Inter-Tube Flow Pattern And Outside-Tube Heat Transfer Coefficient Distribution Of Horizontal Tube Falling Film

The horizontal tube falling film evaporator has many advantages such as high heat transfer efficiency,small heat transfer temperature difference,and low operating cost.It is widely used in desalination,nuclear energy chemical industry,air conditioning and refrigeration,sewage treatment and other fields.The research on the falling film flow and heat transfer outside the horizontal tube is conducive to understanding the interaction between falling film flow and heat transfer,and to develop the horizontal tube falling film heat transfer technology with wide applicability.The inter-tube flow pattern and the tube-surface liquid film interact,which determine the heat transfer state outside the tube together.At present,the cognition of the intertube flow pattern is not comprehensive,and it remains on the basic flow pattern and transitional flow pattern.Especially,the cognition of droplet flow is still unclear.In order to understand the inter-tube flow pattern and dripping phenomenon fully and profoundly,it is necessary to conduct an in-deep study of the inter-tube fluid flow.Most of researches on the horizontal tube falling film heat transfer focus on the average heat transfer coefficient,and researches on the local convective heat transfer coefficient are obviously insufficient,especially the lack of local convective heat transfer coefficient correlations that can be applied to engineering practice considering multiple parameters.Therefore,it is necessary to conduct in-depth research on the local convective heat transfer coefficient of horizontal tube falling film.In this paper,experimental and numerical simulation methods were used to study the intertube flow pattern,the tube-surface liquid film and the outside-tube local convective heat transfer coefficient distribution of the horizontal tube falling film.The main research contents of this paper are described as follows:The effect of operating parameters and structural parameters on the inter-tube flow pattern were studied.A horizontal tube falling film flow experiment platform was built,and the intertube fluid flow was observed by using a high-speed camera.Based on the analysis of dynamic images of the inter-tube fluid flow,the inter-tube flow patterns were identified.The effects of spray density,tube spacing,tube diameter,fluid temperature and fluid physical properties on inter-tube flow pattern were revealed.An inter-tube flow pattern distribution map of pure water was established considering flow rate,tube spacing and fluid temperature.The droplet flow between horizontal tubes was studied.A high-speed camera was used to observe the inter-tube droplet flow,quantitatively proposing the four processes of the droplet flow:droplet formation,droplet falling,droplet impact and neck break and retraction,revealing the effect of the flow rate,tube spacing and fluid physical properties on the droplet flow.Based on the four proposed processes,and according to different droplet morphological characteristics,the droplet flow is further divided into complete droplet flow,pendant droplet flow,incomplete droplet flow,accumulated droplet flow and incompletely droplet flow.The droplet flow pattern distribution map of pure water has been established,revealing the transition law of droplet flow with flow rate and tube spacing.The flow characteristics of droplet flow were analyzed,such as droplet falling velocity,droplet diameter,detaching length,liquid neck contraction,overhanging column and satellite droplet evolution,and dropping site spacing.The multiple models of complete droplet flow were established,including the droplet escape length model,the main droplet diameter model,the droplet detaching length model,the liquid neck contraction model and the dropping site spacing model.The local convective heat transfer coefficient of the horizontal tube falling film was studied.An experimental platform for measuring the local heat transfer coefficient of the falling film of a horizontal tube was built.The rotation method was used to measure the wall temperature distribution of the column flow and the sheet flow.The distribution law of the local heat transfer coefficient of column flow and sheet flow were summarized.The effects of spray density,tube spacing,average heat flux and inlet fluid temperature on the distribution of local heat transfer coefficients of column flow and sheet flow were revealed.The effects of the above four parameters on the local heat transfer coefficient were as follows:inlet fluid temperature>tube spacing>spray density>average heat flux.The local heat transfer coefficient of the sheet flow along the circumferential direction decreases sharply at frist,then decreases slowly,and finally increases slightly.According to the distribution law of local heat transfer coefficient,three heat transfer zones on the horizontal tube surface were defined:impingement heat transfer zone(about θ=0°-40°),heat diffusion transfer zone(about θ=40°-150°)and tail detachment heat transfer zone(about θ=150°-180°).According to the heat transfer mechanism in different regions,considering the influence of spray density,tube spacing and fluid physical properties on heat transfer in different regions,the correlations of local heat transfer coefficient of falling film flow outside the tube were established.In order to reveal the effect of operating parameters and structural parameters on the thickness distribution and fluctuation of liquid film,numerical simulation studies were carried out for the flow and heat transfer of the horizontal tube falling film.The 3D column flow and 2D sheet flow models were established respectively to simulate the spreading process,the liquid film thickness and fluctuation,and the local heat transfer coefficient distribution.The effects of spray density,tube spacing,heat flux,and inlet fluid temperature on the liquid film thickness and fluctuation,and the local heat transfer coefficient distribution were revealed.The extent of the influence of the above four parameters on the liquid film thickness is as follows:spray density>inlet fluid temperature>tube spacing>heat flux.