| Different from the cooling dehumidification and solid adsorption dehumidification,liquid desiccant is a new method of regulating the air humidity,which utilizes the hygroscopicity of the salt solution to reduce the air humidity.This technology bears a considerable application prospect in use of low-grade thermal energy to achieve refrigeration,dehumidification,air conditioning and other aspects.Moreover,studies have shown that the salt solution can remove harmful substances such as volatile organic compounds,bacteria,and dust.However,some scholars have proposed that the direct contaction between the salt solution and the treated air could cause carryover droplets that degrade the indoor air quality and pose a risk of corrosion.In addition,the unsatisfactory mass transfer effect during the dehumidification and regeneration processes makes the device bulky.Although the many advantages of liquid desiccant air conditioning method,there is still no widespread application.Therefore,studying how to improve the performance of heat and mass transfer while seeking advantages and avoiding disadvantages as much as possible on the impact on air quality is an important topic to promote the development of liquid desiccant air conditioning method.At present,the optimization research of liquid desiccant air condition system mainly focuses on the optimization of working fluid,equipment and operation strategy.The falling-film flow of the solution is an important part during dehumidification and regeneration processes.The area of the falling-film determines the efficiency of heat and mass transfer.The breaking and splashing of the liquid film and the purification of micro-particles affect the air quality.Therefore,this paper mainly studies the wettability,droplet entrainment and purification of micro-particles during the process of solution falling-film.The effect of different conditions on wettability,droplet entrainment and purification of micro-particles is researched,which provides theoretical support for the further optimization of liquid desiccant air-conditioning system.The detailed results are shown as follows:Firstly,in order to study the effect of the solution properties on falling-film plate wettability,this work used VOF method to investigate the effect of the temperature and concentration of Li Cl solution on the wetting ratio during dehumidification and regeneration processes.It is found that the solution concentration and temperature have a non-linear relationship with wettability,and the most unfavorable conditions for the wettability of the solution in the dehumidification and regeneration process are obtained.By studying the wetting rate of the most unfavorable conditions under different amount of spray,the recommended amount of spray is obtained,which is 0.619 m3/（m·h）for dehumidification and 0.662 m3/（m·h）for regeneration.The effects of dynamic properties of liquid desiccant on the wettability are studied to explain the influence of the solution temperature and concentration on the wettability.The results show that the solution viscosity and surface tension have opposite influence trends on wettability and the solution density has little effect.Thus,when the working conditions of the solution change,the change trend of the wetting rate mainly depends on the result of the confrontation between the effect of solution viscosity and surface tension.Based on the simulation results,this paper establishes the correlation equation of the effect of Li Cl solution properties on the falling-film wettability during dehumidification and regeneration processes,which is a theoretical reference for the selection of the amount of spray during actual operation.Secondly,for the surface conditions of the falling-film plate,this paper studies the effect of tilt angles,contact angles and surface morphologies of the filler surface on wettability and the critical amount of spray using Li Cl-H2O by simulation.The results show that the smaller the contact angle of the filler surface,the better the wettability and the smaller critical amount of spray required to reach the full-film flow.The critical amount of spray increases the most significantly when the contact angle of the filler surface changes from 40°to 60°.The smaller the surface tilt angle,the smaller the fluctuation of the solution film surface,and the better the wettability.The wettability of the wavy filler surface is significantly improved compared to that of the flat filler surface under small amount of spray.When the amount of spray is 0.288m3/（m·h）,The wetting ratio of the wavy filler surface with 5mm wave height is more than 50%larger than the flat surface.Although the wetting ratio increases with the increasing wave height,the growth of the wetting ratio is rarely only about 5%.In addition,because the stability of the liquid film on the wavy surface is poor,the critical amount of spray required for the wavy surface is larger than the flat surface.Thirdly,based the on compressed air drying system using liquid desiccant,this paper studies the effects of air velocity,air flow pattern and pressure on wettability and critical amount of spray.The results show that the main reason of the air velocity affecting the wettability is that the shear stress generated by the direction of the air velocity disperses the direction of the surface tension and weakens the effect of the surface tension on the liquid film distribution.The surface tension will lead the liquid film to converge toward the center to reduce the wetting ratio in the falling-film process,so the air velocity increases the wetting rate significantly during dehumidification for parallel flow pattern.Besides,for counter flow pattern,the air flow blocks the liquid film longitudinal spreading and destroys the stability of the liquid film at the liquid outlet,which increases the critical amount of spray.The pressure distribution is similar under different operating pressures,thus the pressure has little effect on the wettability.In addition,the simulation results under 8atm are compared with the experimental results.It is found that the sudden increase in the amount of moisture removal when the amount of spray changes from0.05 to 0.1 m3/（m·h）in the experiment is caused by the change of the liquid film flow state from drop flow to multiple streams flow.Finally,in view of the impact of the falling film process on air quality,this paper experimentally investigates how dehumidification and regeneration using Li Cl solution affect the mass concentration of micro-particles in the treated air.The mechanism of the solution’s action on the micro-particles and the target particle size are analyzed.The results show that dehumidification tends to remove micro-particles with sizes of 0-2.5μm.The main absorption mechanisms of the solution are diffusiophoresis and thermophoresis between the solution interface and the air near the interface during the solution dehumidification process.Larger micro-particles with sizes of 2.5-10μm are generated during regeneration.The main mechanism for the release of micro-particles is that water migrates from the solution to the air and then condenses into larger micro-droplets during the regeneration process,which are entrained and flowed out by the air.This article also studied the effects of different working conditions on the amount of micro-particles purified and released by solution.The results shows that increasing the rate of air or solution flow increases moisture migration during regeneration to release more micro-particles.The amount of micro-particles released during counter flow is less than that during parallel flow,because the larger particles settle down during the former.The increase of the concentration of the solution will strengthen the effect of diffusiophoresis and thus increase the absorption of the micro-particles by the solution.Using Monte material as a filler instead of the flat falling film surface can effectively improve the inertial collision and interception between the solution and the micro-particles,thereby improving the absorption of the micro-particles by the solution. |
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