Study On Freezing Heat Transfer Performance Of Ground Source Heat Exchanger With Around Soil In Cold Areas

In the cold climate area,the soil initial temperature is lower than other regions.The inlet temperature of the buried tube will be less than 0? when the ground source heat pump(GSHP)system runs long time for great heat load,so the soil around the buried tube will be frozen.The freezing soil thermal conductivity increases and the water in soil release latent heat after freezing,which can improve the heat transfer performance of ground source heat exchangers(GSHEs).Therefore,we should consider the effect of soil freezing when design the GSHEs in cold areas.In this paper,we conducted sandbox experiments,and established the frozen heat transfer model to calculated,to study on the freezing heat transfer performance of GSHES under low temperature condition.Firstly,based on the theoretical analysis,we set up a sandbox experimenta to simulat the freezing heat transfer process of GSHEs,and analysed the effect of water content and fluid temperature on the freezing heat transfer performance of GSHEs.The experimental results showed that,with the the fluid temperature decreasing or water content increasing,the freezing radius increases,the soil temperature decreases and the heat transfer rate increases.Besides,the fluid temperature was the main factor affecting the freezing heat transfer of GSHEs compared with the soil water content.When the fluid temperature reduced from-2? to 10?with decreased by 400%,the freezing tadius increased from 3.7mm to 21.4mm which increased by 475.81%,and the frozen soil temperature reduced from-0.7? to-6.4? with 814.29% decreased.And the heat transfer time reduced from 10810 s to 8560 s with reduced by 20.81%.While the water content increased from 5% to 25%,increased by 400%,the frozen radius increased from 11.1mm to 16.9mm,increased by 52.16%;The frozen soil temperature decreased from-2.7? to-3.9? with a decrease by 42.99%,and the heat transfer time decreased from 11090 s to 8790 s,reduced by 20.74%.Then,the mathematical model is used to calculate according to the experimental conditions,and the calculated values were used to compare with the experimental value to verify the accuracy of the mathematical model.The results show that the maximum error of the calculated results and the experimental results was 8.87%,which showed that the mathematical model in this paper is reliable.Based on the validated mathematical model,we studied the influence of soil physical parameters on the freezing heat transfer performance of GSHEs.The results show thatThe soil initial temperature and heat capacity had the same influence on the freezing heat transfer performance of GSHEs.With the soil initial temperature and heat capacity increasing,the freezing radius decreased and the soil temperature increased.While with the soil thermal conductivity increased,the freezing radius increased and the soil temperature decreased.The soil physical parameters will be changed when the soil was frozen,so the freezing heat transfer model should be used for simulating calculation.Under the same conditions,we calculated by the freezing heat transfer model and the unfrozen heat transfer respectively,to analyzed the calculated errors.The results show that,when the freezing time was 8000 s and the soil moisture content was 20%,the error of calculating the temperature,resistance,thermal diffusivity and heat transfer time of the soil were 1.54%,7.14%,36.56% and 1.92% respectively,and the maximum error was in the thermal diffusivity.Therefore,the freezing heat transfer model should be used to calculate the freezing heat transfer process.The relevant conclusions in this paper can provide reference for the design of GSHEs in cold area.