Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

This study presents a three‐dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn‐shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn‐shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high‐turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20261     

Numerical simulation of asymmetrical flow and heat transfer behind a hill in shear flows

Three‐dimensional numerical simulations of asymmetrical flows and heat transfer around a hill in shear flows were performed. When shear velocity distributions are introduced at the inlet, a vortex pair is formed asymmetrically to the spanwise direction behind the hill. Further, an asymmetrical hairpin vortex is periodically generated downstream. The leg of the asymmetrical hairpin vortex on the high‐speed side collapses first. Further downstream, the asymmetrical hairpin vortex breaks down earlier than the symmetrical hairpin vortex, and streamwise vortices appear on the high‐speed side. These streamwise vortices increase the heat transfer downstream. In contrast, no hairpin vortex appears in the case of a strong shear velocity distribution, but instead a streamwise vortex appears. The heat transfer decreases downstream since the turbulence generated by streamwise vortices is weak. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20223     

Turbulence and Heat Transfer of a Hairpin Vortex Formed Behind a Cube in a Laminar Boundary Layer

We report numerical simulations of three-dimensional unsteady separated flow and heat transfer around a cube mounted in a laminar boundary layer. The separated shear layer rolls up and a hairpin vortex is generated periodically behind the cube. A horseshoe vortex is also formed ahead of the cube. Heat transfer around the cube is high due to the horseshoe vortex. Since the hairpin vortex interacts with horseshoe vortices downstream of the cube, the heat transfer increases around the center of the spanwise direction. The hairpin and horseshoe vortices generate local areas of high turbulence.     

A spatially advancing turbulent flow and heat transfer in a curved channel

Direct numerical simulation was performed for a spatially advancing turbulent flow and heat transfer in a two‐dimensional curved channel, where one wall was heated to a constant temperature and the other wall was cooled to a different constant temperature. In the simulation, fully developed flow and temperature from the straight‐channel driver was passed through the inlet of the curved‐channel domain. The frictional Reynolds number was assigned 150, and the Prandtl number was given 0.71. Since the flow field was examined in the previous paper, the thermal features are mainly targeted in this paper. The turbulent heat flux showed trends consistent with a growing process of large‐scale vortices. In the curved part, the wall‐normal component of the turbulent heat flux was twice as large as the counterpart in the straight part, suggesting active heat transport of large‐scale vortices. In the inner side of the same section, temperature fluctuation was abnormally large compared with the modest fluctuation of the wall‐normal velocity. This was caused by the combined effect of the large‐scale motion of the vortices and the wide variation of the mean temperature; in such a temperature distribution, large‐scale ejection of the hot fluid near the outer wall, which is transported into the near inner‐wall region, should have a large impact on the thermal boundary layer near the inner wall. Wave number decomposition was conducted for various statistics, which showed that the contribution of the large‐scale vortex to the total turbulent heat flux normal to the wall reached roughly 80% inside the channel 135^° downstream from the curved‐channel inlet. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20275     

Numerical simulation of separated flow transition and heat transfer around a two‐dimensional rib

Numerical simulations of separated flow transition and heat transfer around a two‐dimensional rib mounted in a laminar boundary layer were performed. The separated shear layer becomes unstable due to the Kelvin–Helmholtz instability and generates a two‐dimensional vortex. This vortex becomes three‐dimensional and collapses in the downstream part of the separation bubble. As a result, transition from laminar to turbulent flow occurs in the separated shear layer. Streamwise vortices exist downstream of the reattachment flow region. The low‐frequency flapping motion and transition of the separated shear layer are influenced by three‐dimensional dynamics upstream of the separation bubble. Large‐scale vortices around the reattachment flow region have substantial effects on heat transfer. Downstream of the reattachment point, the surface friction coefficient and Nusselt number are different from their profiles in the laminar boundary layer and approach the distributions seen in the turbulent boundary layer. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(8): 513–528, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20177     

Coherent structures in a fully developed stage of a non‐isothermal round jet

Direct numerical simulation (DNS) was performed for a non‐isothermal air jet with a Reynolds number of 1200 in order to reveal coherent structures of the developed jet. A fourth‐order central finite difference was applied to the simulation. An effort was also made to enable experimental visualization (dye mixing and PTV) to support the validity of the instantaneous structures by DNS. Computational results for two types of inlet profiles suggested that nozzle conditions scarcely affect the turbulence statistics and the coherent structures in a jet‐established stage. Two‐point correlations of velocity and temperature show that similar distributions denoting the temperature can be used as an indicator of a vortex. A conceptual model of a hairpin‐shaped vortex was proposed and validated by two‐point correlations and PDF analysis for vortex alignment. The hairpin‐shaped vortex stands with legs inclined downstream. The inclination angle and the tilting angle between the two legs are ?45° and 40°, respectively. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 342–356, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20014     

Flow felid and heat transfer enhancement investigations by using a combination of corrugated tubes with a twisted tape within 3D circular tube based on different dimple configurations

Different dimple geometrical configurations with a combination of corrugated tubes and twisted tape are numerically investigated. Water is used as a working fluid for constant heat flux heat transfer conditions at the pipe wall. The dimensionless diameter of the dimples (d/D) used in this study is 0.09, 0.18, 0.27, and 0.36. However, the corrugation configuration diameter is 1 mm. The numerical simulations are carried out at the Reynolds number in the range of 1500–14,000. The outcomes reveal that the friction factor (f) and Nu number are augmented as the dimple diameter increases. The Nu number ratio of 1.25 is found for a dimple pipe tube with a diameter of 4 mm. The numerical outcome presented more mixing, secondary, and vortex produced in the main flow direction and near the pipe wall to the rotating flow induced by twisted tape. Moreover, mixed, secondary vortices and rotational flow originate behind and near the dimple, twisted tape, and corrugation surfaces. These rotational and vortices can promote mixing in flow between the thermal boundary layer and velocity boundary flow layer. So, increase the heat transfer enhancement. The improved pipes with different dimple diameters produce a maximum performance evaluation factor of is more than 1.25.     

Experimental study on heat transfer enhancement on natural convection in a vertical plate by using longitudinal vortex generators arranged in rows

Longitudinal vortices are capable of producing beneficial effects in heat transfer enhancement. Experiments in natural convection heat transfer enhancement were done on a vertical flat heating plate using delta‐winglet longitudinal vortex generators (LVGs) arranged in rows. In an experimental range of Rayleigh number, the height and width of the winglet of the longitudinal vortex generator (LVG), the array form of the longitudinal vortex generators on the heat transfer performance were experimentally investigated, and the best height of the winglet of the longitudinal vortex generator was obtained. The results showed the change of the array form of the longitudinal vortex generators could affect the heat transfer effect. Finally by arranging some longitudinal vortex generator arrays with the appropriate interval, the whole heat transfer effect of the interval could reach a prime value. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 351–358, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20119     

Heat transfer enhancement of turbulent duct flow by vortex generators attached to a large eddy break-up manipulator

An experimental study was conducted to investigate the characteristics of wall heat transfer and momentum loss for turbulent duct flow disturbed by insertion of a complicated body composed of a Large Eddy Break-Up (LEBU) plate and winglet-type vortex generators. It was found that the LEBU plate reduces the wall heat transfer in the region downstream of the insertion position and that this suppression of heat transfer could be recovered by attaching vortex generators to the LEBU plate, i.e., conspicuous heat transfer enhancement was achieved over a large streamwise distance. The spatial distribution of the heat transfer coefficient obtained shows the same features as that observed in a previous study of a flat plate turbulent boundary layer. Therefore, the flow and thermal field structure of the turbulent duct flow downstream of the inserted body should be basically the same as those in the same region of the turbulent boundary layer. The effect of a notch, open in the LEBU plate behind the vortex generator, on heat transfer and pressure drop was also examined. The notch simulates the hole of the LEBU plate to be produced in a practical application when a vortex generator is produced by punching from the original plain LEBU plate. It was found that a vortex generator with an open notch works best in augmenting the wall heat transfer and also in suppressing the increase of momentum loss. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 189–200, 1999     

Numerical study on laminar convection heat transfer in a rectangular channel with longitudinal vortex generator. Part A: Verification of field synergy principle

This study presents numerical computation results on laminar convection heat transfer in a rectangular channel with a pair of rectangular winglets longitudinal vortex generator punched out from the lower wall of the channel. The effect of the punched holes and the thickness of the rectangular winglet pair to the fluid flow and heat transfer are numerically studied. It is found that the case with punched holes has more heat transfer enhancement in the region near to the vortex generator and lower average flow frictional coefficient compared with the case without punched holes. The thickness of rectangular winglet can cause less heat transfer enhancement in the region near to the vortex generator and almost has no significant effect on the total pressure drop of the channel. The effects of Reynolds number (from 800 to 3000), the attack angle of vortex generator (15°, 30°, 45°, 60° and 90°) were examined. The numerical results were analyzed from the viewpoint of field synergy principle. It was found that the essence of heat transfer enhancement by longitudinal vortex can be explained very well by the field synergy principle, i.e., when the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient, the heat transfer in the present channels will be enhanced. Longitudinal vortices (LVs) improve the synergy between velocity and temperature field not only in the region near LVG but also in the large downstream region of longitudinal vortex generator. So LVs enable to enhance the global heat transfer of channel. Transverse vortices (TVs) only improve the synergy in the region near VG. So TVs can only enhance the local heat transfer of channel.     

Heat transfer augmentation in a plate‐fin heat exchanger using a rectangular winglet

This study presents numerical computation results on laminar convection heat transfer in a plate‐fin heat exchanger, with triangular fins between the plates of a plate‐fin heat exchanger. The rectangular winglet type vortex generator is mounted on these triangular fins. The performance of the vortex generator is evaluated for varying angles of attack of the winglet i.e., 20, 26, and 37° and Reynolds number 100, 150, and 200. The computations are also performed by varying the geometrical size and location of the winglet. The complete Navier–Stokes equation and the energy equation are solved by the (Marker and Cell) MAC algorithm using the staggered grid arrangement. The constant wall temperature thermal boundary conditions are considered. Air is taken as the working fluid. The heat transfer enhancement is seen by introducing the vortex generator. Numerical results show that the average Nusselt number increases with an increase in the angle of attack and Reynolds number. For the same area of the LVG, the increase in length of the LVG brings more heat transfer enhancement than increasing the height. The increase in heat transfer comes with a moderate pressure drop penalty. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20318     

Numerical Study of Forced Convection of Nanofluid Flow Over a Backward Facing Step With a Corrugated Bottom Wall in the Presence of Different Shaped Obstacles

In this paper, a numerical study of laminar forced convection of nanofluid flow over a backward facing step with a corrugated bottom wall in the presence of different shaped obstacles placed behind the step was performed. The bottom corrugated wall of the channel downstream of the step is isothermally heated and the other walls of the channel and obstacle surface are assumed to be adiabatic. The governing equations are solved with a finite-element method. The influences of the Reynolds number (between 10 and 200), solid volume fraction of the nanoparticle (between 0 and 0.05), and obstacle type (circular, square, and diamond shaped) on fluid flow and heat transfer are numerically investigated. It is observed that among different obstacles, the diamond shaped obstacle provides better local heat transfer enhancement characteristic in the vicinity of the step compared to the circular or square obstacle at high Reynolds number. Heat transfer enhancement of 6.66% is achieved in terms of maximum values with a diamond shaped obstacle compared to the no-obstacle case of the corrugated channel. Adding an obstacle deteriorates heat transfer in terms of average values for the backward facing step geometry with a corrugated wall. When the solid volume fraction of nanoparticle is increased, maximum and average heat transfer rate increase. Heat transfer enhancements of 7.45%, 7.42%, 6.94%, and 6.64% are obtained for the average values for circle, diamond, square, and no-obstacle cases, respectively, when solid volume fraction of 0.05 is compared to pure fluid.     

Developing turbulent heat transfer in a 360° bend of square cross section

This paper reports the computational results on the developing turbulent heat transfer in a 360° bend of square cross section. The centrifugal force acting on the fluid in a bend flow induces strong cross-stream motion. It was found from the computations that the counterrotating vortex pair caused by the centrifugal force are broken into a multicell pattern after the θ = 90° station of the bend and the continuous arising and ceasing of the vortices directly affects the variation of heat transfer through the walls of the bend. Particular attention was paid to the developing process of the vortices because they exert the most significant effects on the convective heat transfer through the bend walls. A low Reynolds number second moment turbulence closure was employed to simulate the near wall turbulence behaviors of the bend. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 77–88, 1999     

Enhancement of Heat Transfer Using Delta-Winglet Type Vortex Generators with a Common-Flow-up Arrangement

Simulations of a coolant air flowing in a heat exchanger with delta-winglet type vortex generators in common-flow-up configuration have been performed to unveil the salient heat transfer characteristics. The heat exchanger is approximated as a periodic rectangular channel with heated walls and a pair of built-in tubes near the inlet and outlet. The heat transfer characteristics of the heat exchangers with vortex generators near the inlet, outlet, and both inlet and outlet have been compared. The Navier-Stokes equations together with the energy equation are solved employing unstructured finite volume method. The simulations reveal a significant enhancement in heat transfer because of the strong swirling motion originating from the streamwise longitudinal vortices behind the pair of delta winglets. The spiraling flow entrains air into the core and causes intermixing of the fluid layers to disrupt the growth of the thermal boundary layer. A parametric study on the angles of attack identifies the conditions under which enhancement in heat transfer can lead to significant miniaturization of the heat exchangers. The analysis also reveals interesting flow structures behind the winglets and correlates them to the mechanism of heat transfer.     

Microscopic phenomena and macroscopic evaluation of heat transfer from plate fins/circular tube assembly using naphthalene sublimation technique

Flow and heat transfer in a plate fins/circular tube assembly is examined using naphthalene sublimation technique. The examined parameters are the gap (δ) to tube diameter (D) ratio δ/D, the Reynolds number Re_D and the tube location l/D for a single tube.A preliminary flow visualization shows large recirculating twin vortices and a weak downstream oscillatory streakline. The local heat/mass transfer coefficient is large at the leading edge of the plate and also in front of the tube. It is relatively small behind the tube and it approaches the fully developed asymptotic value far downstream. The high heat/mass transfer coefficient in front of the tube is considered to be due to the so-called horseshoe vortex. When the Reynolds number is as large as 2660, a smaller subsidiary horseshoe vortex is attached to the upstream of the main one. The positive effect of the horseshoe vortices is prominent when the tube is placed in the downstream region. In this case, the total heat/mass transfer rate increases up to 25%.     

Effects of film cooling hole shape on heat transfer

The effects of hole shapes, secondary injection Reynolds numbers, and blowing ratios on the heat transfer downstream of film cooling holes have been investigated by using a large‐scale low‐speed loop wind tunnel. The test model consists of five film cooling holes. Experiments on dustpan‐shaped holes, fan‐shaped holes, and round holes have been conducted with injection Reynolds number ranging from 10,000 to 25,000 and blowing ratio ranging from 0.3 to 2.0. Measurements are taken under 26 conditions. Results show that the critical blowing ratio is 1.3 for the dustpan‐ and fan‐shaped holes, 0.7 for the round holes. The turbulence generated by air injection through round holes is stronger than those through dustpan‐ and fan‐shaped holes. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 73–80, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20005     

Heat transfer in channel flow around a rectangular cylinder

Heat transfer and flow visualization experiments have been made in a channel with a rectangular cylindrical section having various width-to-height ratios. Vortices were observed to shed periodically from the cylinder and then reattach to the channel wall. This reattachment of the vortices induces a periodic fluctuation in heat flux at the wall and enhances the heat transfer in the downstream region of the cylinder. The streamwise position of the maximum Nusselt number moves downstream with decreasing width-to-height ratio, b/h, of the cylinder. When b/h = 2.0, however, the heat flux periodicity disappears because the wake narrows intermittently owing to reattachment of the separated flows to the upper and lower surfaces of the cylinder. © 1998 Scripta Technica. Heat Trans Jpn Res, 27(1): 84–97, 1998     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Heat transfer enhancement in duct with blunt body inserted close to its wall

This paper shows the effects of clearance length between a body and a duct wall, and duct height on the heat transfer characteristics and flow behavior at a downstream region of the body when a blunt body was set in a parallel plate duct with some distance separating it from the duct wall as a turbulence promoter. For the ratio of clearance to body height, C/D = 0.05–01, the heat transfer was characterized by the reattachment of shear flow separated from the body. Furthermore, the heat transfer depended on both the reattachment flow and the separation vortex at C/D = 0.15–0.2, and the side vortex induced by Karman vortex at C/D = 0.25–0.275 was also observed. We found the reattachment flow gives a superior effect to enhance heat transfer at a low Reynolds number, but at a larger Reynolds number, the side vortex induced by Karman vortex becomes more effective to heat transfer enhancement. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(5): 336–349, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20067