Non-Darcian Effects on the Mixed Convection Heat Transfer in a Metallic Porous Block with a Confined Slot Jet

The present numerical investigation addresses non-Darcian effects on the mixed convection heat transfer in a metallic porous block with a confined slot jet. The generalized model of the momentum equation, which is also known as the Forchheimer-Brinkman extended Darcy model, was used in representing the fluid motion inside the porous layer. The local thermal equilibrium condition was assumed to be valid for the range of the thermophysical parameters considered in the present investigation. The transport equations were solved using the finite element formulation based on the Galerkin method of weighted residuals. The validity of the numerical code used was ascertained by comparing our results with previously published results. Our results revealed that the heat transfer performance of the slot jet was 2.4 times as large as that without the presence of a porous block. In addition, the average Nusselt number was found to increase with a decrease in porosity and an increase in the thermal conductivity ratio. The present results illustrate that the average Nusselt number increases with a decrease in the dimensionless height of the porous layer up to H _porous =  0.05 , after which the Nusselt number decreases.     

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.     

Cooling of Laser Slab by Forced Convection through a Heat Sink

The present study addresses a novel cooling scheme for the high-power solid-state laser slab. The scheme cools the laser slab by forced convection in a narrow channel through a heat sink. Numerical simulations were conducted to investigate the thermal effects of a Nd:YAG laser slab for heat sinks of different materials, including the undoped YAG, sapphire, and diamond. The results show that the convective heat transfer coefficient is non-uniform along the fluid flow direction due to the thermal entrance effect, causing a non-uniform temperature distribution in the slab. The heat sink lying between the coolant fluid and the pumped surface of the slab works to alleviate this non-uniformity and consequently improve the thermal stress distribution and reduce the maximum thermal stress of the slab. The diamond heat sink was found to be effective in reducing both the highest temperature and the maximum thermal stress; the sapphire heat sink was able to reduce the maximum thermal stress but not as effective in reducing the highest temperature; and the undoped YAG heat sink reduced the maximum thermal stress but tended to increase the highest temperature. Therefore, cooling with the diamond heat sink is most effective, and that with the sapphire heat sink follows; cooling with the undoped YAG heat sink may not apply if the highest temperature is a concern.     

Heat Transfer and Friction Characteristics of Perforated Fin with a Longitudinal Slot under Forced Convection

Extended surfaces are used in a variety of heat transfer applications owing to their ability in reducing the convection resistance by exposing a large surface area to the surrounding fluid. Surface modification in the form of perforations is a passive method of increasing the heat transfer rates with the additional benefit of weight reduction. This work deals with numerical investigation of heat transfer and friction from a perforated fin (with and without slot) subjected to forced convection. The perforated fin with slot has been found to have a maximum enhancement in heat transfer with the simultaneous increase in frictional losses versus that of a solid fin. Further, the perforated fin without slot has been able to transfer heat at a relatively higher rate with a considerable reduction in energy loss due to friction in comparison to a solid fin.     

Numerical Investigation of Forced Convection Conjugate Heat Transfer from Offset Square Cylinders Placed in a Confined Channel Covered by Solid Wall

Flow over two isothermal offset square cylinders in a confined channel is simulated for different Reynolds numbers to disclose the forced convection heat transfer from the heated square cylinders to the ambient fluid. The spacing between the cylinder in the normal direction and the blockage ratio are fixed. The channel walls are covered by solid walls of thickness equal to the size of the cylinder and conjugate heat transfer is considered by including these walls. Heat transfer from the cylinders to the ambient fluid as well as that conducted within the solid wall through the conjugate interface boundary are investigated in connection with Reynolds number and are reported for both steady and periodic flows. Simulation is carried out for Reynolds number varying from 10 to 100 with air as the fluid. The onset of the vortex begins when the Reynolds number equals 48. The conjugate interface temperature declines when the Reynolds number grows. The isotherms in the solid wall show two dimensionality near the cylinder region.     

Approximate Analytic Estimate of Axial Fluid Conduction in Laminar Forced Convection Tube Flows with Zero-to-Uniform Step Wall Heat Fluxes

The influence of axial fluid conduction on low Péclet number flows in the thermal entrance region of long circular tubes is investigated in this theoretical study. The convective heat transport of viscous fluids relates to a specific condition under which the first part of the tube ( x h 0) is insulated and the second part of the tube ( x > 0) receives a heat flux of uniform intensity. A conjugate one-dimensional lumped model produces a solution of compressed algebraic form that is able to deliver dependable mean bulk temperatures that are in perfect agreement with those obtained numerically by the formal conjugate two-dimensional distributed model. As a by-product of the succession of algebraic calculations within the platform of the lumped model, the critical Péclet number Pe cr has been easily quantified. This number is a figure-of-merit of remarkable importance in the modeling of forced heat convection in tubes because it establishes the threshold between two contrasting situations: one embracing axial fluid conduction (finite Pe) and the other implicating negligible axial fluid conduction (Pe M X ). In addition, the local wall temperatures were calculated with an approximate engineering procedure, showing good agreement with those determined numerically by the formal conjugate two-dimensional distributed model.     

Numerical Simulation of Mixed Convection in a Two-Dimensional Laminar Plane Wall Jet Flow

A two-dimensional, laminar, incompressible mixed convection with plane wall jet is simulated numerically using the stream function–vorticity method. The buoyancy is assisting the main flow. The flow and heat transfer study is carried out for Re = 300–600, Gr = 10³–10⁷, and Pr = 0.01–15. The streamlines, isotherm contours, similarity profiles, vorticity at the walls, and the local and average Nu values are presented and analyzed. In some cases, similarity behaviour is observed. The vorticity profile at the wall is similar to boundary-layer-type flow. However, for high Gr, the wall vorticity increases in the downstream direction. The average Nusselt number increases when Re, Gr, and Pr are increased.     

Laminar Flow Forced Convection Heat Transfer Behavior of a Phase Change Material Fluid in Finned Tubes

The heat transfer behavior of phase change material fluid (PCM) under laminar flow conditions in circular tubes and internally longitudinal finned tubes was studied. An effective specific heat technique was used to model the phase change process. Heat transfer results for a smooth circular tube with PCM fluid were obtained under hydrodynamically and thermally fully developed conditions. Results for the finned tube were obtained using the H2 and T boundary conditions. It was determined that the Nusselt number was strongly dependent on the Stefan number, fin thermal conductivity value, and height of the fins.     

Laminar Forced Convection Heat Transfer From Isothermal Bodies With Unity Aspect Ratio in Coaxial Air Flow

In this article a semianalytical approach is employed to obtain dimensionless heat transfer correlations for forced convection over three geometries—sphere, cone, and cylinder with unity aspect ratio in laminar axial air flow. The comparison of the present results for a sphere with the previous work shows very good agreement. For example, the average difference between the results of the present model and those of Ahmed and Yovanovich and of McAdams are 4.3% and 0.8%, respectively. Validation of the results for cone and cylinder was done based on available data on drag coefficients. Finally, a general correlation is developed for a rough estimate of forced convection from isothermal bodies of unity aspect ratio and coaxial with the airflow.     

Numerical Analysis of Laminar Forced Convection in Corrugated-Plate Channels with Sinusoidal,Ellipse, and Rounded-vee Wall Shapes

Steady, two-dimensional, developing laminar forced convection is computed in channels of ten modules of three out-of-phase (symmetric) wall corrugations: sinusoidal-wavy-shaped (SWS), rounded-ellipse-shaped (RES), and rounded-vee-shaped (RVS). Different geometric configurations of the three shapes are studied. Fluid flow and heat transfer are examined in a typical module in the fully-developed region for Reynolds numbers in the range of 25 to 300 for a Prandtl number of 2.29. Over the ranges of the geometric parameters studied, the SWS corrugation has, in general, the lowest friction factor and highest average Nusselt number values. The RES corrugation has the highest heat transfer per unit pumping power at an inlet Reynolds number of 300.     

散热片基板开孔对自然对流换热的影响

为研究自然对流情况下矩形散热片基板开孔对换热的影响,采用数值模拟方法对基板开孔的散热片的传热性能进行了分析,讨论了开孔后换热强化的物理机制。结果表明:开孔后发热元件的温度降低;通孔破坏了散热片基板上速度边界层的形成,从而强化了局部表面处的对流换热。     

Performance of Counterflow,Parallel Plate Heat Exchangers under Laminar Flow Conditions

Multilayered heat exchangers were analyzed theoretically and their heat transfer characteristics were clarified. The problem was treated as a two-dimensional, conjugated one with three phases-two fully developed laminar flows and the exchanger wall. From numerical results, the exchanger effectiveness was found to be definitely influenced by the following parameters: Graetz number, heat capacity flow rate ratio, dimensionless wall thickness, and conductance ratios between fluid and wall and between both fluids. Examination of mixed-mean temperature distributions in the exchanger showed that longitudinal wall conduction significantly reduces exchanger effectiveness in the low-Graetz-number region. Experimental results were in fairly good agreement with theoretical predictions.     

Heat Transfer Characteristics of Grooved Fin Under Forced Convection

Material removal from an extended surface in the form of perforations and slots is a proven technique to augment heat transfer rates with a considerable reduction in the surface weight. This work presents the outcomes of experimental investigation on heat transfer characteristics of a plate fin having grooves of various configurations on two broad faces. The experimental data pertaining to heat transfer have been collected by varying Reynolds number from 1500 to 5000, for transverse grooved, inclined grooved, V-grooved, and multi-V-grooved fin. The results of the grooved fin are compared with that of a smooth conventional fin to gauge the heat transfer performance of modified fin. The maximum enhancement in Nusselt number corresponds to the inclined groove fin, whereas the highest value of grooved fin effectiveness is obtained for the multi-V-grooved fin. The Nusselt number correlations are presented for different fin configurations tested in this work.     

Finite-Element Solutions for Laminar Forced Convection in the Thermal Entrance Region of Ducts

Abstract

This paper is concerned with velocity and heat transfer characteristics in the thermal entrance region of hydrodynamically developed laminar duct flows. Consideration is given to the general situation in which ducts have arbitrary but constant cross sections. The finite-element method is used for the first time, and with good results, for the solution of this class of problems.     

Effects of a Thick Plate on the Excess Temperature of Iso-Heat Flux Heat Sources Cooled by Laminar Forced Convection Flow: Conjugate Analysis

A conjugate analysis via the finite volume approach is performed to study the effects of a thick plate on the excess (peak) temperature of an iso-heat flux heat source cooled by laminar forced convection flow. A thick plate of temperature-dependent thermal conductivity is placed between the heat sources and the cooling fluid. A cooling fluid flows over the thick plate and removes the heat by laminar forced convection. On account of the two-dimensional heat redistribution in the finite thick plate with one face subjected to iso-heat flux and the other face exposed to forced flow, the interface ceases to be an iso-heat flux and, consequently, reduces the excess temperature of the heat sources. In the numerical analysis, the thickness of the plate is relaxed one by one to search for the optimal thickness that minimizes the excess temperature. It is shown that the reduction in the excess temperature due to the insertion of the thick plate with optimal thickness depends upon the Reynolds number of the fluid flow and the fluid-to-solid thermal conductivity ratio.     

Role of Melt Convection on Optimization of PCM-Based Heat Sink Under Cyclic Heat Load

In this article, we study the thermal performance of phase-change material (PCM)-based heat sinks under cyclic heat load and subjected to melt convection. Plate fin type heat sinks made of aluminum and filled with PCM are considered in this study. The heat sink is heated from the bottom. For a prescribed value of heat flux, design of such a heat sink can be optimized with respect to its geometry, with the objective of minimizing the temperature rise during heating and ensuring complete solidification of PCM at the end of the cooling period for a given cycle. For given length and base plate thickness of a heat sink, a genetic algorithm (GA)-based optimization is carried out with respect to geometrical variables such as fin thickness, fin height, and the number of fins. The thermal performance of the heat sink for a given set of parameters is evaluated using an enthalpy-based heat transfer model, which provides the necessary data for the optimization algorithm. The effect of melt convection is studied by taking two cases, one without melt convection (conduction regime) and the other with convection. The results show that melt convection alters the results of geometrical optimization.     

Heat Transfer from Pulsating Laminar Impingement Slot Jet on a Flat Surface with Inlet Velocity: Sinusoidal and Square Wave

Heat transfer from a pulsating laminar impingement slot jet on a flat surface was investigated numerically and experimentally. Inlet velocity was considered sinusoidal velocity and square wave velocity. Experimental studies were done only for the sinusoidal velocity state. An inverse heat conduction method, conjugated gradient method with adjoint equation, was used for the experimental estimation of the local heat transfer coefficient along the target surface. Effect of the square wave velocity of the laminar impingement slot jet was studied numerically. The results show pulsations in flow change flow patterns and the thermal boundary layer thickness because of the newly forming thermal boundary layer is extremely small each time the flow is resumed. Heat transfer rate in this state enhances due to pulsating inlet velocity in comparison with steady state. Heat transfer increases with increasing pulsation amplitude. Enhancement in mean heat transfer on the target plate for sinusoidal velocity is rather than square wave velocity.     

Unsteady Mixed Convection of a Confined Jet in a Fluid-Superposed High Porosity Medium

Numerical study of a confined jet impingement cooling of a fluid-superposed porous medium heated from below is conducted to investigate the oscillatory mixed convection. The effects of the Rayleigh number (2 × 10⁵ ≤ Ra ≤ 1 × 10⁶) and the Darcy number (1 × 10^?5 ≤ Da ≤ 5 × 10^?4) on the heat transfer are investigated for different Péclet numbers. It is found that, the average Nusselt number increases with the increase in Darcy number or Rayleigh number. The values of average Nusselt number are found to oscillate with time for some combination of Rayleigh numbers (Ra ≥ 4 × 10⁵) and Péclet numbers (200 ≤ Pe ≤ 1000), at which the oscillatory convection occurs. The oscillation of average Nusselt number is investigated for different porous medium height and porous medium-to-fluid heat capacity ratio.     

Analysis of Laminar Forced Convection of Air Crossflow in In-Line Tube Banks with NonSquare Arrangements

ABSTRACT

Numerical analysis is made of forced-convection heat transfer in laminar two-dimensional steady crossflow in banks of plain tubes in square and nonsquare in-line arrangements. A finite-volume method with a nonorthogonal, boundary-fitted grid and co-located variable storage is used to solve the Navier–Stokes equations and energy conservation equation for a tube bundle with five longitudinal rows, including inlet and outlet sections. Local and overall heat transfer and fluid flow results are presented at combinations of transverse and longitudinal pitch-to-diameter ratios of 1.25, 1.5, and 2.0 at Reynolds numbers of 100 and 300 for a Prandtl number of 0.71. A comparison of the present study results with well-established experiments and empirical correlations showed good overall agreement. New equations are proposed for a correction factor for the effects of nonsquare arrangements on average friction factor.     

气体横掠单管强制对流换热的大涡模拟

应用大涡模拟与二价全展开ETC有限元离散格式相结合的方法对气体横掠单管强制对流换热进行了数值模拟,分别计算了气体横掠团管和方管时的温度场,得到了管壁平均换热系数,数值结果与实验关联式符合较好。同时还表明大涡模拟方法善于捕捉温度场以及流场涡系的时间演化过程,非常适合于具有大尺度涡的绕流运动温度场的分析。