Study On Heat Transfer Performance Of A Buried U-tube

ABSTRACT:Ground source heat pump systems have been applied widely with environmental friendly, energy saving and sustainablility. In this work, heat transfer performance of the vertical u-tube in ground source heat pump system is explored by experiment and numerical simulation. Influences of inlet water temperature, flow rate and soil properties on the heat transfer of the buried pipe have been assessed.Thermoproperties of soil, soil blending with sawdust and soil blending with water retaining agent, polyacrylamide(PAM), were obtained experimentally. Effect of inlet temperature, inlet velocity and backfill materials on the heat transfer performance (heat transfer coefficient, criterion equation and soil temperature distribution) of vertical buried U-tube was revealed in a laboratory-scale setup. Heat transfer rate of the buried pipe in the soil blending with sawdust slightly decreased compared with original soil backfill, which decreased by7.3%in the range of4656-7760for Reynolds number. Heat transfer rate of the buried pipe in the soil blending with PAM, increased by about37%compared with original soil in the range of3104-6208for Reynolds number. By comparing with soil, the thermal conductivity and thermal diffusivity of soil blending with sawdust reduced by about15.2%and16.3%, respectively, while specific heat capacity, increased by about7.5%. The thermal conductivity and thermal diffusivity of soil blending with PAM didn’t change with low water contant, while for water contant over20%, the thermal conductivity and thermal diffusivity increased by about11.8%and12.5%respectively, but specific heat capacity didn’t change.The heat transfer characteristics of the buried U-tube are also highlighted by three dimensional transient numerical simulation. Calculation results showed good agreements with experimantal data. Influences of backfill material, U-tube inlet temperature and inlet water flow rate to vertical buried U-tube heat transfer performance has been analysed. At inlet temperature of30℃, the temperature difference decreased by about0.2℃with Re increased from3000to11000. Heat flow rate of the U-tube increased by122W with the thermal conductivity increased from 0.5W/m K to2.5W/m K.