Effect of soil saturation and grain size on coupled hydrothermal flow in fine sands based on X-ray <math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e2507" altimg="si9.svg" class="math"><mi mathvariant="normal">μ</mi></math>CT imaging

Coupled hydrothermal flow can occur in soils, for example in applications such as ground heat storage and nuclear waste disposal. Therefore, approaches to quantitative analysis of water transfer in response to imposed thermal gradients are required, especially in unsaturated conditions. Analysis methods also require validation by laboratory and field data, which can be hard to obtain. This paper explores the possibility of using X-ray CT techniques to observe and quantify water content changes in soils under thermal gradients. Specimens of a fine sand and a silty fine sand were prepared at degrees of saturation between 20% and 50%, before being subjected to heating from their base. Repeated scans, set up to balance image quality and scan duration, were carried out during the heating process, and Gaussian decomposition techniques were used to determine the changing soil phase proportions throughout the experiments. Based on these results and the accompanying numerical simulation of the experiments, it is shown that rapid vapour diffusion plays a more significant role than liquid flow in all cases. The rate of water content and hence degree of saturation change was more rapid in the less saturated specimens, especially for the fine sand. In practical terms, these moisture changes would result in reduction in thermal conductivity, especially in the soils of lower saturation. As well as providing insight into the dominant water transfer processes, the experiments show the feasibility of applying X-ray CT techniques to thermal problems in soil mechanics.

耦合的热液流可能发生在土壤中，例如在地热储存和核废料处理等应用中。因此，需要有对因施加的热梯度而产生的水分迁移进行定量分析的方法，尤其是在非饱和条件下。分析方法还需要通过实验室和现场数据进行验证，而这些数据可能很难获取。本文探讨了利用X射线CT技术观察和量化热梯度下土壤含水量变化的可能性。将细砂和粉质细砂的样本制备成饱和度在20%到50%之间，然后从底部对其加热。在加热过程中进行了多次扫描，扫描设置在图像质量和扫描时长之间取得平衡，并使用高斯分解技术确定整个实验过程中不断变化的土壤相比例。基于这些结果以及对实验进行的相应数值模拟，结果表明在所有情况下，快速的蒸汽扩散比液体流动起着更重要的作用。在饱和度较低的样本中，含水量的变化速率以及饱和度的变化程度更快，尤其是对于细砂而言。从实际角度来看，这些水分变化会导致热导率降低，尤其是在饱和度较低的土壤中。除了深入了解主要的水分迁移过程外，这些实验还表明将X射线CT技术应用于土力学中的热问题是可行的。