Effects of heat and mass transport on the hydrodynamics and stability of liquid desiccant films.

摘要

The continuous increase in the demand of energy and the high costs of fuels in many areas in the world are fostering improvements in energy efficiency in many sectors. Defrosting due to ice formation on the surface of cooling coils is a major source of inefficiencies in the refrigerated warehouse sector which, just in California, accounts for 1,800 million kWh of energy consumption per year. Liquid desiccants have been identified as a means of reducing latent loads in air conditioning and refrigeration systems. However, the interaction of liquid desiccant films and humid air is a subject that has not been analyzed in detail. In the past, significant efforts have been made in understanding liquid-film absorption processes in heat and mass exchangers for several flow configurations, using various numerical and experimental techniques. However, studies concerning liquid desiccant films near freezing conditions with coupled momentum, heat and mass transfer using non-isothermal thermo-physical properties are rare in the literature. The purpose of this dissertation is to investigate the effect of heat and mass transfer in the interaction of a liquid desiccant film in contact with a humid air stream. Velocity, temperature, and concentration distributions, as well as, film stability are studied experimentally and numerically. The results show a delay in the formation of the ice on the cooling coil due to the reduction of the dew point temperature of the evaporator inlet air, thus reducing operating costs. Moreover, the results show that an effectiveness of the internal heat exchanger less than 60% leads to a desorption process which is opposite to the aims of the dehumidification system. The stability analysis of the liquid film is evaluated using the Orr-Sommerfeld equation with methodologies such as the small wavenumber technique and complete orthonormalization method. Most works found in the literature have focused on the particularly simplified cases of absorption with uniform properties, namely, isothermal systems. The techniques used to analyze film stability allow the inclusion of the effects of heat, mass, and temperature dependent properties, which constitutes a major contribution of this work. Stabilizing and de-stabilizing effects of different parameters are discussed in the results.