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Combustion characteristics such as combustion performance and combustion stability have been studied experimentally using a small liquid rocket thrust chamber with 19 liquid-liquid swirl coaxial injectors. Data were obtained from static pressure, temperature, and dynamic pressure sensors installed in propellant manifolds and the combustion chamber. While changing the recess length of the injector, characteristic velocity and pressure fluctuation data were collected and analyzed. In addition, chamber pressure was varied between 42 and 54 bar, which covers the sub-and supercritical pressures of oxygen. The results show that the longer recess length generally promotes combustion performance and the spray interaction between injectors in the multielement combustor increases the characteristic velocity. When the chamber pressure is above the critical pressure of oxygen, the recess length scarcely affects the pressure fluctuation. However, when the chamber pressure is below the critical pressure, the shift from external mixing to internal mixing of oxidizer and fuel sheets by the variation of recess length significantly degrades combustion stability and induces strong low-frequency instability. Accordingly, the effects of both recess length and operating chamber pressure must be taken into consideration when designing liquid-liquid swirl coaxial injectors.

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Uni-element combustion tests were conducted employing double swirl coaxial injectors fed by liquid oxygen and kerosene. In this study, two major experimental parameters have been considered: the recess length and fuel swirl chamber shape. It was revealed that combustion efficiency depends mainly on the mixing mechanism for the present swirl coaxial injectors, and acoustic intensities become reduced with a decrease of the recess length. Low-frequency pressure oscillations around 200 Hz have been observed for all injectors. Dynamic behavior examined by the measurements of pressure fluctuations considerably differs for external and internal mixing based on the recess length. Internal mixing induces a mixture ratio and a total mass flow rate that vary with strong bias at a single frequency, resulting in highamplitude pressure fluctuations generated by combustion of pulsating coherent flow structures. However, the results for external mixing show that the fuel and oxidizer mixture flow carries more dispersed, multiple-wave characteristics due to the broad mixing region as well as disintegration and merging phenomena of propellant sheets before mixing.