Experimental Study On Flow And Heat Transfer Characteristics Of Low-temperature Multiple-effect Evaporation Devic

Low-temperature multi-effect horizontal tube falling film evaporation technology has the advantages of good heat transfer performance,good product quality,not easy to scale and low treatment cost,etc.,and it has been widely used in wastewater treatment,air conditioning and refrigeration and other fields,and has received extensive attention from scholars at home and abroad.The research on inter-tube flow pattern,heat transfer characteristics of tube bundles and overall performance of multi-effect devices can provide technical support and theoretical basis for the development of high-efficiency low-temperature multi-effect wastewater evaporation equipment.This article focuses on low-temperature multiple-effect horizontal tube falling film evaporation technology,the experimental device of three-effect horizontal tube falling film evaporator was designed and built.Using an aluminum alloy tube with an outer diameter of25.4mm with a smooth surface as the heat exchanger tube of the evaporator,the two tube bundle arrangements of positive triangle and corner triangle are adopted,and the three-effect evaporator adopts the method of parallel feeding.The evaporator inter-effect connection is sealed with PTFE gasket and sealant,which ensures vacuum sealing under vacuum degree of-0.095 Mpa.Taking tap water and brine as the research objects,the flow and heat transfer process of raw material liquid in the evaporator were studied.A high-speed digital camera was used to quantitatively observe the change of liquid flow pattern with spray density under different tube spacing.Using low-temperature heated steam as the heat source,the effects of different spray density,evaporation temperature and salinity on the heat transfer coefficient of the outer evaporation side of the first effect,the heat transfer coefficient of the condensing side in the tube,the total heat transfer coefficient,and the water yield and water-making ratio of the system were studied.Flow pattern observation experiments show that with the increase of spray density,the flow pattern of raw material liquid will appear in various flow patterns such as drop-like flow,dropcolumnar flow,columnar flow and column-sheet flow.In the experimental working range,the spray Reynolds number required to transition from drop-like flow to columnar flow was determined to be 170,and when the spray density changed from large to small,the flow pattern transition lag phenomenon was observed and the cause of it was explained.The results of heat transfer experiments show that with the increase of spray density,the heat transfer coefficient and total heat transfer coefficient of the evaporation side will increase,while the heat transfer coefficient of the condensing side will decrease first and then increase,and it is less affected by the spray density.When the evaporation temperature outside the tube increases,the heat transfer coefficient of the evaporation side,the heat transfer coefficient of the condensing side and the total heat transfer coefficient will increase.When salinity increases,the heat transfer coefficient and total heat transfer coefficient on the evaporation side decrease significantly,while the heat transfer coefficient on the condensing side increases slightly.The water yield of the experimental system increased with the increase of spray density and first-effect evaporation temperature,while the water-to-water ratio remained relatively stable.Water production decreases with the increase of salinity,and the ratio of water production decreases.