Transient heat transfer in a heat‐generating fin with radiation and convection with temperature‐dependent heat transfer coefficient

The transient heat transfer in a heat‐generating fin with simultaneous surface convection and radiation is studied numerically for a step change in base temperature. The convection heat transfer coefficient is assumed to be a power law function of the local temperature difference between the fin and its surrounding fluid. The values of the power exponent n are chosen to include simulation of natural convection (laminar and turbulent) and nucleate boiling among other convective heat transfer modes. The fin is assumed to have uniform internal heat generation. The transient response of the fin depends on the convection‐conduction parameter, radiation‐conduction parameter, heat generation parameter, power exponent, and the dimensionless sink temperature. The instantaneous heat transfer characteristics such as the base heat transfer, surface heat loss, and energy stored are reported for a range of values of these parameters. When the internal heat generation exceeds a threshold the fin acts as a heat sink instead of a heat source. © 2012 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21012     

建筑外墙传热系数检测技术的研究

阐述了目前建筑外墙围护结构传热系数检测方法存在的一些问题,分析了含湿量对墙体传热系数的影响,针对存在的问题结合夏热冬暖地区气候条件,提出了一种可操作的科学的试验室检测方法,采用调节室调节的方式以快速达到检测的要求,大大方便了节能验收和工程质量控制。     

对流换热实验中忽略壁面轴向导热的误差分析

以水和空气在钢管中的热充分发展的层流过程为对象,通过数值计算分析了轴向导热给圆管内对流换热的一维壁面导热带来的误差。在二维导热模型的基础上,添加等热流、绝热边界条件,用Fortran语言编写数值计算程序进行计算,定义了相对误差。结果表明,相对误差近似和壁厚与内径的比值成正比,而不同的流体介质数值大小不同。     

Analytical solution of the inverse unsteady wall heat conduction problem and experimental application

Based on the analytical solution of the unsteady heat conduction differential equation, a solution procedure is presented for the inverse unsteady wall heat conduction problem, i.e. for the calculation of the thermal properties of structural elements of existing buildings under real transient conditions, using on-site temperature measurements. Previous procedures, which were based on the finite-difference method, required a considerable number of temperature measurements in space and time within the wall. The advantage of the present analytical procedure is that it requires only two temperature measurements, apart from some information on the outdoor and indoor temperature variations. The two temperature measurements may be taken on the outdoor and indoor wall surfaces at the same time level, or on one of these surfaces at two different time levels. The proposed analytical procedure provides the values of the thermal conductivity and heat capacity of structural elements, and therefore it may be used in practice for ex post checking of the materials used by the constructor, or for load calculation when heating or cooling systems are to be installed in old buildings of unknown wall properties. Experimental examples are presented which show that the proposed analytical procedure may be applied in practice with very good accuracy.     

Application of the variational iteration method to nonlinear non‐Fourier conduction heat transfer equation with variable coefficient

In this paper, a variational iteration method (VIM) has been applied to nonlinear non‐Fourier conduction heat transfer equation with variable specific heat coefficient. The concept of the variational iteration method is introduced briefly for applying this method for problem solving. The proposed iterative scheme finds the solution without any discretization, linearization, or restrictive assumptions. The results of VIM as an analytical solution are then compared with those derived from the established numerical solution obtained by the fourth order Runge–Kutta method in order to verify the accuracy of the proposed method. The results reveal that the VIM is very effective and convenient in predicting the solution of such problems, and it is predicted that VIM can find a wide application in new engineering problems. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20362     

Transient radiation-conductive heat transfer problems: “The quadrupole method”

This paper presents a statement of the works performed in L.E.M.T.A by the members of the thermal and mechanical heterogeneous media research group during the last six years concerning the solving of coupled conductive and radiative heat transfers within a multilayer and semi-transparent “wall”. Out of the authors, this paper allows to take inspiration from the works of D. Maillet, M. Lazard and V. Manias^{[19, 20, 21]}. The aim of these works is to represent in a macroscopic way, with the minimum number of thermophysical parameters, the heat transfers in a plane system composed of semi-transparent media. The approach we propose is semi-analytic (Kernel substitution technique, Laplace transformation) and allow to obtain in the Laplace domain an analytical solution that can be easily used. This method can be applied in two main scopes of applications: the estimation of thermophysical properties (phononic conductivity, optical thickness, Planck number for instance) of semi-transparent materials (glasses, crystals, glass wool, semi-conductors, synthetic diamonds, vitroceramics and so on) and the modelling of processes with semitransparent walls (for instance bottles forming, flat glass production, drying of paper). The method will be first presented and validated and two examples of applications will be then given. This method can be applied to semitransparent walls that emit, absorb and scatter the radiant energy (participating medium). It appears from the principle of a Kernel substitution technique applied to the radiative flux expression and initially introduced by Lick^[1] that allows to change the character of the governing heat equation from the integro-differential form to a purely differential one. In the case of limiting cases of purely scattering and purely absorbing media, the solution of the radiative transfer equation is exact. In the general case, we make a two-flux approximation. In all cases, we assume a linear transfer and use the Laplace transform. The method can be applied to grey or grey by bands media, with isotropic or anisotropic scattering. The advantage of the method is fast computational times for good precision.     

Critical heat flux enhancement in pool‐boiling heat transfer using oxidized multi‐wall carbon nanotubes

In this study, the critical heat flux (CHF) and heat‐transfer coefficient under the pool‐boiling state were tested using multi‐wall carbon nanotubes (MWCNT) CM‐95, CM‐100, and oxidized MWCNT CM‐100. The results showed that the highest CHF increase for both MWCNT CM‐95 and CM‐100 was at the volume fraction of 0.001%, and that the CHF increase ratio for MWCNT CM‐100 nanofluid with long particles was higher than that for MWCNT CM‐95 nanofluid with short particles. In addition, at the volume fraction of 0.001%, the oxidized MWCNT CM‐100 nanofluid indicated a 47.27% higher CHF‐increase ratio as well as an approximately 21.04% higher heat‐transfer coefficient increase ratio compared with the MWCNT CM‐100 nanofluid without oxidation treatment. Copyright © 2015 John Wiley & Sons, Ltd.     

Control of heat transfer on wall with wall jet

A heat transfer experiment on a wall with laminar flow was performed by using a wall jet. The wall jet was generated by a flow control plate placed near the wall. Heat transfer coefficients were measured by a Mach. Zehnder interferometer. Flow patterns and velocities were measured by a smoke-wire method and a laser Doppler velocimeter, respectively. The height of the plates was varied from 2 mm to 8mm. The clearances between the wall and plate were varied from O mm to 7.6 mm. The following results were obtained. The large plate height gave a large, local heat transfer coefficient. The local heat transfer coefficients were enhanced about 7 times as high as that without the place at h = 8 mm, 0 = 30 degrees, and c/(c + h) = 0.15. The optimum wall jet generator angle for large heat quantity was 30 degrees or 45 degrees. © 1997 Scripta Technica, Inc. Heat Trans Jpn Res. 25 (1): 1–11, 1996     

Exergy transfer characteristics of forced convective heat transfer through a duct with constant wall heat flux

The examination of exergy transfer characteristics caused by forced convective heat transfer through a duct with constant wall heat flux for thermally and hydrodynamic fully developed laminar and turbulent flows has been presented. The exergy transfer Nusselt number is put forward and the dependence relationships of the exergy transfer Nusselt number on the heat transfer Nusselt number, Reynolds number and Prandtl number are obtained. Expressions involving relevant variables for the local and mean convective exergy transfer coefficient, non-dimensional exergy flux and exergy transfer rate, etc. have been derived. By reference to a smooth duct, the numerical results of exergy transfer characteristics for fluids with different Prandtl number are obtained and the effect of the Reynolds number and non-dimensional cross-sectional position on exergy transfer characteristics is analyzed. In addition, the results corresponding to the exergy transfer and energy transfer are compared.     

Investigation of conjugate heat transfer in a photovoltaic wall

In this paper, the results of an experimental and theoretical investigation of combined heat transfer in a photovoltaic wall have been reported. The photovoltaic wall is a prototype, which is composed of two pieces of BP PV panels and a Styrofoam board, and part of the light of radiation energy from the indoor lamps can be converted into electricity. Through experiments, the performance of such a photovoltaic wall has been studied. For the convenience of the treatment of heat radiation, a model in terms of the integration of the absolute temperature has been proposed for the numerical simulation of the combined heat transfer in the test wall. By comparison, it is found that with regard to the thermal radiation of lamp surface, good agreement between the results of simulation and experimental data is obtained. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(2): 117–128, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10136     

Numerical study of radiation‐convection heat transfer

In this study, the authors attempted to introduce a simulation technique for radiation‐convection heat transfer in the high‐temperature fields of industrial furnaces, boilers, and gas turbine combustors. The convection effect was analyzed by a differential equation, but the radiation effect was analyzed by an integral equation. Thus, it was not easy to arrange both effects using the same type of equations. Then, the authors introduced the zone method and Monte Carlo method for the integral equation of the radiation effect and the finite difference method for the differential equation of the convection effect. A three‐dimensional analysis of the high‐temperature furnace was performed by this simulation technique to obtain its temperature distribution. Furthermore, another radiation‐convection heat transfer analysis in the low‐temperature living room was performed by the same technique. Finally, the authors tried to develop a computer software for radiation‐convection heat transfer and described their idea of software construction for the above. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 391–407, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10042     

The adjoint-solution technique for the calculation of the thermal properties of layers in multilayer walls under transient heat conduction

The thermal properties of the layers of a wall, whether or not exposed to solar radiation, are calculated provided that the boundary conditions and some values of the transient temperature field within the wall are known. The developed procedure is based on the adjoint-solution technique and is applicable both to walls in operation and to the design of walls that are required to meet certain temperature specifications. In the former case, temperature measurements are needed. Theoretical and experimental tests have proved the accuracy of the method. Applications may be found in energy management and thermal storage in buildings, in the improvement of passive systems and in the design of multilayer slabs forming parts of heat-transfer equipment. © 1997 by John Wiley & Sons, Ltd.     

Time‐dependent flow due to noncoaxial rotation of an infinite vertical surface subjected to an exponential space‐dependent heat source: An exact analysis

The effect of an exponential space‐dependent heat source on heat and mass transfer flow of a viscous fluid past an infinite vertical plate is examined. The flow is generated due to noncoaxial rotation of the infinite plate. The noncoaxial rotation creates sine or cosine oscillation in its plane and the fluid at infinity. The flow is assumed to be laminar and time‐dependent. The mathematical formulation is developed by considering certain physical initial and boundary conditions. The Laplace transform method is utilized to obtain the exact solutions of the concentration, temperature as well as velocity fields. The Sherwood number, Nusselt number, and skin‐friction coefficient are also calculated and presented in tabular form for various embedded parameters. The velocity distributions are obtained for three different cases. The obtained analytical expressions are found to be identical with published results in the limiting sense.     

Experimental Investigation of Transient Heat Transferin the Once-Through Boiler Water Separator

INTRODUCTI0NWaterseparat0risanimportantthickwallc0mpo-nentof600MWsupercriticalpressureboiler.Itsmainfunctionist0ensurethattheevap0rator,superheater,reheaterandeconomizert0becooledfullyandoper-atesafelyduringstart-up.Itisfittedatthe0utletofevaPorator.Whentheb0ilerloadislessthan35%MCR(boilermaJximumc0ntinuousevaporation),sub-cooledwaterorwatersteammixturefromtheevap-orator0fupperc0mbustioncharnberenterstangen-tiallyinthewaterseparator.Thesteam,separatedbycentrifugalforceandgravity,flowsi…     

Laminar heat transfer of gas in a parallel‐plate microchannel with one wall temperature constant and the other adiabatic

Based on “wall‐adjacent layer” effect, the convective heat transfer for developed laminar flow of gas through parallel‐plate microchannel with one wall temperature constant and the other adiabatic was analyzed theoretically. Considering the change in thermal conductivity and viscosity of gas in the region adjacent to the solid wall, mathematical models were built and the dimensionless temperature distribution and the corresponding heat transfer characteristics were simulated numerically. The results indicate that the laminar heat transfer coefficient is less than that of the larger passages, while the dimensionless temperature is greater than that of the larger passages. Compared with the monotonous boundary conditions, the varied heat transfer coefficient amplitude of mixed boundary condition is lower. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 58–64, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10075     

Numerical modeling of conjugate heat transfer through concrete hollow bricks

The purpose of this study is to determine the thermal conductance of concrete hollow bricks, which is necessary for the evaluation of the energy efficiency of a building. The three varieties of hollow concrete bricks that are often used to build walls in Morocco are the subject of this study. A computational model created using the finite volume method is used to evaluate the conjugate heat transfer through concrete hollow bricks. According to the results, the use of hollow brick type A_h3 reduces the heat flux by approximately 86% compared with type A_h1. It is undeniable that hollow bricks type A_h3 with a thermal conductivity of 1 W/m K can improve the thermal characteristics of building walls.     

Exergy transfer characteristics of forced convective heat transfer through a duct with constant wall temperature

The exergy transfer characteristics of fluid flow and heat transfer inside a circular duct under fully developed laminar and turbulent forced convection are presented. Temperature is kept constant at the duct wall. The exergy transfer Nusselt number is put forward and the analytical expressions for exergy transfer Nusselt number are obtained as functions of heat transfer Nusselt number, Reynolds number, Prandtl number, etc. The variations of the local and mean convective exergy transfer coefficient, non-dimensional exergy flux, exergy transfer rate, etc. with operating parameters are presented graphically. By reference to a smooth duct and taking air as working fluid, a numerical analysis of the influence of the Reynolds number and non-dimensional cross-sectional position on exergy transfer characteristics has been conducted. The results show that the process parameters and configuration in the fluid flow and heat transfer inside a duct should be properly selected so that the forced convection process could have the best exergy utilization. In addition, the results corresponding to the exergy transfer and energy transfer are compared.     

A finite line‐source model for boreholes in geothermal heat exchangers

An analytical solution of the transient temperature response in a semi‐infinite medium with a line source of finite length has been derived, which is a more appropriate model for boreholes in geothermal heat exchangers, especially for their long‐duration operation. The steady‐state temperature distribution has also been obtained as a limit of this solution. An erratic approach to this problem that appears in some handbooks and textbooks is indicated. Two representative steady‐state borehole wall temperatures, the middle point temperature and the integral mean temperature, are defined. Differences between them are compared, and concise expressions for both are presented for engineering applications. On this basis the influence of the annual imbalance between heating and cooling loads of the geothermal heat exchangers is discussed regarding their long‐term performance. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 558–567, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10057