1. Edge Instability in Two-jet Impingement

2. 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

3. Experimental Method and Apparatus
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December 8, 2018

4. 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

5. 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.
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December 8, 2018

6. Edge Instability - Method of Measurement
(Impinging angle: 120deg.; Mean jet velocity: 3.54m/s)
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December 8, 2018

7. Edge Instability – Two pre-breakup Stages
Growing stage
(mainly due to capillarity)
Detaching stage
(due to stretching and capillarity)
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December 8, 2018

8. Edge Instability – Breakup Patterns with Low Jet Velocities
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December 8, 2018

9. Edge Instability - Growth Rate versus Drop Size
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December 8, 2018

10. Edge Instability - Growth Rate versus Drop Size
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December 8, 2018

11. 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
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December 8, 2018

12. Edge Instability - Growth Rate versus Mean Jet Velocity
Maximum growth rates in comparison with Rayleigh’s theory
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December 8, 2018

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

14. Edge Velocity - Theoretical Study
(Rewritten from Taylor [1959])
Ibrahim [1991]
Theoretical
calculation
Downstream distance from the impact point
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December 8, 2018

15. 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

16. 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