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Jet flow pattern: a) Re j 1800 (semiturbulent jet, L u a L); b) Re j 7500 (fully turbulent jet, L u L since a 0); c) schematic summary.
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Impingement heat transfer in a circular block of an open-cell metal foam with high porosity (ε = 0.94) is experimentally studied at two distinctive flow regimes, semiturbulent (Rej = 1800 and 2800) and fully turbulent (Rej = 7500 and 15,000) regimes. The influence of jet structures and their variation with jet exit-to-foam spacing (impinging distan...
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... where the jet has no interaction with the surrounding fluid that is seen as "parallel- sided flow for a certain distance from the jet exit." The surrounding fluid begins to entrain and mix with the jet at the end of the laminar length. In the present round jet for Re j 1800, the laminar length is estimated to be a∕D j ∼ 12.0 as visualized in Fig. 3a, where D j is the round jet ...
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... higher jet Reynolds numbers (e.g., Re j 7500), which falls into the "fully turbulent jet" regime (Re j > 3000) [11], the surrounding fluid is entrained into the jet immediately after the jet exit, as visualized in Fig. 3b. It has been established that there exist three distinctive flow regions [19]: 1) the potential core region, 2) the developing region, and 3) the fully developed region. In the potential core region, the jet velocity along the jet centerline maintains its magnitude as high as that at the jet exit. Furthermore, a region where the jet is ...
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... so-called potential core. Martin [1] suggested that the potential core persists up to L 4D j for round jets, whereas Gauntner et al. [48] reported that it typically ranges from L 4.7D j ∼ 7.7D j . After the jet is fully developed via the developing region, the axial velocity profiles become self-similar [49], as schematically illustrated in Fig. 3c. Both flow visualization images (Figs. 3a and 3b) show that the semiturbulent jet has a wider shear layer than that of the fully turbulent jet once entrainment takes ...
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... that the potential core persists up to L 4D j for round jets, whereas Gauntner et al. [48] reported that it typically ranges from L 4.7D j ∼ 7.7D j . After the jet is fully developed via the developing region, the axial velocity profiles become self-similar [49], as schematically illustrated in Fig. 3c. Both flow visualization images (Figs. 3a and 3b) show that the semiturbulent jet has a wider shear layer than that of the fully turbulent jet once entrainment takes ...
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... the length of potential core [3]. Figure 4 depicts that the semiturbulent jet (Re j 1800 and 2800) has the undisturbed region persisting up to L u ∕D j 15.0 (for Re j 1800) and L u ∕D j 8.5 (for Re j 2800). The length of the undisturbed region decreases as the jet Reynolds number increases. As observed from the visualized flow image in Fig. 3a and the schematic summary in Fig. 3c, the undisturbed flow region of the semiturbulent jet consisted of the laminar length a∕D j and the length of the potential core L∕D j . The latter is invisible in the flow image. The length of potential core may be estimated from the flow image (Fig. 3a) to be approximately L∕D j 3.0 for Re j ...
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... 4 depicts that the semiturbulent jet (Re j 1800 and 2800) has the undisturbed region persisting up to L u ∕D j 15.0 (for Re j 1800) and L u ∕D j 8.5 (for Re j 2800). The length of the undisturbed region decreases as the jet Reynolds number increases. As observed from the visualized flow image in Fig. 3a and the schematic summary in Fig. 3c, the undisturbed flow region of the semiturbulent jet consisted of the laminar length a∕D j and the length of the potential core L∕D j . The latter is invisible in the flow image. The length of potential core may be estimated from the flow image (Fig. 3a) to be approximately L∕D j 3.0 for Re j 1800 [i.e., the length of the ...
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... increases. As observed from the visualized flow image in Fig. 3a and the schematic summary in Fig. 3c, the undisturbed flow region of the semiturbulent jet consisted of the laminar length a∕D j and the length of the potential core L∕D j . The latter is invisible in the flow image. The length of potential core may be estimated from the flow image (Fig. 3a) to be approximately L∕D j 3.0 for Re j 1800 [i.e., the length of the undisturbed region (L u ∕D j 15.0) -the laminar length (a∕D j ...
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... the jet becomes fully turbulent, the laminar length disappears (Fig. 3b). The length of the measured undisturbed region in Fig. 4 is equal to that of the potential core, which remains unchanged for Re j 7500 and 15,000, being L∕D j ...
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... reduced as the jet Reynolds number is increased. With a further increase in the jet Reynolds number toward the fully turbulent jet, the longitudinal decay of V c is attenuated, approaching that of the fully turbulent jet (i.e., n 1.0), which results from the observed wider fanning of the shear layer at the lower jet Reynolds numbers as shown in Fig. ...
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... jet, the increased jet Reynolds number causes a delayed peak at the downstream location and a gradual decreasing trend of the average heat transfer with the increasing impinging distance. Less radial divergence of the jet at the higher jet Reynolds numbers may indicate that longitudinal momentum persists longer, as also visibly observed in Figs. 3a and 3b. Once the jet is fully turbulent, the jet Reynolds number plays no part in determining the longitudinal peak location of the overall heat ...
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... Most of the papers found in the literature concern theoretical modelling [2, [4][5][6][7][8][9], experimental measurements [1,[10][11][12][13][14], or both [15][16][17][18][19][20][21] on metallic or polymeric foams, used to enhance or reduce heat transfer in heat exchangers. Some of them are reviews of this subject [2,[22][23][24][25][26]. ...
... The latter resulted in being the most important factor. Kuang et al. [13] measured the heat transfer convection coefficient in different flow regimes and concluded that a characteristic length is the major factor influencing it. Hong et al. [5] analytically evaluated the border effects in the free convection of a heated vertical wall covered by metallic foam: these effects are meaningful only for high porosity foams. ...
This experimental work presents the results of measurements of thermal conductivity λ and convection heat transfer coefficient h on regular structure PLA and aluminium foams with low density ratio (~0.15), carried out with a TCP (thermal conductivity probe), built by the authors’ laboratory. Measurements were performed with two fluids, water and air: pure fluids, and samples with the PLA and aluminium foams immersed in both fluids have been tested. Four temperatures (10, 20, 30, 40 °C) and various temperature differences during the tests ΔT (between 0.35 and 9 °C) were applied. Also, tests in water mixed with 0.5% of a gel (agar agar) have been run in order to increase the water viscosity and to avoid convection starting. For these tests, at the end of the heating, the temperature of the probe reaches steady-state values, when all the thermal power supplied by the probe is transferred to the cooled cell wall; thermal conductivity was also evaluated through the guarded hot ring (GHR) method. A difference was found between the results of λ in steady-state and transient regimes, likely due to the difference of the sample volume interested by heating during the tests. Also, the effect of the temperature difference ΔT on the behaviour of the pure fluid and foams was outlined. The mutual effect of thermal conductivity and free convection heat transfer results in being extremely important to describe the behaviour of such kinds of composites when they are used to increase or to reduce the heat transfer, as heat conductors or insulators. Very few works are present in the literature about this subject, above all, ones regarding low-density regular structures.
... The axial length of the area without any mixing is represented by the laminar length. The radius of the jet is almost identical to the nozzle radius in this area [23]. The jet begins to mix with the surrounding fluid after the laminar length has passed, and the jet's radius gradually grows as well. ...
... La zone de transition entre régime laminaire et régime turbulent a également été étudiée récemment par Kuang et al. (2016) pour un jet axisymétrique issu d'un tube long. Les auteurs observent des jets transitionnels pour des Reynolds d'injection Re j = 1800 et Re j = 2800. ...
... (a) Distribution de vitesse axiale (profils r/d = 0) (b) Re j = 1800 (c) Re j = 7500 Figure 1.7 -Évolution de la structure d'un jet entre un écoulement transitionnel et turbulent (Kuang et al., 2016) La région pleinement développée, qui compose la deuxième partie de la zone de jet libre, commence à l'extrémité du coeur potentiel. Elle est marquée par une diminution de la vitesse axiale à cause de la fusion des couches de cisaillement. ...
... En revanche, pour des jets transitionnels et laminaires (Re j < 3000), Katti et al. (2011) montrent que la distance d'impact n'a que très peu d'influence sur la distribution du nombre de Nusselt et que le second maximum du nombre Nusselt tend à disparaître. Ceci concorde avec les travaux de Kuang et al. (2016) et McNaughton et Sinclair (1966 qui ont caractérisé la structure du jet dans cette plage de Reynolds Re j < 3000 (paragraphe 1.1.2.1). En effet, les auteurs ont mis en évidence une zone laminaire à l'injection avant le coeur potentiel : la vitesse est constante dans cette zone et la couche de cisaillement n'a toujours pas eu le temps de s'amorcer. ...
Cette thèse est consacrée à l’étude du refroidissement par impact d’une rangée de jets issus d’un tube multi-perforé. Cette configuration particulière trouve son application dans le refroidissement du carter de turbine basse pression d’un turboréacteur et se caractérise par un régime d’écoulement transitionnel ou faiblement turbulent (1500 < Rej < 5000), ainsi qu’un écoulement cisaillant entre le cœur du tube multi-perforé et les perforations. Un banc d’essais, échelle 5, a été mis en place afin de reproduire l’impact des jets rencontré dans l’environnement moteur. La structure aérodynamique des jets est déterminée à partir des champs de vitesses instantanées obtenus par Vélocimétrie par Images de Particules (PIV), les transferts thermiques sont estimés à partir de la distribution du nombre de Nusselt obtenue par technique de thermographie infrarouge. La caractérisation des jets et l’efficacité du refroidissement ont été analysées à travers une étude d’influence des paramètres de similitude géométriques (distance d’impact, pas inter-trous)et aérodynamiques (nombre de Reynolds à l’injection, cisaillement de l’écoulement dans le tube). La similitude d’échelle, basée sur le diamètre des perforations, est par ailleurs validée à partir d’un banc d’essais spécifiquement échelle 1. L’étude numérique se base sur une approche aux grandes échelles(LES) par méthode des frontières immergées. La validation du code numérique est effectuée sur une configuration testée expérimentalement. Elle a mis en évidence la capacité du modèle LES à reproduire d’une part, la structure aérodynamique des jets dans les différentes zones caractéristiques, et d’autre part, à estimer correctement la distribution du nombre de Nusselt. La modélisation d’une rangée de jets augmente considérablement les coûts de calcul du fait de la dimension importante du domaine.Une méthodologie de modélisation de tels jets a été présentée à travers une étude sur la définition des conditions limites et sur le maillage.
... In addition to the length of jet potential core, the variation of axial velocity of a given jet along its axis may indicate alternative jet flow patterns that are developed, which impiles diffusion of the jet. Fig. (5) shows the centerline velocity of semi-turbulent round jets and fully turbulent round jet (Barratt et al. [23] and Kuang et al. [24]). The semiturbulent jets have an undisturbed fluid column up to L u /D j = 15.0 for Re j = 1,800 and L u /D j = 8.5 for Re j = 2,800. ...
... ). Variation of the centerline velocity (w 0 /w e ) of a free round jet for three selected jet Reynolds numbers (Kuang et al.[24]). (A higher resolution / colour version of this figure is available in the electronic copy of the article). ...
... .(24). Circumferential distribution of heat transfer on a target convex surface (r c /r j = 5.0) inside the potential core at Re j = 20,000 (Wang et al.[68]). ...
Jet impingement in engineering applications is used because of the capacity to transport high levels of heat flux from a surface of interest for cooling purposes. Thus far, based on a vast database of experiments and numerical simulations, several correlations have been established for local and average heat transfer on target surfaces as functions of relevant fluid properties and geometric parameters. In addition to these correlations, significant efforts have been made to gain fundamental understanding of jet impingement in varying configurations. However, the physics governing heat transfer by jet impingement are conjectured, even unclear. Thus, this article collates and discusses recent advances in fluidic mechanisms underlying the heat transfer by submerged jet impingement on a convex surface. The fluid properties developed on a convex surface due to jet impingement with varied characteristics, including jet-to-target surface spacing, interchange their primary roles in heat transfer from/to a convex surface. Particularly, conjectures associated with relevant fluidic mechanisms that have been widely accepted, are confirmed, clarified, and corrected.
... After which, the centerline velocity of free jets tends to decrease dramatically. The length of potential core for fully turbulent jets is the distance from a nozzle exit to a downstream location where the centerline velocity w 0 reaches either 98% (Giralt et al., 1977) or 95% (Jambunathan et al., 1992) for semiturbulent, of the jet exit velocity w e , as the example data reported by Kuang et al., 2016 are plotted in Fig. 9(a). Figure 9(b) shows the centerline velocity along the z-axis of the present round jet at Re j = 1500. ...
Severe reactor core damage may occur from fuel channel failure as a consequence of excessive heat emitted from calandria tubes (CTs) in a pressurised heavy water (D2O) reactor (CANDU). The heating of the CTs is caused by creep deformation of the pressure tubes (PTs), which may be ballooning or sagging depending on the internal pressure of the PTs. The deformation of the pressure tube is due to overheating as a result of a loss-of-coolant accident (LOCA) and emergency core cooling system (ECCS) failure. To prevent the exacerbation of the LOCA, circulating D2O in the moderator tank may be utilized by forming a secondary jet that externally cools the individual CTs. The buoyant plume develops around the CTs and interacts with the secondary jet at a certain oblique angle with respect to the gravitational axis, depending on the spatial location of the hot calandria tubes (or the hot reactor core region). This study reports on how the local and overall heat transfer characteristics on a calandria tube where the buoyant plume develops, are altered by the obliqueness of the external secondary jet (from a co-current jet to a counter-current jet) in a simplified configuration at the jet Reynolds number of Rej = 1500 for the Archimedes number of ArD = 0.11 and Rayleigh number of RaD = 1.6 × 10⁶ (modified Rayleigh number of 3.0 × 10⁷).
This experimental investigation is aimed to evaluate the effects of convective heat transfer phenomena in an aluminium metal flat disc/plate placed below metal foam using a single round jet of air impinging in a confined region. The Jet impingement technique contains pressurized air, which is configured for certain specific application. It is designed to spray the fluid with the help of a nozzle on the surface of the disc or plate which is being heated or is being cooled. To understand the heat transfer phenomena in a metal plate with metal foams facing round air jet impingement; three metal foams of similar geometries but of different porosities and permeabilities are taken for study. Each experiment is performed with seven different flow rates and at two different heights with two different heat flux values. The recorded measurements are temperatures, pressure differences, flow rates, resistance voltages and currents, and time. The outcomes of the experimental data are analysed and compared results of all four distinct kinds of metal foams with different jet to plate height and at different heat fluxes. It is concluded that heat transfer efficiency may or may not increase depending upon the metal foam used. Overall, the conclusions are extremely useful in engineering.
The effect of the metal foam thickness on the conduction and convection heat transfer for a metal foam flat plate impinged by a circular air jet is investigated. The IR thermography and thin-metal foil technique are used for the measurement of local heat transfer. An open-cell aluminum metal foam is used for the metal foam flat plate. A 3D-printed resin foam and detached metal foam flat plate are used for the appreciation of the conduction and convection heat transfer. The varying parameters are the thickness of the foam, Reynolds number, and the nozzle exit to plate distance. The presence of the metal foam offers a conduction effect. This predominates over the attenuation in the convective heat transfer by foam due to additional hydraulic resistance. The additional hydraulic resistance offered by the porous foam increases with the increase in the foam thickness. The heat transfer of a porous foamed flat plate decreases with the increase in the foam thickness. The local Nusselt number of the resin foam and detached foam flat plate is almost the same. The conduction effect and attenuation in the convection heat transfer of a metal foam flat plate are quantified by attenuation and enhancement factors. The overall augmentation offered by 4, 8, and 12 mm thick metal foam flat plates is 1.71, 1.42, and 1.43 times compared to the smooth flat plate case, respectively. Hence, it is advisable to use a metal foam flat plate with 4-mm-thick metal foam under circular air jet impingement.
The present study experimentally investigates the thermal performance of stainless-steel foil (AISI-304) integrated with copper open-cell metal foam (OCMF) of 10 pores per inch (PPI) and 90% porosity subjected to a circular air-jet impingement using thin foil thermal imaging technique. This investigation studies the stagnation, local, and average heat transfer characteristics of the foamed plate by varying the thickness of the OCMF in the jet flow directions. The effect of dimensionless thickness of OCMF (t/d= 0.83, 1 and 1.16), dimensionless impinging distance (z/d= 1.5, 2, 4, 6, 8 and 10), and Reynolds number (Re= 10000 – 50000) is considered on the stagnation, local and average Nusselt number. The result shows that the presence of OCMF significantly improves the thermal performance of the surface. In addition, the surface with minimum OCMF thickness produces the highest enhancement in heat transfer. For the metal plate integrated with OCMF, the enhancement in the peak value of stagnation Nusselt number is found to be 56.1%, 86%, and 89% by varying the dimensionless foam thickness from 1.16 - 0.83 for Reynolds number 20000, 30000, and 40000, respectively. A correlation based on the experimental results is also proposed to determine the local Nusselt number in terms of dimensionless foam thickness, Reynolds number, dimensionless impinging distance, and dimensionless radial distance. This study provides essential guidelines for selecting OCMF for the foam-based heat sink.
The local heat transfer distribution is measured for the case of a flat plate with a porous metal foam impinged by a rectangular slot air jet. The main objective is to separate the convection and conduction heat transfer effects in a flat plate with metal foam. For this purpose, flat plate with attached and detached metal foam are studied. The effect of the thickness of the metal foam on the convection and conduction heat transfer is also investigated. An open-cell aluminium metal foam having a porosity of 92% and pore density of 20 PPI (pores per inch) is studied. The thickness of the metal foam studied is 4 mm, 8 mm and 12 mm. Reynolds number ranges from 5200 to 12,000 and impinging distance varies from 2 to 10 times the width of the rectangular slot. The local Nusselt number of a flat plate with attached and detached metal foam is independent of the impinging distance irrespective of the metal foam thickness. The impinging jet on the flat plate with metal foam experiences extra hydraulic resistance depending on the thickness of the metal foam. The 12 mm thick metal foam offers the highest extra hydraulic resistance. However, the 4 mm thick metal foam offers negligible extra hydraulic resistance. The flat plate with detached metal foam experiences attenuation in the convective heat transfer due to extra hydraulic resistance. However, the flat plate with attached metal foam experiences conduction heat transfer due to metallic foam along with attenuation in the convective heat transfer due to extra hydraulic resistance. The attenuation in the convection heat transfer and conduction heat transfer due to the presence of the metal foam is quantified by the attenuation factor and enhancement factor respectively. The average attenuation factor is 1.12, 0.96 and 0.84 for 4 mm, 8 mm and 12 mm respectively. The enhancement factor for the metal foam having 4 mm, 8 mm and 12 mm are 1.91, 2.20 and 2.90 respectively. Hence, the effect of the enhancement due to conduction heat transfer dominates over the effect of the attenuation in the convective heat transfer due to extra hydraulic resistance. Under slot jet impingement, a flat plate with 4 mm, 8 mm and 12 mm thick metal foam shows 2.02, 2.12 and 2.41 times overall enhancement in comparison with the smooth flat plate respectively. Region-wise correlations for Nusselt number are developed using multiple regression analysis.
