固井水泥对同轴型换热系统取热效果影响数值分析

摘要

同轴型换热器(CBHE)地热系统换热面积大,可有效开发深部地热资源开发.针对系统内管流体热损失,本文采用了多层结构保温管柱,而前人的研究多将其简化为单层结构,与实际情况不符,因此本文建立了考虑多层结构管柱的非稳态传热模型,通过有限差分法进行数值求解.由于水泥是井筒与地层间的传热热阻,因此重点分析了其对系统取热效果的影响.结果表明,水泥对取热效果影响显著,引入影响因子W定量表征水泥对取热效果影响程度的变化.研究发现,随着地层导热系数、地温梯度及井深的增加,W明显升高,因此对于深部高温地热资源的开发,有必要开展高导热水泥的研究.%The coaxial borehole heat exchanger (CBHE) geothermal system is expected to be used for exploiting deep geothermal resources because of a larger heat transfer area.To reduce heat dissipation of working fluid in central pipe,a multi-layer insulated central pipe is adopted in this paper.However,most previous heat transfer models for BHE system simplified the central tube to be a single-layer structure,which is inconsistent with the actual conditions.Thus,we present an unsteady-state heat transfer model considering the complete structure of central tube.The finite difference method is employed to solve the mathematical model.Subsequently,the effect of cement sheath on the CBHE performance is studied for its thermal resistance.The simulation results depict that the heat conductivity coefficient of cement has a significant influence on the heat extraction performance of CBHE system.In addition,an impact factor named W is introduced to characterize the diversity degree of impact on cement.And the W increases greatly with the increase in the thermal conductivity of the formation,geothermal gradient and well depth.Therefore,it is necessary to carry out the research of cement with high thermal conductivity for the development of deep geothermal resources.