Experimental Study And Numerical Simulation Of Heat And Mass Transfer Process Of Buried Pipe Backfill Materials

Ground source heat pump(GSHP)heating and air conditioning technology has attracted more and more attention due to its characteristics of energy saving and environmental protection.Grouting material,as the heat and mass transfer medium connecting the ground heat exchanger and the soil,can enhance the heat transfer and prevent the cross contamination between the surface water and the aquifer,which is the key component of the ground source heat pump heat exchanger.After the backfilling material is reinjected,the heat and mass transfer will occure between the ground heat exchanger and the surrounding soil under the action of non external force.The backfilling material and soil are porous media in nature,and their comprehensive thermal conductivity,comprehensive density and other physical parameters are affected by the temperature and water content.Therefore,the physical properties of backfilling materials and soil have a certain influence on the actual heat transfer efficeincy of ground heat exchanger system.The construction quality of backfilling material reinjection also has a certain impact on the actual groundwater environment.However,as a concealed project,it is very difficult to inspect the real underground water infiltration,solidification and deformation of backfilling material after the backfill is grouted in the borehole.Most scholars have ignored the porous characteristics of soil and the grouting and assumed the constatnt thermal parameters of grouting without the water migration between the grouting and the soil.Based on the above research background,this paper focuses on the dynamic heat and mass transfer process between the grouting and soil which are assumed to be porous media.The dynamic influence of the temperature and humidity change on the comprehensive effective thermal conductivities of the grouting and soil are disucssed in this paper.The real infiltration,solidification and deformation of backfilling material after grouted were observed by building an experimental platform.In this paper,an experimental platform for heat and mass transfer of backfilling materials was built to monitor the temperature and humidity variations of the soil around the grouting in the process of heat and mass transfer.The thermal conductivity,moisture content,porosity,permeability and other basic parameters of backfilling materials and soil before and after the experiments were measured by measuring instruments.Based on the experimental data,the variation of temperature and humidity of soil around the borehole with time under different backfill materials and the variation of physical parameters of backfill materials before and after the experiment were analyzed.By observing the experimental phenomena,the morphological changes of backfill materials in the solidification process after backfill are described.Based on the COMSOL platform,a numerical heat and mass transfer model of the ground heat exchanger including the grouting and the surrounding soil is established,which takes into account the porous media properties of backfilling material and soil which is taken as a complicated heat and mass coupling transfer problem.The simulation results are compared with the experimental results to verify the correctness of the numerical model.In this paper,four sets of contrast conditions are designed: first,the pure heat transfer model with the heat and mass transfer model;the second is the comparison between the two different conditons of heat extraction and rejection;the third is the comparisons of three kinds of materials: mud,bentonite-based backfilling and cement-based backfilling;the fourth is the comparisons of different initial moisture contents of different backfilling materials.The variation trends of temperature,water moisture,effective thermal conductivity and the system comprehensive thermal resistance under different working conditions,are simulated and analyzed.The numerical simulation results show that the thermal conductivity of porous media is affected by both temperature and moisture content,so the thermal conductivity of backfill materials and soil is no longer a simple linear change with temperature when considering the moisture migration of backfill materials,and the temperature change trend is also related to the heat transfer or heat release conditions of buried pipes,The changing value is also related to different working conditions.By analyzing the total thermal resistance of the simplified ground heat exchanger,it is found that the overall thermal resistance of the heat and mass transfer model is slightly larger than that of the heat and mass transfer model neglecting mass transfer.This is a comprehensive reflection of the decrease of thermal conductivity of backfill material and the increase of thermal conductivity of soil caused by water migration on the heat transfer capacity of ground heat exchanger.Because the water migration mainly occurs in the small area inside and outside the borehole,which has little effect on the thermal conductivity of the whole soil outside the borehole,the increase of the thermal resistance in the backfill material is the main factor,which leads to a slight increase in the overall thermal resistance,but the increase is not large.At the same time,by comparing and considering the effect of mass transfer on the temperature field,it is found that the mass transfer caused by mud from the drilling site as backfill material has little effect on the temperature field of the whole buried pipe,which can be ignored.The experimental results show that: the moisture content of backfill material decreases in the solidification process,which may produce cracks.The porosity will increase due to the cracks in the solidification process,resulting in poor sealing,which not only affects the heat transfer effect of the system,but also has a certain risk of cross contamination of groundwater environment.The mass transfer process between backfill material and soil has a great influence on the heat transfer performance of the buried pipe system at the initial stage of operation and in a certain range near the borehole wall,but has little influence on the overall heat transfer performance.