Effects of operating and geometric parameters on the combustion characteristics of gas-centered swirl injectors for GOX/kerosene

A test platform for a gaseous oxygen/kerosene single-element liquid rocket engine was established to investigate the combustion characteristics of the gas-centered swirl coaxial (GCSC) injectors. The characteristic velocity ( C me ∗ ) and combustion efficiency ( η ) were measured under various oxygen-to-fuel ratios (O/F), total mass flow rates, tangential inlet diameters ( d ₜ), recess ratios, and chamber length-to-diameter ( L c /D c ) ratios. A nonlinear time series analysis method based on phase space reconstruction, recurrence analysis, and multifractal analysis was developed to identify unstable combustion regimes observed in the experiments. Results show that as O/F increases, with chamber pressure ranging from 0.45 to 1.04 MPa, η increases up to 96.21%. As d ₜ increases from 0.4 to 0.9 mm, C me ∗ and η first increase and then decrease, peaking at 0.7 mm (moderate geometry characteristic parameter A ), while larger d ₜ increases instability during startup. Increasing the recess ratio from 0 to 1.82 improves C me ∗ and η , and the stability of chamber pressure buildup, but induces a 210 Hz spray self-pulsation that most likely leads to the medium-frequency instability. Nonlinear analysis indicates that this instability exhibits strong periodicity and weak multifractality. As L c / D c increases from 2.94 to 10 with a non-recessed injector, both C me ∗ and η show a trend of first increasing and then decreasing. High-frequency instability with a characteristic frequency of ∼950 Hz is observed in the combustor with an L c / D c of 10 and a recess ratio of 1.82. The transition from stable combustion to intermittence and then continuous oscillation is captured by nonlinear analysis, indicating that the intermittent state exhibits higher multifractality and fluctuating RATIO (determinism/recurrence rate), while the continuous state shows limit-cycle behavior with stronger monofractal characteristics and lower RATIO . High-frequency instability significantly reduces combustion efficiency.