The Research Of Enhancing Heat Transfer Properties In Falling-film Absorption Process Out Horizontal Tubes In The Lithium Bromide Absorption Refrigeration System

The Lithium Bromide absorption refrigeration system is a kind of system which has many advantages such as it can be driven by low-grade heat and environment-friendly. And it is widely used around the world in recent decades. In the Lithium Bromide(LiBr) absorption refrigeration system, the heat transfer performance of the absorber largely influences the heat transfer performance of the entire refrigeration system, so it is necessary to explore how to improve the absorption effect of the absorber. Falling-film absorption outside the horizontal tube is more recognized around the world compared with other absorption modes. The theoretical analysis and numerical simulation are carried out about the process of the Lithium Bromide solution falling film absorption out horizontal tube and the heat and mass transfer enhancement during this process in this paper.In this paper, the Lithium Bromide solution falling film absorption process out horizontal tube model is established and appropriately simplified. The analytic formulas of the liquid film thickness, the falling-film velocity, and the thermal boundary thickness and so on are obtained. By changing Reynolds number of the solution, the tube radius angle and other parameters, the distribution of the liquid film thickness and the flow velocity out horizontal tube under different conditions are obtained. And the falling film flow process has been simulated by VOF method, and by changing the parameters of the horizontal tube, the thickness distribution and velocity distribution of liquid film have been got. Compared with the results of the theoretical analysis and numerical simulation, the correctness of the calculation has been verified.The Fluent software is a CFD software which is widely used around the world. The user-defined functions can be defined according to the customer’s needs, so as to realize the function of setting material attribute, boundary conditions and improving the existing models. So the Fluent software is chosen as the CFD software to do the numerical simulation in this paper.Based on the analysis of heat transfer properties in falling-film absorption outside the smooth tubes, the theoretical analysis and numerical simulation are carried out about the heat transfer enhanced tubes wrapped with a stainless-steel wire-mesh screen to hold up more liquid film in this paper. This kind of heat transfer enhanced tube can be simplified as the heat transfer tube with rectangular fins outside it. The height of the fin is the diameter of the stainless-steel wire and the spacing between two fins is the mesh diameter. Different kinds of stainless-steel wire are chosen to wrap the horizontal tubes in this paper. The numerical simulation is made to analyze the heat transfer properties of different micro fins outside the horizontal tube. During the process of falling-film absorption, the thickness distribution of the liquid film and the thermal boundary layer under different wire diameters and numbers are analyzed and compared by the CFD software in this paper. Besides, the modes of whole absorber under different kinds of enhanced tubes are established separately. And the temperature distribution of the whole absorber is also simulated by the CFD software. Finally, the thickness distribution of the liquid film and the thermal boundary layer are obtained and the optimal conditions to improve the falling-film absorption performance and the heat transfer efficiency are summarized. Studies find that the measure of wrapping the horizontal tube with a wire-mesh screen can enhance the heat and mass transfer performance obviously. The wire diameter should be chosen to obtain a suitable thickness and a bigger heat transfer coefficient. In this paper, when the wire diameter is equaling to 0.2mm and the mesh number is equaling to 5, the heat transfer performance and the absorption effect are the best. The middle temperature region in the simulated temperature field is the largest and the water vapor absorbed is the most. The heat boundary layer is the thinnest, the thinnest place is only 0.48 mm and the heat transfer coefficient is up to 917 W/(m2·K).