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Title: Lattice boltzmann simulation of heat and mass transfer process in falling film based liquid desiccant air conditioning
Authors: Lu, Tao
Degree: Ph.D.
Issue Date: 2020
Abstract: Indoor thermal environment is quite important for both indoor occupants and indoor building materials, especially in hot and humid regions. Temperature and humidity control of indoor air is the core part of indoor thermal environment. In the conventional HVAC system that removes moisture by condensation, air is cooled and dehumidified simultaneously. However, the overcooling and reheating process is energy-intensive, which leads to low evaporating temperature, a poor COP value for the chiller and higher energy consumption of the system. In the conventional HVAC system, energy consumption on treating fresh air is extensive. About 20-40% of the overall energy consumption of air conditioning system is consumed in fresh air handling process. Moreover, the ratio of sensible load to latent load varies largely due to the changes of outdoor climate, number of indoor occupants, indoor equipment utilization situation and so on. Therefore, the indoor temperature and humidity can hardly be satisfied with condensation by the cooling coil only. Falling film is widely used in heat engineering fields, such as falling film heat exchangers, evaporators and cooling towers, for its simple structure, low temperature difference and considerable heat and mass transfer efficiency. Falling film liquid desiccant air dehumidification is a promising alternative to traditional vapor compression air-conditioning system due to its lower energy consumption and less pollution. It is reported that the liquid desiccant air conditioning system based on this kind of dehumidifier can save about 10-30% of energy compared with the low temperature dehumidification using conventional gas compression system. Many researchers have studied the effect of various design operating parameters and conditions on the performance of the falling film. The factors are evaluated specifically, including the desiccant fluid properties (like density, viscosity, and specific heat capacity and so on), the flow configuration, the desiccant distribution, the inlet flow rate and condition of the desiccant solution, moist air and cooling media, energy store capacity and so on. However, further studies are also needed on some aspects. Firstly, most researches focus on the macroscopic parameter change rather than the microscopic heat and mass transfer process in falling film. Secondly, many researches take the falling film as a single phase problem, which means the surface tension and phase change are neglected. Thirdly, in conventional numerical models, the turbulence flow is usually simulated with semi-experimental turbulence models, which may not have solid physical backgrounds.
The lattice Boltzmann method (LBM), which is a numerical method rooted in kinetic theory, is becoming popular in the fields of computational fluid dynamics in last decades. Due to the particle-based mesoscopic nature which connects the micro and macro worlds, the LBM has an advantage in simulation fidelity and computational efficiency especially for multiphase flows. Generally, there are four popular LBM models so far to solve the multiphase flow problems, including the color-gradient model, the free energy model, the Shan-Chen model and the phase-field model. The phase-field model is more appropriate to solve the falling film problems due to its advantage in handing high density ratio problem. To solve the heat and mass transfer problem in falling film process, thermal phase-field LB models are proposed by many researches. However, in these previous models, there are still some fundamental problems that have not been solved. First, most of the existing works are focused on the lattice Boltzmann (LB) models for Cahn-Hilliard (C-H) equation, but these models suffer from the poor stability, difficulty in simulating the large density ratio problems, and the mass non-conservation. Second, most of the existing LB models are only suitable for steady state flow instead of dynamic problems. Thirdly, most existing LB models still have problems in dealing with the phase change problem. For above problems, we have carried out the following works: (1) A mass conservative lattice Boltzmann model (LBM) is proposed to simulate the two-phase flows with moving contact lines at high density ratio. The proposed model consists of a phase field lattice Boltzmann equation (LBE) for solving the conservative Allen-Cahn (A-C) equation, and a pressure evolution LBE for solving the incompressible Navier-Stokes equations. In addition, a modified wall boundary treatment scheme is developed to ensure the mass conservation. The wetting dynamics are treated by incorporating the cubic wall energy in the expression of the total free energy. The current model is characterized by mass conservation, proper treatment of wetting boundary and high density ratio. We applied the model on a series of numerical tests including equilibrium droplets on wetting surface, co-current flow and a droplet moving by gravity along inclined wetting surfaces. Theoretical analysis and experiments are conducted for model validation. The numerical results show good performances on mass conservation even with a density contrast up to 1000. Furthermore, the results show that the moving contact line can be successfully recovered, which proves that this model is applicable on the study of moving contact line issue and further related applications. (3) Based on the above thermal LB model, we simulated the complete development processes of the falling film from beginning to steady state at different Reynolds numbers. The results are compared with the experiments of previous researches. In conclusion, a thermal multi-phase lattice Boltzmann model is established for simulating the heat and mass transfer process in falling film based liquid desiccant air conditioning. The model can well predict the temperature and concentration distribution of the falling film process and has a better computational efficiency than traditional simulation methods. The model has made contributions in the aspects of two-phase flow interaction, high density ratio flow and phase-change problems. The model can provide suggestions in the future design and optimization of the falling film based liquid desiccant air conditioning.
Subjects: Air conditioning -- Design and construction
Air conditioning -- Energy consumption
Hong Kong Polytechnic University -- Dissertations
Pages: xxv, 206 pages : color illustrations
Appears in Collections:Thesis

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