Heat conduction performance and transfer mechanism of sand-barite powder backfill materials

The ground source heat pump of buried pipeline is a major form of shallow thermal energy utilization, and the thermophysical properties of backfill materials, especially the thermal conductivity, are the key factors affecting the heat transfer efficiency of the heat pump system. In this paper, the variation law of thermal conductivity of the sand-barite powder samples at different barite powder mixing ratio (volume fraction) and saturation was studied. The mechanism of improving the thermal conductivity of backfill materials with barite powder was revealed based on the macroscopic and microscopic experiments such as stereoscope and nuclear magnetic resonance analysis. In addition, the influence of the heat conduction performance of this backfill materials on heat transfer efficiency was studied by numerical simulation. The study shows that barite powder has a significant effect on the thermal conductivity of backfill materials, and the barite powder at the mixing ratio of 20% has best effect on improving the thermal conductivity of backfill materials, which could improve the thermal conductivity by 52.09% at maximum. Water content also has a significant effect on the thermal conductivity, and the thermal conductivity of the saturated sample is significantly increased, which is 4−5 times higher than that of dry sample. The improvement of the thermal conductivity of backfill materials by barite powder is mainly caused by the package and filling effect. Specifically, the package effect is to wrap the sand particles with barite powder, while the filling effect is to fill the pores between sand particles with barite powder. The numerical simulation results prove that it is feasible to improve the heat transfer efficiency of ground source heat pump by enhancing the thermal conductivity of backfill materials with barite powder. Generally, the research results provide reference for the selection of backfill materials of ground source heat pump.