Dynamic Heat Transfer Characteristics And Heat Transfer Enhancement Of Direct-Expansion Carbon Dioxide Downhole Heat Exchanger And Its Array

Direct-expansion borehole heat exchanger（DXBHE）is a technology in which the source-side heat exchanger of the heat pump is directly buried underground and the refrigerant directly exchanges heat with the underground medium.Compared with the conventional second-loop borehole heat exchanger（BHE）,the DXBHE can achieve higher heat transfer temperature difference and higher heat transfer rate for both supercritical-exothermic condition and phase-change-endothermic condition.The unit heat flux of the DXBHE can reach more than 200 W/m.Based on the characteristics of large temperature difference of heat transfer,a novel direct-expansion downhole heat exchanger（DXDHE）using supercritical pressure CO₂ as refrigerant is proposed in this study.The CO₂ DXBHE is placed inside a closed water well,in order that the total thermal resistance can be decreased by natural convection,and the heat transfer can be enhanced.Firstly,based on the comparison of various DXBHE models,the thermal resistance and capacity model（TRCM）,which is faster and more applicable to complex conditions,is selected to construct a 3-D transient heat transfer model for the CO₂DXDHE.The thermal resistance and heat transfer correlations for each node are introduced,the node division method and the iterative solution process are analyzed,and the necessity of importing Jacobi sparse matrix when solving ordinary differential equations is discussed.Then,an complete experimental system was built which including a 100 m～135m deep CO₂ DXDHE,a heat pump and terminal equipment system.The operating characteristics of the DXDHE and the system under cooling and heating conditions are investigated.Experimental results show that the system can achieve long-term stable operation.Compared with the experimental or theoretical research on conventional CO₂DXBHE,the average unit heat flux of cooling and heating conditions of the CO₂DXDHE can be increased by 52.5%and 18.6%,respectively.Then,the experimental data are used to validate the heat transfer models for both supercritical-exothermic condition and phase-change-endothermic condition.The mean absolute percentage error of the unit heat flux under the two conditions are 2.29%and 4.98%,respectively,which can verify the accuracy of the heat transfer model.The model is then used to analyze the dynamic influences of several key parameters under cooling and heating conditions,and a sensitivity analysis is carried out.The thermal short circuit effect and the heat transfer deterioration caused by the drastic change of pseudocritical CO₂ properties are discussedThen,with the newly defined dimensionless thermal short circuit coefficientη_SC,the positive effect of natural convection in reducing the total thermal resistance of the heat exchanger and the negative effect in aggravating the thermal short circuit are both quantitatively analyzed.Two novel methods for heat transfer enhancement are proposed to alleviate thermal short circuit and enhance heat transfer,which are insulation method and artificial well-water circulation method.Quantitative analyses of several key parameters on enhanced effect are carried out for both methods.The average increase ratio of unit heat flux of the two methods can reach up to 35.1%and50.5%.Finally,since the existing TRCMs cannot be applied to the heat transfer simulation of the BHEs array,a novel 3-D transient heat transfer model for the BHEs array that can include the influence of groundwater flow is proposed,which is based on a novel division method of soil side thermal resistance.A finite volume model is used for validation,and the calculation speed of the new model can be increased by more than600 times.The model is then applied to the CO₂ DXDHE,and a new method of using artificial groundwater flow（AGF）to balance the soil temperature and enhance the heat transfer of the BHEs array is investigated.The influences of different key parameters on enhanced effect are analyzed.The results of a 30-day simulation based on a 4×4DXDHEs array show that,by the AGF method,the average unit heat flux of all DXDHEs can be increased up to 21.5%.