Experimental And Numerical Simulation Study On Vertical U-tube Underground Heat Exchanger Of Ground Source Heat Pump

Along with the policy of the energy-conservation and reduction, ground source heat pump (GSHP) technology has become a hot spot research with the features of energy-efficacy and environmental protection in HVAC industry. Focus and difficulties on GSHP system study is the research for the underground heat exchanger system.Study on the characteristics of the heat transfer for the underground heat exchanger in the GSHP system is the key to popularize GSHP system, which can optimize design of GSHP system. Therefore, it is worth making such research on GSHP system.With the advantages of using small area,stable performance,high efficiency of heat transfer ,vertical U-tube GSHP system has been wide concerned and application. This paper aims at a comprehensive understanding of working condition of vertical U-tube GSHP system by using experimental, theory analyzed and numerical method on actual project.The GSHP system used by hospital in Chongqing city is chosen for experimental object, and three test stages are included. Tests on the underground heat exchanger in different working conditions dictate that temperature at test spots between 1m and 9m below surface of soil rises and down sharply;temperatue at test spots among 9m,29m,and 59m below surface of soil decrease with increasing depth, temperature difference within 2℃.It illustrates that heat exchange between underground heat exchange and soil is mainly done in superficial layer of soil and then becomes to be steady with increasing depth.Vertical U-tube heat exchanger model is established by CAD and CFD technique is used to simulate the actual working condition of GSHP system by using above established heat exchanger model. The result of simulation is good with the experiment, that is to verify the mathematics physical model. The factors, which including the length of U-tube, backfill materials, interval space between 2 branches of U-tube, flow rate in U-tube, are studied to analyze their effect on the performance of the underground heat exchanger.Studying on factors which effect on the performance of underground heat exchanger by using PHOENICS, the results indicate that total heat exchange amount of the U-tube with 80m depth is higher than that of the U-tube with 60m depth; while considering heat exchange between underground heat exchanger and soil doesn't exist in whole tube ,it illustrates that the length of U-tube should be chosen reasonably to make GSHP running in the best working condition.GSHP system performance is better by enforcing the thermal conductance of backfill materials, which can also aggravate thermal short circuit. The thermal short circuit can be reduced by lengthening the interval space between two branches of U-tube, which can promote the performance of underground heat exchanger. The thermal performance of underground heat exchanger is various with various flow rates, therefore, that are there should be an optimal flow rate which makes underground heat exchanger performance best.