Research On Horizontal Tube Falling Film Flow And Evaporation Heat Transfer

Horizontal tube falling film evaporation,as an important thermal process in multi-effect evaporation equipment,directly dominates the equipment performance.The influence of the falling film flow spatial-temporal non-uniformity on heat transfer has not been clearly explained.Most of the published studies have not proposed an accurate physical description and mechanism explanation for the flow and heat transfer process in horizontal tube falling film evaporation.Therefore,this investigation is experimentally and numerically performed to further understand the inter-tube flow behavior,liquid film spreading and evaporation heat transfer.The effect mechanism of the flow spatial-temporal non-uniformity on heat transfer process was studied.The main research contents and conclusions are as follows:(1)The experimental and numerical investigations with flow pattern classification and characteristic analysis were carried out for the horizontal tube falling film flow.Based on the experimental flow patterns and evolutions,the typical inter-tube flow patterns were identified and characterized in detail.According to the neighboring droplets falling,the droplet flow pattern can be further classified into asynchronous droplet fall and synchronous droplet fall;the film velocity under the stable column flow pattern has obvious three-dimensional distribution characteristic.Furthermore,sheet flow was classified into incomplete sheet flow and complete sheet flow.The experimental and simulated results were in good agreement.(2)The liquid film thickness distribution characteristics with the effects were summarized and novel correlations for liquid film thickness were proposed.The effects of Re,tube spacing,tube diameter and gas crossflow on inter-tube flow pattern and liquid film thickness distribution were analyzed based on both experimental and numerical results.The three-dimensional liquid film thickness evolutions with time affected by droplet impact and spreading process were summarized.The liquid film distribution under column flow in one “flow element” can be classified into impingement region,stable region,crest region,bottom region and departure region,while the liquid film thickness distribution under complete sheet flow shows two-dimensional characteristics and can be divided into impingement region,spreading region and accumulation region along circumferential direction.The correlations were respectively proposed to predict the film thickness under column flow and complete sheet flow based on the three-dimensional characteristics.(3)Three-dimensional numerical simulations of horizontal tube falling film flow and evaporation heat transfer were carried out focusing on the spatial-temporal characteristics of local heat transfer coefficient and the influence mechanism.It is found that dryout patches appear under droplet flow and reduce the heat transfer before the dryout patches are rewetted by the saddle-shaped waves.The local heat transfer coefficient distribution of the column flow can be divided into three regions: impact influence region,transition region and fluctuation influence region.The local heat transfer coefficient of sheet flow decreases rapidly along the circumferential direction,then increases slightly at the bottom of tube and is uniformly distributed along the axial direction.With the increasing of spray density,the local heat transfer coefficients of droplet flow and column flow both increase,while that of sheet flow increases first and then stays constant.The increment of tube spacing improves the heat transfer on the top half part of the tube.(4)A droplet impacting on a horizontal tube was further experimentally carried out with the influences of dimensionless tube diameter,Weber number,impact eccentricity and the tube surface wettability under the extremely dry condition.The results show that the process of a single droplet impacting the tube surface experiences four stages: impact,spreading,oscillation and stabilization.Central jet flow can be observed when the dimensionless tube diameter is large enough or Weber number is relatively small.Increasing Weber number,decreasing the dimensionless tube diameter or surface contact angle can improve the spreading.Increasing of the impact deviation accelerates the droplet sliding along the tube surface,which results in improving the circumferential spreading length.This study can further improve the understanding of the inter-tube flow pattern,the falling film flow outside the horizontal tube and the evaporation heat transfer mechanism.It also provides a certain theoretical basis for improving the horizontal tube falling film evaporation technology and the performance of multi-effect evaporation equipments.