Falling Film Flow Characteristics On Vertical Plate

The heat and mass transfer of falling fluid film, which are widely applied in industry, isinfluenced significantly by the liquid flow characteristics. Experiments are conducted toinvestigate the characteristics of droplet flow, rivulet flow and falling film flow. The effect ofthe interfacial effect and other factors such as Reynolds number, heat flux, on the droplet andrivulet flow is analyzed. The modal and relativity analyses are introduced to explain thefalling film flow process. It is valuable to analyze and optimize the disturbance effect infalling film flow process.The configuration of rivulet flow is investigated experimentally. Module of rivuletflowing downward along a vertical plate is analyzed, based on the principle of the minimumtotal energy (MTE). The contact angle hysteresis of the solid surface is taken into account inthe boundary conditions of the governing equations. The wavy feature in the direction ofliquid film thicknesses of rivulet is investigated when the flow rate is fluctuating. Theexperimental results show that the real thickness of rivulet is related to the average flow rate,in some range defined by contact angle hysteresis. If the change of flow rate in the rivulet isless than a critical value determined by the contact angle hysteresis and the initial flow rate,then only the rivulet thickness may exhibit a wavy behavior in terms of fluid flow rate;otherwise the rivulet will expand in width as well. The flow rate, surface and liquid propertiesall affect the configuration of rivulet flow.PIV technology is applied in the experiment to measure the flow field of the rivulet. It isproved that eddies occur when the flow rate varies. Hysteresis tension is introduced to analyzethe cause of eddy in rivulet flow sublayer and its strength, in case of variation of flow rate.Moreover, it is convinced both theoretically and experimentally that the interfacial effect isthe primary factor to cause the difference of the liquid film and the rivulet flow field.The optical method and high speed camera are utilized to acquire the experimentalphotos, and then the data of falling film thickness are obtained by self-compiling imagineanalysis software. The film thickness is measured under different disturbances of sound waveand different fluid flow rates. The correlation of the disturbing function and the initial filmthickness is analyzed, and FFT is also applied to the data results. With the introduction ofmodal analysis, the wave frequency and the amplitude of the falling film are analyzed in bothtime domain and frequency domain. The effects of environmental disturbances on the flow feature of the falling film are investigated. A new way to choose a proper disturbance ispresented to increase or decrease effect of the disturbance on the falling liquid film regardingthe heat and mass transfer.The internal flow field of falling droplet is measured and a double-eddy flow profile isobserved, which agrees to the numerical simulation result reported in literature.