Numerical Investigation Of Heat And Mass Transfer Of Falling Film Absorption Process In Different Laminar Flow States

Absorption refrigeration system is widely concerned for its advantages of environmental protection,high efficiency and simple structure.Absorber is the most critical component in the vapor absorption refrigeration system,its performance has a major impact on the performance and cost of the whole system,so it is urgent to improve the absorption performance of the absorber.Falling film absorption is one of the most common forms of mass and heat transfer in an absorption system.In order to facilitate the research,the falling film flow process at small Reynolds number is usually assumed to be smooth laminar flow.However,even at small Reynolds numbers,the surface of the solution flowing vertically downwards will generate small waves,and surface waves have a great influence on the absorption process.Therefore,in-depth study of the evolution of surface waves and the mechanism of strengthening the absorption process have become a very important research subject,which has great significance in improving the performance of absorption refrigeration systems.Based on the analysis of the mass and heat transfer characteristics of the falling film absorption of the lithium bromide solution,taking into account the changes in the physical properties of the lithium bromide solution,a mathematical physical model of the smooth laminar falling film absorption was established using the COMSOL Multiphysics software based on the finite element method.The internal temperature and concentration distribution of the liquid film,the mass and heat transfer flux at the interface,the mass and heat transfer rate,and the liquid mass and heat transfer coefficient were calculated.According to the calculation results,the effects the film flow rate of the solution,and the temperature and concentration of the solution inlet on the absorption process were analyzed.The results show that the average mass and heat transfer coefficient increases with the increase of the film flow rate.The average mass and heat transfer flux increases first and then decreases with the increase of the film flow rate.Compared with lowering the solution inlet temperature,increasing the inlet concentration of the solution can greatly improve the absorption performance.Enhancing the heat and mass transfer performance of the absorber is one of the most effective means to enhance the performance of the entire refrigeration unit.Adding nanoparticles can effectively improve the heat and mass transfer performance of the absorber.Based on a smooth laminar falling film absorption model,copper oxide nanoparticles were added to an aqueous lithium bromide solution.The effects of the volume fraction of nanoparticles on parameters such as mass transfer coefficient,mass transfer rate,and interface mass transfer flux were studied.The results show that the addition of nanoparticles improves the absorption performance,and the effect of nanoparticles is slightly obvious when the concentration of the solution inlet is low.Considering the influence of mass and heat transfer in the flow and the changes in the physical properties of the lithium bromide solution,a strong coupling model of falling film flow,mass transfer and heat transfer in a wave laminar state was established.The changes of surface wave,velocity field,temperature field,concentration field,interfacial mass transfer flux and mass transfer coefficient in natural flow were studied,and the mechanism of surface wave improving mass transfer in natural flow was analyzed.The results show that when the solution flows downward along the wall with a low Reynolds number,it is spread on the wall as a liquid film after a wave.Although the solution will eventually spread on the wall,this process also enhances mass transfer.The mechanism of enhanced mass transfer can be interpreted as changes in concentration gradients and film thicknesses due to surface waves,and changes in velocity fields in solution.By adding disturbances of different frequencies,the falling film absorption process under forced flow was studied.The effects of different frequencies on the surface waves and the effects of different waveforms on absorption processes were analyzed in depth.The results show that the lower frequency makes solitary and capillary waves appear on the solution surface.With the increase of disturbance frequency,the capillary waves gradually decrease until they disappear,forming a quasi-sine wave.All three surface waves have enhanced absorption to varying degrees.The main factors affecting the absorption process are changes in the interface concentration gradient and the increase in the mass transfer area caused by the surface wave of the solution,and the mass transfer caused by fluid motion and component diffusion.