Experimental Study On Regeneration Characteristics Of Salt Solution Under Vacuum

With the increasingly strict requirements for air quality,liquid desiccant air conditioning system was favored by people because of its advantages of high air cleanliness and independent control of temperature and humidity.However,the high energy consumption of regeneration limited the promotion of liquid desiccant air conditioning to a certain extent.If the pressure of regeneration environment was reduced,the dehumidification solution can boil at a lower heat source temperature,and the water was evaporated to realize regeneration process,which can expand the selection range of regeneration heat source.At present,the research on vacuum regeneration methods mostly took the evaporation amount of solution as the evaluation index to evaluate the regeneration effect.In order to further analyzed the boiling regeneration process of LiCl solution under vacuum conditions,the effects of pressure,concentration and heat transfer temperature difference on the heat transfer characteristics of solution under vacuum conditions were studied experimentally in this paper,and the process of heat transfer was analyzed,which can provide a reference for improving the regeneration efficiency of LiCl solution under vacuum.The work mainly included three aspects:(1)Theoretical study of boiling heat transfer process,analysis of the factors affecting boiling process from the mechanism of bubble formation.(2)Set up a test bench for the boiling characteristics of solution vacuum regeneration at a constant concentration,and introduced the main components of the test platform and its selection.(3)The experiments were carried out on aqueous solution and 30%~34% LiCl solution at the pressures of 4kPa~10kPa.The experimental scheme and specific experimental procedure were given,the experimental data were processed and analyzed,and the influence of pressure and concentration on boiling process under vacuum conditions were obtained.The control variables included boiling temperature difference,regeneration pressure and solution concentration.The constant temperature heating water tank was used to adjust the water supply temperature to realize the adjustment of boiling temperature difference,and the water-ring vacuum pump was used to adjust the regeneration pressure.The heat flux density and surface heat transfer coefficient were used to react the boiling process was strong or weak.The influence of regeneration pressure on heat flux density and surface heat transfer coefficient were studied by changing the temperature of hot water supply and the pressure of regeneration tank at constant concentration.The influence of solution concentration on heat flux density and surface heat transfer coefficient were studied by changing the temperature of hot water supply and solution concentration at constant pressure.The experimental data were processed and analyzed and the results showed that:(1)The change of pressure and concentration did not affect the tendency of heat flux and surface heat transfer coefficient increased with the increase of boiling temperature difference,because the increase of temperature difference caused the amount of bubbles increased,the solution was disturbed,and the heat transfer effect was enhanced.(2)At the same temperature difference and concentration,the change of regeneration pressure will affected the heat transfer effect of boiling.As the pressure decreased,the minimum radius of bubbles increased and the amount of bubbles decreased,so the heat flux density and surface heat transfer coefficient decreased with the decrease of pressure.(3)At the same temperature difference and pressure,the change of solution concentration also affected the heat transfer effect of boiling process.With the increase of solution concentration,the viscosity and surface tension of solution increased,the number of vaporization cores decreased,the growth and floatation of bubbles were hindered,so the heat flux and surface heat transfer coefficient decreased with the increase of solution concentration.(4)Reducing the pressure of regeneration tank can reduce the boiling point of solution and expand the selection range of regeneration heat source,but at the same time,the heat surface transfer coefficient will decrease.Therefore,in order to improve the regeneration efficiency of solution under negative pressure,the appropriate pressure should be selected.(5)By comparing the change of concentration with the change of pressure,it was found that the concentration had a greater influence on the boiling process,and the change of heat flux and surface heat transfer coefficient were greater when the concentration of solution was changed.