Investigation of liquid rocket swirl coaxial injection dynamics under self-excited high frequency oscillations
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Robert A Frederick, Jr.
A. Kader Frendi
D. Keith Holingsworth
Fluid mechanics., Liquid propellent rockets.
Liquid-centered swirl coaxial injector elements used within liquid rocket combustion devices can exhibit self-excited and self-sustained oscillations known commonly as self-pulsations. The objective of this dissertation is to experimentally determine under non-reactive conditions how self-pulsation is excited and what fluid oscillators control its frequency. The phenomenon is explored for a study injector element within a range of fluid momentum flux conditions spanning 4 to 93 kPa for water flow and 12 to 585 kPa for gas flow. The recess length of the inner swirl post is also varied from 0 to 2 mm from the injector face. Self-pulsation is diagnosed with fluctuating pressure measurements gathered both upstream of the injector and downstream in the far-field. High speed schlieren imagery of near-field spray behavior is also captured. Spectral analyses of pressure measurements and objective data-based modal decompositions of the imagery are combined for a characterization of oscillations during self-pulsation. For the conditions investigated, self-pulsation is found to occur over a wide range of frequencies of approximately 900 to 4000 Hz for both the non- recessed and recessed injector element. Self-pulsation is more pronounced with recess. Near onset, imagery captures periodic, non-pulsatile stripping of liquid from surface waves that are consistent with characteristics of Kelvin- Helmholtz-type instability. The frequency of these spray patterns is found to correspond within approximately 15% of resonant spray patterns that occur at self-pulsation onset, and indicate that liquid stripping behavior is key to exciting the self-pulsation phenomenon. An analysis of injector eigenmodes identifies fluid oscillator frequencies which could define that of self-pulsation once excited. Longitudinal eigenmodes of the injector are found to correspond in frequency to those of self-pulsation at some conditions. However, these resonant frequencies do not explain the full range of measured self- pulsations. An analytical response analysis is carried out for the internal hydrodynamics of the swirl injector study element and flow conditions investigated. For the majority of conditions, the calculated frequencies of surface wave response within the injector's vortex chamber are found to correlate well with measured frequencies of self- pulsation to suggest that internal hydrodynamics participate as an additional fluid oscillator.
Eberhart, Chad Jonathan, "Investigation of liquid rocket swirl coaxial injection dynamics under self-excited high frequency oscillations" (2016). Dissertations. 107.