| Groundwater source heat pump（GWHP）technology can effectively use groundwater with relatively stable temperature in shallow depth as low-grade heat energy,and is widely used because of its clean and environmentally friendly characteristics.In the process of pumping and recharging,there are complex and changeable heat transfer conditions in the aquifer,and the effect of pore water will have a non-negligible impact on the evolution of the aquifer’s underground temperature field,and may cause imbalance of water and heat,which will affect the utilization of natural aquifer resources.Therefore,considering the influence of the natural flow of pore water on the heat transfer characteristics of water intake is the key to effective use of natural cold and heat sources.In this paper,through theoretical analysis,numerical simulation and calculation analysis,based on Darcy’s law and porous media heat transfer theory,a water-heat coupling model is established,and COMSOL Multiphysics finite element simulation software is used to analyze the heat transfer characteristics of water intake considering the natural flow of pore water,and the influence of pore water flow direction,natural flow rate,and physical parameters of porous media on the aquifer underground temperature field is studied.Take the time of thermal breakthrough,the degree of heat diffusion and the heat transfer as the standard to measure the heat transfer characteristics of water intake,and the optimization analysis of the three structural characteristics of the well groups design aspects of well diameter,pumping and recharging well spacing,and pumping and recharging well layout mode are carried out to further improve the theoretical basis of GWHP in practical engineering applications.By analyzing the influence of pore water on the heat transfer characteristics of water intake in three different flow directions（concurrent flow,countercurrent flow,and cross flow）,it is concluded that the average temperature drop at the pumping well at the end of the operation,the countercurrent flow direction is 0.32%of the concurrent flow direction and 6.67%of the cross flow direction;compared with the heat transfer under ideal conditions of no heat loss,the heat loss rates of the concurrent flow,cross flow and countercurrent flow,the extraction heat transfer are 63.4%,3%and 0.05%,respectively.Therefore,in actual projects,the arrangement of wells in the countercurrent flow direction is preferred,and the pumping wells are arranged on the upstream side of the recharge wells.By analyzing the influence of pore water on the heat transfer characteristics of water intake at five natural flow rates,it is concluded that as the natural flow rate of the pore water increases in the concurrent flow direction,the thermal breakthrough occurs earlier and the heat loss increases.As the natural flow rate of the pore water increases,the countercurrent flow direction delays the change of the average temperature at the pumping well;when the natural flow rate reaches 4×10-6m/s,the temperature at the pumping well is not affected by the recharge water.When the natural flow rate of the pore water reaches 8×10-7m/s,thermal breakthrough occurs in the cross flow direction;as the natural flow rate increases,the heat loss generally tends to decrease,but the migration process of the recharge water to the pumping well is more complicated and cannot be generalized.Analyzing the influence of porous media physical parameters on the heat transfer characteristics of the water intake by the controlled variable method,it is concluded that when k=70m/d,the heat transfer amount is 1.12,1.07 and 1.02 times when k=7m/d,k=15m/d and k=35m/d,respectively.The larger the permeability coefficient,the more beneficial it is.When M=40m,the heat transfer amount is 1.26,1.15 and 1.02 times when M=15m,M=20m and M=30m respectively.The thicker the aquifer,the more beneficial it is.The thermal conductivity and porosity have almost no effect on the heat transfer of water intake.In order to better optimize the natural flow of pore water in the coarse gravel aquifer at the gentle hydraulic slope into the design scheme of the GWHP system under hydrogeological conditions,the analysis shows that in the application of large-scale GWHP projects involving a large number of pumping and recharging wells,the heat transfer can be increased by increasing the size of the well diameter,reasonably increasing the spacing of the pumping and recharging wells according to the site conditions,and adopting the staggered layout of the pumping and recharging wells. |
