Transient formulation for evaluating convective coefficients in regenerative exchangers with hygroscopic channels

Abstract

A transient two-dimensional model for coupled heat and mass transfer in channels with hygroscopic walls has been developed. The formulation employs a generalized form that is valid for both circular tubes and parallel-plates channels with walls composed of a desiccant material that can physically adsorb humidity from a moist airstream. The model equations are normalized in terms of physically relevant dimensionless groups, and then solved using a computational implementation based on the Numerical Method of Lines (NUMOL) using the Finite Volumes Method (FVM) for the spatial discretization. Validation results are presented by comparing the periodic solutions of the proposed formulation with experimental data for two different cases, showing very good agreement. Finally, illustrative results of the proposed formulation are presented for two typical situations found in desiccant wheel applications: dehumidification and regeneration. An analysis of the simulated data shows that the convective coefficients can strongly depend in both space and time, which suggests that lumped models based on constant convective coefficients for simulating desiccant and enthalpy wheels should be used with care.