Numerical study of natural convection from a heat generating element using a locally divergence free FEM and comparison with experiment

A locally divergence free numerical scheme based on a hybrid finite element-finite volume method together with a restricted domain approach is used for the numerical solution of laminar conjugate natural convection in a vertical channel containing a short planar heat generating element. Numerical simulations have been carried out for modified Rayleigh numbers in the range 1 × 10⁵–8.1 × 10⁷. The numerically evaluated temperature rise above the ambient of the heat generating element is found to agree well with experimental data. A correlation for dimensionless temperature rise as a function of dimensionless volumetric heat generation is presented. Natural convection, Hybrid FEM-FVM method, Restricted domain approach, Volumetric heat generation.     

MHD-conjugate heat transfer analysis for a vertical flat plate in presence of viscous dissipation and heat generation

In this paper, the effects of magnetic field, viscous dissipation and heat generation on natural convection flow of an incompressible, viscous and electrically conducting fluid along a vertical flat plate in the presence of conduction are investigated. Numerical solutions for the governing momentum and energy equations are given. A discussion is provided for the effects of magnetic parameter, viscous dissipation parameter and heat generation parameter on two-dimensional flow. Detailed analysis of the velocity profile, temperature distribution, skin friction, rate of heat transfer and the surface temperature distribution are shown graphically.     

Numerical study of flow and heat transfer characteristics of alumina-water nanofluids in a microchannel using the lattice Boltzmann method

In the present study, mathematical modeling is performed to simulate force d convection flow of Al₂O₃/water nanofluids in a microchannel using the lattice Boltzmann method (LBM). Simulations are conducted at low Reynolds numbers (Re ≦ 16). Results indicate that the average Nusselt number increases with the increase of Reynolds number and particle volume concentration. The fluid temperature distribution is more uniform with the use of nanofluid than that of pure water. Furthermore, great deviations of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The results of LBM agree well with the classical CFD method for predictions of flow and heat transfer in a single channel and a microchannel heat sink concerning the conjugate heat transfer problem, and consequently LBM is robust and promising for practical applications.     

BEM/FDM conjugate heat transfer analysis of a two-dimensional air-cooled turbine blade boundary layer

A coupled boundary element method （BEM） and finite difference method （FDM） are applied to solve conjugate heat transfer problem of a two-dimensional air-cooled turbine blade boundary layer. A loosely coupled strategy is adopted, in which each set of field equations is solved to provide boundary conditions for the other. The Navier-Stokes equations are solved by HIT-NS code. In this code, the FDM is adopted and is used to resolve the convective heat transfer in the fluid region. The BEM code is used to resolve the conduction heat transfer in the solid region. An iterated convergence criterion is the continuity of temperature and heat flux at the fluid-solid interface. The numerical results from the BEM adopted in this paper are in good agreement with the results of analytical solution and the results of commercial code, such as Fluent 6.2. The BEM avoids the complicated mesh needed in other computation method and saves the computation time. The results prove that the BEM adopted in this paper can give the same precision in numerical results with less boundary points. Comparing the conjugate results with the numerical results of an adiabatic wall flow solution, it reveals a significant difference in the distribution of metal temperatures. The results from conjugate heat transfer analysis are more accurate and they are closer to realistic thermal environment of turbines.     

Numerical study on the parabolic flow of MHD fluid past a vertical plate in a porous medium

A numerical investigation on MHD fluid flow in parabolic mode has been performed to point out its significant properties. Thermal radiation, porous medium, heat generation, chemical reaction, and thermal diffusion along with variable temperature and concentration are taken into consideration in the analysis. The novelty of the work is the inclusion of heat generation and thermal diffusion along with exponentially varying temperature and concentration. The constituent governing equations are solved by using finite difference schemes in explicit form. The fluctuations in velocity, concentration, and temperature are observed and discussed with the help of graphs as well as numerical data. Their gradients are also calculated and analyzed the flow properties by using numerical tables. The existence of heat generation, as well as viscous dissipation, creates an increment in the temperature. The gradient of heat transfer rises with the impact of Prandtl number and decay in it is examined under the existence of a source of heat and viscous dissipation.     

一种新型集热介质热工性能的试验与分析

通过实验测定了一种新型集热介质的凝固温度、膨胀系数、比热容及传热系数等性能参数,并与水进行了比较。结果表明,该介质无腐蚀性,在零下40℃不会凝固结冰,其比热容小于水,升温快,传热性能好,在对数温差较小时传热系数大于水,因此可用作太阳能集热器的循环介质,应用于太阳能建筑一体化采暖、热水供应系统,能够保证系统在冬季正常运行,不会冻结且节能高效。     

Numerical investigations of flow and heat transfer enhancement in a corrugated channel using nanofluid

In this paper, heat transfer and pressure drop characteristics of copper–water nanofluid flow through isothermally heated corrugated channel are numerically studied. A numerical simulation is carried out by solving the governing continuity, momentum and energy equations for laminar flow in curvilinear coordinates using the Finite Difference (FD) approach. The investigation covers Reynolds number and nanoparticle volume fraction in the ranges of 100–1000 and 0–0.05 respectively. The effects of using the nanofluid on the heat transfer and pressure drop inside the channel are investigated. It is found that the heat transfer enhancement increases with increase in the volume fraction of the nanoparticle and Reynolds number, while there is slight increase in pressure drop. Comparisons of the present results with those available in literature are presented and discussed.     

Investigation of fluid flow and heat transfer in a vertical channel heated from one side by PV elements, part I - Numerical Study

The impetus of this paper is to analyse numerically the fluid flow and heat transfer characteristics of buoyancy-driven convection between two vertical parallel walls, heated from one side. Both convection and radiation heat exchanges are considered as the heat transfer mechanisms by which the thermal energy is transferred into the air. A steady-state two-dimensional model is used for the analysis. Numerical results are derived for a channel of 6.5 m in height and different widths of the channel. Various heat fluxes are considered in order to show the effect of the input heat on the heat transfer across the air layer. Detailed studies of the flow and thermal fields in the air are presented in order to explore the thermal behavior of air in the channel. Velocity and temperature profiles of the outlet air and the surface temperature of the heated and insulated wall are presented. In Part II of this paper the findings from an experimental study are reported.     

Transient two-dimensional model of heat and mass transfer in a PEM fuel cell membrane

In the present work, a numerical study of heat and mass transfer within the membrane of a proton exchange membrane fuel cell is presented. The electrolyte membrane is considered an isotropic porous medium and ideal insulator for electrons and reactants. The adopted model in this study is based on the assumption of single-phase and multi-spices flow, supposed two-dimensional and unsteady. For the water transport, the major considered forces are; the convective force, resulting from the pressure gradient, the osmotic force, due to the concentration gradient and the electric force caused by the proton migration from the anode to the cathode. Based on a one-dimensional model, found in the literature, a transient two-dimensional one was proposed. The set of governing equations, written in velocity–pressure formulation, is solved by the implicit finite difference method. An alternating Direct Implicit scheme was used for the calculation. The numerical resolution gives the time- and space-dependent temperature and water concentration. The main focus lies on the influence of different cases of boundary conditions on water concentration and heat transfer variation with the intention of testing the reliability of the proposed computational fluid dynamic (CFD) code.     

Parametric optimization of a solar-driven Braysson heat engine with variable heat capacity of the working fluid and radiation–convection heat losses

An irreversible solar-driven Braysson heat engine system is presented, in which the temperature-dependent heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the irreversibilities resulting from heat transfer and non-isentropic compression and expansion processes are taken into account. Based on the thermodynamic analysis method and the optimal control theory, the mathematical expression of the overall efficiency of the system is derived and the maximum overall efficiency is calculated, and the operating temperatures of the solar collector and the cyclic working fluid and the ratio of heat-transfer areas of the heat engine are optimized. By using numerical optimization technology, the influences of the variable heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the multi-irreversibilities on the performance characteristics of the solar-driven heat engine system are investigated and evaluated in detail. Moreover, it is expounded that the optimal performance and important parametric bounds of the irreversible solar-driven Braysson heat engine with the constant heat capacity of the working fluid and the irreversible solar-driven Carnot heat engine can be deduced from the conclusions in the present paper.     

Development of a finite element based heat transfer model for conduction mode laser spot welding process using an adaptive volumetric heat source

Numerically computed results of weld pool dimensions in conduction mode laser welding are sensitive to the estimated value of the actual beam energy absorbed by the substrate. In a conduction based heat transfer analysis, the incorporation of the laser beam induced energy as a surface only heat flux fails to realize enhanced heat transfer in weld pool as molten material attains higher temperature and convective transport of heat becomes predominant. An alternate is to include fluid flow analysis considering phenomenological laws of conservation of mass and momentum that greatly increases the complexity in modeling. Uncertainty of material properties such as effective thermal conductivity and viscosity in the weld pool also impedes such extensive fluid flow analysis. A simpler and tractable approach can be to consider a volumetric heat source within weld pool in a conduction based heat transfer analysis. Earlier efforts to accommodate volumetric heat source such as double-ellipsoidal form remained unpopular since the size of the final weld pool shapes is required to be known to begin with the calculation. The present work describes an improved approach where a volumetric heat source is defined adaptively as the size of the weld pool grows in size within the framework of a conduction based heat transfer analysis. The numerically computed results of weld pool dimensions following this approach have shown fair agreement with the corresponding measured values for laser spot weld samples.     

A numerical investigation of conjugated-natural convection heat transfer enhancement of a nanofluid in an annular tube driven by inner heat generating solid cylinder

Laminar conjugate heat transfer by natural convection and conduction in a vertical annulus formed between an inner heat generating solid circular cylinder and an outer isothermal cylindrical boundary has been studied by a numerical method. It is assumed that the two sealed ends of the tube to be adiabatic. Governing equations are derived based on the conceptual model in the cylindrical coordinate system. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. Results are presented for the flow and temperature distributions and Nusselt numbers on different cross sectional planes and longitudinal sections for Rayleigh number ranging from 10⁵ to 10⁸, solid volume fraction of 0‹φ‹0.05 with copper-water nanofluid as the working medium. Considering that the driven flow in the annular tube is strongly influenced by orientation of tube, study has been carried out for different inclination angles.     

基于有限元方法的溴化锂降膜吸收传热传质的数值研究

吸收器是吸收式制冷系统的重要部件.溴化锂溶液的降膜吸收是吸收器中最常见的传质传热形式之一.通过对溴化锂溶液在降膜吸收过程中传质和传热特性的分析,使用基于有限元法的COMSOL Multiphysics软件,建立了溴化锂溶液和水蒸汽降膜吸收的物理数学模型,计算了液膜内部温度和质量分数的分布、界面处传质通量、界面处传热通量...     

流体流过带折流板通道时的流动及传热数值研究

对常物性流体在通道内的周期性充分发展层流流动和换热特性进行了二维数值计算分析。所研究的通道是由两平行平板布置于中心线位置的一系列折流板构成。平行平板保持温度恒定，折流板则分成完全导热和绝热两种情况，对不同几何参数，Re数和Pr数下的流动和换热性能进行了数值研究。文章还给出了系统流函数图和局部换热系数分布情况。     

Numerical study on heat transfer enhancement of a proton exchange membrane fuel cell with the dimpled cooling channel

As the power density of a proton exchange membrane fuel cell (PEMFC) increases, the problems of internal heat accumulation and non-uniform temperature distribution are becoming significant. In this paper, a novel cooling channel with dimple structures is designed and a three-dimensional PEMFC numerical model is established. When comparing to the conventional channels, the heat transfer performance of dimpled channel is 10% higher than the smooth one, and the pressure loss is almost 13% lower than that of wavy channel. In addition, the optimization of dimple structure parameters is investigated based on the index of uniformity temperature (IUT) and performance evaluation criteria (PEC) of heat transfer. It is found that a diameter-to-depth ratio of 4 is recommended when the dimple diameter is less than 0.80 mm. Furthermore, the clock-wise vortex observed inside the dimple is considered to be the main reason affecting heat exchange. This study will contribute to the design of cooling channels for high-power density PEMFCs in the future.     

Numerical study of heat transfer enhancement and fluid flow with inline common‐flow‐down vortex generators in a plate‐fin heat exchanger

Three‐dimensional numerical simulations are performed on a plate‐fin heat exchanger (with triangular fins as inserts between the plates) to evaluate the laminar heat transfer and fluid flow characteristics with longitudinal vortex generators (LVGs). The effect with an inline rectangular winglet pair (RWP) with a common‐flow‐down (CFD) configuration is studied. The numerical results indicate that the application of inline LVGs effectively enhances the heat transfer of the channel. The heat transfer further increases with the increase in the Reynolds number from 200 to 500 and angle of attack from β = 15° to 22.5°. The computations are also performed to find the best location for the second RWP. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20414     

Numerical study on unsteady heat transfer and fluid flow in a closed cylinder of reciprocating liquid hydrogen pumps

Modeling and optimization of liquid hydrogen (LH₂) pumps require accurate in-cylinder heat transfer correlations. However, the applicability of existing correlations based on gas mediums to LH₂ remains to be verified. In this paper, the unsteady heat transfer and fluid flow in a closed LH₂ pump cylinder are numerically studied by adopting the gas spring model. The phase shifts and temperature distribution in the closed pump cylinder are investigated. LH₂ is less affected by in-cylinder heat transfer and has a more uniform temperature distribution compared to nitrogen gas, while a low-temperature zone appears near the piston face at 120 rpm. Finally, the validity of Lekic's correlation in predicting the heat flux of the LH₂ compression process in the closed pump cylinder is verified, and the efficiency decrement versus rotational speed is analyzed based on the correlation. This work would be useful for selecting a proper in-cylinder heat transfer model for predicting the thermodynamic process in reciprocating LH₂ pumps.     

Numerical study of heat and mass transfer in MHD flow of nanofluid in a porous medium with Soret and Dufour effects

This article models the transport mechanism of mass and heat energy under temperature and concentration gradients. Mathematical models in the form of partial differential equations based on conservation laws for fluid flow and transfer of heat and mass subjected to thermal diffusion and diffusion thermos, heat generation porous medium, and buoyancy forces are developed under boundary layer approximations. These models along with models of nanostructures are solved numerically using the shooting method with the Runge–Kutta method of order five. Convergent solutions are obtained and are used for parametric analysis regarding thermal enhancement of a working fluid having nanoparticles of CuO, Al₂O₃, and TiO₂. Numerical experiments are performed and it is observed that the transport of heat is accelerated when the compositional gradient is increased. Similarly, a significant rise in the transport across concentration is noted when the temperature gradient is increased. The magnetohydrodynamic flow experienced retardation when the porous medium parameter and Hartmann number are increased. The temperature increased when the friction force produced heat and that heat is distributed to the particles of the fluid. Hence, viscous dissipation is responsible for widening the thermal boundary layer region.