Influence Of Gas Stream On Falling Film Flow And Heat Transfer Of Horizontal Tube

In order to meet the demands of energy and sustainable environmental development,the application of falling film technology in industrial processes has gained widespread popularity due to its high heat transfer efficiency and reduced refrigerant charge.However,the falling film process is influenced by various factors,particularly the effect of gas stream,which can lead to anomalous phenomena in falling film heat exchangers.While some studies have focused on the influence of longitudinal gas stream on falling film,less attention has been given to the investigation of cross and oblique gas stream,and the mechanisms behind their impact remain unclear.Therefore,exploring the effects of gas stream on the flow and heat transfer characteristics of horizontal falling film evaporators is of paramount importance for optimizing their design.In this study,a three-dimensional model was established to analyze the influence of different liquid film flow rates,gas stream directions(cross,co-current,counter-current,and oblique),and gas stream velocities on the flow and heat transfer characteristics of horizontal falling film.Key results regarding the effects of gas stream were obtained,including the liquid film thickness,heat transfer coefficient,and liquid film velocity during the falling film process.Empirical correlations were derived through fitting to capture these relationships.The main findings are as follows:(1)In the horizontal falling film flow process,the distribution of liquid film thickness and heat transfer coefficient exhibits non-uniformity,with circumferential and axial variations featuring valleys and peaks.In the absence of gas stream,the circumferential distribution of liquid film thickness and heat transfer coefficient is centrally symmetrical,with the liquid film flow rate being the primary influencing factor.(2)The influence of gas stream on falling film flow and heat transfer characteristics is closely related to the liquid film flow rate,gas stream velocity,gas stream direction,and position of the heat transfer surface.A higher liquid film flow rate helps resist highspeed cross gas stream,thereby maintaining the flow and heat transfer performance of the falling film.The gas stream direction is a dominant factor in the variations observed,while the magnitude of gas stream determines the degree of these variations.(3)Under the effect of cross gas stream,significant deviations occur in the liquid film,resulting in the formation of new stagnant and separation regions on the downwind side.This disrupts the symmetrical distribution of circumferential liquid film thickness and heat transfer coefficient,particularly at higher gas stream velocities.As the gas stream velocity increases,the variations in heat transfer coefficient distribution differ on the upwind and downwind sides.The average heat transfer coefficient distribution exhibits different trends with increasing cross gas stream velocity at different liquid film flow rates.(4)Under the influence of longitudinal gas stream,the circumferential distribution of liquid film thickness and heat transfer coefficient maintains its symmetry.Co-current and counter-current gas stream respectively accelerate and decelerate the liquid film flow,thereby affecting the contact time of the liquid film.With an increase in co-current gas stream velocity,the liquid film becomes thinner in the region of the thin film,while it becomes thicker in the region of the thick film,leading to a significant enhancement of heat transfer coefficient near the impact point.Counter-current gas stream has a small effect on the liquid film thickness in the upper part of the external tube,while it generally increases the liquid film thickness in the lower part,resulting in a significant increase and larger fluctuations in heat transfer coefficient near the separation region.(5)Under oblique gas stream,the liquid film experiences significant displacement,and the symmetrical distribution of flow and heat transfer characteristics is disrupted.The average heat transfer coefficient on the horizontal tube surface generally increases.The variations in axial average heat transfer coefficient on the upwind and downwind sides exhibit significant differences.In general,as the angle of oblique gas stream approaches90 degrees,the impact on the distribution of liquid film thickness and heat transfer coefficient becomes more pronounced.(6)By establishing correlation equations,it is possible to predict the film thickness,local heat transfer coefficient,and average heat transfer coefficient on the outer surface of a horizontally smooth single tube.This provides a predictive model for flow and heat transfer in non-phase change situations.