A Study On Layered Heat Transfer Model Of Vertical Double U-tube Ground Heat Exchangers

Ground source heat pump system, which utilizes rock-soil body as low temperature reservoir, is a kind of air-conditioning system that is made up of heat pump unit, ground heat exchangers and components inside the building. Taking full advantage of low-grade energy, it can be used for cooling and heating efficiently, which makes it known as a promising air-conditioning system that uses renewable energy in the 21 st century. Ground heat exchangers are used for heat transfer between the heat-transfer agent and rock-soil body and the heat transfer performance of the ground heat exchangers is a key factor in the development of ground source heat pump system. The heat exchange efficiency can be enhanced by fully understanding the heat transfer process and designing the heat ex-changers reasonably. Thus the system can function well and the energy consumption can be reduced, which are good to the development and application of the system.In a pracitcal project in Chongqing, the heat transfer performance of ground heat exchangers are investigated by theoretically analyzing and measuring and ground heat exchangers are set up in places where there is underwater seepage and the soil consists of several geological layers. On account of these factors, geological layered model of heat transfer of U-shaped vertically ground heat exchangers is developed. According to the thermal response test in practical project and actual operating functions, the comprehen-sive thermophysical parameter of each layer is solved by making theoretical calculation and programming. Taking measured layered thermophysical parameters as boundary con-ditions, the correctness and reliability of the model are fully analyzed by creating a dy-namic numerical simulation with CFD to campare the relevant factors, including velocity, backfill material and buried depth in the heat removal and rejection conditons. The re-searcher compares the data of regular homogeneous heat transfer model and layered heat transfer model in this paper and the functioning data in practical projects. Thus, the level of each factor can be determined by analyzing the effects of each factor, including velocity, backfill material, buried depth and well space, with the proved layered heat transfer model. On the basis of orthogonal test theory, the experimental program is reasonably launched. Heat transfer performance of ground heat exchangers in different conditions and the sig-nificance of the relevant factors are analyzed. Finally, on the basis of layered model of buried pipes, simplified layered model is developed by analyzing and optimizing its ap-plication methods.The practical research shows that thermal parameters in different geological layers, even in the same layer, are different. Layered thermal resistance varies from 0.1344(m·K)/W to 0.1717(m·K)/W and the rangeability is more than 20%. So it is nec-essary to build a layered model.It is found by measuring and simulating that the solid thermal-conduction resistances are different in one geological layer, which results from the direction of groundwater seepage, velocity and underground porosity. Thus, it is feasible to take geological strati-fication and test point arrangement into consideration and build a vertically layered model. By camparing the practical data and simulation results in different conditions, it is found that the average fractional error of tube wall temperature in each depth is only 1.82%. The layered heat transfer model, which takes geological stratification and ground water seepage into consideration, is more accurate and reliable.The research shows that there exists a number of differences between the results of regular homogeneous heat transfer model and practical measurement. The error of quan-tity of heat exchange for unit well depth of the two models are 1.56% and 7.83% respec-tively and the value of layered model is more closed to the measured value.Thus, the influence of velocity in heat transfer of buried pipes is the most siginficant, followed by buried depth and well space. Heat conductivity coefficient of backfill meter-ial is not a significant factor.According to heat response test theory, thermophysical parameter of the solid can be accurately analyzed with simplified layered heat transfer model. It can be used to solve unit well depth and the length of buried pipes correctly. Layered heat transfer model is prospective in engineering.