Numerical simulation of heat and mass transfer in desiccant coated rotary dehumidifiers

 Author: Zhang, Huan Title: Numerical simulation of heat and mass transfer in desiccant coated rotary dehumidifiers Degree: Ph.D. Year: 2004 Subject: Hong Kong Polytechnic University -- DissertationsHumidity -- Control -- Mathematical modelsAir conditioning -- Control -- Mathematical modelsDrying agents Department: Dept. of Building Services Engineering Pages: xxxiii, 247 leaves : ill. ; 30 cm Language: English OneSearch: https://www.lib.polyu.edu.hk/bib/b1762026 URI: http://theses.lib.polyu.edu.hk/handle/200/1940 Abstract: A numerical model that predicts the heat and moisture transfer during the adsorption and desorption processes in a desiccant-coated, rotary dehumidifier has been developed from fundamental principles. All the assumptions are explained. The model is one-dimensional and transient, and includes all important parameters that affect the performance of desiccant wheels. The influential parameters include energy and mass storage in both the air and the matrix, convection, the axial thermal conduction through both the desiccant and the support material, the axial molecular diffusion within the desiccant coating, and the energy transfer between the air stream and the matrix. No existing models of desiccant coated rotary dehumidifiers include all the above-mentioned important parameters, but literature review indicated a need to accurately model heat and mass transfer in a desiccant-coated, rotary dehumidifier. The model developed in this research can be used in designing or optimizing the desiccant wheels, and in predicting the performance of desiccant enhanced air-conditioning systems. The model was based on the four governing equations for coupled heat and mass transfer in desiccant wheel. Discretization of the governing equations was performed and all the dependent variables and properties were solved at the nodes. The transient terms were solved using the implicit formulation. The numerical solution scheme was fully implicit, with all dependent variables expressed in their respective current time step values in the algebraic equations. The discretized moisture conservation equations were solved using the Thomas algorithm, also called the Tridiagonal Matrix Algorithm (TDMA), and the discretized energy conservation equations were solved using the Tridiagonal Matrix Algorithm for double variables. For validation of the numerical model, its predictions were compared with experimental data obtained with a test rig built in accordance with ASHRAE Standard 139 - 1998, Standard Method of Testing for Rating Desiccant Dehumidifiers Utilizing Heat for the Regeneration Process. The accuracy of the simultaneous heat and moisture transfer numerical model of desiccant coated rotary dehumidifier developed in this study has been confirmed by the good agreement with experimental data. The simulated moisture removal capacity agrees with the test data within experimental uncertainty. The effect of certain parameters on the predicted performance of a desiccant coated rotary dehumidifier was studied. The parameters include regeneration air inlet temperature and humidity ratio, process air inlet temperature and humidity ratio, and mass flow rate of process airstream. The optimal rotation speed was then found. The variations in parameters of process and regeneration air with angular position and with depth into the tubes are simulated. The desiccant-coated rotary dehumidifier model developed in this thesis was also applied to predict the performance of desiccant dehumidification and evaporative cooling (DDEC) systems. A two-stage DDEC system using low-temperature heat was proposed, and its performance was predicted. A comparison between a two-stage system and a single-stage system was then made. The required regeneration air inlet temperature of the two-stage DDEC system with intercooling was found to be 35.0C lower than of the single-stage system. The lowered regeneration temperature will make it possible to utilize low-quality thermal energy such as solar energy and waste heat. Direct evaporative air coolers are components of DDEC systems. The theoretical and experimental analysis of evaporative coolers filled with corrugated holed aluminum foil fillers (CHAF fillers) is detailed in this thesis. The experimental results show that the direct evaporative cooling effectiveness 摨▄c of the evaporative air cooler using CHAF fillers was about 0.8, and the air flow resistance was less than 200Pa for an air velocity of less than 3m/s. Therefore, CHAF filler is a good pad material.

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