in unshrouded axial flow turbines. The relative motion between the blade tip and the casing was simulated by moving the casing in a direction from the suction side to the pressure side of the stationary blade. Baseline flat tip geometry and squealer type geometries namely double squealer or cavity and suction side squealer were considered at a clearance gap of 1.6%C. The computations were performed using a single blade with periodic boundary conditions imposed along the boundaries in the pitchwise direction. Turbulence was modelled using the SST k-ω model. The flow conditions correspond to an exit Reynolds number of 2.3×105 . The results were compared with those obtained without the relative casing motion reported in part I of this paper. In general, the effect of relative casing motion was to decrease the tip leakage mass flow and the average heat transfer to the tip due to the decrease in leakage flow velocity caused by a drop in driving pressure difference. Compared to the computations with stationary casing, in the case of all the three geometries considered, the average heat transfer to the suction surface of the blade was found to be larger in the case of the computations with relative casing motion. At a larger clearance gap of 2.8%C, in case of flat tip, while the tip leakage mass flow decreased due to relative casing motion only a smaller change in the average heat transfer to the tip and the suction surface of the blade was noticed.
This paper presents an experimental study of the flow mechanisms of tip leakage across a blade of an unshrouded turbine rotor. It shows the design of a new one-and-1/2-stage, unshrouded turbine configuration, which has been developed within the Turbomachinery Laboratory of ETH Zurich. This test case is a model of a high work (Delta hlu(2) =2.36) axial turbine. The experimental investigation comprises data from Unsteady and steady probe measurements, which has been acquired around all the bladerows of the one-and-1/2-stage, unshrouded turbine. A newly developed 2-sensor Fast Response Aerodynamic Probe (FRAP) technique has been used in the current measurement campaign. The paper contains a detailed analysis of the unsteady interaction between rotor and stator blade rows, with particular attention paid on the flow in the blade tip region. It has been found that the interaction of the rotor and the downstream stator has an influence on the development of the tip leakage vortex of the rotor The vortex is modulated by the stator profiles and shows variation in size and relative position to the rotor trailing edge when it stretches around the stator leading edge. Thereby a deflection of the tip leakage vortex has been observed, which expresses in a varying circumferential distance between two neighboring vortices of 20% of a rotor pitch. Furthermore, a significant influence of quasi-stationary secondary flow features of the upstream stator row on the secondary flow of the rotor has been detected. The geometry and flow field data of the one-and-1/2-stage turbine will be available to the turbomachinery community for validation and improvement of numerical tools.
In axial turbine the tip clearance flow occurring in rotor blade rows is responsible for about one third of the aerodynamic losses in the blade row and in many cases is the limiting factor for the blade lifetime. The tip leakage vortex forms when the leaking fluid crosses the gap between the rotor blade tip and the casing from pressure to suction side and rolls up into a vortex on the blade suction side. The flow through the tip gap is both of high velocity and high temperature, with the heat transfer to the blade from the hot fluid being very high in the blade tip area. In order to avoid blade tip burnout and a failure of the machine, blade tip cooling is commonly used. This paper presents the physical study and an improved design of a recessed blade tip for a highly loaded axial turbine rotor blade with application in high pressure axial turbines in aero engine or power generation. With use of three-dimensional Computational Fluid Dynamics (CFD), the flow field near the tip of the blade for different shapes of the recess cavities is investigated. Through better understanding and control of cavity vortical structures, an improved design is presented and the differences to the generic flat tip blade are highlighted. It is observed that by an appropriate profiling of the recess shape, the total tip heat transfer Nusselt Number was significantly reduced, being 15% lower than the flat tip and 7% lower than the baseline recess shape. Experimental results also showed an overall improvement of 0.2% in the one-and-1/2-stage turbine total efficiency with the improved recess design compared to the flat tip case. The CFD analysis conducted on single rotor row configurations predicted a 0.38% total efficiency increase for the rotor equipped with the new recess design compared to the flat tip rotor.
In this paper, numerical methods have been applied to the investigation of the effect of rotation on the blade tip leakage flow and heat transfer. Using the first stage rotor blade of GE-E3 engine high pressure turbine, both flat tip and squealer tip have been studied. The tip gap height is 1% of the blade height, and the groove depth of the squealer tip is 2% of the blade height. Heat transfer coefficient on tip surface obtained by using different turbulence models was compared with experimental results. And the grid independence study was carried out by using the Richardson extrapolation method. The effect of the blade rotation was studied in the following cases: 1) blade domain is rotating and shroud is stationary; 2) blade domain is stationary and shroud is rotating; and 3) both blade domain and shroud are stationary. In this approach, the effects of the relative motion of the endwall, the centrifugal force and the Coriolis force can be investigated respectively. By comparing the results of the three cases discussed, the effects of the blade rotation on tip leakage flow and heat transfer are revealed. It indicated that the main effect of the rotation on the tip leakage flow and heat transfer is resulted from the relative motion of the shroud, especially for the squealer tip blade.
The detailed development of tip clearance loss from the leading to trailing edge of a linear turbine cascade was measured and the contributions made by mixing, internal gap shear flow, and endwall/secondary flow were identified, separated, and quantified for the first time. Only 13 percent of the overall loss arises from endwall/secondary flow and of the remaining 87 percent, 48 percent is due to mixing and 39 percent is due to internal gap shear. All loss formation appears to be dominated by phenomena connected with the gap separation bubble. Flow established within the bubble by the pressure gradient separates as the gradient disappears and most of the internal loss is created by the entrainment of this separated fluid.
The effects of squealer rim height on three-dimensional flows and aerodynamic losses downstream of a high-turning turbine rotor blade have been investigated for a typical tip gap-to-chord ratio of h/c=2.0%. The squealer rim height-to-chord ratio is changed to be hst/c=0.00 (plane tip), 1.37, 2.75, 5.51, and 8.26%. Results show that as hst/c increases, the tip leakage vortex tends to be weakened and the interaction between the tip leakage vortex and the passage vortex becomes less severe. The squealer rim height plays an important role in the reduction of aerodynamic loss when hst/c⩽2.75%. In the case of hst/c⩾5.51%, higher squealer rim cannot provide an effective reduction in aerodynamic loss. The aerodynamic loss reduction by increasing hst/c is limited only to the near-tip region within a quarter of the span from the casing wall.
An overview of the science and technology involved in today's turbine engines is presented with specific focus on the critical rotational-to-stationary interfaces comprising axial turbine blade tips. The purpose is to provide a concise informative review of turbine blade tip functional, design, and durability issues. Neither a historical account nor a bibliography is presented. Attention is paid primarily, to the most challenging blade tips in high-pressure, high-temperature gas turbine systems, although most of the science discussed applies equally well to blade tips in low-pressure turbines, as well as steam turbines. As such, a wide range of both aircraft engine and power generating turbine systems are considered. Basic functional requirements, turbine systems design aspects, and transient operational considerations affecting blade tips and affected by blade tips are discussed in light of the multidisciplinary tradeoffs; involved in a successful design. The three dominant design philosophies for blade tips in practice today are presented with detailed examination of the aerodynamics, heat transfer, and cooling benefits and detractors. Finally, the in-service durability aspects of turbine blade tips are noted.
The local heat/mass transfer characteristics on the tip and shroud were investigated using a low speed rotating turbine annular cascade. Time-averaged mass transfer coefficients on the tip and shroud were measured using a naphthalene sublimation technique. A low speed wind tunnel with a single stage turbine annular cascade was used. The turbine stage is composed of sixteen guide plates and blades. The chord length of blade is 150 mm and the mean tip clearance is about 2.5% of the blade chord. The tested Reynolds number based on inlet flow velocity and blade chord is 1.5×105 and the rotational speed of the blade is 255.8 rpm at design condition. The results were compared with the results for a stationary blade and the effects of incidence angle of incoming flow were examined for incidence angles ranging from −15 to +7 degree. The off-design test conditions are obtained by changing the rotational speed with a fixed incoming flow velocity. Flow reattachment on the tip near the pressure side edge dominates the heat transfer on the tip surface. Consequently, the heat/mass transfer coefficients on the blade tip are about 1.7 times as high as those on the blade surface and the shroud. However, the heat transfer on the tip is about 10% lower than that for the stationary case due to reduced leakage flow with the relative motion. The peak regions due to the flow reattachment are reduced and shifted toward the trailing edge and additional peaks are formed near the leading edge region with decreasing incidence angles. But, quite uniform and high values are observed on the tip with positive incidence angles. The time-averaged heat/mass transfer on the shroud surface has similar a level to that of the stationary cases.