Study On Convective Heat Transfer Characteristics Of Thin Liquid Film Outside Horizontal Tube

Falling film evaporators have caused wide public concern while it was designed.It has many advantages such as higher thermal efficiency,less land occupation,lower operating temperature,less liquid storage compared with submerged evaporators,easier to recovery of waste heat,and it can be combine with other systems.There are two forms:horizontal tubes and vertical tubes.The heat transfer coefficient of horizontal tube falling film evaporator is two or three times of the heat transfer coefficient of the vertical tube falling film evaporator,which is about five times of that of the traditional heat exchanger.Therefore,the study of falling-film evaporation on horizontal tubes has shown great value.In view of superiority of this system,horizontal-tube falling film evaporator was studied theoretically and experimentally.The purpose of this thesis is to explore the distribution of liquid film thickness and the factors what affect the heat transfer in horizontal-tube falling film evaporator.First,the N-S equations are simplified according to the boundary conditions,and then the distribution of the liquid film thickness along the circumferential angle and the relationship of liquid film thickness and the spray density were obtained.The liquid film thickness decreased first and then increased with the increase of circumferential angle.The minimum value was obtained while the angle is 90 degrees.The film thickness was proportional to the spray density.Then,according to the balance of force,dryout caused by the increase of heat flux or the decrease of spray density was analyzed.The main factors that affected the critical liquid film thickness were the spray density and the physical properties of the fluid.The falling film evaporator was mainly used in seawater desalination,evaporation and crystallization.In these processes,the fluid viscosity was larger,and it was easier to accumulate on the horizontal tube.So it is easier to dry out.In this paper,we hopeed to establish a model for this situation to guide the practice.Due to the small film thickness,it was difficult to measure the film thickness.The distribution of the axial and circumferential liquid film thickness of the horizontal tube was measured by the probe.In virtue of Taylor instability,the liquid film fluctuates along the axial direction and circumferential direction.The experimental results showed that the liquid film thickness was not symmetrical in 90 degrees along the circumferential direction,and the influence of the impact would form an arch between two drops in the axial direction.After that,the heat transfer experiments were carried out on a single horizontal tube.A heating rod was placed in a horizontal tube to heat the film.In order to ensure that the tube heat evenly along axial and circumferential direction,the gap between the heated pipe and the heating rod was filled with Mg uniformly.The effect of spray density,heat flux,inlet temperature and circumferential angle on heat transfer coefficient were measured.But at this time the liquid film did not meet the boiling conditions,it can be determined that this was not caused by the dryout due to boiling and evaporation.At the same time,it was found that it was different with different inlet temperature.Only when the inlet water temperature was low,it may dry out.This is due to thermal capillarity.Because of the increase of temperature,the physical properties of the fluid were changed:the surface tension decreased;the viscosity decreased and the evaporation made the liquid film become thinner,the gravity decreased,but the surface tension decreased less tian the gravity speed.The decrease of surface tension would make the film produce a shear stress.Surface tension was greater than gravity until the film dry out.With the increase of temperature,when the surface tension is equal to the gravity,the shear force caused by the change of the surface tension leads to the contraction of the liquid film,and the film dry out.The spray density and heat flux density were measured in the experiment,and then the author compared with the theoretical model and the experimental data.