Analysis On Heat Exchange Performance Of PHC Geothermal Pile

Energy pile is a new building energy-saving technology derived from traditional ground source heat pump technology.It is one of the most important forms of energy underground structure.It can fully utilize the shallow geothermal energy while meeting the conventional mechanical carrying capacity of the building.Compared with traditional ground source heat pump technology,energy pile system has obvious technical and economic advantages.PHC piles are an important type of piles for energy piles.However,relatively few scholars at home and abroad have done enough research on PHC geothermal pile.There is no effective and comprehensive evaluation of the heat transfer performance of PHC geothermal pile heat exchange systems.A comprehensive study of the heat transfer performance of PHC geothermal pile was conducted using a combination of field tests and numerical simulations.Firstly,this paper introduced the experimental research of PHC geothermal pile in the field.In order to study the temperature field characteristics of the energy pile better in the heat exchange process,a distributed optical fiber sensing technology was introduced based on the conventional thermal response test to monitor the temperature distribution and change of the pile along the depth.Firstly,the construction process of field measurement system for PHC geothermal pile was introduced comprehensively.The thermal response test of the field heat exchange hole was conducted.The initial temperature distribution of the site was measured using the optical fiber in the borehole and the comprehensive thermal conductivity of soil in the site was calculated according to the line heat source model.Then,the thermal response tests of the single U-tube and double U-tubes of PHC geothermal pile were conducted.The temperature and strain of the pile were measured during the test by using distributed optical fiber monitoring in the center and wall of the pile.The temperature change characteristics in time and depth reveal the heat transfer efficiency of PHC geothermal pile.The heat transfer per unit length of the traditional buried pipe heat exchanger and the PHC geothermal pile was calculated.It was found that the heat exchanging efficiency of the PHC geothermal pile was better than the borehole buried pipe,and the double U-shaped buried pipe was superior to the single U-shaped buried pipe.According to the on-site test conditions,the factors influencing the heat transfer performance of PHC geothermal pile were analyzed,which provided references for the design and application of energy piles in practical projects.A three-dimensional numerical model of the PHC geothermal pile was established using COMSOL numerical analysis software.The heat transfer process in the form of single U-shaped and double U-shaped buried pipes was simulated according to field test conditions.The simulation results were compared with the actual field measurements and the reliability of the model simulation was verified.Using the validated model,a comprehensive simulation analysis of the factors influencing the heat transfer performance of the PHC geothermal pile was performed.The heat exchange capacity of the unit pile length heat exchange and the distribution of the temperature field of the pile and soil were evaluated based on the simulation.The simulation results show that the form of buried pipe and the spacing of the buried pipe and the thermal conductivity of the backfill and the inlet temperature have a great influence on the heat transfer performance for the PHC geothermal pile system,while the effects of the circulating water flow rate and the diameter and length of the pile are relatively small.The corresponding suggestions are given in practical engineering applications for different factors.The results of this study revealed the heat transfer performance of the PHC geothermal pile and discussed its applicability.The PHC thermal pile was optimized and analyzed according to the results of the research,which can provide important reference for the design and application of PHC geothermal pile heat exchange systems.