Experimental study on droplet flow of falling film between horizontal tubes

Abstract

Droplet flow is one of the most significant flow patterns in horizontal tubes falling film flow, which is widely observed in industrial equipment. Flow visualization of water/ethanol/30% glycerol-water solution falling in 8/23/38 mm tube spacing using high-resolution and high-speed digital video camera is presented here. The image edges detection algorithm of Sobel is developed to quantify falling distance and velocity for each frame of a sequence. The results illustrate that the droplet flow between tubes can be divided into the following four processes: droplet formation, droplet falling, droplet impact, neck break and retraction. The “escape length” is proposed in droplet formation process. The falling process can be divided into three stages according to the appearance and break of liquid neck. The droplet in different tube spacing displays different falling stages, which affect the processes of droplet impact and neck break and retraction eventually. The droplets falling velocity increases linearly with the falling distance in the first and second stages of falling process. The increase of viscosity can retard the droplets falling and the decrease of surface tension can predate the appearance of the droplet detaching. In addition, a new correlation has been suggested to predict the primary droplet diameter. When 9mm < s < 15 mm and 25ms < τa < 40 ms, there is a phenomenon of ``liquid globule rebound".