Edge Instability in Two-jet Impingement

Multiphase Flows and Spray Systems Laboratory Introduction When two cylindrical jets of equal diameters collide they form an expanding sheet in the plane at a right angle to the plane containing the axes of the two jets. Multiphase Flows and Spray Systems Laboratory December 8, 2018

Experimental Method and Apparatus Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Generation of Drops A cylindrical mass was built up on the edge. The cylindrical mass was roughened by small disturbances, which resulted in bead-like shapes. The beads kept growing while moving along the edge. After growing to some extent, the beads became drops attached to the sheet edge via ligaments. Drops first separated from ligaments, and the further breakup of ligaments produced small droplets. Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Bead Interval Bead intervals were continuously changing while moving on the edge. Small beads tended to move faster than big ones at the same location. Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Method of Measurement (Impinging angle: 120deg.; Mean jet velocity: 3.54m/s) Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability – Two pre-breakup Stages Growing stage (mainly due to capillarity) Detaching stage (due to stretching and capillarity) Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability – Breakup Patterns with Low Jet Velocities Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Growth Rate versus Drop Size Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Growth Rate versus Drop Size Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Growth Rate versus Mean Jet Velocity The edge thickness at the area on the sheet were measured for all the four jet velocities, and was used to substitute for in Rayleigh’s formula. Smallest drop radii in comparison with Rayleigh’s theory Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Instability - Growth Rate versus Mean Jet Velocity Maximum growth rates in comparison with Rayleigh’s theory Multiphase Flows and Spray Systems Laboratory December 8, 2018

Multiphase Flows and Spray Systems Laboratory Drops Shedding Angle Multiphase Flows and Spray Systems Laboratory December 8, 2018

Edge Velocity - Theoretical Study (Rewritten from Taylor [1959]) Ibrahim [1991] Theoretical calculation Downstream distance from the impact point Multiphase Flows and Spray Systems Laboratory December 8, 2018

Multiphase Flows and Spray Systems Laboratory Edge Velocity The speed of small bead was assumed to represent the fluid velocity within the local edge. (Impinging angle: 120deg.; Mean jet velocity=4.48 m/s) Multiphase Flows and Spray Systems Laboratory December 8, 2018

Multiphase Flows and Spray Systems Laboratory Conclusions The process of bead growth on the edges of the sheet was divided into two distinct stages, growing stage and detaching stage. The growing stage was found exponential with time, and was studied by relating the growth rate to the radius of resultant drop. Two trends were revealed. 1) Small beads tend to grow faster than big beads, because they more quickly reach higher fluid-velocity field than big ones. 2) The growth rate varies with the mean jet velocity. This trend probably indicates the relation between the growth rate and the initial bead interval. The edge velocity of the sheet was studied both theoretically and experimentally. Both the theory and the experiment showed that the edge velocity increases with decreasing the angle on the sheet. Multiphase Flows and Spray Systems Laboratory December 8, 2018