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Heat Transfer Performance of Cellular Materials and Optimization Design of Heat Dissipation Structure

Author: ZhangYongCun
Tutor: LiuShuTian
School: Dalian University of Technology
Course: Engineering Mechanics
Keywords: Ultralight porous material Radiative heat transfer Effective thermal conductivity of Convective heat transfer Metal honeycomb Insulation Radiator Optimization Model Topology optimization Configuration theory Bionic Design
CLC: TB383.4
Type: PhD thesis
Year: 2008
Downloads: 1288
Quote: 5
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Abstract


Thermal management exists widely in engineering fields such as aerospace, energy power and electronics. The heat insulation and active coolings are all needed in the thermal barrier issue of space vehicle, the temperature control of temperature-sensitive equipment, and heat dissipation of electronic devices. The cellular materials (including the truss-material) with light-weight can be easily tailed to a multifunctional application that demands not only heat transfer capacities but also other roles. The close-celled cellular materials with low conductivity are often applied as the heat insulation material and the open-celled cellular material with voids enabling fluid flow in one direction is used to active cooling. The researches on the heat transfer performance of cellular materials and the optimal heat dissipation structure have big significances in theories and application.For the demands of practical applications above, the methods for predicting the heat transfer performance of cellular materials are investigated: the multi-scale analysis method for thermal conductivity of close-celled cellular with radiation is proposed, and the transfer matrix method and the rapid numerical algorithm for heat transfer efficiency of active cooling by open-celled cellular material with one easy flow direction are presented. The mathematical formulations and the corresponding solving methods for the configuration of the optimal heat dissipation structure based on the topology optimization and bionic idea are studied. The main content and results are given in the following paragraphs:(1) Multi-scale analysis method for thermal conductivity of cellular material with radiation. The close-celled cellular material is often applied in the high temperature entironment as the heat insulation material. A multi-scale method for the predicting the effective thermal conductivity with radiation of close-celled cellular material is presented, it also considers the effect of geometry and distribution of pores. Using the homogenization method to solve the pure conductive problem of porous materials with periodic structure, the effective thermal conductivity without radiation is predicted, and the temperature field in a local domain of a unit cell is obtained. The temperature field is taken as the good approximation of the real temperature distribution and the radiative thermal conductivity is obtained. Furthermore, the effect of the microstrcutre, the distribution and geometry of pores on heat transfer porous materials is discussed. (2) The new methods for heat transfer efficiency of active cooling by metal honeycomb structures: Transfer Matrix Method and Rapid Numerical Algorithm. Theopen-celled cellular materials have potential for simultaneous load bearing and active cooling. The transfer matrix method and the rapid numerical algorithm for heat transfer performance of metallic honeycomb structure under the forced convection conditions are presented. The transfer matrix method only used to analyze the heat transfer performance of sandwich metallic honeycombs can overcome the approximations in the corrugated wall model. The heat transfer efficiency predicted by this new method is consistently lower than that predicted by the corrugated wall model, higher than that by the effective medium model, and closer to the numerical simulation results. It indicates that this method is accurate. Motivated in the engineering applications of a variety of structure filled with honeycombs and optimization design of non-uniform honeycomb structure, the rapid numerical algorithm is presented by inspiration from the finite method and the transfer matrix method. It’s about 3-4 order of magnitudes improvement in computational velocity of this method compared with the finite volume method. The validity and applicability of the method is proved by two examples.(3) Design method for the optimal conducting paths based on topology optimization. The new method based on the topology optimization is presented to solve the volume-to-point heat conduction problem which is the fundamental problem in electronics and cooling architecture. The performance of the conducting paths obtained by the present method has great improvement relative to the design of bionic optimization, and can break down the design limit of construct method. Furthermore, the configuration of the optimal conducting paths is more similar to the natural tree than those obtained by other methods.(4) The optimization model of the heat conduction structure. Generally, the highest temperatures in a package must not exceed a specified value, an optimization model which considers a novel thermal performance index as the objective function is proposed for minimizing the highest temperature. Firstly, the performance of the conventional heat conduction optimization model with the dissipation of heat transport potential capacity as the objective function is evaluated by a one-dimensional heat conduction problem in a planar plate exchanger. Then, a new thermal performance index, named by the geometric average temperature in this paper is introduced. The new heat conduction optimization model with the geometric average temperature as the objective function is developed. The results show that the geometric average temperature is an ideal thermal performance index and the solution of the new model is close to the theoretical optimal solution.(5) Structural design of forced convection cooled tree-like channels heat sink based on bionic idea. For the demand of industrial cooling, the improved design of fractal channel net based on the bionic idea that can meet the demand for cooling of electronic chip with arbitrary ratio of length to width is presented. A theory model is proposed to estimate the performance of heat transfer and pressure drop approximately. It is found that the best total branching level is 7. If the surface area for cooling is fixed, it is the best choice when the ratio of length to width of heat sink is 1.87. These results are instructive to design the fractal branch net of rectangular shapeThe work of this dissertation is supported by National Natural Science Foundation of China through the Grant No.s(10332010, 90205029,10421202), by the national key basic research program of china (Grant No.2006CB601205), and by the program for new century excellent talents in university of china (NCET-04-0272).

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