Study On Heat Transfer Characteristics Of High Thermal Conductivity Permeable Concrete Energy Pile

Statistics show that in 2018,the energy consumption in the building run phase accounted for about 46.5%of the total energy consumption in the whole construction process,accounting for about 21.7%of the total energy consumption in.Therefore,the development of new ground source heat pump technology represented by energy piles and the use of geothermal energy to reduce energy consumption in the construction run phase are important measures of achieving the strategic goal of"double carbon".However,the thermal conductivity of traditional energy pile concrete is low,and the thermal convection of groundwater cannot be fully utilized,so the heat transfer efficiency of energy pile is difficult to further improve.In view of the existing problems of energy piles,this paper proposes a kind of high thermal conductivity permeable concrete energy pile suitable for groundwater conditions.The permeable pile greatly increases the contact area between the pile body and groundwater,and it can make full use of the thermal convection of groundwater.The thermal conductive material added to the pile body also improves the heat exchange efficiency between the pile and soil.The heat transfer characteristics of high thermal conductivity permeable concrete energy pile are studied by means of model test and numerical simulation.The main work and conclusions are as follows:(1)The mix proportion test of high thermal conductivity permeable concrete was carried out to study the mix proportion,mechanical properties and thermal conductivity of high thermal conductivity permeable concrete.With the 1200 mesh silicon carbide powder as thermal conductive reinforcement material,the water-binder ratio,silicon carbide content and design porosity were selected for the mix proportion test by orthogonal test method.The compressive strength,effective porosity and permeability coefficient of permeable concrete were measured,and the consistency,adhesion and thermal conductivity of cement slurry were measured.The results show that the larger the design porosity of permeable concrete is,the lower the compressive strength is,and the compressive strength decreases with the increase of water-binder ratio.The weight loss rate of concrete is negatively correlated with the compressive strength of concrete.The lower the weight loss rate is,the higher the compressive strength is.The incorporation of silicon carbide can effectively improve the thermal conductivity of cement stone.When the content of silicon carbide increases from 5%to 15%,the thermal conductivity of cement stone increases by about 43.53%.Considering the thermal conductivity and mechanical properties,the water-binder ratio of high thermal conductivity permeable concrete used for making energy piles is 0.29,the designed porosity is 9%,and the silicon carbide content is 15%.(2)The unit pile test of high thermal conductivity permeable energy pile is carried out.The effects of soil saturation around the pile,operating parameters of energy pile and groundwater seepage on the heat transfer efficiency of high thermal conductivity permeable energy pile are mainly studied.The results show that in dry sand,the heat exchange efficiency of high thermal conductivity permeable energy pile is about 12.12%lower than that of high thermal conductivity impermeable energy pile,and it is about 11.54%higher than that of ordinary permeable energy pile.When the soil around the pile is from dry to saturated,the heat transfer efficiency of the high thermal conductivity permeable energy pile increases by about 24.14%.In saturated sand,the increase of circulating liquid temperature or circulating liquid flow rate can improve the heat transfer performance of high thermal conductivity permeable energy pile.When groundwater is from static to flowing,the heat exchange efficiency of high thermal conductivity permeable energy pile increases by about 19.44%.When groundwater is flowing,the heat transfer efficiency of high thermal conductivity permeable energy pile is about 13.16%higher than that of high thermal conductivity impervious energy pile and 22.86%higher than that of ordinary permeable energy pile.When the groundwater is static,the intermittent operation mode of high thermal conductivity permeable energy pile can improve the operation efficiency,but when the groundwater is flowing,the heat exchange efficiency of continuous operation mode is higher than that of intermittent operation mode.(3)The simulation study on the heat transfer characteristics of high thermal conductivity permeability energy piles was carried out,and the thermal-flow coupling model of energy piles with groundwater static or flowing was established,and the model was verified by the test results.On this basis,adjusting of seepage field,when the groundwater is flowing,the influence mechanism of the pile concrete permeability and the pile concrete thermal conductivity about the high thermal conductivity permeable energy pile on the heat transfer performance is studied in depth.The influence of the pile concrete thermal conductivity and the groundwater flow rate on the heat transfer performance with the high thermal conductivity energy pile is analyzed.The results show that when groundwater is flowing,the heat exchange efficiency of the high thermal conductivity permeable energy pile is 16.25%higher than that of the high thermal conductivity impermeable energy pile,and 62.96%higher than that of the ordinary permeable energy pile.Increasing the matrix thermal conductivity and groundwater flow rate can significantly improve the heat transfer efficiency of high thermal conductivity permeable energy pile system.In addition,when the groundwater level is in the middle of the energy pile or shallower buried depth,the heat transfer performance of the high thermal conductivity permeable energy pile is better than that of the high thermal conductivity impervious energy pile,and the groundwater seepage can significantly improve the long-term operation efficiency of the energy pile.