线热源法测量冻土热参数的适用性分析

提出了应用线热源法测量冻土热参数存在的问题,冻土中未冻水含量随温度的变化是导致线热源法测量误差产生的根本原因。对二维无限大——点热源热传导方程进行简化得到一维中心对称——热流边界热传导方程。采用双针法对不同温度的环刀土样进行了测试并对冻土传热过程进行数值试算,反演得到不同温度的相变热容和未冻水含量。结果表明:当温度大于-0.7℃时,相变热容值远远大于冻土体积热容并且随温度急剧变化,线热源法测量的结果严重失真;当温度介于-0.7~-4℃时,线热源法测量结果仍有较大的误差;当温度低于-4℃时,相变热容值很小且比较稳定,线热源法测量的热参数能满足一定的精度要求。

Applicability of line heat source method in measuring thermal parameters of frozen soil

The problem in the line heat source method in measuring the thermal parameter of frozen soil is put forward. The variation of unfrozen water content with respect to temperature is the primary cause for the measuring error in the heat source method. The 2-D heat conduction equation for infinite zone-point heat source is simplified as the 1-D one for central symmetry-heat flow boundary. The dual needle method is used to measure the soil samples in cutting ring at various temperatures, and the corresponding numerical simulations for heat conduction are conducted. The volumetric heat capacity due to phase change and unfrozen water content is obtained through back calculation. The results show that as the temperature of soil is higher than -0.7 ℃, the volumetric heat capacity due to phase change is much greater than the volumetric heat capacity of frozen soil, and it varies with the temperature drastically, hence the heat source method loses efficacy; as the temperature of soil is ranging from -0.7~-4 ℃, the measured error remains larger in the volumetric heat capacity. As the temperature of soil is lower than -4℃, the volumetric heat capacity due to phase change is little and steady so that the measured thermal parameters meet the requirements of accuracy to a certain degree.