Study On Falling Film Evaporation And Heat Transfer Of Horizontal Tube

Freshwater shortage has become a hot issue around the world.Seawater desalination is an effective approach to this problem by extracting freshwater from the sea.Thanks to its utilization of waste heat and high quality product,Multi-Effect Distillation(MED)is attracting lots of attentions.However,studies about the distribution strcture of heat exchanger tube bundles and its influence on the scaling formation,the device performance and the heat transfer are rare.This kind of studies in this research are meaningful to the design and operation of the MED system.This research begins with the study on the spray and distribution characteristics of a swirl liquid nozzle with hollow structure.And the variation tendencies of the spray angles,the liquid drop dimension along with the liquid flow were obtained.Two uniform distribution structures were developed according to the different layouts of horizontal tube bundles.The influence of several parameters such as spray density,evaporation temeperature,heat transfer difference and noncondensable gas content on the heat transfer coefficient was analyzed based on an experimental study.The heat transfer coefficient increase with the increase of spary density or evaporation temperature.Decreasing the heat transfer difference and extract the noncondensable gas in time lead to higher heat transfer coefficient.A mathmatical model of horizontal tube falling film evaporation was established.The variation tendencies of salinity,heat transfer coefficient and evaporation rate along with the tube number were studied.Results indicate that salinity increases with the increase of tube number,while heat transfer coefficient and evaporation rate change in an opposite direction.When the number of heat transfer tube rises,the performance of heat transfer and evaporation degrades,the scaling tendency seems to be more serious.The developed liquid distribution structure impoved the performance of horizontal tube bundles indeed by decreasing the highest salinity and increasing the evaporation rate.This research is helpful to the design and operation of MED system.