Modeling Heat and Mass Transfer under Local Thermal Non-Equilibrium Conditions in Structured Soils: A Dual-Permeability Approach for Infiltration and Freeze-Thaw Dynamics

Infiltration events, such as rain-on-dry soil or snowmelt over frozen ground, often introduce water at a temperature different from the surrounding soil and air. In such cases, Local Thermal Non-Equilibrium (LTNE) conditions arise, where water, air, ice, and the solid matrix maintain distinct temperatures over extended periods. This is especially true in macropores systems, where rapid flow enhances thermal decoupling.

We present a novel dual-permeability model that resolves water, air, and solid temperatures independently under LTNE conditions. The framework captures the dynamic behavior of macropores and micropores during infiltration and freeze-thaw cycles and is validated against controlled laboratory experiments.

In the first stage, we simulate unsaturated infiltration into soils featuring laboratory-defined macropore configurations under non-isothermal boundary conditions. Sensitivity analyses identified the mass exchange coefficient (γ=10^-4 , 10^-2 [s^-1]) and the macropore volume fraction (ω_Ma= 0.2, 0.3 [-]) as key parameters controlling thermal equilibration between pore domains. Results show that thermal disequilibrium persists significantly longer in macropores than in micropores, reflecting the dominance of advective transport in larger pore structures. To expand this investigation, we apply the model to freezing and thawing scenarios in cold-region soils. By integrating a three-phase formulation (liquid, ice, solid) and freezing point depression, we reproduce key phenomena such as delayed freezing fronts, preferential flow paths during thaw, and pore-wall ice formation in macropores. These results demonstrate the importance of domain-specific phase dynamics and the need for LTNE frameworks in frozen soil simulations.

This work provides a numerical approach for calibrating thermo-hydraulic dual-permeability models, highlighting how structural features like macropores influence the transient thermal regime during both infiltration and freeze-thaw cycles. Our approach can be extended to support multi-scale modeling and soil temperature prediction under climate-sensitive scenarios.