Study on the coupled flow, heat and mass transfer processes in a liquid desiccant dehumidifier

Abstract

At present, a large amount of energy is needed to create a livable indoor environment. It is estimated that the buildings sector consumed 28% ~ 30% of the total final energy in China. Among all the power-driven equipment in buildings, chillers for air conditioning are well-known for their high power consumption in buildings. This study concentrates on solar energy based liquid desiccant air-conditioning to reduce latent cooling load of central air-conditioning systems. Due to the separate control of temperature and humidity, a liquid desiccant air conditioning system can save up to 40% of energy compared to a conventional vapor compression system. Thus, it has drawn more and more attention. Even though many researches have been carried out about the dehumidifier by simulation and experiment, there are still some limitations. It has been found that most of the previous researches focus on the macroscopic parameter changes of the fluids and the effect of various operating parameters. However, the flow situations and their impacts on heat and mass transfer in the dehumidifier interior are seldom studied. Therefore, this thesis aims to study the coupled flow, heat and mass transfer processes in a typical dehumidifier numerically and experimentally. Firstly, numerical predictions were conducted to investigate the flow dynamics in the liquid desiccant dehumidifier with the CFD software Fluent. The developed model was validated by existing data from literatures. With the model, the mechanism of the gas-liquid flow in the dehumidifier was illustrated. The velocity profiles, the minimum wetting rate, the effective interfacial areas between the solution and air, the average and local film thickness at different conditions were investigated. Based on the calculation results, it was concluded that the simulation model could predicate the dynamic and local flow conditions in the dehumidifier interior microscopically. It was found that the counter-flow air did change the velocity profile of the LiCl solution along the film thickness due to the drag force. And when the air inlet velocity reached 3.0 m s⁻¹, the impact became very distinct. Under that situation, the air became the dominated factor to decide the velocity field at the interface. Meanwhile, the importance of a suitable solution and air flow rates could be highlighted. The results also explained the reason of the enhancement of mass transfer with film flow. Then, by adding suitable sources files to Fluent, the coupled flow, heat and mass transfer processes were described comprehensively. In the model, the effect of the velocity field on the heat and mass transfer process has been considered. Meanwhile, the variable physical properties of the desiccant and air, which were taken as constant in almost all existing models, render the simulation more in line with real conditions. In addition, the penetration mass transfer theory was employed to make it possible to observe the dynamic process in the dehumidifier interior. With the established model, the parametric studies were conducted in a range of different flow conditions. Through the simulation, it was found that the air velocity played a critical role on the performance of the dehumidifier, which had to be matched with the channel geometric size for optimization. Besides, titling the plate to keep certain mass transfer gradient in the channel was also essential. Meanwhile, the coupled flow, heat and mass transfer phenomenon was also analyzed. Secondly, to investigate the hydrodynamics of falling film in the dehumidifier experimentally, the film thickness was measured by a capacitance probe and the flow morphology was captured by a camera. The wetting area under different solution flow rates and plate surface temperatures were investigated. It was found that the minimum solution flow rate to prevent the breakout of the falling film was 0.068 kg m⁻¹ s⁻¹, which was of the same order of magnitude of the simulation results (0.071 kg m⁻¹ s⁻¹). The local and temporal film thicknesses under different air velocities were recorded and analyzed for both the upper and lower parts of the falling film. The results showed that the wave was intensified along the flow direction, which also agreed well with the simulation results. In addition, the influence of the air velocity on the film thickness was also investigated. The average film thicknesses were calculated and compared with those of the Nusselt empirical formula. It was found that the Nusselt empirical formula underestimated the film thicknesses in present condition. In addition, the pattern and shape of the typical film flow were presented in the thesis. Experiments were also designed to investigate the real dehumidification process in a single channel internally-cooled dehumidifier. The absolute humidity change and overall mass transfer coefficient were chosen as the indices to evaluate the performance of the dehumidifier. The influences of various parameters were analyzed in depth. It was found that the overall mass transfer coefficient was higher than that reported in the previous literature due to much higher humidity of the inlet air in the present work. The result fully demonstrated the feasibility and advantages of adopting liquid desiccant dehumidification in Hong Kong. The increase of air velocity in the dehumidifier resulted in the increase of the overall mass transfer coefficient and yet the decrease of the absolute humidity change. The experimental results showed the same tendency with those of the simulation model, which verified the correctness of employing the penetration mass transfer theory. Academically, the numerical research provides a novel simulation model to predict the coupled flow, heat and mass transfer processes in a liquid desiccant dehumidifier in this thesis. The model considered the effect of the flow and the variable physical properties of the fluids on the dehumidification process, and enabled the dynamic observation by adopting the penetration mass transfer theory. Secondly, a single channel experimental setup was fabricated to investigate the performance of the dehumidifier. The hydrodynamics of the falling film was studied by investigating the local and temporal film thickness and the flow patterns. Then the effects of various parameters on the performance of the dehumidifier were analyzed in depth. The experimental results verified the correctness of the simulation model to some extent. Thus, the engineers and researchers can employ the model to predict the wetting situation, optimize the operation conditions and design dehumidifiers.