Numerical Simulation for Non-Equilibrium Heat and Mass Transfer during Drying by Two User-Defined Scalar Transport Equations in ANSYS Fluent

Abstract

In this paper, we have discussed one method of non-equilibrium numerical solution of heat and mass transfer processes during drying using UDS and UDF in FLUENT. A mathematical model of fluid flow and non-equilibrium heat and mass transfer in porous media is proposed. The governing equations are presented for fluid, solid and porous region. Special consideration is given to reflect moisture transfer from porous media to surrounding moist air flow. In order to solve these equations by using a commercial CFD package, FLUENT, reforming of type of govern equations into FLUENT’s type is presented, accounted for “variation” of physical properties of solid in regard with reforming of equation type. Transient simulation of drying process of porous media placed in 2D channel by using FLUENT is presented. The results demonstrate the mathematical model and reforming of equations’ type presented in this work is capable of simulating non-equilibrium heat and mass transfer in porous media. The results show that numerical predictions follow the expected trends with respect to temperature and velocity at inlet.

Introduction

Fluid flow and heat transfer in porous media are important in applications such as drying, filtration, and fluidized bed reactors. In systems with high porosity (e.g., fluidized beds, textiles, metal foams), the temperature difference between the solid matrix and fluid requires two separate energy equations for accurate modeling.

Extensive research has been conducted on non-equilibrium heat and mass transfer in porous media using Computational Fluid Dynamics (CFD). Studies by Amiri and Vafai, Nakayama et al., and Betchen et al. investigated effects such as local thermal non-equilibrium, non-Darcian flow, variable porosity, and interface treatment between fluid and solid regions. Furqan Ahmad Khan et al. developed a 3D numerical model for conjugate fluid/solid/porous systems, accurately simulating unsteady drying processes.

Recent works have used both in-house CFD codes and commercial software (FLUENT) to simulate non-equilibrium heat and mass transfer. However, most prior studies assumed local thermal equilibrium and treated the problem as conjugate heat transfer. Few have developed non-equilibrium drying models in FLUENT.

The objective of the present study is to create a numerical formulation for modeling local non-equilibrium heat and mass transfer in porous media using FLUENT. The model reformulates the solid matrix’s moisture and energy transport equations as user-defined scalar (UDS) equations, with additional properties and coefficients defined via User Defined Functions (UDFs). The model’s predictions will be validated against experimental data.

The mathematical model involves solving the mass, momentum, species, and energy transport equations in a 2D domain where moist air flows through a wet porous medium. Moist air, modeled as a mixture of dry air and water vapor, exchanges mass and heat with the solid matrix. The species transport equation includes the water vapor mass fraction (Yv), which is related to specific humidity (ωspec) and the partial pressures of dry air and vapor. The energy equation determines the temperature distribution of the moist air.

Conclusion

In the present study, CFD method for analyzing non-equilibrium heat and mass transfer in regular constructional porous media was proposed. A formulation capable of simulating non-equilibrium heat and mass transfer in porous domains was presented, with special consideration towards reforming the heat and mass transfer equations in order to make it possible in FLEUNT. The results show that our predictions follow the expected trends with respect to temperature and velocity at inlet. As such, the present model reforming is shown to be capable of simulating a large class of problems associated with non-equilibrium heat and mass transfer in porous media by using FLUENT. In the future these problems will be investigated with regarding the change of pore space in porous media during drying.

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Copyright © 2025 Song Gun Choe, Chung Hyok Kim, Chol Jin Jon. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.