Heat Transfer Performance Optimization And Thermal-mechanical Characteristics Research Of Energy Pile

Energy pile,as a new energy-saving technology that utilizes renewable and clean energy,is of great significance to solve the current serious energy shortage and environmental pollution problems in China.It uses shallow geothermal energy to place the underground heat exchange pipes of the traditional ground source heat pump directly in the pile foundation to form a heat exchanger,which plays the dual role of carrying and heat exchange.Its special working environment puts forward higher requirements on the performance of concrete materials,which requires higher thermal conductivity and heat storage performance.Therefore,studying the improvement of thermal properties of energy pile materials is of great practical significance for the efficient use of thermal energy of energy pile structures.In this paper,based on the modification of the pile material,the heat transfer performance and thermodynamic characteristics of the heat conduction enhanced energy pile and the phase change modified energy pile are studied by numerical simulation.For heat conduction enhanced energy piles,the heat transfer models of energy single piles and group piles were first established,and the heat transfer performances of the soil temperature changes around the heat conduction enhanced energy piles,water outlet temperature,and heat exchange efficiency were analyzed;The pile thermal-mechanical coupling model analyzes the thermodynamic characteristics of the heat conduction enhanced pile.For the phase change modified energy pile,a new type of energy storage phase change material was compounded into ordinary concrete to construct the phase change energy pile.Using the finite element numerical model of the energy pile section,the transmission of the ordinary energy pile and the phase change modified energy pile was compared and analyzed.Thermal difference;then based on the current experimental conditions,the phase change material suitable for the energy pile is selected to prepare the composite phase change material.FBG fiber grating sensing technology was used to simultaneously measure the temperature and strain changes of the magnesium phosphate cement added with phase change materials during the hydration stage,and to study its effect on the early hydration shrinkage of the cement.The heat transfer and thermal-mechanical coupling simulation analysis of energy-consumption enhanced energy piles show that: no matter group piles or single piles,the stronger the heat conductivity of energy piles,the faster the heat transfer between the heat exchange tube and the pile body and the soil around the pile,the heat exchange efficiency can be effectively improved The heat transfer enhancement of some of the piles in the group shows that the heat transfer ability of the diagonal pile and side pile enhanced heat conduction is better than that of the center pile and side pile.The thermal conductivity is enhanced,and the rapid increase in the temperature of the pile body causes the thermal expansion effect of the pile body to be more obvious.The displacement of the pile body upwards and downwards increases,and the displacement zero point has a slight upward movement trend;enhancing the thermal conductivity of the pile body will increase The stress near the zero point of large displacement has little effect on the stress value of the pile head and pile end,and the lateral friction resistance of the upper and lower parts of the pile body increases,and the lateral friction resistance at the displacement zero point is almost zero.Combining the two models,heat conduction enhancement has a positive significance for heat transfer efficiency,but it has increased the adverse mechanical effects on the pile body due to thermal load.In the design of energy piles,heat transfer and load must be considered comprehensively to play the optimal role of energy piles.Through the heat transfer simulation analysis of the phase change energy pile model,it is found that when the heat exchange tube absorbs heat,the addition of the phase change material can significantly delay the temperature rise and fall speed of the energy pile in the phase change temperature range.To study the direct utilization effect of phase change materials,arranging the phase change materials directly in the periphery of the heat exchange tubes and encapsulating them with casing can significantly delay the temperature rise of the pile body and the soil around the pile and reduce the thermal influence radius of the energy pile.The early hydration experiment results of magnesium phosphate show that the addition of phase change materials can significantly reduce the hydration temperature of magnesium phosphate cement,delay the hydration reaction rate,and reduce the early shrinkage microstrain.