Date of Award


Document Type


Degree Name

Master of Science in Engineering (MSE)


Mechanical and Aerospace Engineering

Committee Chair

Robert A. Frederick, Jr

Committee Member

David Lineberry

Committee Member

Gabriel Xu


Liquid propellant rockets., Liquid propellants--Combustion., Injectors.


Liquid-gas swirl coaxial injectors in liquid rocket engines can produce self- excited, and self- sustained oscillations called self- pulsations that result in spray oscillations and high-intensity pressure fluctuations. The objective of this thesis is to determine the onset of self-pulsations, marked by a stability boundary over a range of liquid and gas momentum flux values. Cold-flow injector tests are performed to investigate the self- pulsation phenomenon in a dual- swirl coaxial injector using water and nitrogen to simulate liquid and gas propellants used in liquid rocket engines. Three geometric configurations are tested: non-recessed, and recesses of 1 mm and 2 mm. The recess length is the length that the inner swirl post is recessed from the injector face. The parameter space investigated encompasses propellant momentum flux values from 12.67 to 71.83 kPa for the liquid, and 6.7 to 648 kPa for the gas. Sound measurements and high-speed videography images are used to determine the onset of self-pulsations, which marks the lower stability boundary. Self-pulsations are observed for the dual-swirl coaxial injector element for both, recessed and non-recessed configurations. For the two recessed configurations, as the recess length increases, the self-pulsation zone becomes wider because the onset of selfpulsations is observed at lower gas momentum flux values. For 1 mm recess, the gas to liquid momentum flux ratio at the stability boundary covers a range of 2.6 to 8.0, while for v 2 mm recess, the gas to liquid momentum flux ratio at the stability boundary covers a narrower range of 1 to 3.5. Thus, between the two recessed configurations, the 1 mm recess configuration is preferable as it provides a wider range of stable operating conditions. The lower stability boundary for the non-recessed configuration lies between the boundaries of the recessed configurations, with gas to liquid momentum flux ratios ranging from 2.5 to 4.4 at the boundary. The trend with increasing liquid momentum flux is similar to that recorded for the 2 mm recess configuration, however, unlike the 2 mm recess configuration, the gas momentum flux value for the non-recessed configuration has to be at least twice that of the liquid momentum flux value in order to incite self-pulsations. This allows for a marginally wider stable zone, resulting in more stable operating conditions. The uncertainty of the water and nitrogen momentum flux measurements were calculated to be 7% and 14% respectively. The inner swirl post is modeled as a quarter-wave resonator to determine if it has potential to incite and exacerbate self-pulsations due to resonance. The calculated first quarter-wave resonant frequency, 1420 Hz, lies within the dominant acoustic frequency range of 1000 to 3400 Hz, which is determined by an FFT analysis of the sound emissions across the self-pulsations zone for the recessed configurations. The swirl post, therefore, may act as a potential internal oscillator that could act by itself or through coupling with other internal/external oscillators to excite and sustain self- pulsations.



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