Design Of Gradient Cellular Material As Thermal Conductivity Enhancer For PCM-based Temperature Controller

Phase change materials can realize cooling of short-time high power devices by phase change absorption.And the temperature control is realized by heat absorption and heat release in large wave thermal environment.The temperature control mechanism is of great significance to the design of temperature control system of high precision equipment in harsh environment with high power and large fluctuation.But the low thermal conductivity of phase change material limits the thermal diffusion,and seriously affects the temperature control ability.Ultra-light porous materials(such as metal foam,micro-truss material,etc.)have the advantages of large surface area,high thermal conductivity and strong mixing ability is an ideal heat transfer enhancement.It has attracted wide attention.However,the current research focuses on the direct application of existing materials.The greatest advantage of porous metals is that they are programmable.The optimum gradient design of porous metal medium as phase change temperature control and thermal conductivity enhancement is studied from experiment,numerical analysis and optimization design.In this paper,the bottom heating of rectangular temperature control device is studied.A gradient design scheme for analytic expression is given.And compared with the optimal gradient design scheme,the effectiveness of the proposed gradient design scheme is verified.Researches are as follows:(1)Experime ntal study on phase change te mpe rature control of e mbedded gradient porous metal me dium.Experiment is the most reliable method to verify the validity of the design scheme.Firstly,an experimental platform for phase change heat trans fer is designed and constructed.The change curve of the critical point temperature with time during the whole phase transition process can be obtained.Secondly,porous metallic materials in the form of micro trusses are designed.The change of porosity was realized by changing the diameter of the rod and the technology of additive manufacturing was used to prepare the test piece.The paraffin wax is filled therein to form the corresponding test piece.Finally,the temperature control of two typical test pieces under different heat distribution conditions was tested by the experimental platform.The results show that the distribution of porosity has significant influence on the temperature control under the same quality.Therefore,through reasonable gradient design,the effect of phase change temperature control is expected to be improved.(2)Effect of natural convection on the temperature control of PCM embedded metal foam porous medium.The simulation of phase change heat transfer process,considering the influence of natural convection of liquid phase change material,is a fluid-solid coupling heat transfer problems with time.Due to the large amount of calculation,it is difficult to achieve the process of iterative optimization analysis.How to consider the influence of natural convection heat transfer is the key to model simplification.The object of this paper is the phase change heat dissipation device with bottom heating.The suppression of natural convection of liquid phase change materials by metal foams is investigated by numerical method.The influence of natural convection under different porosity and pore density on the temperature control performance was discussed.The results show that porosity is the main factor affecting the temperature control performance.Natural convection has little influence on temperature control performance,and can be neglected in analysis.(3)Optimum gradient design of porous metal medium for unidirectional phase change heat transfer.The optimum design model is established by using the phase change heat sink(unidirectional phase change heat transfer)with the bottom heating as the object.An explicit analytical expression for the optimum grad ient design of porous metallic media is proposed by the hierarchical optimal design.The results show that the effect of phase change temperature control can be significantly improved by gradient design of porosity.The research provides a reference for the design of practical engineering.