Study Of Heat Transfer And Liquid Wetting Characteristics Of Falling Liquid Film

The evaporative condenser is an efficient and compact heat exchanger,which combines the advantages of air-cooled and water-cooled condensers.Through the evaporation of the spray water on coil pipes,the condensation heat is discharged into the environment.In the traditional horizontal tube evaporative condenser,the condensate will accumulate at the bottom of the horizontal tube when the spray water will also accumulate at the bottom of the horizontal tube on the outside surface,which depresses the thermal performance of the evaporative condenser.In this paper,combining the the falling film technique,a vertical tube evaporative condenser is designed,in which the refrigerant is condensed inside the vertical tube and the spray water flows and evaporates on the outside surface.The thermal performance of the vertical tube evaporative condenser was studied experimentally,and the effects of spray water flow rate,inlet air mass flow rate and air wet bulb temperature on the performance of the refrigeration system were explored.The empirical correlations of the liquid film coverage and the convective heat transfer coefficient of falling films were also proposed.To predict the heat transfer rate and the state of and outlet fluid under different experimental conditions,a theoretical model of the heat and mass transfer process in the vertical tube evaporative condenser was put forward.The falling liquid film coverage on the heat surface was found to be of great significance to improving the performance of the falling film equipment in the study of the vertical tube evaporative condenser.The effect of the surface hydrophilic coating and vertical rib on improving the coverage of the falling film on the vertical plate was investigated with the infrared thermal imager.The coverage rate of falling film on the untreated copper plate,the hydrophilic copper plate and the vee-grooved plate was measured under the experimental conditions with various liquid film flow rates,cross-flow air velocities and heat fluxes.The empirical correlations of the falling film coverage under different working conditions was fitted with the experimental results.Experiments has proved that the vertical grooved surface can promote the uniform distribution of the liquid film on the vertical plate,and improve the liquid film coverage.To study the expansion behavior of the liquid film on the vee-grooved surface,this paper experimentally and theoretically investigated the growth and expansion wetting behavior of the droplet on the horizontal concentric vee-grooved surface,based on the hypothesis that the diameter of the droplet is smaller than the capillary length.A high-speed camera and a CCD camera were used to capture the growth and the expansion process of the droplet on the horizontal grooved surface,and the critical droplet volume and critical angle for the expansion wetting process of the droplet were measured.A thermodynamic method based on the energy barrier to calculate the critical volume and critical angle was also presented,the theoretical results of which can better predict the transition behavior of the droplet on the grooved surface than the Gibbs theory.Finally,the minimum wetting rate of the falling liquid film on the vertical plate was studied.The minimum wetting rate is defined as the critical flow rate that ensures the vertical surface is completely wetted by the liquid film,which is of great significance to ensure the efficient and safe operation of the falling film equipment.Based on the theoretical study of the droplet expansion on the horizontal grooved surface,this paper puts forward a theoretical method to calculate the minimum wetting rate of the falling liquid film on the vertical vee-grooved surface.The relationship between the minimum wetting rate and the groove structure and the equilibrium contact angle was summarized and the empirical correlation of the minimum wetting rate on different grooved surface was proposed.According to the experimental study on minimum wetting rate of the falling film on the vertical grooved surface with different groove structures,the correction factor was determined to include the effect of the wave on the liquid surface.