Experimental And Numerical Study On Thermal Performance Of Heat Exchanger For Ground Source Heat Pump System

Ground heat exchanger(GHE)is a vital part of the ground source heat pump system,which plays an important role in heat transfer efficiency of the whole ground source heat pump systems.Thermal performance of GHE was studied by full-scale experimental tests(pipes buried underground with a depth of 100 m)in SOOCHOW University and CFD(computational fluid dynamics)simulation,i.e.,different shapes of pipes(smooth pipe and pipes with different petals)and different materials of pipes(PE pipes and steel ones).Further investigation on thermal performance of heating and cooling model and overall heating system and the coefficient of performance(COP)of overall heating system of different type of GHE.Firstly,we have established a full-scale ground source heat pump systems laboratory.Thermal performance and coefficient of performance COP of different types of GHE were compared by experiment and analysis.Next,GHE model(U-tubes buried with a depth of100m)for all type of pipes were established.We validated the numerical simulation using k-?RNG model through experiments.Finally,CFD simulation tool was mainly employed for analysis the factors influencing thermal performance of GHE.The results of the experiments indicated that cooling thermal performance of all types of GHE in summer was better than heating in winter.The Q_L of steel pipe GHE in cooling model was 13.8%larger than heating model.The Q_L of PE pipe GHE in cooling model was26%larger than heating model.The Q_L of steel and PE pipe GHE was declined with the operation of heat pump both in heating and cooling conditions.The COP of steel pipe GHE was 16%larger than PE pipe GHE.The numerical simulations studies shown that the radial temperature presented notable changes near the wall of the tube,but small far from the tube.The radial temperature presented notable changes near the wall of the tube,but small far from the tube.Besides,the effects of thermal short circuiting between two legs of the U-Tube were noticed.Increase of inlet flow velocity would lead to the improvement of heat exchanger effects for all types of heat exchangers.The numerical simulations and experiments studies shown that after validating with experimental results,CFD simulation is proved to be a feasible method for the predictions of the thermal performance of heat exchanger unit.Although changing the shape of U-tube would increase the turbulent heat diffusion effects,the flow resistance is also increased due to the increase of pressure loss coefficient,further leading to the declined advection effects on heat exchange performance.Thermal resistance R of steel type was smaller than the PE one with the differences up to 7%.Thus,the steel pipe always showed better thermal performance compared to PE pipe for different inlet velocities,with Q_L increased up to 36%with inlet flow velocity of 0.8m/s.The conclusions of this manuscript can provide the guidance to design the GHE.Thus,the initial investment of ground source heat pump system can be reduced and promote the development of ground source heat pump technology.