Analysis of transient heat transfer in multilayer thin films with nonlinear thermal boundary resistance

Transient energy transport in thin-layer films with a nonlinear thermal boundary resistance is analyzed theoretically within the framework of the dual-phase-lag heat conduction model. An iterative finite difference numerical method is used and is verified using a derived semi-analytical solution of the problem. Effects of the thermo-physical properties on energy transport when a two-layer film is exposed to a thermal pulse of certain duration and strength are presented. The thermal boundary resistance, the heat flux and temperature gradient phase lags and the thermal conductivities and heat capacities all are important factors that characterize energy transport through the interface and the temperature distribution in the two layers. The maximum interfacial temperature difference that takes place in the transient process of thermal pulse propagation is found to be the proper choice to measure the perfect-ness of the interface with a finite thermal boundary resistance. The results show that even with high values of the thermal boundary resistance the maximum interfacial temperature difference can be very small when the thermal pulse propagates from a high-thermal conductivity and heat capacity layer to a low-thermal conductivity and heat capacity layer. For a certain range of the thermal conductivities and heat capacities, the maximum interfacial temperature difference approaches zero even with high values of the thermal boundary resistance. Thermal conductivities and heat capacities are much more important in characterizing transient heat transfer through the imperfect interface than the phase lags of the heat flux and temperature gradient.     

Three-dimensional transient analysis of FGM cylindrical shell subjected to thermal and mechanical loading

In this article, transient analysis of functionally graded material (FGM) cylindrical shell subjected to thermomechanical load is performed. Mechanical and thermal properties of the shell are assumed to be graded in radial direction according to power law distribution. In the case of simply supported edge condition, problem is solved analytically using Fourier series expansions for stresses and displacements along the axial direction and state space technique along the radial direction and Laplace transformation technique for time domain. For other boundary conditions, we use a semianalytical method by applying differential quadrature method along the axial direction and the state space method along radial direction. Accuracy of this approach is validated by comparing the results with the results reported in the literature. Moreover, influence of edge boundary conditions, length to mid radius ratio, FGM direction and time on stresses, and displacements is studied.     

Three-dimensional thermal weight function method for the interface crack problems in bimaterial structures under a transient thermal loading

Different from previous two-dimensional thermal weight function (TWF) method, a three-dimensional (3D) TWF method is proposed for solving elliptical interface crack problems in bimaterial structures under a transient thermal loading. The present 3D TWF method based on the Betti's reciprocal theorem is a powerful tool for dealing with the transient thermal loading due to the stress intensity factors (SIFs) of whole transient process obtained through the static finite element computation. Several representative examples demonstrate that the 3D TWF method can be used to predict the SIFs of elliptical interface crack subjected to transient thermal loading with high accuracy. Moreover, numerical results indicate that the computing efficiency can be enhanced when dealing with transient problems, especially for large amount of time instants.     

Comparison of experimental and theoretical results for the transient heat flow through multilayer walls and flat roofs

This study deals with comparison of experimental and theoretical results of transient temperature variations in multilayered building walls and flat roofs, and heat flow through the building structures. Experimental and theoretical models are presented to find the transient temperature variations in these structures and heat flow through these elements, which depends on inside surface and room air temperatures. Instantaneous inside and outside air temperatures, and surface temperatures of each wall and roof layers are measured by using the experimental model consisted of two rooms, cooling units, measuring devices and computers. A computer program based on the theoretical model is developed to perform numerical calculations. Hourly temperature variations of the nodal points are computed numerically over a period of 24 h by using the hourly measured ambient air temperatures and solar radiation flux on a horizontal surface for the city of Gaziantep (37.1°N), Turkey, and also by using thermophysical properties of the structures. Results obtained from the experimental and theoretical models are compared with each other, and validation of the theoretical model is verified in this paper. Computations for various multilayer building walls of briquette, brick, blokbims, and autoclaved aerated concrete (AAC), which are commonly used in Turkey are repeated for finding heat gain through these structures, and results are compared to determine suitable wall material. It is observed that AAC and blokbims are more suitable wall materials than briquette and brick due to heat flow through these elements.     

A test procedure for extruded polymeric solar thermal absorbers

An indentation test is proposed to study the degradation of extruded polymeric solar absorbers. The thermal degradation caused by accelerated aging is investigated. The results are compared with the thermal and mechanical impacts during the operation of the solar collector.     

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.     

A thermal formulation for single-wall quenching of transient laminar flames

Improving our knowledge of flame-wall interaction is of relevance to performing near-wall combustion calculations. Quenching distance is to be determined accordingly, as a major parameter of flame quenching. For this purpose, an equation describing the behavior of single-wall flame quenching has been derived from a simplified model of laminar flame-wall interaction. It allows evaluating quenching distance from wall heat flux and mixture properties; a significant advantage of this formula is the absence of any empirical coefficient. To assess its reliability, the results computed with this equation have been compared to experimental data concerning laminar flame-wall interaction. For this purpose, single-wall quenching parameters have been recorded in both head-on and sidewall configurations. Quenching distance and wall heat flux have been measured simultaneously, during the combustion of quiescent methane-air mixtures in a constant-volume vessel. Quenching distance is determined through direct visualization, whereas wall heat flux is processed from the time evolution of wall surface temperature. The equation has been verified over the pressure range 0.05-0.35 MPa in stoichiometric and lean mixtures. It shows good agreement with experimental data at first order, with less than 20% variation.     

A simple analytical model for calculating the effective thermal conductivity of nanofluids

A simple mathematical model for calculating the effective thermal conductivity of nanofluids has been developed based on the thermal resistance approach. The model is developed by considering both effects of a solid‐like nanolayer and convective heat transfer caused by Brownian motion which have not been considered simultaneously by most available models in the literature. In addition the correlation of Prasher and Phelan for the convective heat transfer coefficient is modified to take into account the effect of the solid‐like nanolayer. In addition a general value for n (different from the one presented by Tillman and Hill) is introduced to modify the thickness of the solid‐like nanolayer. The latter is done by considering both conduction and convection heat transfer mechanisms. Comparisons with previously published experimental results and other mathematical models show that the presented model could well predict a nanofluids effective thermal conductivity as a function of the nanoparticles mean diameter, volume fraction, and temperature for different kinds of nanofluids. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20290     

机车轴重转移和最佳牵引高度计算的通用可视界面

分析了机车轴重转移的计算原理,利用VB程序制作良好的可视化界面,实现现有适用于各种型式、各种牵引方式的机车的轴重转移和理想牵引高度计算的通用方程的计算。通过该界面可以快速方便研究包括一系、二系的悬挂方式及刚度、牵引高度、轴距、牵引电动机的布置方式,还有最大牵引力、车钩高度、轴数等对轴重转移的影响。     

Thermochemical cooling for large thermal load applications

The requirement to reject heat within a small envelope has stunted the development and deployment of high heat flux applications. Thermochemical cooling is a new innovative approach, inspired by engine cooling in aircraft. In aircraft, sensible heat is often rejected to the fuel, without altering its chemical composition, and dissipated through the wings. However, sensible heat absorption is limited to 431 kJ/kg for JP8. Using endothermic reactions, JP8 can absorb as much as 11,688 kJ/kg of heat when converted to hydrogen, carbon monoxide, and other gases. A common refrigerant, such as R134A has an enthalpy of vaporization of 209 kJ/kg. However, after evaluating multiple fuels and reforming literature, it was determined that optimum lower temperature performance could be achieved with a methanol water mixture. This endothermic reaction is a strong candidate for heat absorption. Preliminary test data demonstrated heat absorption as low as 300 °C, with peak absorption at 400 °C. At its peak (R = 400 °C), the reactor is capable of absorbing 3411 kJ/kg. Future efforts will evaluate the use of JP8, which has a larger endothermic reaction potential.     

A new method for numerical simulation of thermal contact resistance in cylindrical coordinates

In this paper, a random numbers model using an innovative equiperipheral grid in cylindrical coordinates has been proposed to predict the contact spot distribution of two rough surfaces at various loads. The ability of this method to predict the contact spot distribution has been proven through comparison with results using a conventional equiangular grid. Further, a network method using such an equiperipheral grid has been developed in order to solve a three-dimensional heat conduction problem where two cylindrical specimens were connected to each other along the longitudinal direction. A uniform heat flux is given at the bottom surface of specimen I, a uniform temperature is maintained at the top surface of specimen II, and thermal insulation is assumed at the outer radius of the two specimens. The present numerical results have been compared to calculations using conventional equiangular grids and to experimental results obtained for cylindrical brass specimens. The present results are shown to compare much more closely with experimental measurements than previous calculations using conventional numerical models.     

A new transient thermal fouling probe for cross flow tubular heat exchangers

The present probe is developed in order to accurately estimate in situ not only the convective exchange coefficient but also the fouling thickness of heat exchangers from a reliable transient state estimation method.The originality of the estimation method consists in considering a global response time of the system in fouling conditions to be compared to clean conditions. The sensitivity function is then built from the experimental signal without precise knowledge about the model or the absolute thermophysical properties. The reliability of the method is demonstrated in theoretical cases and with calibrated experiments.     

Analytical solution for transient temperature and thermal stresses within convective multilayer disks with time-dependent internal heat generation,Part I: Methodology

This work deals with the exact solution for asymmetric transient problem of heat conduction and accordingly thermal stresses within multilayer hollow or solid disks which lose heat by convection to the surrounding ambient. The combination of the separation of variables method (SVM) and Duhamel's theorem is applied to the heat conduction problem which provides a versatile technique. The temperature distribution is obtained by the SVM which concerns the heat conduction problem with time-independent internal heat generation. Applying Duhamel's theorem on the previous solution, temperature distribution with time-dependent internal heat generation can be achieved. Accordingly, assuming plane stress condition, radial and tangential stresses are obtained which are incorporated into the equivalent tensile stress formulation to calculate von Mises stress. The comprehensive methodology described here can be useful addition for many new emerging fields in which both transient and steady-state temperature distributions and thermal stresses for composite disks are important.     

A three-dimensional theoretical model for predicting transient thermal behavior of thermoelectric coolers

A simulation model is developed and used to predict transient thermal behavior of the thermoelectric coolers. The present model amends the previous models, in which the P–N pair is simply treated as a single bulk material so that the temperature difference between the semiconductor elements was not possible to evaluate. Based on the present simulation model, the thermoelectric cooler is divided into four major regions, namely, cold end (region 1), hot end (region 2), and the P-type and N-type thermoelectric elements (regions 3 and 4). Solutions for the three-dimensional temperature fields in the P-type and the N-type semiconductor elements and transient temperature variations in the cold and the hot ends have been carried out. The magnitude of the coefficient of performance (COP) of the thermoelectric cooler are calculated in wide ranges of physical and geometrical parameters. To verify the numerical predictions, experiments have been conducted to measure the temperature variations of both the cold and the hot ends. Close agreement between the numerical and the experimental data of the temperature variations has been observed.     

Analytical solution for transient temperature and thermal stresses within convective multilayer disks with time-dependent internal heat generation,Part II: Applications

This work investigates the analytical solution for transient temperature and thermal stresses within three circular geometries. First, the transient temperature and thermal stresses within a composite disk are addressed. Then, two examples regarding transient temperature and thermal stresses throughout circular heaters are analyzed. Pulsed and sinusoidal internal heat generations are incorporated into the second and third examples, respectively. For the composite hollow-disk example, merely the separation of variables method (SVM) is used to overcome the energy partial differential equation. For the other two examples, the combination of the SVM and Duhamel's theorem are adopted to solve the partial differential equations. Accordingly, assuming plane stress formulation, the transient thermal stresses within structures are obtained.     

Thermal load levelling of heat flux through an insulated thermal storage water wall

This communication presents an investigation of the thickness distribution of a given total thickness of the insulation inside and outside a thermal storage water wall for acheiving the maximum load levelling of the heat flux entering through the wall. Analysis is based on the solution of the heat conduction equation for the temperature distribution in the insulated wall subjected to periodic solar radiation and atmospheric air on one side and in contact with room air at constant temperature (corresponding to air-conditioned rooms) on the other side. an explicit solution for a temperature distribution satisfying the apporpriate boundary conditions at the surface has been derived to obtaing a periodic heat flux through the storage water wall. It is found that for a given total thickness (cost) of insulation the thicknesses of outside and inside insulation must be equal for best load levelling. Moreover, more load levelling is achieved when the whole of the insulation is outside rather than inside the thermal storage water wall.     

A procedure for the approximated response history analysis of linear thermoelastic structures

This article presents a procedure for the approximated response history analysis of linear thermoelastic structures subjected to pure thermal loading based on eigenvalue analysis. The underlying assumption is that the mechanical system response does not affect heat transfer. Typically, the mechanical response of a structure subjected to pure thermal loading is such that inertia forces can be neglected but there are instances, however, where this is not the case. An approach to make this determination is also presented for a generic pair of retained mechanical and thermal modal coordinates and then extended to the multiple degree-of-freedom case.     

A simple procedure for assessing thermal comfort in passive solar heated buildings

The Fanger thermal comfort equation is linearized and used to develop a procedure for assessing thermal comfort levels in passive solar heated buildings. In order to relate comfort levels in non-uniform environments to uniform conditions, a new thermal index called the “equivalent uniform temperature” is introduced.