Study On Heat Transfer Performance Of Screw Type Heat Exchanger

Ground source heat pump (GSHP) is an energy-saving and environmentally friendly technology for heating and cooling by using the vast thermal storage capacity of soil. In recent years it is widely concerned and used with the emergence of global energy crisis and environmental problems. The heat performance and investment of ground heat exchangers are important factors of the promotion of GSHP system. Because of high costs and hard construction, the conventional U-tube ground heat exchangers restrict the promotion of GSHP. As a new kind of ground heat exchangers, the heat transfer performance of coil ground heat exchangers are excellent with low costs and easy construction, so they are gradually concerned by industry insiders. In this paper, the heat transfer performance of coil ground heat exchangers inside foundation piles and slinky-coil horizontal ground heat exchangers which are widely used are investigated in different conditions by experimental study and numerical simulation.The heat transfer models of coil ground heat exchangers inside foundation piles and slinky-coil horizontal ground heat exchangers are developed to model the heat transfer processes of the ground surrounding the heat exchangers. The influences of foundation pile diameter, foundation pile depth and the number of buried pipes on the heat transfer performance of the heat exchanger inside foundation piles are investigated. The results indicated that increasing foundation pile diameter, foundation pile depth and the number of buried pipes can develop the heat transfer performance of heat exchangers. And the effects of soil type on the two coil ground heat exchangers are investigated, it can be found that high specific heat and high thermal diffusivity would improve the heat performance of heat exchangers and sandstone is the most favorable to the two coil ground heat exchangers among sand, stone and clay. To slinky-coil horizontal ground heat exchangers, when groundwater level is higher than 4.2m, the heat transfer performance will be improved with the groundwater level, velocity of the groundwater or water content of ground increasing; when groundwater level is lower than 4.2m, groundwater seepage has little impact on the heat exchangers.Two model experiment systems of coil ground heat exchangers inside foundation piles and slinky-coil horizontal ground heat exchangers are established on the basis of the similarity theory. The heat transfer performance of coil ground heat exchangers inside foundation are investigated in different conditions. The results indicated that increasing the fluid inlet temperature can develop the heat transfer performance of the two coil ground heat exchangers, but the velocity and range of the soil temperature change will be higher, hence fluid inlet temperature should be determined on the basis of the actual operating conditions. The intermittent operation condition is good for the recovery of soil temperature around two coil ground heat exchangers, and it can enhance heat transfer efficiency of pipes. To coil ground heat exchangers inside foundation piles, reducing the pitch between pipes can develop the heat transfer performance of heat exchangers, but the velocity and range of the soil temperature change will be higher and the heat transfer efficiency of pipes will decrease, so the pitch between pipes can’t be infinitely reduced. The heat transfer performance of coil ground heat exchangers inside foundation will be improved with the specific heat of pile foundation material increasing, but increasing the specific heat of pile foundation material increasing will delay soil temperature recovery. To slinky-coil horizontal ground heat exchangers, the heat transfer performance will be better with center distance of coil pipe decreasing, but the heat transfer efficiency of pipes will decrease. The heat transfer performance of a slinky-coil horizontal ground heat exchanger will be better with the velocity of wind increasing because the air will take away more heat.The aim of the paper is to investigate the influence factors of heat transfer performance and the measures for heat transfer enhancement for coil ground heat exchanger inside foundation piles and slinky-coil horizontal ground heat exchanger, and provide theoretical guidance and technical support for promotion and application of them in the actual project.