Study on the heat transfer performance of boiling in vertical buried tube of direct expansion ground source heat pump

Abstract

Two kinds of physical phenomena of phase change heat transfer and gas-liquid two-phase flow co-exist in the buried tube of the direct expansion ground source heat pump system. The performance of boiling heat transfer process in vertical U-tube was discussed in this study. Firstly, the models of the refrigerant flow equation and the phase change heat transfer process in the vertical U-tube, the heat transfer process internal and outside well heat transfer were established. Based on the models, the changes of the refrigerant parameters along the length of tube were simulated and analyzed during boiling heat transfer process. The simulation results showed that the pressure and temperature of refrigerant rise in the downward tube, while the pressure and temperature of refrigerant declined rapidly in the upward tube. So the evaporation kept slow rate in the downward tube but fast in the upward tube. The effect of heat transfer in upward tube is better. At the same time, the influence of the soil initial temperature is simulated. as the soil initial temperature declining, the changes of the refrigerant temperature and pressure in the buried tube are smaller. And the changes of the soil temperature field with the running time and the different depth of the borehole are also simulating studied. The simulation results show that the temperature around the buried tube decreases along with the operating time, and the temperature diffusion radius around the buried tube increases in a descent gradient.