中深层套管式埋管换热器逆向传热现象的研究

Research on the reverse heat transfer phenomenon of the coaxial deep borehole heat exchangers

中深层地埋管换热器取热过程中,在浅层埋管区可能存在循环液向岩土传热的逆向传热现象,对其开展研究将为提高中深层地埋管地源热泵工程系统能效提供指导。文章利用数值传热模型分析了取热时间、循环液进口温度与流量以及大地热流等参数对逆向传热深度和埋管取热损耗率的影响,讨论了在逆向传热区设置保温层对换热性能的影响。结果表明:为保持地埋管换热器达到最大取热量,建议进口温度选为10℃,即等于地表环境温度;地埋管总取热量随着大地热流以及循环液流量的增加而增加,对应的逆向传热深度和取热损耗率则逐渐降低;根据模拟结果,给出了适用于工程设计的逆向传热深度拟合计算公式,经验证其具有较高的拟合相关性。

In the practical engineering application of the deep borehole heat exchangers(DBHE),the depth of reverse heat transfer zone may exist in the shallow deep area which has an important influence on the thermal performance of DBHE.The research of reverse heat transfer zone will provide guidance for improving the energy efficiency of ground source heat pump engineering system.In this study,the influences of the heat extraction time,inlet temperature and flow rate and ground heat flux on the reverse heat transfer zone are analyzed with the numerical heat transfer model.The effects of the insulation layer in the reverse heat transfer zone is discussed.The results show that to maintain the maximum heat output of the ground heat exchanger,it is recommended that the inlet temperature be 10℃,i.e.equal to the surface ambient temperature.The total heat extracted from the underground increases with the increase of ground heat flux and fluid mass flow rate and the reverse heat transfer depth and heat loss ratio decrease correspondingly.Based on the simulation results,a fitting formula for inverse heat transfer depth with a high fitting correlation is given,which is suitable for engineering design.