Theoretical And Experimental Study On Heat And Mass Transfer For Falling Film Absorption

Absorption at surfaces of flowing liquid is the most widely used absorption mode in absorption refrigeration and heat pump systems.The absorbent is distributed in the form of a liquid film on the absorber heat exchange surface,with the advantage of large gas-liquid contact area,high heat transfer efficiency,small flow rate and easy control and so on.According to the absorption refrigeration absorber structure,that is,the spatial distribution of liquid film,the absorption of the absorber is divided into the following categories:horizontal tube falling film absorption,vertical tube/plate falling film absorption,spiral tube falling film absorption,inclined plate falling film Absorption,etc.,of which vertical and horizontal tube falling film absorption is the most common.The vertical flow of the liquid film is usually assumed to be uniform and smooth laminar flow.However,in the actual process,the laminar flow only occurs at the extremely small Re number,or within an extremely short distance from the entrance.For a certain length of descending tube,the liquid film will develop into a wave-like flow.For a typical vertical falling film absorber size,the liquid film will develop into Long wavelength waves or solitary waves.The absorption process affected by the liquid film's waviness can not be reflected in most theory,and even reflected,the deviation is relatively large.Therefore,it is generally expressed in the form of experimental correlation,and experimental correlation can not reflect the actual absorption state of the liquid film in the mechanism,including the distribution of the temperature and the concentration in the liquid film,the thermophysical properties of the liquid film along the axial direction of the falling film tube.In this paper,the stability of the falling film flow is analyzed theoretically.By solving the Kuramoto-Sivashinsky equation,the solution to the differential equation of the wavy falling film flow is obtained.By analyzing the stability of the falling film flow,Compared with the initial value,we can deduce the relationship of the occurrence of the waviness in the dimensionless fluid and the flow state.Secondly,the Lattice Boltzmann method is used to simulate the liquid film flow in two cases,ie random perturbation and forced perturbation.The numerical simulation method has the advantages of obvious gas-liquid interface,explicit physical mechanism and high computational efficiency.By this method,the velocity field distribution of the wavy liquid film at a certain Reynolds number is obtained.Thirdly,on the basis of the laminar falling film absorption results,we add the wavy velocity distribution term and use the finite difference method(FDM)to simulate the falling film absorption process.Through the simulation,the temperature distribution and the concentration distribution inside the wavy liquid film were obtained.Finally,visualized falling-film flow and falling-film absorption experimental rigs were set up,and the flow test rig was used to carry out the falling-film flow of water and absorbent solution respectively.The absorption experiment rig was used to test the heat and mass transfer of the water vapor absorpiton.Experimental results and numerical results were compared.In this paper,through physical modeling and numerical simulation,heat transfer and mass transfer processes with the wavy flow pattern,in the conventional range of mass flow rate of falling film absorbers in absorption chiller are studied.Similar to previous studies,the waviness has a significant strengthening effect on the process of falling film absorption.In this paper,the effect is expressed in the form of transversal velocity component,which can predict the heat and mass transfer performance of the absorber well.Based on this,we can further propose an optimized theoretical method for falling film absorber.