Study On Flow Characteristics And Mechanisms Of Horizontal-tube Falling Film Evaporation

The horizontal tube falling film evaporation flow pattern and flow mechanism have an important influence on the design of the actual heat exchange equipment application system.In this paper,multi-effect evaporation technology in seawater desalination is used as the application background.A horizontal tube falling film experimental platform is established to analyze the changes of flow patterns and the causes of special flow patterns under different operating conditions.A three-dimensional horizontal tube falling film flow model was established to simulate the flow characteristics and provide basic theoretical support for the application of horizontal tube falling film evaporation technology in the field of seawater desalination.In the experimental study of falling film evaporative flow in horizontal tubes,mainly analyzes the influence of different physical properties,tube center distance and tube diameter on falling film flow pattern and explains the flow pattern.Analyzes the causes of film rupture,droplet slippage,droplet retraction and hysteresis.The simulation study focuses on different flows outside the tube under multiple operating conditions,analyzes the effect of liquid film velocity,tube center distance,and the spray density on falling film flow,conducts detailed liquid film velocity analysis on the formation of special flow patterns and understands droplet drops change in speed during falling and pulling.A detailed analysis of the drop-column flow regime was conducted to investigate the difference in liquid film velocity and liquid film thickness between the first and second rows of tubes and the formation of droplets between the tubes.The results show that the conclusion is obtained in the experiment: the working fluid is 30% glycerol,and the columnar flow shows a "downstream" flow state,while the working fluid is water,the column discharge does not have a "downstream" state.The tube center distance has a greater influence on the transition of column curtain flow than the drop-column flow.The change of critical transition Re of column curtain flow is larger than that of drop-column flow.Under the tube diameters of d=19 mm and d=25.4 mm,it will not affect the flow pattern of the falling film significantly.Under the working condition that the working medium is water,the critical point of the flow state is inconsistent as the flow rate increases and decreases,showing a lag phenomenon.The increase of the tube center distance will delay the time point of the flow pattern change.The hysteresis is related to the surface tension.The greater the surface tension,the more obvious the hysteresis.From the simulation results: The liquid film spreads on the first row of tubes,the axial distance L * = 0.5,the greater the circumferential angle,the greater the liquid film speed,and the thinner the liquid film thickness.In the drop-column flow state,the liquid film spreading can be divided into a liquid film formation process and a liquid column steady state process.During the steady state of the liquid film,the circumferential angle increases,and the speed of the liquid film on the second row of tubes fluctuates downward in a pulsating manner,showing a certain angle trend.With the increase of the flow time,the thickness of the liquid film spreading on the second row of tubes exhibits a wave-like alternation.When the wake area is reached,with the change of the axial distance,the thickness of the liquid film on the first row of tubes changes in a "U" shape,and the thickness of the liquid film on the second row of tubes changes in a stepped manner.