氨水竖管降膜蒸发实验和理论研究

Experimental and Theoretical Study of Ammonia Water Falling Film Evaporation in a Vertical Tube

为了增强氨水竖管降膜蒸发过程中的热质传递效果,建立了降膜蒸发过程的数学模型,基于有限容积法对模型进行了求解,并通过实验验证了模型的准确性,用数学模型计算了溶液的速度场、浓度场、膜厚和换热量,得出结论:降膜蒸发过程的入口段小于100 mm,在入口段,溶液的流速和膜厚都发生剧烈的变化;最佳的布膜厚度是由溶液进口流量决定的;单位管长氨气的蒸发量随管长由上而下逐渐减小,前3 m管长的蒸发量占整个蒸发量的80.64%。基于建立的数学模型,可以确定不同流量下的布膜器最佳布膜厚度和管道最佳长度,为降膜换热器的优化设计提供参考。

To enhance the heat mass transfer effectiveness during the ammonia water falling film evaporation in a vertical tube,established was a mathematical model for the falling film evaporation process and the finite volumetric method was based to seek solutions to the model in question.Finally,the correctness of the model was verified through a test and the speed field,concentration field,film thickness and heat exchange quantity of the liquid solution were calculated by using the model.The authors have arrived at a conclusion that the inlet section of the falling film evaporation process is confined to a length of less than 100 mm,in which the speed and film thickness of the liquid solution will undergo a drastic change.The analytic and calculation results show that the optimal film thickness is determined by the flow rate of the solution at the inlet;the evaporization quantity of ammonia in a unit tube length will gradually decrease with an increase of the tube length from the top to bottom.The evaporation quantity in the first 3-meter length of the tube accounts for 80.64% of the total evaporation quantity.Through a calculation by using the model in question,the optimal film thickness obtained by using the film distributor and the optimal tube length can be determined at various flow rates,thus providing a useful reference and underlying basis for optimized design of falling film heat exchangers.