机械蒸汽再压缩降膜蒸发器的沸腾传热特性

Boiling heat transfer characteristics on falling film evaporator of mechanical vapor recompression

降膜蒸发器的传热性能是影响机械式蒸汽再压缩(MVR)系统高效运行的关键因素,为了提高蒸发器的传热效率,搭建了降膜管蒸发实验系统,以氯化钠溶液作为系统循环工质进行了实验研究。分别探究了降膜蒸发传热系数随热流密度(3.58—8.07 k W/m^2)、流量(30—150 L/h)、真空度(6.3—26.3 k Pa)和液体进料温度(91.7—98.2℃)的变化规律。结果表明:随着液体进口流量的增大,对流传热系数由层流的先下降后转变为湍流的增大;随着热流密度的增加,层流状态时,对流传热系数减小,湍流状态时,对流传热系数增大;液体进口温度的升高和真空度的增加,有助于提高液膜的对流传热系数。研究结果为MVR降膜蒸发器在工业实践中的高效运行奠定了基础。

Heat transfer performance of falling film evaporator is key factor affecting efficient operation of Mechanical Vapor Recompression(MVR)system.In order to improve heat transfer efficiency of evaporator,an experimental system of falling film tube evaporation was built,and sodium chloride solution was used as circulating working medium of system.Law of change in heat transfer coefficient of falling film evaporation with heat flux(3.58-8.07 kW/m 2),flow(30-150 L/h),vacuum degree(6.3-26.3 kPa)and liquid feed temperature(91.7-98.2℃)was investigated respectively.Results showed that:With increase of liquid inlet flow,convection heat transfer coefficient decreased firstly and then changed to turbulent flow.With increase of heat flux,convection heat transfer coefficient decreased in laminar flow and increased in turbulent state.Increase of liquid inlet temperature and vacuum would help to improve convection heat transfer coefficient of liquid film.Results laid a foundation for efficient operation of MVR falling film evaporator in industrial practice.