中深层同轴套管式地埋管换热器传热性能分析

Heat transfer performance analysis of the middle-deep coaxial casing ground heat exchanger

为了探讨不同因素对中深层同轴套管式地埋管换热器传热性能的影响,采用了一种中深层同轴套管式地埋管换热器数值模型,利用交替方向隐式法和追赶法,并结合邯郸地区现场测试数据,对该模型进行了求解.分析了环腔流体流速、管径比、内管导热系数、回填材料导热系数、岩土导热系数及井孔深度对套管换热器换热量及出口水温的影响规律.结果表明,增加环腔流体流速、减小管径比、减小内管导热系数、增大回填材料及岩土导热系数、增加井孔深度,可提高换热量;减小管径比、减小内管导热系数、增大回填材料及岩土导热系数、增加井孔深度,可提高出口水温.因此,在实际工程中,应选择高热阻的管材作为内管、选择导热系数较大的回填材料、岩土导热系数较大的地区有利于地热能开发、选择管径之比较小的套管、环腔流体流速的大小在允许范围内宜取大、较深井孔宜作为地热开采井,同时要综合考虑换热量、循环水泵功耗、钻井成本和实际负荷需求及热泵机组性能匹配.

A numerical model of the middle-deep coaxial casing ground heat exchanger was adopted to study the influence of different factors on the heat transfer performance.Combined with the field test data in the Handan area,the model based on the alternating direction implicit method and the chasing method was solved.The effects of annular tube fluid flow rate,pipe diameter ratio,thermal conductivity of inner tube,thermal conductivity of backfill material,thermal conductivity of rock-soil and depth of borehole were analyzed on heat exchange and outlet water temperature of casing heat exchanger.The results show that the increase of the thermal conductivity of backfill material,the thermal conductivity of rock-soil or the depth of borehole can increase the heat exchange and the outlet water temperature.The decrease of the pipe diameter ratio or the thermal conductivity of inner tube also increase the heat exchange and the outlet water temperature.Moreover,the increase annual tube fluid flow rate can also increase the heat exchange.Therefore,in actual engineering,selecting high thermal resistance of tube as the inner tube to reduce the heat losing between the inner tube and annular tube of circulating fluid increase the heat exchange and the outlet water temperature.Selecting large thermal conductivity of backfill materials to increase the heat transfer rate of the heat storage fluid into the annular tube fluid improves capacity of heat exchange of middle-deep coaxial casing heat exchanger.The area with large thermal conductivity of rock-soil is beneficial to the development of geothermal energy.Small pipe diameter ratio of casing heat exchanger and large velocity of annular tube fluid was selected within the allowable range.Deeper borehole is more suitable for improving the performance of geothermal well.At the same time,the heat exchange,power of circulating water pump,the drilling cost,actual load demand and performance matching of heat pump unit should be comprehensively considered.