Investigation of the thermal properties of a Li-ion pouch-cell by electrothermal impedance spectroscopy

Electrothermal impedance spectroscopy (ETIS), is introduced as a new measurement method and thermal parameters derived from a pouch-type lithium-ion cell are presented. ETIS is a valuable tool for (i) the determination of the thermal impedance and (ii) the validation of thermal models. The excitation signal applied to the cell during measurement does not cause a change in entropy, thus facilitating the parameter identification of a thermal model for heat conduction and thermal capacity.ETIS can be applied to measurements in time domain and in frequency domain. Both approaches are presented and a combination further improves measurement time and accuracy.     

Simultaneous measurement of thermal properties by thermal probe using stochastic approximation method

A novel thermal probe method is proposed for the simultaneous measurement of the thermal properties by the Monte Carlo stochastic approximation method. In this method, thermal capacity of probe and thermal contact resistance between probe and sample are considered. An experimental system is set up with the method to validate the measurement accuracy of the method. The thermal properties of several liquid samples as well as solid samples are measured. The results show that: (1) the thermal conductivity and the volumetric heat capacity can be measured with an error of less than 1.2% and 3% respectively, therefore, the measurement accuracy by the method is much higher than the conventional method and (2) the thermal contact resistance has a great effect on thermal conductivity for solid sample, while little influence on thermal conductivity for liquid sample and volumetric heat capacity.     

Heat conduction in nanofluids: Structure–property correlation

We examine numerically the effects of particle-fluid thermal conductivity ratio, particle volume fraction, and particle morphology on nanofluids effective thermal conductivity and phase lags of heat flux and temperature gradient, for six types of nanofluids containing sphere, cube, hollow sphere, hollow cube, slab-cross and column-cross nanoparticles, respectively. The particle’s radius of gyration and the non-dimensional particle-fluid interfacial area are found to be two characteristic parameters for the effect of particles’ geometrical structure on the effective thermal conductivity. The nanoparticles with larger values of these two parameters can change fluid conductivity more significantly. Due to the enhanced particle-fluid interfacial heat transfer, the nanofluid effective thermal conductivity can practically reach the Hashin–Shtrikman bounds when the particle-phase connects to form a network and separates the base fluid into a dispersed phase. The particle aggregation can effectively affect the effective thermal conductivity when the separation distance among particles is smaller than about one fifth of the particles’ dimension. For the nanofluids considered in the present work, the phase lags of heat flux and temperature gradient scale with the square of particle dimension and range from 10^?11 s to 10^?7 s; the effect of cross-coupling between the heat conduction in the fluid and particle phases is weak; the phase lag of temperature gradient is larger than that of heat flux such that the heat conduction in them is diffusion-dominant and their effective thermal conductivity can be well predicted by the predictive models developed in the present work based on the classical diffusion theory for two-phase systems.     

Optimization of a thermal manufacturing process: Drawing of optical fibers

The optimization of thermal systems and processes has received much less attention than their simulation and often lags behind optimization in other engineering areas. This paper considers the optimization of the important thermal manufacturing process involved in the drawing of optical fibers. Despite the importance of optical fibers and the need to enhance product quality and reduce costs, very little work has been done on the optimization of the process. The main aspects that arise in the optimization of such thermal processes are considered in detail in order to formulate an appropriate objective function and to determine the existence of optimal conditions. Using validated numerical models to simulate the thermal transport processes that govern the characteristics of the fiber and the production rate, the study investigates the relevant parametric space and obtains the domain in which the process is physically feasible. This is followed by an attempt to narrow the feasible region and focus on the domain that could lead to optimization. Employing standard optimization techniques, optimal conditions are determined for typical operating parameters. The study thus provides a basis for choosing optimal design conditions and for more detailed investigations on the feasibility and optimization of this complicated and important process.     

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.     

A NEW THERMAL INERTIA MODEL BASED ON EFFUSIVITY

In the framework of the PASCOOL project, theoretical and experimental studies have been performed to improve the quantitative understanding of the presence and distribution of massive building elements for the passive cooling of buildings. A new thermal model, based on effusivity, a parameter that can be analytically derived from the solution of the heat equation for a semi-infinite wall, was developed. The transient thermal response of a variety of building zones to a series of steps in heat gain has been measured for the validation of the thermal inertia models. It was observed that, for each step in gain, the mean surface temperature of the spaces increases proportionally to the square root of time, as predicted by the effusivity model. The results allowed the formulation of simple passive cooling design guidelines for buildings.     

简捷计算法在300MW火电机组热经济性分析中的应用

不同容量的火电机组,其热经济性分析模型与分析方法不同。文中基于热力系统筒捷计算理论,以内蒙古某电厂300MW火电机组热力系统为研究对象,对其热经济指标（汽耗量、汽耗率、热耗率、标准煤耗率、全年标准煤耗量）进行了计算,可为热力系统优化运行方式及机组热经济性诊断提供参考。     

On the role of radiation view factor in thermal performance of straight-fin heat sinks

The present study is aimed to experimentally investigate the importance of the effects of thermal radiation and its corresponding view factor on the thermal performance of a straight-fin heat sink designed for electronic cooling under natural convection. The convection heat transfer coefficient between the fins and the ambient air is evaluated in conjunction with the results obtained through the experimental investigation. Three different models are developed to investigate the effects of thermal radiation and its pertinent view factor on the convection coefficient as well as the fin performance of the heat sink. The deviations based on different operating conditions for these models are analyzed and the importance of the effects of thermal radiation and view factor in the thermal analysis of fin arrays are identified and discussed. It can be concluded that the practice of neglecting the radiation view factor in the thermal analysis of fin arrays should be prohibited based on the fact that the errors generated are noticeably larger than those of solely neglecting thermal radiation.     

Measurements of thermal properties of insulation materials by using transient plane source technique

The paper reports on the measuring technique and values of the measured thermal properties of some commonly used insulation materials produced by local manufacturers in Saudi Arabia. Among the thermal properties of insulation materials, the thermal conductivity (k) is regarded to be the most important since it affects directly the resistance to transmission of heat (R-value) that the insulation material must offer. Other thermal properties, like the specific heat capacity (c) and density (ρ), are also important only under transient conditions. A well-suited and accurate method for measuring the thermal conductivity and diffusivity of materials is the transient plane source (TPS) technique, which is also called the hot disk (HD). This new technique is used in the present study to measure the thermal conductivity of some insulation materials at room temperature as well as at different elevated temperature levels expected to be reached in practice when these insulations are used in air-conditioned buildings in hot climates. Besides, thermal conductivity values of the same type of insulation material are measured for samples with different densities; generally, higher density insulations are used in building roofs than in walls. The results show that the thermal conductivity increases with increasing temperature and decreases with increasing density over the temperature and density ranges considered in the present investigation.     

Degradation of a stratified thermocline in a solar storage tank

The thermal stratification behaviour in a solar storage tank is simulated and analysed using a theoretical model based on an integral transform technique. A comparison with available experimental and theoretical data is used to validate the present theoretical results. The accuracy of the model in simulating the thermal behaviour of stratification is reasonably good, especially when consideration is given to the complexity of the physical mechanisms involved, and the relative simplicity of the model. The effect of the heat loss parameter is investigated and it is found that initially it is strongly spatially and temporally dependent. Therefore, a functional form that accurately represents the heat loss parameter is needed for closer agreement with experimental results. However, after a relatively long time, the assumption of a constant heat loss parameter is adequate to produce acceptable predictions.     

Reconstruction of thermal conductivity and heat capacity using a tomographic approach

We consider the estimation of the volumetric heat capacity and the thermal conductivity as distributed parameters. The measurement scheme consists of sequentially heating the boundary of the object in different source locations and measuring the induced temperature evolutions in different measurement locations on the boundary. The estimation of the distributions of volumetric heat capacity and thermal conductivity based on these boundary data is an ill-posed inverse boundary value problem. We propose an approach which is based on transient data on the boundary and the modelling of the unknown coefficients as Markov random fields. The intended applications are non-destructive retrieval of defects as well as the estimation of macroscopic characteristics of novel materials. We evaluate the proposed approach by a numerical simulation.     

Thermal management of electronic components with thermal adaptation composite material

Thermal adaptation composite material is a kind of composite material with required thermal conductivity or coefficient of thermal expansion through the selection and design of its components. A kind of thermal adaptation composite material that has excellent thermal conductivity and heat storage capacity is prepared by absorbing paraffin into expanded graphite. An electronic cooling experimental system based on the thermal adaptation composite material is built. The temperature variations of the simulative chip are respectively measured in this system and the traditional cooling system to investigate the effect of the thermal adaptation composite material on electronic cooling. At the same time, the impacts of composite material dosage and combining active cooling manner on the performance of electronic cooling are also studied. The experimental results show that the apparent heat transfer coefficients of the electronic cooling experimental system are 1.25–1.30 times higher than those of the traditional cooling system. It also can be found that the dosage of composite material has positive impact on the performance of electronic cooling. By combining active cooling manner, it can compensate the deficiency of cooling capacity in phase change thermal control.     

Effective pipe-to-borehole thermal resistance for vertical ground heat exchangers

The effective pipe-to-borehole thermal resistance of a vertical ground heat exchanger is investigated numerically. An analysis is carried out to determine the dimensionless geometrical parameters affecting such resistance. The heat transfer rates between the U-shaped pipes and the borehole are determined numerically and compared with some well-known limiting analytical solutions. A best-fit correlation for the effective pipe-to-borehole thermal resistance is presented in dimensionless form. The results are compared against approximate analytical solutions that represent the U-shaped pipes as a single pipe of equivalent diameter and against experimental data available in the literature. It is found that the available models do not accurately represent the effective pipe-to-borehole thermal resistance.     

A fast non-iterative method for the evaluation of the thermal response of wall elements

This paper describes the theoretical development and numerical solution of a non-iterative scheme for the calculation of the thermal response of all elements. The method considers the effect of the variation in external surface temperature, thermal resistance and heat capacity on the internal temperature of a heat storage zone. The method offers potential saving in computation time, in comparison to finite difference methods, without any loss in the accuracy of the computation.     

Artificial neural network models for predicting soil thermal resistivity

Thermal properties of soils are of great importance in view of the modern trends of utilizing the subsurface for transmission of either heated fluids or high power currents. For these situations, it is essential to estimate the resistance offered by the soil mass in dissipating the heat generated through it. Several investigators have tried to develop mathematical and theoretical models to estimate soil thermal resistivity. However, it is evident that these models are not efficient enough to predict accurate thermal resistivity of soils. This is mainly due to the fact that thermal resistivity of soils is a complex phenomenon that depends upon various parameters viz., type of the soil, particle size distribution and its compaction characteristics (i.e., dry density and moisture content). To overcome this, Artificial Neural Network (ANN) models, which are based on experimentally obtained thermal resistivity values for clay, silt, silty-sand, fine- and coarse-sands, have been developed. Incidentally, these soils are the most commonly encountered soils in nature and exhibit entirely different characteristics. The thermal resistivity of these soils, corresponding to their different compaction states, was obtained with the help of a laboratory thermal probe and compared vis-à-vis those obtained from the ANN model. The thermal resistivity of these soils obtained from ANN models and experimental investigations are found to match extremely well. The performance indices such as coefficient of determination, root mean square error, mean absolute error, and variance account for were used to control the performance of the prediction capacity of the models developed in this study. In addition to this, thermal resistivity of these soils obtained from ANN models were compared with those computed from the empirical relationships reported in the literature and were found to be superior. The study demonstrates the utility and efficiency of the ANN model for estimating thermal resistivity of soils.     

高热流密度量热探针的设计与实验

以热等离子体射流冲击平板传热的热流密度测量为背景,分析比较了文献中报道过的各种测量高热流密度的量热探针的结构及实验方法;结合特定实验条件,设计了新型量热探针并对其测量高热流密度的方法误差进行了分析,描述了实际测量采用的方法,给出若干初步实验结果。     

Semi-analytical solutions for the dual phase lag heat conduction in multilayered media

A semi-analytical solution procedure for transient heat transfer in composite mediums consisting of multi-layers within the framework of the dual phase lag model is presented. The procedure is then used to derive solutions for the temperature-, temperature gradient-, and heat flux distributions in a two-layer composite planar slab, a bi-layered solid-cylinder and sphere. The solutions obtained are applicable to the classical Fourier heat diffusion, hyperbolic heat conduction, phonon–electron interaction, and phonon scattering models with perfect or imperfect contact and with layers of different materials. The interfacial contact resistance, the heat flux and temperature gradient phase lags, thermal diffusivities and conductivities, initial temperatures of the composite medium and a general time-dependent boundary heat flux enter the solutions as parameters, allowing the solutions obtained to be applicable to a wide range of arrangements including perfect and imperfect contact. Analysis of thermal wave propagation, transmission and reflection in planar, cylindrical and spherical geometries with imperfect interfaces are presented, and geometrical—as well as the temperature gradient phase lag—effects on the thermal lagging behavior in different layered media are discussed.     

Estimation of soil and grout thermal properties through a TSPEP (two-step parameter estimation procedure) applied to TRT (thermal response test) data

In this paper, a versatile TSPEP (two-step parameter estimation procedure) based on a three-dimensional numerical model of a geothermal system is presented. The procedure is applied to both simulated and experimental TRT (thermal response test) data in order to restore the grout and soil thermal conductivities and volumetric heat capacities. The TSPEP is essentially a two-step process. The first step uses the parameter estimation procedure, in the early transient regime to restore the grout thermal conductivity and volumetric heat capacity. The values from the first step are used as the input values in the second step, in which the parameter estimation procedure is applied to the late transient regime to restore the soil thermal conductivity and volumetric heat capacity. Further iterations of these two steps can be used to improve the accuracy of the procedure and are discussed in this paper. The time separation used between the estimation of the soil properties and the estimation of the grout properties partially uncouples the two problems and makes the estimation of these four parameters feasible. A criterion to select the time separation is discussed and validated in this paper.     

An optimization study on the finned tube heat exchanger used in hydride hydrogen storage system – analytical method and numerical simulation

Metal hydrides show great potential for hydrogen storage. However, for efficient hydrogen storage, thermal management is the technical barrier. Among the different heat exchangers proposed in the literature, finned tube heat exchangers are of great technological interest due to their adaptability to wide range of practical applications, high compactness and high heat transfer efficiency. In the present paper, the optimization of finned heat exchanger considering both enhanced heat transfer and vessel volume efficiency is conducted. A semi-analytical expression of heat transfer rate from a single fin is derived. The effects of fin dimension (fin thickness and radius) on the heat exchanger performance are studied. It was shown that the thermal resistance of the whole heat exchanger can be reduced by increasing the fin radius and decreasing the fin thickness, while the fin volume is kept fixed. In the second part of the study, a 2-D numerical simulation was performed in order to validate the results of the analytical study. The effects of two parameters (cooling tube diameter, the fin length) on the hydrogen charging time were highlighted. The increasing in the tube diameter from 2.5 mm to 5 mm results to 25% reduction of the charging time, which is very noticeable. On the other hand, given a reactor radius, increasing the length of fin reduces the overall thermal resistance of the reactor-heat exchanger. The results showed that the decreasing of the thermal resistance of 13% leads to a decreasing in charging time of 42%. Finally, it was found that the results of the numerical simulation agreed qualitatively with those of analytical study. Therefore, the analytical solution presented can be used for a quick assessment of the finned tube heat exchanger design without significant errors.     

Transient thermal behavior of porous media under oscillating flow condition

An analytical characterization of the heat transfer in an oscillating flow through a porous medium is presented in this work. Based on a two-equation model, two important dimensionless parameters are identified as the ratio of the thermal capacities between the solid and fluid phases and the ratio of the interstitial heat conductance between the phases to the fluid thermal capacity. The analytic solutions are obtained for both the fluid and solid temperature variations, and the heat transfer characteristics between the phases are classified into four regimes. In addition, a criterion for the validity of the local thermal equilibrium is suggested in a simple form as the ratio of the two time scales intrinsically involved in any transient heat transfer in porous media, namely the time scale relevant to the thermal inertia of porous media and the time scale pertinent to the transient variation of the boundary condition.