Research On Flow Patterns And Two-phase Flow Characteristics Of Falling Film Evaporation On Horizontal Tubes

In the field of desalination,as one of the three main technologies,low-temperature multi effect evaporation technology has drawn widely attention of researchers for its low operating temperature,high heat transfer efficiency,less scaling,high safety performance.The core equipment of this technology,horizontal tube falling film evaporator,has also been developed rapidly.This paper focuses on the theoretical analysis and experimental study of the flow pattern and two-phase flow characteristics of the horizontal fall ing film evaporator.In this paper,the flow patterns between horizontal tubes is studied firstly,and a falling film flow experiment platform is built and observed.The flow patterns between tubes are divided and the internal mechanism is discussed.The fact ors affecting the flow patterns between tubes are analyzed to establish the criterion of flow pattern determination.According to the observation of flow pattern,it was shows that as the spray density increase s,the flow pattern between the tubes appeared dr oplet,droplet-column,column,column-sheet and sheet.The column-shaped flow has the most stable shape and the widest continuous range.By studying the influencing factors of the critical Reynolds number of the flow pattern transformation,it indicted that the critical Reynolds number is h ardly affected by the tube material.In addtion the tube spacing has a greater influence on the critical Reynolds number.And as the tube spacing increases,the critical Reynolds number shows an increasing trend;The critical Reynolds number is slightly increased.Secondly,an experimental analysis of the hysteresis of the flow between the tubes and the separation length of important paramet ers that affect the flow is performed.The analysis results show that the hysteresis occurs in each flow stage,and the greater the spray density,the greater the hysteresis interval.The experimental value fluctuates around the theoretical value,the experimen tal value increases with increasing temperature.In actual operation,the horizontal airflow in the horizontal tube falling film evaporator will cause the fluid displacement between the tubes,resulting in uneven distribution of the liquid film or even the i nability to form a complete liquid film,which affects heat transfer.In view of this phenomenon,this paper theoretically explores the phenomenon of fluid displacement outside the tube under the action of lateral airflow,and analyzes the displacement characteristics of droplets and liquid column.The results show that as the lateral airflow velocity increases,the droplet and liquid column offset distance also increases;the critical velocity decreases with increasing tube spacing and temperature;when the lateral airflow medium is water vapor,the critical flow rate is much greater than air.