Research On Heat And Mass Transfer Performance Of Internally-cooled Falling-film Dehumidification Process Of LiCl Solution

Liquid desiccant air conditioning is to achieve dehumidification by directly contacting the air with the solution easy to absorb the moisture in the air.Compared with the condensation dehumidification method of traditional air conditioning,this dehumidification method does not need to cool the air temperature below the dew point temperature and then heat it up.It has low energy consumption and good thermal comfort.It is easy to achieve zone control and total heat recovery,so this dehumidification method has great potential for popularization and application.This paper takes the important equipment dehumidifier in the liquid desiccant air conditioning system as the research object,and the heat and mass transfer characteristics of vertical internally-cooled falling-film dehumidification process of LiCl solution were studied by the combination of numerical simulation and orthogonal test optimization.In order to study the coupling of dehumidification effect and solution physical properties,a mathematical model of vertical internally-cooled falling-film dehumidification process of LiCl solution was established based on the solute permeation theory and VOF model.This model considered the change of physical properties of LiCl solution with concentration and temperature during dehumidification.The influence of the inlet parameters of dehumidification solution and wet air on the dehumidification capacity and efficiency of dehumidifier was studied by using the control variable method.The law of air moisture content and temperature along the dehumidification channel was analyzed.The results show that the dehumidification capacity of dehumidifier increases with the increase of solution inlet concentration,solution inlet velocity,air inlet water vapor mass fraction and air inlet velocity,and decreases with the increase of solution inlet temperature and air inlet temperature.The dehumidification efficiency increases with the increase of solution inlet velocity,and decreases with the increase of solution inlet concentration and air inlet velocity.The temperature of the solution inlet,the steam content of the air inlet and the air inlet temperature have little effect on the dehumidification efficiency.The mechanism of heat and mass transfer in the dehumidification channel was analyzed by the distribution of air moisture content and temperature in the dehumidification channel.The correlation formula between the dehumidification capacity and dehumidi-fication efficiency with the dehumidifier inlet parameters is obtained by using the MATLAB software.Through the design of orthogonal test,the influence of the inlet parameters of dehumidification solution and wet air on the dehumidification capacity and efficiency was studied,and the optimal parameters combination of dehumidification effect was obtained.Through the analysis of variance,it is found that the influence degree of the dehumidifier inlet parameters on the dehumidification capacity from large to small is as follows:air inlet velocity,water vapor mass fraction of air inlet,solution inlet velocity,solution inlet temperature,solution inlet concentration,air inlet temperature;the degree of influence on dehumidification efficiency from large to small is in the order of air inlet velocity,solution inlet velocity,solution inlet concentration,solution inlet temperature,air inlet temperature,water vapor mass fraction of air inlet.Through range analysis,the optimal combination of parameters for dehumidification capacity is:Ts,in=298 K,xs,in=42%,us,in=0.25 m/s,Ta,in=299 K,xa,in=2.4%,Ua,in=0.42 m/s.The optimal parameter combination for dehumidification efficiency is:Ts,in=299 K,xs,in=36%,us,in=0.25 m/s,Ta,in=305 Ka,in=1.8%,Ua,in=0.02 m/s.The single variable method was used to study the influence of the structure parameters of dehumidifier on the dehumidification capacity and dehumidification efficiency.With other parameters unchanged,the dehumidification capacity and dehu-midification efficiency increased by 53.85%and 55.16%respectively with the passage length of dehumidifier increasing from 60 mm to 140 mm.The calculation formulas of the outlet moisture content and temperature of dehumidifier changing with the passage length were obtained,and the fitting error was within ± 4%.When other conditions remain unchanged,as the width of the dehumidification channel increases from 6 mm to 22 mm,the dehumidification capacity and dehumidification efficiency are reduced by 48.28%and 62.13%,respectively.Compared with the flat plate,the dehumidification capacity and dehumidification efficiency of corrugated plate with wave height of 1.5mm increased by 42.31%and 44.17%,respectively,On the one hand,the wall ripple will enhance the disturbance in the liquid film and air side,thus enhancing the heat and mass transfer.On the other hand,because of the support of the ripple surface,the gas-liquid contact time and contact area will also increase.As the wave pitch of the corrugated plate was reduced from 15 mm to 5 mm,the dehumidification capacity and dehumidification efficiency increased by 72.13%and 72.88%,respectively.