Abstract:

An analytical heat transfer model was established for a borehole heat exchanger (BHE) with single U-tube in stratified soils based on the moving finite line heat source model, taking the factors such as soil stratifications and groundwater advection into account. This model was validated by the data obtained in an on-site soil thermal response test. The soil temperature responses to the BHE heat injection were obtained by using this model and homogeneous model. Comparison of these two sets of results indicates that the soil temperature around the BHE along axial direction is not uniformly distributed due to the characteristics of soil stratification. It takes different length of time for the borehole wall temperature to reach stable and to maintain at a different stable temperature in every layer, due to the difference of physical properties of soil in each layer. Three cases are calculated and analyzed for homogeneous soil, stratified soil with groundwater advection in whole soil domain, and stratified soil with groundwater advection in some soil layer. It shows that the heat transfer efficiency coefficient is underestimated up to 6.3% if ignoring the soil stratification and the influence of groundwater advection. The thermal effect distance deviation in each soil layer may be up to 43% for cases with and without groundwater advection. In order to assess the soil temperature distribution characteristics around the BHE with the influence of groundwater advection, it is suggested that the stratified soil model is applied to calculate the thermal effect distance in each soil layer and their maximum value is chosen as the borehole spacing. Otherwise, the borehole spacing may be too small for the design of multiple BHEs.

Key words: borehole heat exchanger, heat transfer, thermal response test, moving line heat source, soil stratified model, groundwater advection, experimental validation

摘要：

基于移动有限长线热源理论,考虑土壤分层及存在渗流现象,建立了埋管传热解析模型。利用热响应试验验证了该模型的正确性。对比分层模型与均质模型所给出的埋管散热过程的土壤温度响应表明:埋管周围土壤沿轴向分层使其温度分布也呈现分层而非均匀的特点,各层土壤中钻孔壁处温度趋于稳定所需的时间及数值因各层物性的差异而不同。对整个埋管区土壤均质或分层但存在渗流作用以及部分土壤层存在渗流作用等3种情况进行了计算分析,发现:若整个分层土壤中存在渗流,而视土壤为均质并忽略渗流的影响,则对换热能效系数低估可达6.3%;埋管散热在各土壤层中的热作用距离因存在渗流与否偏差可达43%。可见,为了准确评估钻孔周围渗流作用下分层土壤中的温度分布特性,应利用分层模型计算各土壤层中的热作用距离,以最大值确定管间距,否则会导致管群布置时管间距选取偏小。

关键词: 地埋管换热器, 传热, 热响应试验, 移动线热源, 土壤分层模型, 地下水渗流, 试验验证