Large eddy simulation of flow and heat transfer in a channel with a detached rib array

Ribbed channels are widely used to enhance heat transfer in various heat exchange equipment. However, the heat transfer is locally deteriorated immediately behind the rib due to the flow separation. To overcome this shortcoming, a detached rib array has been proposed recently. In the present study, large eddy simulations (LES) of turbulent flow and heat transfer in a channel with a detached rib array have been conducted. The no-slip and no-jump conditions on the rib surface are satisfied in the Cartesian coordinates by using an immersed boundary method. Experiments are conducted as well to validate the simulation. The velocity and temperature fields are obtained by a hot wire and a thermocouple, respectively. The surface heat transfer is measured using the thermochromic liquid crystal with a high spatial resolution. Compared with the attached rib, the detached one enhances the heat transfer underneath the rib, whereas the channel wall downstream of the rib shows lower heat transfer rate. By investigating the effect of the clearance between the rib and the wall, we have found an intermediate flow pattern, where the counter-rotating vortex pair and the separation bubble coexist. Including the new flow pattern, we discuss the flow characteristics such as the wake length and the locus of the saddle points, and the flow physics behind the local heat transfer distribution on the channel wall based on the instantaneous flow and thermal fields. The turbulence data near the solid surface are also presented, which have not been provided by previous experiments.     

Vortical structures and heat transfer augmentation of a cooling channel in a gas turbine blade with various arrangements of tip bleed holes

Abstract

This study investigates the internal cooling processes affected by the tip bleed holes in gas turbine blades. Double bleed holes are fixed at the center of the blade tip near the pressure side and suction side, respectively. Five different arrangements of the holes along the center line of the tip are studied. The purely double holes are set as the Baseline. The purpose of the present study is to provide a new perspective of the tip film cooling to understand the internal flow processes, vorticity evolution and the mechanism of the heat transfer augmentation. A topological analysis and the boundary layer analysis methods are introduced to better understand the tip heat transfer. The total extraction area and volume is kept at the same level for all the studied cases. The results show that the Dean vortices and the near-wall vortices induced by the secondary flow contribute to the high heat transfer coefficient on the tip surface. The mixing effect of the Dean vortices and the hole extraction helps to enhance heat transfer upstream of the tip. Different arrangement of the bleed holes can affect the internal flow processes and heat transfer performance. The suction effect of the center-line bleed hole can accelerate the near-hole flow and reduce the thickness of the boundary layer. The center-line hole fitted at the middle of the tip affects significantly the rear side of the hole. Thus, the holes aligned in the middle of the tip provide the highest heat transfer and thermal performance. The thermal performance is enhanced by up to 4.7% compared with the Baseline.     

Effect of moving distance of a moving block on heat transfer in a channel flow

A numerical investigation of heat transfer rate of an insulated block moving on a heated surface in a channel was studied. The study mainly investigated the effect of block moving distance on the heat transfer rate of heated surface. This subject belongs to a kind of moving boundary problems, and the modified Arbitrary Lagrangian Eulerian method is suitable for solving this subject. The results show that the block moving distance affects the flow and thermal fields remarkably. The heat transfer rate of heated surface increases proportionally to the increment of the block moving distance, when the block moving distance is larger than a critical value.     

MHD oscillatory flow with heat transfer in a flat plate channel

An analysis of the oscillatory MHD channel flow of an electrically conducting, viscous, incompressible fluid under a transverse magnetic field is presented. The transient velocity and the transient temperature are exhibited graphically, whereas the numerical values of the amplitude, phase of the skin-friction and the first and second harmonics of the rate of heat transfer are displayed in tabular form. Also discussed are the effects of M (the Hartmann number), e (the loading parameter), ω* (the frequency), and E (the Eckert number).     

Turbulent heat transfer in a channel flow with arbitrary directional system rotation

Arbitrary directional system rotation of a channel flow can be decomposed into simultaneous componential rotations in the three orthogonal directions. In order to study its effect on turbulent heat transfer, three typical cases, i.e., combined spanwise and streamwise (Case I), streamwise and wall-normal (Case II), and wall-normal and spanwise rotations (Case III), are simulated with two of the three coordinate-axial rotations imposed on the system. In Case I, the effect of spanwise rotation dominates the heat transfer mechanism when the two componential rotation rates are comparable. However, if the streamwise rotation is much stronger than the spanwise rotation, the turbulent heat transfer can be enhanced on the two walls, but more strikingly on the suction side. In Case II, even though no explicit spanwise rotation is imposed on the system, the combined rotations also bring the enhancement/reduction of turbulent heat transfer on the pressure/suction side, respectively, which is similar to that in a spanwise rotating channel flow. In Case III, the spanwise rotation effect is still obvious, however, its effect is reduced somewhat due to the redirection of the mean flow by the wall-normal rotation.     

Turbulent flow characteristics and heat transfer enhancement in a rectangular channel with elliptical cylinders and protrusions of various heights

This numerical study investigates turbulent flow characteristics and heat transfer augmentation in a rectangular channel with elliptical cylinders and protrusions. The height of protrusions is changed to quantify interactions with surrounding elliptical cylinders. Flow structures, temperature distributions, and heat transfer characteristics are obtained with a standard k ? ε turbulence closure model. Cases are run at Reynolds numbers of 15,000 and 20,000. Combinations of protrusion and elliptical cylinder geometries give higher heat transfer rates than a case without protrusions. Additionally, Nusselt numbers increase with increases in height of protrusions and an optimum height-to-footprint diameter ratio is found.     

哥氏力对旋转方通道内流动与换热的影响

在雷诺数亿为6000,旋转数助为0～0．26内,数值模拟了旋转光滑径向出流通道的内流动与换热分布,分析了哥氏力对旋转管流的作用机理。计算结果表明,切向哥氏力推动了通道截面内的双涡二次流,径向哥氏力则使得近侧壁流体加速和中心流体减速。换热计算结果从定性趋势上吻合公开文献中的实验现象。反映了旋转附加力的基本影响规律。     

Calculation of turbulent flow and heat transfer in a porous-baffled channel

This study presents the numerical predictions on the turbulent fluid flow and heat transfer characteristics for rectangular channel with porous baffles which are arranged on the bottom and top channel walls in a periodically staggered way. The turbulent governing equations are solved by a control volume-based finite difference method with power-law scheme and the k-ε turbulence model associated with wall function to describe the turbulent structure. The velocity and pressure terms of momentum equations are solved by SIMPLE (semi-implicit method for pressure-linked equation) method.The parameters studied include the entrance Reynolds number Re (1×10⁴-5×10⁴), the baffle height (h=10, 20 and 30 mm) and kind of baffles (solid and porous); whereas the baffle spacing S/H are fixed at 1.0 and the working medium is air. The numerical calculations of the flow field indicate that the flow patterns around the porous- and solid-type baffles are entirely different due to different transport phenomena and it significantly influences the local heat transfer coefficient distributions. Relative to the solid-type baffle channel, the porous-type baffle channel has a lower friction factor due to less channel blockage.Concerning the heat transfer effect, both the solid-type and porous-type baffles walls enhanced the heat transfer relative to the smooth channel. It is further found that at the higher baffle height, the level of heat transfer augmentation is nearly the same for the porous-type baffle, the only difference being the Reynolds number dependence. As expected, the centerline-averaged Nusselt number ratio increases with increasing the baffle height because of the flow acceleration.     

Laminar flow and heat transfer in a periodic serpentine channel with semi-circular cross-section

Computational fluid dynamics (CFD) has been used to study fully developed laminar flow and heat transfer behaviour in periodic serpentine channels with a semi-circular cross-section. The serpentine elements are characterised by their wavelength (2L), channel diameter (d) and radius of curvature of bends (R_c), with results reported for Reynolds numbers (Re) up to 450, as well as for a range of geometric configurations (3 < L/d < 12.5, 0.525 < R_c/d < 2.25) at Re = 110. The flow in these channels is characterised by the formation of Dean vortices following each bend. As the Reynolds number is increased, more complex vortical flow patterns emerge and the flow domain becomes increasingly dominated by these vortices. Alignment of flow with vorticity leads to efficient fluid mixing and high rates of heat transfer.Constant wall heat flux (H2) and constant wall temperature (T) boundary conditions and a range of fluid Prandtl numbers (0.7 < Pr < 100) have been examined. High rates of heat transfer and low pressure loss are found relative to fully-developed flow in a straight pipe, with heat transfer enhancements greater than 10 for a Prandtl number of 100.As part of this work, we also obtain an accurate value for the Nusselt number for fully-developed flows in straight semi-circular passages with constant wall temperature, Nu_T = 3.323(±0.001).     

Enhancement of heat transfer in a turbulent channel flow with insertion of a flat body

Experiments have been performed for turbulent channel flow obstructed with a flat body. The local heat transfer coefficient and the wall static pressure were measured on two kinds of flat bodies for which the trailing edge shape differed. The length of the body, the thickness of the body, and the distance between the wall and the body were changed in several steps. The total performance between heat transfer and pressure drop was estimated under conditions of equal pumping power. The total performance hardly changed, even if the trailing edge shape and length of the bodies were different. The averaged heat transfer coefficient increased with increasing thickness of the bodies. However, as the friction factor increased, the performance became worse. When a comparatively thin body was installed near the heating surface, good performance was obtained. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(4): 354–366, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10100     

Confined flow and heat transfer across a triangular cylinder in a channel

In this paper, fluid flow and heat transfer across a long equilateral triangular cylinder placed in a horizontal channel is studied for Reynolds number range 1–80 (in the steps of 5) and Prandtl number of 0.71 for a fixed blockage ratio of 0.25. The governing Navier-Stokes and energy equations along with appropriate boundary conditions are solved by using a commercial CFD solver FLUENT (6.3). The computational grid is created in a commercial grid generator GAMBIT. The flow and temperature fields are presented by stream-line and isotherm profiles, respectively. The wake/recirculation length, mean drag coefficient and average Nusselt number, etc. are calculated for the above range of conditions studied here. The critical value of the Reynolds number (i.e., transition to transient) is found to lie between Re = 58 and Re = 59. The average Nusselt number and the wake length increase with increasing value of the Reynolds number; however, the mean drag coefficient decreases with increasing value of the Reynolds number. Finally, simple correlations for wake length, mean drag coefficient and average Nusselt number are obtained for the range of conditions studied here.     

Analysis of fluid flow and heat transfer in a channel with staggered porous blocks

Fluid flow and heat transfer characteristics in a channel with staggered porous blocks were numerically studied in this paper. The Navier–Stokes and Brinkman–Forchheimer equations were used to model the fluid flow in the open and porous regions, respectively. Coupling of the pressure and velocity fields was resolved using the SIMPLER algorithm. The local thermal equilibrium model was adopted in the energy equation to evaluate the solid and fluid temperatures. The effect of Darcy number, Reynolds number, porous block height and width on the velocity field were studied. In addition, the effects of the above parameters as well as the thermal conductivity ratio between the porous blocks and the fluid on the local heat transfer were analyzed. The pressure drops across the channel for different cases were discussed. The results show that the flow behavior and its associated local heat transfer are sensitive to the variation of the above parameters. It is predicted by the present study that an increase in the thermal conductivity ratio between the porous blocks and the fluid results in significant enhancement of heat transfer at the locations of the porous blocks.     

Experimental study of heat transfer enhancement in a drag-reducing two-dimensional channel flow

In this paper, drag reduction and heat transfer enhancement were studied in a fully developed two-dimensional water channel flow. Surfactant solutions at different concentrations, namely 30, 70, 80 and 90 ppm, were used to examine the influence of surfactant additives on the skin friction drag and heat transfer coefficient. The magnitudes of the maximum achievable drag reduction at the above four different surfactant concentrations are about 7%, 30%, 50% and 55%, respectively. The present results show that there is no heat transfer reduction when 30 ppm of surfactant is added to the flow. With the increase of surfactant concentration to 90 ppm, heat transfer rate was reduced by about 55%. The critical Reynolds number for loss of heat transfer reduction increases with the increase of surfactant concentration. The effect of the low-profile vortex generators on heat transfer rate was examined for the surfactant concentration of 90 ppm. The results show that the averaged Nusselt number is enhanced by 180%, 160% and 150% for the Reynolds numbers of 7000, 12,000 and 16,200, respectively, as compared with that obtained in the surfactant solution without the use of vortex generators and yet the pressure drop penalty for heat transfer enhancement is rather small.     

Laminar periodic flow and heat transfer in a rectangular channel with triangular wavy baffles

This paper presents a numerical analysis of laminar periodic flow and heat transfer in a rectangular constant temperature-surfaced channel with triangular wavy baffles (TWBs).The TWBs were mounted on the opposite walls of the rectangular channel with inline arrangements.The TWBs are placed on the upper and lower walls with attack angle 45?.The numerical is performed with three dif-ferent baffle height ratios (BR=b/H=0.05 0.3) at constant pitch ratio (PR) of 1.0 for the range 100 ≤ Re ≤ 1000.The computational results are shown in the topology of flow and heat transfer.It is found that the heat transfer in the channel with the TWB is more effective than that without baffle.The in-crease in the blockage ratio,BR leads to a considerable increase in the Nusselt number and friction factor.The results indicate that at low BR,a fluid flow is significantly disturbed resulting in inefficient heat transfer.As BR increases,both heat transfer rate in terms of Nusselt number and pressure drop in terms of friction factor increase.Over the range examined,the maximum Nu/Nu0 of 7.3 and f/f0 of 126 are both found with the use of the baffles with BR=0.30 at Re=1000.In addition,the flow structure and temperature field in the channel with TWBs are also reported.     

Radiative heat transfer to flow of a combustible mixture in a vertical channel

The paper studies the flow of a combustible mixture in a vertical channel in the presence of radiative heat transfer as a model for biomass moving bed gasifiers operating in the temperature range 750–1500 K. The simplistic binary reaction A → B is assumed, and both the optically thick (high density gas) and the optically thin (low density gas) situations are considered for the radiative heat transfer. Analytical and numerical solutions are obtained and discussed quantitatively.     

Developing fluid flow and heat transfer in a channel partially filled with porous medium

A three-dimensional computational model is developed to analyze fluid flow in a channel partially filled with porous medium. In order to understand the developing fluid flow and heat transfer mechanisms inside the channel partially filled with porous medium, the conventional Navier–Stokes equations for gas channel, and volume-averaged Navier–Stokes equations for porous medium layer are adopted individually in this study. Conservation of mass, momentum and energy equations are solved numerically in a coupled gas and porous media domain along a channel using the vorticity–velocity method with power law scheme. Detailed development of axial velocity, secondary flow and temperature field at various axial positions in the entrance region are presented. The friction factor and Nusselt number are presented as a function of axial position, and the effects of the size of porous media inside the channel partially filled with porous medium are also analyzed in the present study.