Experiments reveal effects of particle and pore-scale heterogeneity on thermal dispersion during porous media heat transport

Abstract

The growing applications of heat transport in engineering and hydrogeology, including ground source heat pump systems and the assessment of water flow and sediment thermal properties, necessitate precise thermal dispersion modeling. Despite various factors like particle size, shape, and distribution impacting dispersion in porous media, a lack of experimental data and analysis has limited our grasp of the relationship between flow velocity and thermal dispersion. To address this gap, we conducted solute and heat tracer experiments using sands of different grain sizes, employing analytical models based on a universal power law relationship for dispersion. Additionally, we systematically compiled dispersion data from the scientific literature to assist in interpreting of the effects of particle size, shape, and pore-scale heterogeneity on dispersion. Our findings reveal that solute and thermal longitudinal dispersion align for high flow velocities in the dispersion-dominated flow regime (Pe > 5), where the velocity-dispersion relationship is linear. However, in the transition regime (advection approx. equal to diffusion, 0.1–0.3 < Pe < 5), the observed deviation from linearity (R2<0.9) is linked to increased pore-scale heterogeneity, particularly in smaller particle sizes. This study underscores the pivotal role of thermal dispersion and underscores the need for caution when estimating thermal dispersion based on solute dispersion, given the intricate nature of real-world conditions and the potential influence of factors such as grain size distribution and pore-scale heterogeneity in natural porous materials.