Optimal geometries for one- and two-faced symmetric side-wall booster mirrors

Moderate concentration in east-west aligned non-tracking, infinitely long, trough-like solar energy collectors is examined. Two designs are evaluated in detail. They are the one-and two-facet plane side-wall configurations. The maximum performance designs are shown not to be the most practical designs since they tend to require disportionately large reflectors. A significant reduction in reflector area can be made with only a small degradation of performance. At solstice a 9° acceptance angle is necessary for a minimum of eight hours of collection at optimum performance. Under this restriction the practical concentration ratios are limited to about 2 and 2.6 for the one- and two-facet designs, respectively.     

Analytical heat diffusion models for different micro-channel heat sink cross-sectional geometries

This study explores heat diffusion effects in micro-channel heat sinks intended for electronic cooling applications. Detailed analytical models are constructed for heat sinks having micro-channels with rectangular, inverse trapezoidal, triangular, trapezoidal, and diamond-shaped cross sections. Solutions are presented for both monolithic heat sinks and heat sinks with perfectly insulating cover plates. The analytical results are compared to detailed two-dimensional numerical models of the same cross-sections over a broad range of cover plate thermal conductivities for different micro-channel aspect ratios, fin spacings and Biot numbers. These comparisons show the analytical models provide accurate predictions for Biot numbers of practical interest. This study proves the analytical models are very effective tools for the design and thermal resistance prediction of micro-channel heat sinks found in electronic cooling applications.     

A distance-function-based Cartesian (DIFCA) grid method for irregular geometries

A distance-function-based Cartesian grid (DIFCA) method is presented for conduction heat transfer in irregular geometries. The irregular geometries are identified by distance functions. The finite-volume method is used to discretize the heat conduction equation. Non-zero departure from regular geometries terms are added to the discretization equations for the control volumes bisected by irregular boundaries. With these additional departure terms, the existing Cartesian finite-volume solver can be modified easily to model heat conduction in irregular geometries. Given boundary temperatures, given boundary fluxes and convective heat transfer at irregular boundaries are considered. Non-zero heat generation is also modeled. The proposed procedure is validated against eight test cases where good agreements are achieved.     

Optimization of heat sink geometries for impingement air-cooling of LSI packages

This paper describes the use of our previous study's prediction procedures for calculating thermal resistance and pressure drop. The procedures are used in the optimization of heat sink geometries for impingement air-cooling of LSI packages. Two types of heat sinks are considered: ones with longitudinal fins and ones with pin fins. We optimized the heat sink geometries by evaluating 16 parameters simultaneously. The parameters included fin thickness, spacing, and height. For the longitudinal fins, the optimal fin thicknesses were found to be between 0.12 and 0.15 mm, depending on which of the four types of fans were used. For pin fins, the optimal pin diameters were between 0.39 and 0.40 mm. Under constant pumping power, the optimal thermal resistance of the longitudinal fins was about 60% that of the pin fins. For both types of heat sinks, the optimal thermal resistance for four off-the-shelf fans was only slightly (maximum about 1%) higher than the theoretical optimum for the same pumping power. When manufacturing cost performance is considered, the most economical fin thickness and diameter are about 5 to 10 times higher than the optimal values calculated without respect for manufacturing costs. These values almost correspond to the actual limits of extrusion and press heat-sink manufacturing processes. © 1999 Scripta Technica, Heat Trans Asian Res, 28(2): 138–151, 1999     

污泥加热处置的热经济性分析

对污泥加热处置的干燥或干燥一焚烧、直接焚烧等技术进行了理论分析与计算,并与工程应用数据进行对比,结果表明,采用干燥或干燥焚烧技术对污泥进行热处置,具有显著的节能和环保效益.     

Meshless method for solving radiative transfer problems in complex two-dimensional and three-dimensional geometries

A meshless method is presented to solve the radiative transfer equation in complex 2D and 3D geometries. In order to avoid numerical oscillations, the even parity formulation of the discrete ordinates method is used. A moving least squares approximation meshless method is used to solve the second order partial differential equations. Prediction results of radiative heat transfer problems obtained by the proposed method are compared with reference in order to assess the correctness of the present method.     

基于逐级裂解气质联用和热重质谱联用技术的杉木热解特性研究

利用逐级裂解气质联用(stepwise Py-GC/MS)和热重质谱联用技术(TG/MS)对杉木的热解特性进行了研究。结果表明,杉木热解主反应温度为250～430℃,随着升温速率的增大,热解起始温度和终止温度均向高温侧移动。对杉木慢速热解过程进行了动力学分析,得到3种升温速率下对应的活化能分别为83.99,88.87,91.98kJ/mol。杉木逐级裂解主要液体产物有乙酸、1-羟基-丙酮、糠醛、雪松醇、左旋葡萄糖和4-羟基-2-甲氧基肉桂醛等,各温度段产物分布各不相同。在杉木慢速热解条件下,左旋葡萄糖发生二次反应,液体产物中存在很多芳香类物质。     

Development of a suitable model for characterizing photovoltaic arrays with shaded solar cells

The aim of this study is to investigate the effects of non-uniform solar irradiation distribution on energy output of different interconnected configurations in photovoltaic (PV) arrays. In order to find which configuration is less susceptible to mismatch effects, a PV module model is developed. This model can take into consideration the effects of bypass diodes and the variation of the equivalent circuit parameters with respect to operating conditions. The proposed model can provide sufficient degree of precision as well as solar cell-based analysis in analyzing large scale PV arrays without increasing the computational effort. In order to produce more reliable and robust simulations, improved and extended algorithms are presented. Some results are discussed in detail and some recommendations are extracted by testing several shading scenarios.     

A methodology for characterizing fault tolerant switched reluctance motors using neurogenetically derived models

This paper examines the feasibility of using artificial neural networks (ANNs) and genetic algorithms (GAs) to develop discrete time dynamic models for fault free and faulted switched-reluctance-motor (SRM) drive systems. The results of using the ANN-GA-based (neurogenetic) model to predict the performance characteristics of a prototype SRM drive motor under normal and abnormal operating conditions are presented and verified by comparison to test data.     

Thermal decay of an initially hot isothermal water body with application to thermal energy storage

The thermal decay of an initially hot isothermal water body contained in a tank was studied. Analytical solutions (including a group-theoretic solution) were obtained for the one-dimensional heat conduction equation with heat loss from the sides of the tank. The convective heat transfer coefficient was assumed not to be constant over the surface of the tank but to vary with space and time. A very good agreement was obtained between the group-theoretic solution and a numerical solution using the Crank-Nicholson finite difference scheme. The analytical results are discussed in terms of the underlying physical mechanisms.     

Transient heat conduction analysis for distance-field-based irregular geometries using the meshless weighted least-square method

The paper analyzes the transient heat conduction problem with the irregular geometry using the meshless weighted least-square method (MWLS). The MWLS as a meshless method is fully independent of mesh, a discrete function is used to construct a series of linear equations, which avoided the troublesome task of numerical integration. First, irregular geometries are represented by the signed distance field. Then sampling the distance field, discrete nodes are obtained for MWLS analysis. The effectiveness and accuracy of the approach are illustrated by several numerical examples. Numerical cases show that a good agreement is achieved between the results obtained from the proposed meshless method and available analytical solutions or commercial software ANSYS.     

Methods for Conceptual Thermal Design

This article describes three generic models and application methods that can be used to analyze temperature development in an electronic product during conceptual design. The models are based on generally known heat transfer and resistor–capacitor network theory and are theoretically and numerically approximated. The result is three easy-to-use tools for conceptual thermal design. Application of the models in design practice has been assessed using a usability experiment and several in-depth interviews with industrial design engineers from the field.     

The dual-mode heat flow meter technique: A versatile method for characterizing thermal conductivity

The dual-mode heat flow meter technique was developed for steady-state thermal characterization by optimizing the curve fit between the experimental temperature profile and the prediction from a simple analytical solution taking into account conductive as well as radiative heat transfer. The validation results were seen to be in good agreement with published literature values and demonstrated the versatility of the method. Moreover, the technique is ideally suited for characterization of anisotropic materials without necessitating any additional information about the nature of anisotropy and lends itself as an attractive alternative for characterization of novel materials with engineered transport properties.     

Approximate analytical solutions for the solidification of PCMs in fin geometries using effective thermophysical properties

Heat transfer enhancement of phase change materials (PCMs) is essential in order to achieve high charge and discharge powers of latent heat storage systems. The utilisation of highly conductive fins is an effective method to improve heat transfer. In the presented paper, solidification times of two fundamental geometries are examined by analytical modelling and numerical computation. These geometries are the finned plane wall and a single tube which is radial-finned on the outside. The paper describes approximate analytical solutions based on the effective medium approach which include the boundary conditions, as well as material and geometric parameters.     

Optimal pipeline geometries and oil temperatures for least rates of energy expenditure during crude-oil transmission

A mathematical model has been developed to describe the energy behaviour of a hot oil being pumped through a thermally insulated pipeline. With this model, for each specified application, predictions can be made of the interrelated optimal pipe size and temperature distribution along the pipeline corresponding to which ensues the least overall steady-state rate of energy expenditure due to thermal and hydraulic losses. In particular, the evolved iterative computational technique has been applied to the design of long (>100 km) crude oil pipelines. It is also relatively simple to extend the presented analysis in order to determine the optimal numbers of reheating and booster pumping stations necessary along the pipeline for any specific application.     

Evaluation of the coupling scheme of FVM and LBM for fluid flows around complex geometries

Systematic research has been carried out to evaluate the performance of the proposed coupling between finite volume method and lattice Boltzmann method (CFVLBM), including both accuracy and efficiency aspects. Three numerical examples, e.g., fluid flow around a circular cylinder, NACA0012 airfoil and porous media are employed. In the first two examples, careful comparisons are done between the results of CFVLBM and the results of previous publications as well as multi-block LBM, ISLBM and FLUENT software. The good agreement indicates the CFVLBM model is reliable and accurate. Meanwhile, the figures of residual history depict the better computational efficiency of CFVLBM than that of multi-block LBM. Finally, a practical application of CFVLBM for flow past a porous-medium core is simulated. The results prove the present method can be used to the fluid flows around complex geometries.