Numerical Simulation Of Falling Film Flow And Heat Transfer On Horizontal Tube Under Steam Traverse Condition

Low temperature multi-effect seawater desalination system with horizontal tube falling film evaporator is a highly efficient heat exchanger,it has the advantages of low evaporation temperature,small heat transfer temperature difference and high heat transfer coefficient.Horizontal tube falling film evaporation mainly includes the steam condensation process in the horizontal tube and the liquid evaporation process outside the tube,which involves the complex phenomena such as fluid flow,gas-liquid two-phase flow and fluid-solid coupling.The study on the flow and heat transfer of the falling tube evaporator is beneficial to the design and improvement of the horizontal tube falling film evaporator.In this paper,a 3-D single horizontal tube falling film flow evaporation model with a wavelength period is established.The CFD software based on the finite volume method is used to capture the gas-liquid interface by the VOF two-phase flow model.The PISO algorithm is used to simulate the falling-And heat transfer process.By changing the wind speed at the entrance and changing the running conditions of the spray hole,the heat transfer process of the liquid film outside the horizontal pipe under different working conditions is studied,and the relevant conclusions are drawn.First,the flow distribution process of liquid film outside the horizontal tube is studied: After the liquid column impinges on the wall of the tube,the falling film liquid flows in opposite directions,and finally the center of the horizontal tube is merged to form the annular liquid film peak,and the liquid film on both sides of the peak is symmetrically distributed to form the wrong discharge;The vertical velocity value increases first and then decreases with the increase of the circumferential angle,and the maximum value appears at about 110 °,and the velocity value is not related to the spray density;The thickness of the film in the peak region increases with the increase of the spray density,and this phenomenon does not exist in the non-peak region.And then,this paper analyzes the effect of the flow of steam on the liquid film distribution and heat transfer: With the increase of the wind speed,the flow velocity of the film in the windward surface changes obviously,the film thickness decreases as a whole,the thickness of the leeward mask increases,and the thickness of the two films tends to be uniform;In the role of steam around the tube,the thinnest of the liquid film appears in a liquid column section,150° circumferential angle in the windward side;The heat transfer coefficient of the tube is mainly affected by the thickness of the film and the degree of fluctuation in the liquid film,which decreases first and then decreases,and the minimum appears between 160° and 170°;Spray density has no obvious monotonous effect on heat transfer coefficient.Finally,the influence of circulating steam on the liquid column is analyzed: The characteristics of the actual liquid column shift are found,and the shortcomings of the theoretical derivation formula of the relationship between the liquid column and the incoming steam are pointed out;The greater the spray density,the more stable the liquid column;In the small spray density,the critical deviation angle of the liquid column is about 50% of the geometric critical angle.With the increase of the spray density,the ultimate wind speed of the incoming steam increases gradually,and the critical deviation angle decreases small.