Study On Heat Transfer Performance Of A Vertical Ground U-shaped Internally Helically Grooved Tube Heat Exchanger

ABSTRACT:As the conventional energy increasingly decrease, the ground source heat pump system is being paid more and more attention because of its environmental protection, energy saving and renewable. In this paper, the influences of the backfill types, imported water temperature and Re on heat transfer performance of the ground tube heat exchanger were investigated by means of experiment and numerical simulation. The experimental study revealed the influences of the temperature and initial moisture content on internal heat and moisture transfer for the soil, sand, soil with PAM, and sand with PAM.The experimental results of the heat and moisture transfer in soil and sand showed that the scope of the heat and moisture transfer in soil and sand increased with the increase of the temperatures and initial moisture, the scope of sand was larger than that of soil. The average temperatures of soil and sand increased respectively about 21.4% and 7.6%, and also the scopes of moisture migration of soil and sand with PAM decreased respectively about 39.6% and 13.4%, which revealed that the soil and sand with PAM had a good ability of water retentivity, and the water retention ability of soil with PAM was better than that of sand with PAM. The average temperature and average moisture of soil with 3%PAM increased about 0.24% and 0.13%, while the average temperature and average moisture of sand with 3%PAM increased about 0.53 and 0.21%.The experimental results showed that heat transfer of ground tube increased obviously after addition of PAM into soil or sand when imported water temperatures are 30℃,35℃,40℃,45℃ and Re were 4000,6000,8000,10000. The average peripheral soil temperatures of soil and sand increased by 1% and 0.04%, the average temperature of tube wall increased 1.8-2.2℃ and water temperature drops increased 0.4℃ at most. The heat transfer coefficients in the tube increased 60% and 36% when PAM was added into raw soil or sand. The dimensionless heat transfer correlations of ground U-shaped internally helically grooved tube in soil, sand, soil with PAM, and sand with PAM were obtained.Based on numerical simulation, the heat transfer performance of internally helically grooved tube was further studied. The values of calculation matched well with experiment values. The data of heat transfer performance of the smooth tube showed that the water temperature drops of internally helically grooved tubes increased by 0.33℃,0.53℃,0.63℃with three different fin heights (0.1mm,0.15mm,0.2mm) compared with smooth tube, while heat transfer coefficients increased by 26%,52%, 66%. The flow line of smooth tube paralleled to the axial direction of tube, while that of internally helically grooved tube was perpendicular to the axial direction of tube outside the mainstream, and the flow field density of internally helically grooved tube was obviously larger than that of smooth tube. The heat transfer enhancement factors of three internally helically grooved tubes decreased with the increase of Re. The results of this paper could provide some basic data for the study on strengthening heat transfer of ground heat exchanger.