Numerical Analysis On Microscopic Heat Transfer Behavior Of Horizontal-tube Falling Film Evaporation Based On VOF Method

Low temperature multi-effect falling film evaporation of horizontal tubes is widely employed in industries such as sea water desalination, chemical refining, petroleum distillation and refrigeration, due to its principal advantages of low temperature differences, high thermal efficiency and maintenance expenses, etc. The low temperature multi-effect device shows different heat transfer performance under distrinct operating conditions, which illustrates the complexity and specific characteristic of horizontal-tube falling film evaporation technology. This study simulated the dynamic and heat transfer characteristic of falling film evaporation to predict the heat transfer mechanism within the thin film.In this work, a 2-D, Computational Fluid Dynamic model was developed using ANSYS FLUENT 14.5 to simulate the horizontal tube falling film evaporation. The model was carried out for the study of the flow and heat transfer characteristics with different inlet conditions, including the details about the profiles of film thickness, film velocity, film heat transfer coefficient, the development of thermal boundary layer and the ways of heat transfer within the film, etc. Volume of Fluid (VOF) model was used to track the free interface between gas and liquid phases.In this thesis, the study focusing on numerical simulation is performed for a single horizontal tube with the diameter of 25.4mm, the feeder height of 5mm, the saturation and the wall super heat temperature of 333.15K and 2K, the Reynolds number range was from 600 to 1250. Results of liquid film thickness, velocity, temperature profiles, the variation of wall shear stress and heat transfer coefficient were obtained. It is observed that the minimum film thickness tend to be located at a cylinder position between 90-135°. The highest film velocity is found to be at the position of 90° where the effects of wall shear stress and gravity are the most significant. Circulating flow exists at the position of 5° as the absolute value of film normal velocity is the maximum. It is also investigated that the heat transfer coefficient has a sharp decrease first and then the reduction is slowing down to stable trend from the tube top point where the film heat transfer is dominated by convection.It can be indicated from the numerical simulation results that increasing Reynolds number does not give a great assistance to a better heat transfer performance for laminar flow. The keys to promote the convective heat transfer intensity are the magnitude of film normal velocity and the existence of circulating flow. Therefore a few methods as the tube outer surface treatment and the enhancement of fluctuation intensity could be taken to obtain an improvement of heat transport performance.