The effect of particle size on thermal and solute dispersion in saturated porous media

Thermal dispersion, caused by fluid velocity and temperature fluctuations in the pore space and the effects of hydrodynamic mixing on the temperature field, controls convective heat transport in saturated porous media. While the thermal dispersion coefficient, a governing parameter in the thermal equilibrium model (TEM), has been investigated for natural systems, the dependence of the thermal dispersion coefficient on particle size remains undetermined. Previous research found that the relationship between the thermal dispersion coefficient and flow velocity follows a power law and that there may be a temperature difference between the solid and fluid phase (thermal non-equilibrium). However, experiments are limited to discrete particle sizes and comparison of the dispersion-velocity relationship is impeded by different experimental approaches. We conducted a series of separate heat and solute transport experiments in a column filled with uniform porous media consisting of different sized glass spheres for a range of flow velocities. The thermal and solute dispersion coefficients obtained from experimental measurements were correlated with flow velocities through the thermal or solute Péclet number. Our results demonstrate that, while solute dispersion is independent of particle size, the dependence of the TEM based thermal dispersion coefficient on flow rates is influenced by the particle size. This is caused by the fact that, unlike solute transport, heat exchanges between fluid and particles and that this induces thermal non-equilibrium between both phases. The results have significant implications for quantifying forced convective heat transport in natural porous media because thermal non-equilibrium between the phases is not considered. The porous media particle size must be considered when selecting appropriate values for the thermal dispersion coefficient.