Research On Heat And Mass Transfer Performance Of LiBr Solution Falling-Film Absorption Process Outside Circular Tubes With Micro Fin

Due to the lithium bromide absorption chillers can use low-grade thermal energy as the driving force,no harm to the environment,and it has irreplaceable economic efficiency in the use of waste heat and also has the unique superiority in reducing environmental pollution,using energy reasonably and relieving the power shortages in summer,lithium bromide absorption refrigeration technology has attracted more and more attention.However,compared with the electric refrigeration,low performance coefficient(COP)is an important factor which restricts its development.Therefore,it is very important to improve the performance coefficient of lithium bromide absorption refrigeration system.Absorber is one of the most important components in lithium bromide absorption refrigeration system,and it is of great significance to study on enhanced heat and mass transfer of absorber to improve the efficiency of the whole absorption chiller.In this paper,through the establishment of a two dimensional model of the enhanced heat transfer tube wrapped with a wire-mesh screen,the theoretical analysis and numerical simulation are carried out about the process of the lithium bromide solution falling film absorption on the surface of the enhanced heat transfer tube.In this paper,firstly,the physical process and enhancing heat and mass transfer mechanism of the lithium bromide solution falling film absorption outside the heat transfer tube wrapped with a wire-mesh screen is studied.Then,the heat transfer tube wrapped with a wire-mesh screen can be simplified to a heat exchange tube with rectangle micro fin.And a twodimensional physical and mathematical model for the process of the lithium bromide solution falling film absorption outside micro fin tube was established,the numerical simulation was implemented with the aid of computational fluid dynamics software FLUENT,and the distribution of temperature,velocity and concentration of lithium bromide solution in the process of falling film absorption was analyzed.Furthermore,the numerical simulation is carried out about the process of the lithium bromide solution falling film absorption outside heat transfer tube wrapped with different wire diameter and mesh number wire-mesh screen.Through the analysis of heat and mass transfer law,what can be concluded is the most favorable conditions for improving the heat and mass transfer efficiency of heat exchanger tubes wrapped with a wire-mesh screen.Finally,the model of falling film absorption of smooth surface heat transfer tube is established and simulated is carried out.Through comparing and analyzing the heat and mass transfer law of smooth surface heat transfer tube and the process of the lithium bromide solution falling film absorption outside heat transfer tube wrapped with a wire-mesh screen.In addition,based on the previous model,the influence of absorber structure and the physical properties of lithium bromide solution on the heat and mass transfer of falling film absorption process was studied.The study results show that:(1)By comparing with the simulation results of the smooth heat exchanger tubes,heat exchange tube with micro fin can obviously intensify the effect of heat and mass transfer in the process of lithium bromide solution falling film absorption.The heat transfer rate of the falling film absorption process of the heat exchange tube with micro fin(wire-mesh screen which wire diameter is 0.3mm and the mesh number is 30)is about 30% higher than that of the smooth surface heat transfer tube,and the water vapor absorption is increased by about 35%.(2)When the heat exchange tube wrapped with wire-mesh screen which wire diameter is 0.3mm and the mesh number is 30,the heat transfer effect and absorption effect of falling film absorption process is the best.(3)The heat transfer quantity and vapor absorption quantity of the falling film absorption process increases with the increase of the ratio of the pitch and the diameter of tube(S/D)and tube diameter(D).(4)When the solution turns to be variable property,the heat transfer quantity and vapor absorption quantity of the falling film absorption process is smaller than constant properties solution.But it does not change a lot.