Experimental Study On Wettability And Heat Transfer Characteristics Of Liquid Falling Film Evaporation In A Vertical Tube

The application of falling film evaporation technology in refrigeration and air conditioning systems can greatly reduce the refrigerant filling capacity,thereby making it possible to promote the application of new low-GWP refrigerants with flammability problems.Rivulet is a very common form of flow during the refrigerant falling film evaporation process which cannot completely cover the solid wall surface,thus forming a three-phase contact line.As we know,the liquid evaporation mainly occurs on the contact line.As the wall temperature increases,the evaporation in this area intensifies.The flow characteristics and heat and mass transfer mechanisms involved are closely related to the wettability of the liquid film.In order to reveal the wettability of the refrigerant during the falling film evaporation process,a visual falling film evaporation experiment system was designed to observe the flow characteristics of R134 a.The cross-sectional profile of the liquid film is obtained using a camera equipped with a charge-coupled device(CCD HV1351UM),after which the edge detection technology and the shape image analysis method were used to measure the falling film flow contact angle under the wall superheated conditions(0.2-1.8?).The results show that the measured contact angles lie close to the exponential curve:(?) = 12.12?0.343.In addition,the liquid film profile parameters such as contact angle,wetting width,average liquid film thickness,and profile area were used to establish a profile model for the three different profile shapes of the liquid film inside the tube: concavoconvex,planoconvex,and biconvex.At the same time,a rivulet heat transfer model was established by combining the profile characteristics,and the initial liquid film characteristics were analyzed.And by calculating the distribution of the heat transfer coefficient of the liquid film at different mass flow rates,drop heights,and wall superheats,the law of the liquid film heat transfer coefficient is summarized.Finally,a preliminary experimental study was conducted on the falling film flow characteristics of water on the copper surface coated with super-hydrophilic coating.The results show that when the temperature of the substrate is close to the saturation temperature or even a certain degree of superheat occurs,the violent evaporation of the liquid film will cause the liquid film to shrink.Macroscopically,the wetting width becomes narrower and the contact angle increases.