Research On The Distribution Of Liquid Film Thickness And Heat Transfer Characteristics On Horizontal Falling Film Of Semi-Elliptical Tube Under Forced Convection

Horizontal falling film evaporation technology has many advantages such as improving the heat transfer efficiency,optimizing the equipment size of the heat exchanger,and reducing the consumption of working fluid.In practical applications,it is often used under forced convection.However,there are relatively few related studies at present,and the types of tube adopted are mainly traditional round tubes.As a new type of special-shaped tube,the semi-elliptical tube exhibits its unique advantages in liquid film thickness,heat transfer efficiency,and flow stability.Therefore,this paper conducts an in-depth study and analysis on the thickness and heat transfer characteristics of the semi-elliptical tube under forced convection.The research adopts a combination of experiment and numerical simulation.An experimental platform was built under cold counter-current airflow,combined with digital image processing,to study the influence of different spray density and different counter-current airflow speed.The distribution of the liquid film thickness of the semi-elliptical tube was compared with that of a round tube and an elliptical tube with the same tube section outer circumference.Compared with experimental data,the error of the cold numerical simulation is 3.1%,indicating that the accuracy of the model meets the requirement.Through numerical simulation,the effect of different air directions and different airflow speed on the heat transfer characteristics of the horizontal falling film of the semi-elliptical tube was studied under hot conditions with constant wall temperature.It showed the difference of the distribution of the velocity field and temperature field of the liquid film and the air outside the pipe under the conditions of counter-current airflow and forward airflow.The experimental results under cold conditions show that:(1)With the increase of the circumferential angle under the condition of no airflow,the thickness of the liquid film decreases first and then increases,and the thinnest point is around 90°.With the increase of the counter-current airflow speed,the thickness of the liquid film remains unchanged in the upper half of the tube;the peak of the distribution curve in the lower half shifts to the position with a smaller circumferential angle,and the fluctuation is more severe.The average liquid film thickness increases first and then decreases with the increase of the counter-current airflow speed.When the airflow speed is equal to the critical point,it reaches the maximum.(2)The liquid film thickness distribution curve of the round tube is the steepest and its average liquid film thickness is the largest,followed by the elliptical tube,and the liquid film distribution of the semi-elliptical tube is more uniform and its average liquid film thickness is the smallest.(3)Based on regression analysis,the correlation of the liquid film thickness of a single semi-elliptical tube under the counter-current airflow condition was obtained.The error between the fitted value and the actual data is not more than 15%,indicating that the accuracy of the correlation is high.The numerical simulation results under hot conditions show that:(1)With the increase of the counter-current airflow speed,the size of the vortex formed by the counter-current airflow in the upper region remains unchanged,and the vortex in the tail region first increases and then decreases to disappear;the average wall temperature first increases,then decreases,and then rises sharply.(2)Under the condition of forward airflow,the thickness of the liquid film first decreases and then increases along the circumferential angle,and the thinnest point is around 90°.When the forward airflow speed increases,the thickness of the liquid film gradually decreases in the upper half of the tube,and it first increases and then decreases in the lower half;the average liquid film thickness decreases;the average wall temperature decreases.(3)Under the same working condition,the fluctuation of the liquid film and the wall temperature distribution curve under the forward airflow condition are more stable and their change ranges are smaller.