Experimental Study of the Heat Transfer Enhancement from a Circular Cylinder in Laminar Pulsating Cross-flows

Experiments were designed to investigate the heat transfer enhancement from a single heated circular cylinder in laminar pulsating cross-flows. Several parameters were explored, including Strouhal number between 0.18 and 2.80, Reynolds number between 205 and 822, and pressure amplitude p_rms between 40 Pa and 276 Pa. It was observed that the heat transfer enhancement factor increased up to 2.10, and an empirical correlation between the convection heat transfer coefficient and pulsating flows was developed. Results found that the heat transfer enhancement factor decreases with Strouhal number. Increasing Reynolds number was found to have a negative impact on the heat transfer enhancement factor when the pulsating frequency is relatively low, but with a higher pulsating frequency, an increasing Reynolds number increases the heat transfer enhancement factor. Larger pressure amplitude was found to continuously produce larger heat transfer enhancement factor when both Strouhal number and Reynolds number are kept constant.     

ENHANCEMENT OF HEAT TRANSFER IN TURBULENT SEPARATED AND REATTACHING FLOW BY LOCAL FORCING

This article presents numerical solutions for solving the problem of a mixed convective micropolar fluid flow and heat transfer along a vertical wavy surface with a discontinuous temperature profile. The overall surface is equally divided into a heated section succeeded by an unheated section alternately. The problems in the present study have been formulated by using a simple transposition theorem and the cubic spline collocation method. Eringen has applied the spline alternating direction implicit (SADI) procedure to solve the governing momentum, angular momentum, and energy equations those formulated. Along the wavy surface, the velocity, temperature, and microrotation profiles are presented. The influences of micropolar parameters R, u , geometry, and Gr/Re 2 number on the skin friction coefficient and Nusselt number have been studied in this work. The results demonstrate that the skin friction coefficient consists of a mixture of two harmonics in micropolar fluids and in Newtonian fluids. As the vortex viscosity parameter (R) increases, the heat transfer rate decreases, but the skin friction increases. In addition, when the spin gradient viscosity parameter ( u ) increases, the skin friction decreases. Comparisons between a Newtonian fluid and a micropolar fluid are also discussed.     

Visualization of heat flow in a vertical channel with fully developed mixed convection

A study on visualization of heat flow in three channels with laminar fully developed mixed convection heat transfer is performed. The first channel is filled with completely pure fluid; the second one is completely filled with fluid saturated porous medium. A porous layer exists in the half of the third channel while another half is filled with pure fluid. The velocity, temperature and heat transport fields are obtained both by using analytical and numerical methods. Analytical expression for heat transport field is obtained and presented. The heatline patterns are plotted for different values of Gr/Re, thermal conductivity ratio, Peclet and Darcy numbers. It is found that the path of heat flow in the channel strongly depends on Peclet number. For low Peclet numbers (i.e., Pe = 0.01), the path of heat flow is independent of Gr/Re and Darcy numbers. However, for high Peclet numbers (i.e., Pe = 5), the ratio of Gr/Re, Darcy number and thermal conductivity ratio influence heatline patterns, considerably. For the channels with high Peclet number (i.e., Pe = 5), a downward heat flow is observed when a reverse flow exits.     

Heat transfer downstream of a 3D confined square cylinder under flow pulsation

Abstract

This 3D study deals with the effect that a low Reynolds number (Re) pulsating water flow has on both the heat transfer to the walls of a square section channel in which a square section prism was located, and the associated pumping power. The prism blockage ratio was 2.5/1. Re, based on the prism cross-section height, varied from 100 up to 200. The inflow velocity profile was sinusoidal. Taking St₀ as the Strouhal number of the vortex shedding downstream of the prism at Re 100, the inflow Strouhal of the pulsating cases varied between 0.125 St₀ and 4 St₀. The results show that, for this particular geometry and parametric space, flow pulsation generally enhances heat transfer although its extent depends on Re and pulsation frequency. In the most favorable case, heat transfer doubles as compared to the clean channel case at the expense of tripling the pumping power.     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]     

Comparison of Mixed Convection in a Square Cavity with an Oscillating versus a Constant Velocity Wall

This article presents a numerical investigation of unsteady laminar mixed convection heat transfer in a two-dimensional square cavity. The cavity is configured such that one of the vertical walls is cooled and slides either with a constant speed or with a sinusoidal oscillation. A portion of the opposite stationery wall is heated by a constant temperature heat source while, the remaining walls of the cavity are thermally insulated. Different configurations of sliding wall movement and a series of Richardson numbers and Strouhal numbers are tested. The results indicate that the direction and magnitude of the sliding wall velocity affect the heat transfer rate. At low Richardson numbers, the average heat transfer rate for the cavity with an oscillating wall is found to be lower compared to that for the cavity with a constant velocity wall. In addition, at a fixed Richardson number, as the Strouhal number decreases the oscillation frequency of average Nusselt number on the vertical walls decreases; however, the oscillation amplitude of average Nusselt number increases.     

空冷凝汽器椭圆翅片椭圆管束外空气的流动与传热特性

研究空冷凝汽器椭圆翅片椭圆管管束外空气的流动与传热特性,对火电站空冷岛的设计与运行具有重要意义.通过CFD模拟,获得了椭圆翅片椭圆管管束外冷却空气的流场和温度场,计算得到了空冷凝汽器冷却空气对流换热平均Nu和摩擦系数f随Re的变化规律,并采用最小二乘法拟合得到了相应的关联式.结果表明:随冷却空气流动Re的增大,Nu增大,f减小.     

Thermal performance criteria of elliptic tube bundle in crossflow

In this work, the thermofluid characteristics of the elliptic tube bundle in crossflow have been investigated. Experimental and numerical investigations of the turbulent flow through bundle of elliptic tubes heat exchanger are carried out with a particular reference to the circular tube bundle. The investigation covers the effects of key design parameters of Reynolds numbers (5600–40,000), minor-to-major axis ratios (0.25, 0.33. 0.5 and 1) and flow angles of attack (0–150°). Five bundles of elliptic tube heat exchangers with different axis ratios were designed and manufactured in staggered manner. Numerical CFD modeling using finite volume discretization method was conducted to predict the system performance extensively. Four methods were presented to resort a metric that expresses the thermal performance criteria of the elliptic tube bundle. The results indicated that, increasing the angle of attack clockwise until 90° enhances the convective heat transfer coefficient considerably. The maximum thermal performance under constraint of a fixed pumping power or a mass flow rate was obtained at a zero angle of attack and the minimum thermal performance occurred at an angle of attack equals 90°. The best thermal performance of the elliptic tube heat exchanger was qualified with the lower values of Reynolds number, axis ratio and angle of attack.     

Optimization of the angle of attack of delta-winglet vortex generators in a plate-fin-and-tube heat exchanger

Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy-carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks. In this study optimal angles of attack of the delta-winglets are investigated based on the Pareto optimal strategy. The optimization process combines a CFD analysis, genetic algorithms and the response surface methodology. The angle of attack of a pair a delta-winglet-type VGs mounted behind each tube is varied between β = ?90° and +90°. Three circular tube rows with inline and staggered tube arrangements are investigated for Reynolds numbers from 200 to 1200 (based on the inlet height and inlet velocity). The flow structure and heat transfer behavior is analyzed in detail for certain cases and the staggered and the inline tube arrangements are compared. Finally, for each of these arrangements the optimal sets of angles of attack for different Reynolds numbers are presented.     

Heat transfer to the transpired turbulent boundary layer

This paper contains a summarization of five years work on an investigation on heat transfer to the transpired turbulent boundary layer. Experimental results are presented for friction coefficient and Stanton number over a wide range of blowing and suction for the case of constant free-stream velocity, holding certain blowing parameters constant. The problem of the accelerated turbulent boundary layer with transpiration is considered, experimental data are presented and discussed, and theoretical models for solution of the momentum equation under these conditions are presented. Data on turbulent Prandtl number are presented so that solutions to the energy equation may be obtained. Some examples of boundary layer heat transfer and friction coefficient predictions are presented using one of the models discussed, employing a finite difference solution method.     

Effects of winglets to augment tube wall heat transfer in louvered fin heat exchangers

The louvered fin heat exchanger, a type of compact heat exchanger, has been used heavily in the automotive and air conditioning industries for the last several decades. The majority of past research, aimed towards improving louvered fin exchanger efficiency, has focused on optimizing various parameters of the louvered fin. The experimental study presented in this paper concentrates instead on augmenting the heat transfer along the tube wall of the compact heat exchanger through the use of winglets placed on the louvers. The experiments were completed on a 20 times scaled model of an idealized louvered fin exchanger with a fin pitch to louver pitch ratio of 0.76 and a louver angle of 27°. The Reynolds numbers tested, based on louver pitch, were between 230 and 1016. A number of geometrical winglet parameters, including angle of attack, aspect ratio, direction, and shape, were all evaluated based on heat transfer augmentation, friction factor augmentation, and efficiency index (combination of both augmentations). In an attempt to optimize these winglet parameters, tube wall heat transfer augmentations as high as 39% were achieved with associated friction factor augmentations as high as 23%.     

不同纵向涡发生器流动传热的数值模拟

利用三维数值模拟的方法对带有3种异形纵向涡发生器的H型翅片椭圆管换热器的空气侧流动传热特性进行研究。基于H型翅片椭圆管束,讨论了在不同雷诺数下,纵向涡发生器的摆放位置、摆放攻角和形状对空气侧流动传热的影响。研究表明:纵向涡发生器能够将高能量的流体引向流速较低的壁面区域,使冷热流体之间的混合加剧,增强流体的湍流动能,进而达到强化传热的效果;与无纵向涡发生器的管束相比,带纵向涡发生器管束的传热效果有明显的提高;当纵向涡发生器后置时,换热器的传热效果最优;在雷诺数相同,攻角为30°时,流体的传热性能和阻力特性均达到最优;相同攻角摆放时,椭圆角矩形发生器的传热性能和阻力因子均优于其他两种形式的发生器。研究结果为烟气余热回收系统换热器传热性能强化提供理论依据。     

双向扭曲管传热与流阻特性及其多目标优化

以水为工质,对双向扭曲管湍流状态下传热与流阻特性进行数值计算和准确性实验验证,分析了其强化换热机理及截面尺寸a与导程S对其传热流阻的影响,并在其基础上对双向扭曲管结构参数进行多目标优化以及努塞尔数和阻力系数的关系式拟合。结果表明:相同工况下,双向扭曲管的综合换热性能优于光管和扭曲管,在Re小于20 000时具有较好的强化效果,且高出扭曲管3.92%;流体旋转流动的方向周期性改变,进一步强化了对流传热;当a增加到一定程度,管内的湍流程度不再增加;相同Re下,η、Nu和f对导程S的变化反应较灵敏。在Re=2 300～20 000、介质为水时,拟合关系式具有较高的准确性,且通过优化得到了结构的最优组合。     

Investigation on laminar convection heat transfer in fin-and-tube heat exchanger in aligned arrangement with longitudinal vortex generator from the viewpoint of field synergy principle

3-D numerical simulation results are presented for laminar flow heat transfer of the fin-and-tube surface with vortex generators. The effects of Reynolds number (from 800 to 2000), the attack angle (30° and 45°) of delta winglet vortex generator are examined. The numerical results are analyzed from the viewpoint of field synergy principle. It is found that the inherent mechanism of heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, the second flow generated by the vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. In addition, the heat transfer enhancement of delta winglet with the attack angle of 45° is larger than that of 30°, while the delta winglet with the attack angle of 45° results in an increase of the pressure drop, however, the delta winglet with the attack angle of 30° results in a slight decrease.     

Computational analysis of coupled radiation-convection dissipative non-gray gas flow in a non-Darcy porous medium using the Keller-box implicit difference scheme

The effects of thermal radiation parameter (F), transpiration (γ), Eckert number (Ec), Prandtl number (Pr), buoyancy (Grashof number Gr), a Darcy parameter (Re/Gr Da) and a Forcheimmer inertial parameter (Fs Re/Gr Da) on two-dimensional free convective flow of an optically thin, near-equilibrium, non-gray gas past a vertical surface in a non-Darcy porous medium, are studied using the robust Keller finite-difference technique incorporating Newtonian quasilinearization and block-tridiagonal elimination. The Darcy–Brinkman–Forcheimmer inertial-viscous flow model is used for the momentum equation and the Cogley–Vincenti–Giles formulation is adopted to simulate the radiation component of heat transfer. The one-dimensional thermal radiation model works successfully for gases in the optically thin limit. Pseudo-similarity transformations are employed to simplify the highly non-linear partial differential equations for momentum and heat transfer into numerically manageable pseudosimilar ordinary differential equations which are solved with Keller's box method. Effectively, the radiation contribution is seen to take the form of a linear temperature term Fθ coupled with the streamwise pseudo-similar variable ξ. Local wall shear stress and local heat transfer rates are systematically computed for a wide selection of radiation parameter F values. The results are presented graphically for different gases. © 1998 John Wiley & Sons, Ltd.     

NUMERICAL INVESTIGATIONS OF HEAT TRANSFER OVER A MOVING SURFACE DUE TO IMPINGING KNIFE-JETS

Turbulent flow field and heat transfer from an array of impinging horizontal knife jets on a moving surface have been investigated using large eddy simulation (LES) with a dynamic subgrid stress model. The surface velocity directed perpendicular to the jet plane is varied up to two times the jet velocity at the nozzle exit. Performance of a horizontal knife jet with an exit angle of 60° is compared with the standard axial jet. It has been observed that increasing surface motion reduces heat transfer for both types of jets. However, the amount of heat transfer from the knife jets is more than that from the axial jets when the surface velocity is within the order of half the jet velocity at the nozzle exit. For further increase in surface velocity, heat transfer from the knife jets is, however, less than that in the case of axial jets if the Reynolds number (Re) is low. For higher Re and higher surface velocity, the heat transfer from either type of jets is of comparable magnitude.     

Conjugate Mixed Convection Heat Transfer in Plane Laminar Wall Jet Flow

A numerical investigation of heat transfer from a uniformly heated slab of finite thickness by plane laminar wall jet flow under combined forced and natural convection, i.e., mixed convection, is presented. The problem has been solved for two classical cases such as Pr ? 1 and Pr ? 1. The effects of the Grashof number (Gr), Reynolds number (Re), Prandtl number (Pr), and thermal conductivity ratio (R_k) between the slab and fluid medium are investigated on the heat transfer characteristics, i.e., local Nusselt number, interface temperature, and average Nusselt number.     

Numerical investigations of developing flow and heat transfer in raccoon type microchannels under inlet pulsation

The present study investigates numerically the simultaneously developing unsteady laminar fluid flow and heat transfer inside a two dimensional wavy microchannel caused by a sinusoidal varying velocity component at an inlet. The flow was both thermally and hydro dynamically developing while the channel walls were kept at a uniform temperature. The simulation was performed in the laminar regime for Prandtl number 7(water) and Reynolds number ranging from 0.1 to 100. A Wavy microchannel having non-dimensional hydraulic diameter 1 with varying pulsating amplitude and frequency represented by the Strouhal number was designed for the given Reynolds number range. Based on the comparison with steady flow in a wavy channel it was found that imposed sinusoidal velocity at the inlet can provide improved heat transfer performance at different amplitudes (0.2, 0.5, 0.8) and frequencies (1, 5, 10) while keeping the pressure drop within acceptable limits.     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.