Performance Investigations On Heat Exchanger Of Free-piston Stirling Engine

Due to the deterioration of global pollution and exhaustion of global energy, much attention has been focused on the emerging renewable energy such as solar energy, wind energy and geothermal energy. Here, much attention has been paid to Stirling engine due to its broad applicability and excellent thermal performance. While the performance of the heat exchanger which is a critical component of the Stirling engine and directly influence its performance and power ratio. Therefore, study on the heat exchanger is of crucial significance.For gas flows back and forth in the heater and cooler of Free-Piston Stirling Engine(FPSE), therefore in this paper the flow and heat transfer performances were analyzed through establishing oscillatory flow model to serve as references for the theoretical research of the heater and he cooler as well as the design optimization.In this paper, preliminary design of the size and structure of miniwatt FPSE(300W) and initial estimation and optimization of its power were presented firstly, the geometry of the heater and cooler were obtained, then the model simplification about the heater and cooler were made, meanwhile physical modeling were carried out by GAMBIT software and numerical solution about models of the heater and the cooler were also obtained through commercial software FLUENT. The inlet adopted mass-flow-inlet and the outlet employed pressure-outlet, with the addition of UDF files, the oscillatory flow in the inner flow field of the heater and the cooler were achieved and the flow characteristic and heat transfer characteristics under different circumstances were analyzed.In order to achieve the goal of enhancing heat transfer of the heater, triangular fins, rectangular fins and curved fins were respectively added to the heat wall, and the foam metal porous media was also be used in the oscillatory flow model of heater. Then the changes of the temperature and flow field in a oscillation period were analyzed. The results show that the flow resistance loss and heat transfer should be taken into comprehensive consideration so as to select the length of the channel under oscillatory flow. The triangular fin among fins structure is of optimal effect of heat transfer enhancement, curved fin is next, and the rectangular fin is the worst. The higher the fin, the stronger the disturbance of the flow field and the greater the heat transfer amount, at the same time, the loss of flow resistance loss is also increased. Considering the heat transfer amount and Performance Evaluation Criteria(PEC) values comprehensively, the optimal height of the fin is 0.2-0.3 mm within the channel of 5 mm width. And the foam metal porous media is of the best heat transfer enhancement effect, its PEC value is significantly greater than that of the fins structure, its porosity increases, the heat transfer coefficient decreases, while the pressure drop also reduces, and the maximal value will be gained at about 0.9.Triangle fins, rectangle fins, curved fins and the method of cooling the heat pipe are adopted to achieve the purpose of enhanced cooling. The velocity field and temperature field are obtained by numerical simulation. The study finds that the heat flux and heat transfer rate are changed periodically over time, and the heat pipe is of the best heat transfer efficiency E. The cooling effect of staggered heat pipes is superior to that of inline heat pipes. When the windward interval s is 2 mm, the optimum cooling effect will be gained, and factors such as heat transfer amount, pressure drop and heat transfer efficiency should be comprehensively considered for the selection of downwind interval l1.