On the validity of lumped capacitance approaches for the numerical prediction of heat and mass transfer in desiccant airflow systems

Abstract

In the present work, a detailed model of simultaneous heat and mass transfer through a desiccant micro-porous medium of a channel wall of compact heat and mass exchangers, such as desiccant wheels, was developed. The relevant phenomena considered by this model within the porous medium are surface diffusion of the adsorbed water, Knudsen diffusion of water vapour, heat conduction and the sorption process. A onedimensional formulation of this model is used to investigate the validity of two simplifying approaches based on the lumped-capacitance method. One consists of neglecting the transversal heat and mass transfer resistances within the porous medium, the other of cancelling only the thermal resistance.Results are presented for a wide range of values of the layer thickness of the porous medium. It is concluded that the hypothesis of nullresistances in the cross direction is valid only for layer thicknesses lower than 0.1 mm, approximately, while the approach based on the thermal lumped capacitance model is valid for layer thicknesses lower than 5 mm. Finally, a dimensional analysis of interfacial balances leads to the conclusion that the Biot number for surface diffusion is several orders of magnitude higher than the corresponding thermal Biot number.

Simplified Heat and Mass Transfer Model for Cross-Flow and Countercurrent Flow Packed Bed Tower Dehumidifiers with a Liquid Desiccant System

“…(1) Humid air is the mixture of an ideal gas, and Dalton's law of partial pressures is true [35]. (2) One-dimensional transient heat and mass transfer are considered [36].…”

“…In order to understand performance of dehumidifier, it is necessary to consider effects of component operation in a transient state. Ruivo et al [35], based on the lumped-capacitance method used a one-dimensional formulation to investigate the validity of two simplifying approaches of neglecting the transversal heat and mass transfer resistance within the porous medium and cancelling only the thermal resistance on desiccant wheel. They showed that the Biot number for surface diffusion is several orders of magnitude higher than the corresponding thermal Biot number.…”

Experimental study on heat and mass transfer characteristics for a desiccant-coated fin-tube heat exchanger

“…where D s is the surface diffusivity, and calculated by [30][31][32], q d is the density of the desiccant particle, and C w is the moisture content of the desiccant. D 0 is a constant that depends on the adsorbent and b depends on the type of adsorption bond.…”

Comparative performance of desiccant wheel with effective and ordinary regeneration sector using mathematical model

“…They also discussed the effect of some practical issues such as wheel purge, residual water in the desiccant and the wheel supporting structure on the wheel performance. Ruivo et al [14] developed 1D transient heat and mass transfer model for desiccant wheel and presented two approaches. In the first approach, the model was valid for thickness lower than 0.1 mm, neglecting the transversal heat and mass transfer resistance in micro porous desiccant while in the second approach, the model was valid for thickness lower than 5 mm, neglecting only thermal resistance.…”