Numerical Simulation Of Heat Transfer Performance Of The UHESSPORP

Nowadays,the environment and energy problems are increasingly serious,The vertical buried ground source heat pump system(GSHPS)becomes the most common application of a heat pump system.,because of its minimal footprint,higher efficiency of cooling and heating air compared with the air source heat pump.Ground heat exchanger as a core component of the GSHPS,its initial investment has a great impact on the total system investment.Therefore,optimizing the structure and improving the heat transfer efficiency of the ground heat exchanger is an important way to reduce the initial investment and promote the development and the application of the GSHPS.In order to reduce the heat short-circuit between the water supply and return pipes in the drilling,the heat exchange between the return pipe and the soil and increase the heat transfer proportion,a new type of underground heat exchanger is proposed,which consists of several supply pipes and one central return pipe,while the return pipe use a vacuum double-walled tube for insulation.Taking the underground heat exchanger with several supply pipes and one return pipe(UHESSPORP)which the number of the supply pipes is 2 ~8 and the traditional single-U and double-U tube as the research object in this paper,analyzes whether the UHESSPORP can replace the traditional single-U and double-U tube in certain conditions and increase the heat transfer efficiency and heat exchange amount.In this paper,the formula for calculating the thermal resistance in the borehole of the UHESSPORP is deduced,and the total thermal resistance of the heat exchanger is calculated.It is found that the thermal resistance of the UHESSPORP has a certain degree of reduction compared with the traditional single-U and double-U tube.The theoretical model and numerical model of the UHESSPORP are established,and the numerical model is validated which has high degree of reliability.The heat transfer performance in summer and winter of a 48 hours continuous working condition of the heat exchanger is simulated by the FLUENT software,the results show that considering the heat transfer performance and economy of the heat exchanger,as a substitute for a single-U tube and a double-U tube the most appropriate flow rate is 0.4 m/s.As a substitute for a single-U tube the proper number of the supply pipe of the UHESSPORP is 3 to 5 and as a substitute for a double-U tube the proper number of the supply pipe of the UHESSPORP is 5.By analyzes the results of simulating the continuous operating conditions of the UHESSPORP,the proper number of the water supply pipes is 2 ~ 6.Then simulated the discontinuous operation of 10 d of the UHESSPORP and the traditional single-U and double-U tube.The results show that,the average heat quantity per well depth of the UHESSPORP under the flow rate of 0.4 m/s is 18.73% ~ 41.63% higher than that of the single-U tube under the flow rate of 0.8 m/s,while it is 2.31% ~ 11.54% higher than that of the double U under the flow rate of 0.6 m/s;the average heat quantity per well depth of the UHESSPORP under the flow rate of 0.6 m/s is 23.73% ~ 45.61% higher than that of the single-U tube under the flow rate of 0.8 m/s,while it is 5.7% ~14.67% higher than that of the double U under the flow rate of 0.6 m/s,and the increment increases with the number of water supply pipes.So the traditional single-U and double-U tube can replaced by the UHESSPORP under a low flow rate and the effect is real good.Through the research of this paper,it is proved that the heat transfer efficiency of the UHESSPORP is improved to a certain extent compared with that of single-U and double-U tube under certain conditions,which reduces the number of boreholes and saves the floor space,but also increases the water flow and the pump power consumption,so the UHESSPORP are suitable for the high-cost area of drilling.The UHESSPORP in this article has applied for a patent and is public,but now it is not appeared in the actual project yet,so there still further work to do to optimize the design and finishe product in order to promote the application of the actual project.And the return pipe located in the center of the heat exchanger causes some difficulties in putting the UHESSPORP in the borehole.It is necessary to explore the appropriate way to solve this problem and improve the feasibility of the feasibility.