The influence of simplified line source on ATRT's calculation of soil thermal conductivity——Numerical simulation research
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摘要: 不同深度土体的导热系数是精细评价浅层地热能的关键参数，基于内加热光缆的热响应测试（ATRT）是获取原位土体分布式导热系数的有效方式之一。利用有限元数值模拟软件ComsolMultiphysics，建立了二维有限元多孔介质传热模型，探究了U型布设内加热光缆条件下内加热光缆间距对实施ATRT计算导热系数的影响。结果表明：ATRT的热响应过程可以分为3个阶段，分别是光缆影响阶段、回填料影响阶段和土体影响阶段。U型布设光缆会增加土体影响阶段达到稳定时的加热时间，不利于提高ATRT计算导热系数的效率；由于简化计算的影响，U型光缆测试结果计算所得的导热系数在加热初期出现了明显高于土体导热系数的峰值，随着加热时间的增加，U型光缆布设条件下温升速率逐渐接近理想线热源温升速率，计算结果也越接近于真实值，缩小间距可以加速这个过程；随着间距增大，将误差控制在10%以内的建议加热时间将快速增大，故在实际工况中为了提高测试效率应尽量减小U型光缆的布设间距。Abstract: The thermal conductivity of soil at different depths is a key parameter for the evaluation of shallow geothermal energy. A thermal response test based on the actively heated optical cable (ATRT) is one of the effective ways to obtain the in-situ distributed thermal conductivity. Compared with traditional thermal response test (TRT) and distributed thermal response test (DTRT), ATRT has better test efficiency. One of the key issues that determines the error and the effect of ATRT is how to effectively lay a heat source that satisfies the assumption in the line source model. Using the finite element numerical simulation software ComsolMultiphysics, a two-dimensional finite element heat transfer model of porous media was established, and we explored the influence of U-shaped internal heating cable's spacing on the calculated thermal conductivity. The results show that the thermal response process of ATRT can be divided into three stages, namely the cable-influence stage, the grout-influence stage, and the soil-influence stage. U-shaped laying of the cable will increase the heating time when the soil influence stage is stable, which is not conducive to improve the efficiency of ATRT. Due to the influence of simplified calculation, the thermal conductivity calculated from the test results of the U-shaped optical cable has a peak value significantly higher than the thermal conductivity of the soil at the initial stage of heating. With the increase of heating duration, the temperature rise rate under the condition of U-shaped optical cable layout gradually approaches the ideal line heat source temperature rise rate, the calculation result is closer to the true value, and reducing the spacing can speed up this process. As the spacing increases, the recommended heating duration will increase rapidly in order to control the error within 10%. Therefore, to improve the test efficiency, the distance between the U-shaped optical cables should be minimized in the field.
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