The Flow Characteristics And Heat Transfer Enhancement For Horizontal Tube Falling Film At Low Spray Density On Superhydrophilic Surface

Thin film evaporation heat transfer process is widely used in petrochemical,light industry,energy,aerospace,electronics and many other fields,due to the advantages of small heat transfer temperature difference,high heat flux density,high heat and mass transfer coefficient,good temperature uniformity,simple structure and low power consumption,.The horizontal tube liquid film evaporation technology is widely used in large-scale industrial production due to its advantages of high heat transfer efficiency,anti-fouling,and easily to use the low-grade residual heat and elimination non-condense gas,etc.It is a high-efficiency heat exchange technology that solves the problems of large energy consumption and serious emission of pollutants.In recent years,with the increasing demand for environmental protection and energy conservation,the use of low flow rates and small temperature differences to achieve efficient heat transfer technology has increasingly attracted the attention of scholars.The evaporation process of the falling film process on the horizontal tube at low spray density is a complex and complicated interface evolution process with the partition features that interact with and interact with each other in the liquid droplet.Therefore,a deeper understanding of the partition characteristics of droplets and liquid films and their effect on heat transfer characteristics during the evaporation process of the falling film has significance influence on further explore the low spray density heat transfer control mechanism and efficient heat transfer enhancement technology for the falling film evaporation process.This article focuses on the falling film evaporation process on horizontal tube under low spray density.Through theoretical analysis,the key factors affecting the evaporation heat transfer performance at low spray densities are obtained.The theoretical model of the minimum spray surface density of the iquid film falling down in the horizontal tube was established by using the force balance model.Moreover,the enhanced method of heat transfer deterioration under low spray density was obtained by introducing the liquid film thickness formula on partially wetted surfaces.Thus,two kinds of hydrophilic surfaces with different wettability are prepared by oxide etching method.The film flow patterns on the tubes with different wettability were observed and the minimum spray density was determined.Additionally,a minimum spray density model of the downhole liquid film with good agreement with the experimental results was obtained by introducing a correction factor.A three-dimensional model was established in numerical simulation for both of drop model and jet model to accurate presence of intermittent droplets and continuous liquid jets in experiments for superhydrophilic surface.The spreading characteristics and ripple fluctuation characteristics of drop model and jet model were emphatically analyzed.The cause of the saddle-shaped film and the formation of the "interaction ring" at the impingement zone for adjacent liquid jets were revealed.The effect of Re on film thickness and spreading velocity was also investigated.It was found that Re had little effect on the thickness and spreading characteristics of the liquid film in droplet model but had a significant effect on the jet model.The evolution of the liquid film thickness in the axial and circumferential directions was discussed.The results show that the liquid film was in the form of a valley-peak-valley distribution for the entire axial direction.The film was thickest at the middle convergence area and nearly double times thicker than the other locations.A high-resolution thermal imaging camera and a high-speed camera were used to study the temperature partition and fluctuation characteristics of the falling film process at low spray density.The effects of spray density,initial liquid film temperature,and heating power on the temperature distribution of liquid film under different flow patterns were analyzed.The result showed that the film temperature in droplet model presented a periodic distribution.The temperature of the liquid film in the impact area rised significantly,while non-impact area was basically unchanged.At the same time,the impact area and the non-impact location were not been fixed,there were changes in a periodic.A high-temperature annular structure had formed in the impact area,which was about 0.3-0.6K higher than the body area temperature and the temperature rise of that region occurd when the spreading liquid film reached the maximum.The temperature of the liquid film in the jet model was uniformly distributed.There was a clear low-temperature boundary between adjacent liquid column junctions,which was about 0.2K lower than the region temperature.The transitional flow is between droplet and jet model provided two kinds of temperature distributions-uniform distribution and periodic distribution.The temperature fluctuation periodic of the heat exchange tube decreased with the increasing of the spray density and the initial film temperature.The intensity of the fluctuation significant decreased as the increasing of spray density.That corresponded to the conversion of the flow pattern from the droplet model to the droplet/jet model.Combined with infrared tracing technique and statistical analysis,the multiple droplets spacing transfer rule under low spray density on superhydrophilic surface was studied.It was found that the production of droplets was not random,but there was a certain periodicity.This typical periodicity did not appear in the entire range,but only existed in a few special flows and the number of repeat units decreased as the flow increases.The energy minimization theory and statistical analysis were used to obtain the reasons for the periodic appearance of the droplet.A correlation prediction model was given and it was in good agreement with the experimental results.Based on this,the distribution of averaged liquid film thickness under for droplet model was constructed.Finally,a horizontal tube falling film evaporation experiment platform was designed and built.The surface wettability on evaporation heat transfer effect of the horizontal tube falling film process was investigated.The effects of spray density,inlet flow,inlet temperature and saturation temperature on the heat transfer performance were investigated.The result shows that the heat transfer coefficient for superhydrophilic tube was higher than that of the hydrophilic tube and the smooth copper tube under lower spray density and offered a better heat patience over a wide range of temperature.The maximum heat transfer coefficient was approximately 3 times higher than that of the smooth surface,thus,heat transfer enhancement at low spray density was achieved.It provides theoretical guidance and experimental basis for the design and development of new enhanced heat transfer surfaces.