Local thermal equilibrium or not? Reconciling the groundwater heat transport mechanisms in heterogeneous sediments at different scales

中欧的浅层沉积含水层是重要的淡水水库，同时也因其储存大量热能的能力而受到关注。了解和控制这些含水层的热条件对于量化流动状态、估计地热利用潜力和保持地下水质量至关重要。虽然基本的热传输机制（即扩散和平流）已经很好地建立起来，但在尺度相关非均质性的影响下，解释它们的联合效应仍然没有解决。天然材料本质上是不均匀的，从小（例如，粒度混合物）到大（例如，水力导电性分布）。在水文地质学中，传统上采用假设颗粒与周围液体之间局部热平衡（LTE）的体积平均方法来描述热传递，但这种方法既没有得到验证，也没有定义其有效性的条件。此外，这种最先进的方法忽略了非均质性对不同尺度热输运的可变影响。这些影响包括局部热不平衡（LTNE）和宏观热色散的明显标度。本项目旨在通过结合实验室、现场、分析和数值方法，调和从颗粒（毫米）到地质非均质性（几十米）尺度上的热传输机制。在小尺度上，将进行特定的流动柱实验，以研究自然粒径分布对LTNE和热分散的影响。在大尺度上，真正的含水层类似物将在热传输模型中进行测试，以仔细检查沉积结构的作用。在已知地下非均质性的既定测试场地，现场实验将揭示LTNE的发生和强度，并量化热分散作为地质尺度非均质性的函数。实验室和现场实验将用于验证详细的数值热传输模拟。这些将随后用于阐明高级方面，如晶粒尺寸混合物与传热系数之间的关系，以及地质非均质性对LTNE和热分散的影响。最后，这些发现将被共同解释以调和不同尺度的热传输机制。该结果将为模拟具有自然非均质性的沉积物中的热输运提供一个新的和通用的框架。预期发展出来的知识将促进理论和实践方面的标准，并支持改进规划和管理，例如地下水的热示踪和浅层地热系统。

Shallow sedimentary aquifers in Central Europe are important freshwater reservoirs while also gaining attention for their ability to store enormous amounts of thermal energy. Understanding and controlling the thermal conditions in such aquifers is vital for quantifying flow regimes, estimating the potential for geothermal use and maintaining groundwater quality. While the fundamental heat transport mechanisms (i.e., diffusion and advection) are well established, interpreting their combined effects under the influence of scale-dependent heterogeneity remains unresolved. Natural materials are inherently heterogeneous at scales from small (e.g., grain size mixtures) to large (e.g., hydraulic conductivity distribution). In hydrogeology, a volume-averaging approach assuming local thermal equilibrium (LTE) between grains and the surrounding liquid has traditionally been applied to describe heat transport, yet this approach has neither been verified nor have conditions for its validity been defined. Further, this state-of-the-art approach ignores the variable effects of heterogeneities on heat transport at different scales. Such effects include local thermal non-equilibrium (LTNE) and an apparent scaling of macroscopic thermal dispersion. This project aims to reconcile the heat transport mechanisms at scales from grain (millimetres) to geological heterogeneity (tens of metres) by combining laboratory, field, analytical and numerical methods. At the small scale, specific flow-through column experiments will be conducted to investigate the influences of natural grain size distributions on LTNE and thermal dispersion. At the large scale, real aquifer analogues will be tested in heat transport models to scrutinize the role of sedimentary structures. At an established test site with well-known subsurface heterogeneity, field experiments will reveal the occurrence and intensity of LTNE and quantify thermal dispersion as a function of geological scale heterogeneity. Both laboratory and field experiments will be used to validate detailed numerical heat transport simulations. These will subsequently be deployed to elucidate advanced aspects, such as the relationship between mixtures of grain sizes and heat transfer coefficient as well as the influence of geological heterogeneity on LTNE and thermal dispersion. Finally, the findings will be jointly interpreted to reconcile heat transport mechanisms at different scales. The outcome will provide a novel and universal framework for modelling heat transport in sediments with natural heterogeneities. It is anticipated that the developed knowledge will advance standards in theory and practice and support improved planning and management, for example of heat tracing in groundwater and shallow geothermal systems.