Hybrid Turbulence Modeling of Liquid Spray Impingement on a Heated Wall with Arbitrary Lagrangian Eulerian Method

Numerical simulation of two phase spray impingement on a heated wall was carried out. Hybrid turbulence modeling was used for analysis where large eddy simulation was employed away from the wall, and a k-epsilon model was employed near the wall. The effect of vortex motion on turbulent heat flux values was analyzed using different Reynolds numbers of impingement and at different angles. It was observed that the turbulent heat flux attained maximum values with high vortex formation. The ejection of hot fluid from the surface was predominant when compared to the down sweep motion of the cold fluid. The Nusselt number plot indicated high heat transfer rates for higher Reynolds number.     

Convection Heat Transfer Enhancement on Recirculating Flows in a Backward Facing Step: The Effects of a Small Square Turbulence Promoter

This work addresses the numerical simulation of incompressible turbulent recirculating channel flows in a backward-facing step. The effects of a small square turbulence promoter on convection heat transfer are evaluated through a parametric study. The governing equations comprise the time-averaged mass, linear momentum, and energy conservation principles in conjunction with the two-equation k–epsilon turbulence model. The study is focused on the assessment of the local and global Nusselt numbers at the channel stepped wall. The main results indicate that a maximum increment around 15% on the average Nusselt number can be achieved by using a small turbulence promoter to disturb the flow. Furthermore, it was found that the peak of the local Nusselt number on the stepped wall is located in the region where the turbulent diffusion is maximum in the near wall region.     

Conjugate Heat Transfer from Sudden Expansion Using Nanofluid

Conjugate heat transfer from sudden expansion using nanofluid is studied numerically. The governing equations are solved using unsteady stream function-vorticity formulation method. Results are compared with zero nanoparticle fluid to exhibit the role of nanoparticle. The effect of volume fraction of nanoparticles and type of nanoparticles on heat transfer are examined and found to have a significant impact. Local Nusselt number and average Nusselt number are reported in connection with various nanoparticle, volume fraction, and Reynolds number for expansion ratio 2. Two dimensionality is more pronounced in the solid wall up to recirculation length. Local Nusselt number reaches peak values near the reattachment point and reaches asymptotic value in the downstream. Bottom wall eddy and volume fraction show significant impact on average Nusselt number. The wall thickness causes larger temperature gradient at the conjugate interface boundary, which leads to larger average Nusselt number.     

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.     

Effects of Heating Location and Size on Natural Convection in Partially Heated Open-Ended Enclosure by Using Lattice Boltzmann Method

Natural convection heat transfer in an square enclosure, consisting of a partially heated west wall with east end open to ambient, is investigated numerically by using an in-house computational fluid dynamics solver based on thermal lattice Boltzmann method. In particular, the influences of Rayleigh number (10³–10⁶), heating location (bottom, middle, and top) on west wall, and dimensionless heating length (0.25–0.75) on momentum and heat transfer characteristics of air are presented and discussed. The streamline patterns show bifurcation at the lowest Rayleigh number for bottom and middle heating, whereas at the highest Rayleigh number, all heating positions yield bifurcation and elongation of flow patterns with a secondary vortex near the lower side of open end. The middle and bottom heating locations show a linear increase in Nusselt number with heater size, whereas inverse dependence is seen for top heating. The maximum heat transfer is observed in the case of middle heating. As expected, average Nusselt number increased with increasing Rayleigh number. Finally, the functional dependence of the average Nusselt number on flow governing parameters (Rayleigh number and heating length) for different heating locations is presented as a simple predictive empirical relationship.     

Large eddy simulation of coherent structure of impinging jet

The flow field of a rectangular exit, semi-confined and submerged turbulent jet impinging orthogonally on a flat plate with Reynolds number 8500 was studied by large eddy simulation (LES). A dynamic sub-grid stress model has been used for the small scales of turbulence. The evolvements such as the forming, developing, moving, pairing and merging of the coherent structures of vortex in the whole regions were obtained. The results revealed that the primary vortex structures were generated periodically, which was the key factor to make the secondary vortices generate in the wall jet region. In addition, the eddy intensity of the primary vortices and the secondary vortices induced by the primary vortices along with the time were also analyzed.     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

螺旋管内水和蒸汽局部传热特性研究

以高温气冷堆蒸发器为背景,采用FLUENT软件模拟了单相水和蒸汽在不同尺寸螺旋管内部的流动和传热过程,研究了壁面局部传热特性。计算结果表明,远离螺旋中心线一侧局部传热较强而靠近螺旋中心线一侧传热较弱,壁面Nu周向分布非常不均匀。管径与螺旋直径之比是主要影响因素,当其值增大时截面温度极值点向螺旋中心线外侧移动,加剧了温度分布和Nu分布的不均匀性。在层流向湍流过渡区内,Re的增大使截面各点温度梯度均有所增加,同时也增大了Nu周向分布的不均匀程度,但在旺盛湍流区内Re对Nu分布无明显影响。壁面热边界条件形式对局部Nu周向分布没有显著影响。给出了局部Nu的估算式。     

Turbulence Models for Fluid Flow and Heat Transfer Between Cross-Corrugated Plates

Three-dimensional numerical predictions of fluid flow and heat transfer between cross-corrugated plates were obtained for the same geometry and grid using eight turbulence models, i.e., LBKE, SKE, RKE, RNGKE, RSM, KW, SST and LES, for the purpose of model performance evaluation. The average Colburn factor j, friction factor f, and local Nusselt number distribution were presented and compared with available experimental data. The velocity, temperature, and turbulent viscosity ratio distributions were recorded and discussed. It has been found that all models can predict practically satisfactory j and acceptable f within the current Reynolds number range. LBKE and SST provide the best overall agreement with experimental data and thus are highly recommended for application. Taking into account its robustness and economy, SKE with enhanced wall treatment is also recommended for CC channel flow and heat transfer simulation. In addition, near wall treatment approach seems to be significant for the current wall-bounded flow simulation. Since some models predict similar j and f values but very different velocity and temperature distributions, it seems not quite sufficient to only compare the overall heat transfer and pressure drop data between simulation and experiment for comprehensive model evaluations.     

倾斜射流对移动平板表面紊动和传热特性的影响

采用雷诺应力湍流模型和Simplic算法对半封闭槽道内倾斜射流冲击移动平板的流动和传热特性进行了数值模拟,研究了不同射流角度和不同平板移动速度下平板近壁湍动能和板面努塞尔数的变化.结果表明：射流角度和平板运动速度对平板近壁湍动能和表面努塞尔数值分布影响显著;当入射角与平板运动方向相同时,板速的升高提高了近壁面的湍动能,但是降低了冲击区域的局部努塞尔数值;平板表面的平均努塞尔数值随板速的提高先降低后大幅升高,高速下角度对平板表面的平均传热效果影响较小;当入射角为80°,平板运动方向与入射方向相反且板速和射流速度相同时,在移动平板表面能够获得较佳的紊动和传热效果.     

Numerical simulation of vortex structures and heat transfer behind a hill in a laminar boundary layer

The present study examines a three‐dimensional numerical simulation of vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. A vortex pair is formed symmetrically in the separation bubble behind the hill, and a hairpin vortex is periodically shed in the wake. The hairpin vortex moves downstream with time, and the gradient of the head of the hairpin vortex increases. Further downstream, the hairpin vortex is deformed to an Ω‐shaped structure. In the growth process of the hairpin vortex, horn‐shaped secondary vortices grow near the wall. The dissipation rate of the temperature fluctuation around the hairpin vortex increases because the heated fluid near the wall is removed to the free stream by Q2 ejection. Heat transfer increases due to the legs of the hairpin vortex and secondary vortices. These vortices generate high turbulence in the flow field and also contribute to an increase in Reynolds shear stress and turbulent heat flux. © 2008 Wiley Periodicals, Inc. Heat Trans Asian Res, 37(7): 398–411, 2008; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20217     

Numerical prediction on laminar heat transfer in square duct with 30° angled baffle on one wall

A numerical investigation on periodic laminar flow and heat transfer behaviors in a three-dimensional isothermal wall square duct fitted with 30° angled baffles on lower duct wall only is presented. The computations based on a finite volume method with the SIMPLE algorithm have been conducted for the fluid flow in terms of Reynolds numbers ranging from 100 to 2000. The angled baffles with attack angle of 30° are mounted periodically on the lower duct wall to generate a longitudinal vortex flow through the tested duct. Effects of different baffle height and three pitch length ratios on heat transfer and flow characteristics in the duct are investigated. The study shows that the longitudinal vortex flow created by the baffle helps to induce impinging flows over the baffle trailing end sidewall and the inter-baffle cavity wall resulting in drastic increase in heat transfer rate over the test duct. The computational results reveal that the Nusselt number ratio and the maximum thermal enhancement factor values for using the angled baffle are, respectively, found to be about 7.9 and 3.1 at Re = 2000, BR = 0.3 and PR=1.5.     

刚性壁面法向振动对流动换热的影响

采用理论推导与数值模拟结合的方法,通过求解流动方程及能量方程得到壁面法向振动下流体流动特性变化规律,并分析振动对换热的影响.结果表明:层流状态下,低强度振动使壁面附近流体质点产生法向速度,但影响范围很小,对换热影响有限;随着振动强度的增大,流场逐渐转变为湍流,导致换热系数提升.通过数值模拟计算壁面平均努塞尔数随振动强度...     

Effect of wall proximity on forced convection in a plane channel with a built-in triangular cylinder

A numerical investigation has been carried out to analyze the effect of wall proximity of a triangular cylinder on the heat transfer and flow field in a horizontal channel. Computations have been carried out for Reynolds numbers (based on triangle width) range of 100–450 and gap widths (a/h) 0.5, 0.75 and 1. Results are presented in the form of instantaneous contours of temperature, vorticity, with some characteristics of fluid flow and heat transfer; such as time-averaged and instantaneous local Nusselt number, skin friction coefficient along bottom channel's wall, and drag coefficient. Results show that approaching triangular cylinder in the wall, removes vortex shedding and subsequently the heat transfer rate decreases at low Reynolds number. By decreasing the vortex shedding, drag coefficient decrease as triangular cylinder approaches the wall of the channel. The variation of vortex formation has a more significant suppression effect on the skin friction coefficient than the Nusselt number.     

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     

FLUID DYNAMIC BEHAVIOR OF HEAT EXCHANGERS WITH ACTIVE CAVITIES: A NUMERICAL STUDY

Numerical simulations have been performed on a series of cavities (called active cavities here) with a peculiar, purpose-fitted geometry, located on the external surface of tubular heat exchanger passages and on one side of the active wall in a flat plate heat exchanger. An incompressible fluid with a flow Reynolds number of 42,000 was considered and three turbulence models were compared: k- k , Spalart--Allmaras and large eddy (LES). The purpose of the cavities is to increase the heat transfer on the surface on which they are applied: some limited comparisons with an independent set of experimental data are also presented.     

Convective heat transfer to water near the critical region in a horizontal square duct

Numerical analysis has been carried out to investigate forced convective heat transfer to water near the critical region in a horizontal square duct. Near the critical point convective heat transfer in the duct is strongly coupled with large variation of thermophysical properties such as density and specific heat. Buoyancy force parameter has also severe variation with fluid temperature and pressure in the duct. There is flow acceleration along the horizontal duct resulted from fluid density decrease due to the heat transfer from the wall. Local heat transfer coefficient has large variation along the inner surface of the duct section and it depends on pressure. Nusselt number on the center of the bottom surface also has a peak where bulk fluid temperature is higher than the pseudocritical temperature and the peak decreases with the increase of pressure. Flow characteristics of velocity, temperature, and local heat transfer coefficient with water properties are presented and analyzed. Nusselt number distributions are also compared with other correlations for various pressures in the duct.     

Near-wall modeling of an isothermal vertical wall using one-dimensional turbulence

One-dimensional turbulence (ODT) modeling approaches are considered for describing the heat transfer from a vertical isothermal wall. In this approach, near wall gas-phase conduction processes are treated exactly while the effect of turbulent mixing is accounted for using triplet mapping stirring events. A new buoyancy generation production term is proposed, based on the vorticity transport scaling arguments, to account for the generation of large scale eddy mixing events. Both temporal and spatial implementations of ODT are explored and compared to the experimental data of Tsuji and Nagano. Comparisons of velocity, temperature, and Nusselt number show overall excellent agreement of simulation results to experimental data and to established inner and outer scaling laws for buoyancy driven boundary layers.     

Large Eddy Simulation of Turbulent Heat Transfer in a Channel With a Square Cylinder

Three-dimensional turbulent channel flow and heat transfer in the presence of a square cylinder were investigated numerically for a Reynolds number of 3000. Turbulent flow was simulated using large eddy simulation in which the eddy viscosity was evaluated by a subgrid-scale model of structure function and selective structure function. The attached wall boundary layer of the channel starts to separate if an insulated cylinder is placed in the channel, and a subsequent recirculation zone appears downstream of the cylinder. Nusselt number distribution along the floor of the channel showed an increase in the downstream part of the cylinder with a relative maximum slightly downstream of the reattachment point. The position of the cylinder in cross-stream direction affects the heat transfer from the channel wall. This quantity increases with decreasing the distance between the cylinder and the channel wall. A correlation was developed for the mean total Nusselt number over the floor of the channel with the position of the cylinder in the channel.     

NUMERICAL PREDICTIONS OF HEAT TRANSFER AND FLUID FLOW CHARACTERISTICS FOR SEVEN DIFFERENT DIMPLED SURFACES IN A CHANNEL

Heat transfer and fluid flow characteristics for seven different dimpled surfaces on one surface of a channel are predicted numerically using version 6.1.18 of FLUENT. The turbulent model employed is a realizable κ–? model without a wall function. The different dimples investigated are spherical dimples, tilted cylinder dimples, cylinder dimples, in-line triangular dimples, reverse in-line triangular dimples, staggered triangular dimples, and reverse staggered triangular dimples. Results show the existence of a centrally located vortex pair and vortex pairs near the spanwise edges of each dimple for the three circular dimple types, which augment local magnitudes of eddy diffusivity for momentum and eddy diffusivity for heat. Advection of reattaching and recirculating flows from locations within the spherical-type dimple cavities, as well as strong instantaneous secondary flows and mixing within the vortex pairs, are especially apparent. For the four triangular types of dimples, only one primary flow circulation zone generally is present within individual dimples. In all cases, regions of augmented streamwise vorticity show approximate correspondence to locations where eddy diffusivities for momentum and heat are increased. Overall, the highest heat transfer augmentations, and the most significant local and overall increases to eddy diffusivity for momentum and eddy diffusivity for heat, are produced by the spherical dimples and the tilted cylinder dimples.