Thermal behavior (in cooling) of soil mass with varying fine and water contents: Experimental and numerical investigation

This paper presents a preliminary study on the thermal behavior and response of soil samples with varying water content to assess their significance in addressing the nighttime urban heat island (UHI) effect. For this reason, the experimental program and the analysis focus on the cooling phase of the heated soil samples. To ensure a representative analysis of field conditions, the soil samples were prepared in the laboratory with varying coarse-grained (sand) and fine-grained (kaolin) soil particles. The thermal properties of the soil samples, including thermal conductivity and transmissivity, decrease with increasing fine content and particle packing. In contrast, the thermal conductivity and transmissivity values increase with water content. The thermal diffusivity followed a similar behavior but was relatively less prominent. On the other hand, the specific heat capacity is higher in samples with higher moisture and clay contents but lower with dry unit weight. However, having a higher specific heat capacity also resulted in samples retaining the heat relatively longer and releasing it slowly, which could contribute to delayed nighttime UHI. The numerical models considering natural convective conditions and their analysis further confirm that the water content of the soil samples has a relatively more prominent effect on the soil's specific heat capacity than the increasing clay contents. To this end, it is noted that the measurement, analysis, and estimation of heat transfer and cooling mechanisms in the soil requires careful consideration of the soil type, water content, and the surrounding environmental conditions.