Numerical Study On The Formation And Evolution Of Dry Spots Induced By Liquid Film Evaporation

The heat transfer technology or coating technology of thin liquid films on different surfaces can be found in many engineering applications: from heat exchangers to falling film evaporators;from cooling equipment for microelectronic devices to coating of precision instrument circuit boards and sensitive electronic components,these applications are all closely related to the flow process of the liquid film on different surface.However,the appearance of dry patches in the liquid film affects the efficiency and application of this technology.For example,for the falling film evaporator,local drying of the liquid film will lead to the decrease of the convective heat transfer coefficient,the local surface temperature of the heat transfer tube rises rapidly and the deterioration of the overall heat transfer efficiency;dry patches on the coating products of precision instrument circuit boards can cause electrical leakage and burn out the whole circuit,resulting in additional economic losses;dry patches on the coating products of sensitive electronic components can reduce the life,safety and reliability of electronic products.In view of the importance of avoiding and predicting the formation of dry patches,this paper makes a preliminary attempt and exploratory research on the rupture of a thin liquid film evaporating on different surface,aiming at exploring the mechanism of dry patches in the liquid film and the influence of various working parameters on dry patches formation,and perfecting the coating technology of heat transfer technology of the liquid film on different surface.In this paper,according to the law of conservation of mass,the one-dimensional drying model outside the horizontal circular tube and on the inclined plate is established respectively.The first two models mainly consider the influence of evaporation,then,the concept of thermal capillary force is introduced to increase the dimension of the drying model outside the horizontal circular,in the other word,the two-dimensional drying model tube is established.Aiming at the evolution of liquid film drying out,MATLAB software was used to numerically calculate the governing equations of the three models,and the influences of spray density,spray temperature,tube diameter and inclined angle of plate was discussed in detail.Aiming at the one-dimensional drying model on the inclined plate,it is mainly discussed in three parts: the evolution of the liquid film drying on the inclined plate;the heat transfer characteristics of the liquid film;effects of various working parameters on drying of liquid film.Increasing the flow rate can not only improve the heat transfer coefficient of the liquid film,but also reduce the area of dry patches and even avoid the appearance of dry patches.The higher the spray temperature,the earlier the dry patches appeared,and the larger stable dry patches area.The effect of the inclined angle of the plate on the dry patches area is minimal.As for the dry-out phenomenon of the liquid film outside the horizontal circular tube,the numerical studies have found that increasing the flow rate or reducing the diameter of the circular tube not only improves the convective heat transfer coefficient,but also increases the thickness of the liquid film so as to reduce the dry patches area and even avoid dry out.The dry patches area in the liquid film is directly proportional to the spray temperature,and the time required for dry patches to appear is inversely proportional to the spray temperature.Compared with the liquid film affected only by evaporation,the liquid film under the combined action of thermal capillary force and evaporation will rupture in advance.The stable dry patches shape is parabolic,and the stable speed of dry patches area on the inclined plate is faster than that outside the horizontal circular tube.