A conjugate transient computational analysis of flow, heat, and mass transfer in desiccant-airflow systems

The objective of this study is to advance the understanding of gas dehumidification by desiccants and desiccant regeneration to: (1) propose significant improvements in these processes, and (2) develop better design tools to configure the desiccant-carrying device. Two models are developed to solve numerically a conjugate transient flow, heat and mass transfer problem in a desiccant-airflow system (DAS). Model 1 describes a transient laminar and turbulent 2-D and 3-D humid airflow, including the heat and mass transfer phenomena, over porous desiccant-lined finite flat plates and inside a two parallel plates channel, and square, circular and triangular ducts. To improve the process description, Model 2 replaces the Model 1 desiccant heat and mass transport equations with a semi-heuristic Darcy momentum equation, and convective/diffusive heat and mass transfer equations.; The main contributions of this study to the state of the art are: (1) modeling and solution of the conjugate transient 2-D and 3-D laminar flow, heat and mass transfer equations numerically in the DAS with a finite thickness desiccant (Model 1): previously published work solved the model assuming conjugate transient 2-D heat and mass transfers between a solid desiccant and a humid steady-laminar airflow. Experimental results and non-conjugate numerical models developed previously studied transient 1-D heat and mass transfer in a thin packed bed of silica gel particles exposed to humid air. The non-conjugate numerical model assumed the silica gel bed to be thin enough to consider the internal temperature and concentration constant, and empirical heat and mass transfer coefficients at the desiccant-air interface were assigned; (2) solution of the above model for turbulent flow; (3) use of these models to determine and investigate the effects of the primary parameters, as listed above, on the dehumidification and regeneration processes; (4) modification of the heat and mass transport equations in the solid desiccant by including the flow through the porous desiccant and the associated convective heat and mass transfer (Model 2) and (5) development of ‘effective diffusion coefficient’ correlations as a function of particle diameter and a free humid airflow Re for practical use in adsorption process evaluation.; Guidelines are formulated for heat and mass transport enhancement for the above described range of parameters. (Abstract shortened by UMI.)...