Simulation Study Of Thickness And Flow Field For Falling Film Evaporationg On A Horizontal Tube Under Vacuum

Comparing with the traditional heat transfer evaporation,horizontal tube falling film evaporationhas many advantages, including low temperature difference of heat transfer, high heat transfer coefficient and capability of heat transfer under low temperature. It is widely applied in the system of refrigeration, HVAC, chemical industry etc.Single horizontal tube falling film evaporation under vacuum is researched. The inside of the tube is a condensation heat transfer process, while the outside of the tube is a falling film evaporation process. The flow field inside and outside of the tube are are both gas-liquid two-phase flows. And there is latent heat exchange during the process which is complicated.A horizontal falling film evaporation and condensation outside and inside the tube theory model is set up.The tube evaporation and condensation heat resistance, the tube wall thermal resistance, the heat resistance of fouling and the tube falling film evaporation resistance are calculated and analyzed. The results instigated that the tube falling film evaporation resistance accounted for the largest proportion, which is two times larger than the tube evaporation and condensation heat resistance. Therefore, study on the theory of tube falling film evaporation heat transfer plays an important role in improving the thermal performance of the falling film evaporators.The tube falling film model and the unit simulation model are set up. Heat transfer boundary conditions of the unit simulation model are loaded. VOF method and evaporation and condensation model from ANSYS FLUENT are used to simulate the heat and mass transfer performance of the tube falling film flow under different spray density.Simulation results show that with the spray density, the tube falling film evaporation heat transfer coefficient outside the tube increases. When spray density changes from 0.013 s)kg/(m ? to 0.058 s)kg/(m ? the heat transfer coefficient increases from 716 k)w/(m2? to 2698 2k)w/(m ?, increased by 4 times. The thickness of the falling film and the flow field of horizontal tube falling film evaporation process are calculated and analyzed, including x-velocity, y-velocity, the falling film thickness distribution of different range of tube circumferential angle and the falling film thickness distribution of different spray water density.In the light of previous studies, when the liquid impacted on the tube wall, the liquid film was thicker, owing to the accumulation of liquid. Subsequently the liquid film will be spread on the tube wall within the range of 5o~30o, which have the liquid film thinner with a great extent. Owing to the gravity and surface shearing stress of wall, the liquid film is getting thin at low speed within the range of 30o~90o,which is the thinnest. Over the range of 90o~155o liquid film gets little thicker. Attribute to the confluence of bilateral liquid film and detachment from tube wall, liquid film accumulate within the range of 155o~180o. As a consequence, the thinnest liquid film of horizontal is 0.001 m at the range of 90°when the spray density is at 0.4 s)kg/(m ?, evaporation temperature is at 80℃ and the tube temperature is at 85℃.