Study On Thermal-mechanical Properties Of Phase Change Steel Ball Energy Pile In Saturated Sand Foundation

Phase change steel ball energy pile includes two new technologies: phase change steel ball concrete and energy pile.Phase change steel ball concrete is a new type of concrete formed by encapsulating phase change materials through hollow steel balls and mixing it into concrete.energy pile is a new type of ground source heat pump technology formed by replacing the underground heat exchanger in the traditional ground source heat pump with pile foundation buried tube heat exchanger.The phase change steel ball energy pile combines the advantages of these two technologies,so that the building pile foundation not only makes full use of shallow geothermal energy,but also has a certain heat storage capacity.In this paper,through Abaqus finite element analysis method and outdoor model test,the heat transfer law of phase change steel ball energy pile and soil around pile in sand foundation is deeply studied and analyzed under the conditions of different temperature load,different upper transfer structure load,different steel ball replacement rate and so on.the main work and research results are as follows:The main results are as follows:(1)the phase change steel ball concrete with different content of phase change steel ball is made,and the effect of the content of phase change steel ball on the compressive strength and thermal conductivity of phase change steel ball concrete is studied.the results show that the compressive strength of phase change steel ball concrete decreases linearly with the increase of phase change steel ball content.When a small amount of phase change steel ball is added,the thermal conductivity of concrete increases and the thermal resistance decreases because of the good thermal conductivity of the steel ball.With the increase of the content of phase change steel ball,the phase change material inside the steel ball begins to absorb a large amount of heat,which leads to the decrease of the thermal conductivity of concrete and the increase of thermal resistance.(2)the phase change steel ball energy pile model with different steel ball stone replacement ratio is established by using the finite element software Abaqus,and the whole process temperature change at the contact between the short-term inlet pipe wall and the pile body and the long-term temperature change of the whole model are simulated.The simulation results show that when the steel ball replacement rate of phase change steel ball concrete is 20%,the corresponding phase change steel ball energy pile has the best heat transfer effect.(3)the energy pile model test system of outdoor saturated sand foundation is designed to compare the heat transfer effect of ordinary energy pile and phase change steel ball energy pile under different upper loads in summer.The test results show that the temperature variation law of the soil around the pile shows the temperature lag effect,and the strength of the temperature lag effect is related to the distance between the measuring point and the pile body along the horizontal direction.the farther the distance between the measuring point and the pile body is,the stronger the temperature lag effect is.and the phase transformation steel ball energy pile shows a stronger temperature lag effect than the ordinary energy pile.At the same time,it is found that the long-term heat exchange between the ordinary energy pile and the soil around the pile is easy to produce the phenomenon of soil heat accumulation,which makes the soil temperature around the pile high,thus affecting the heat transfer efficiency of the energy pile.Due to the phase change effect of the internal phase change material,the temperature lag effect of the phase change steel ball energy pile is stronger,and the temperature change rate of the pile and the soil around the pile is lower,so the addition of the phase change steel ball greatly improves the thermal stability of the pile and the soil around the pile.this characteristic provides the feasibility for the phase change steel ball energy pile to solve the problem of soil thermal accumulation.At the same time,the phase change steel ball energy pile needs the corresponding intermittent working environment in order to achieve the best use effect.