Date of Award

2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Committee Chair

Robert Frederick Jr.

Committee Member

Keith Hollingsworth

Committee Member

Kader Frendi

Committee Member

Jason Cassibry

Committee Member

David Lineberry

Subject(s)

Liquid propellant rockets., Jet nozzles.

Abstract

This research explored how the jet breakup length to impingement distance ratio affects the spray characteristics of like-doublet injectors and consisted of cold-flow experiments using water at atmospheric pressure. A combination of three impingement angles, four jet velocities, and four jet breakup length to impingement distance ratios between one-half and two were tested. The breakup characteristics, sheet lengths and ligament wavelengths were determined from high-speed videos of the spray while droplet statistics were collected with a Phase Doppler Particle Analyzer. The sheet breakup characteristics were altered when the ratio transitioned from greater than one to equal to one and dramatically changed when the ratio became less than one. A robust impingement and `steady' sheet formed when the ratio was greater than one. While an `unsteady' sheet formed when the ratio equaled one due to intermittent jet breakup at the impingement point. Finally, no sheet was formed for ratios less than one. The flat sheet experienced two breakup modes separated by a transition Weber number. Empirical sheet breakup correlations based upon the Weber number and impingement angle were determined for both breakup modes. The mean wavelength between the shed ligaments was equal for all jet velocities and impingement angles tested. This produced a mean atomization frequency that was near to and parallel with the Hewitt stability threshold for liquid rockets demonstrating the importance of injector primary atomization characteristics on combustion instability. In addition, the droplet diameter and droplet distribution width was found to be inversely proportional to Weber number and impingement angle and was described by a single empirical correlation. Finally, dynamic mode decomposition (DMD) was used to characterize the spray. The DMD analysis was able to extract modes and frequencies corresponding with both the impact waves on the surface of the sheet and the ligaments shed from the end of the sheet. The impact wave modal structure was also observed on the two jets suggesting that the disturbances present on the two impinging jets are the source of impact waves. This idea was solidified by observation of the high-speed videos that clearly showed that impact waves are formed by the local momentum imbalance between the two turbulent jets at the impingement point. Also, composite contour plots using all of the calculated dynamic modes produced a contour of the spray remarkably similar to what is seen in a high-speed photograph indicating the capability of DMD to extract the relevant modes that influence the complex impingement and breakup process.

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