Study On Heat Transfer Enhancement Of Loop Thermosyphon Under High Heat Flux

With the increasingly sophisticated manufacturing accuracy of electronic chips and higher power,compared with the traditional gravity heat pipe,the loop thermosyphon can work stably under high heat flux,so it has good application prospect and development potential.At present,in the research of loop thermosyphon,it is found that the heat transfer characteristics of loop thermosyphon depend on many factors,such as different liquid filling rate,two-phase flow state in the pipe,evaporation and condensation conditions and the role of many external factors.However,at present,the research on the influence of various factors on the heat transfer characteristics of loop thermosyphon is not clear.In this paper,the heat transfer and flow characteristics of loop thermosyphon with different liquid filling rate are analyzed.It is proved that the loop thermosyphon with high liquid filling rate has better heat transfer ability under high heat flux density;The effects of different external factors on the heat transfer performance of loop thermosyphon with high liquid filling rate are compared.The main research contents are as follows:Based on the traditional gas-liquid phase change model,a moderate correction is made,so that the condensation frequency can be adjusted after each time step iteration in the numerical simulation process,so as to ensure the conservation of mass in the evaporation and condensation process.The modified gas-liquid phase change model is applied to the gravity heat pipe,and the simulated data are compared with the experimental data in the reference to verify the reliability of the evaporation and condensation correction model.The physical model of loop thermosyphon is established,and the loop thermosyphon under different liquid filling rates is numerically simulated.The flow and heat transfer characteristics of loop thermosyphon under four liquid filling rates are systematically studied.The velocity vector diagram and two-phase volume fraction diagram in the pipe are obtained and analyzed.The results show that the loop thermosyphon can form a stable gas-liquid two-phase circulation under high heat flux and high liquid filling rate,and the loop thermosyphon can not produce circulation under medium and low liquid filling rate.Under the condition of high heat flux and high liquid filling rate,the effects of different inclination angle,pipe diameter,working medium and cooling water temperature on the heat transfer characteristics of loop thermosyphon were studied.The wall temperature of loop thermosyphon under different external factors is systematically compared,and the corresponding non-uniform temperature coefficient and thermal resistance of heat pipe are calculated.The results show that the reduction of inclination angle and too large or too small pipe diameter will increase the thermal resistance and non-uniform temperature coefficient of heat pipe,which is not conducive to improving the heat transfer limit of loop thermosyphon;When the loop thermosyphon is filled with liquid water and R134 a respectively,due to the differences of specific heat,latent heat,surface tension and viscosity of the two different working fluids,the loop thermosyphon shows better heat transfer performance when the filling working fluid is R134a;By reducing the cooling water temperature of the loop thermosyphon,the wall temperature at the condensing end of the loop thermosyphon can be reduced,but the temperature uniformity of the heat pipe will become worse.Therefore,the selection of the cooling water temperature of the heat pipe should take into account the actual temperature control demand and the stability of the heat pipe when working,and reasonably select the cooling water temperature of the loop thermosyphon.In this paper,the traditional phase change model is moderately modified to solve the problem of non conservation of evaporation and condensation mass in the actual simulation process.The operation mechanism,flow and heat transfer characteristics of loop thermosyphon under different liquid filling rates and external conditions are systematically analyzed.It is of certain significance to the practical application and heat transfer limit of loop thermosyphon.