Effect of annulus width on combustion characteristics of liquid oxygen/methane swirl coaxial injectors

A parametric investigation was conducted based on an experimental approach to evaluate the effect of annulus width on spray and flame dynamics in liquid oxygen/methane swirl coaxial injectors. By employing dual-camera laser background light imaging and visualization of a rectangular combustor, a comprehensive range of combustion characteristics were captured, including spray morphology, flame structure, and combustion stability. The results demonstrate that the combustion process of the liquid oxygen/methane engine can be classified into two distinct categories: quasi-steady (QS) combustion and quasi-steady converting to unstable (QSCU) combustion, as the annulus width increases. The spray and flame of QS combustion exhibits a conical shape, whose spray length, spray cone angle, and flame spreading angle are rise with increasing the annulus width. During QSCU combustion, a portion of the liquid oxygen inside the injector vaporizes, leading to oscillations in both the spray and the flame. These oscillations, in turn, induce combustion instabilities within the frequency range of 43.3 to 48.3 Hz. The flame morphology undergoes a series of transformations, beginning with a stable cone, then evolving into a “double cone”, and ultimately manifesting as a “tadpole”. Subsequently, the flame exhibits periodic flashback and filling motion. During shutdown, the spray cone angle and flame spreading angle gradually increase. This is followed by the emergence of combustion oscillation, with an oscillation frequency distributes between 337.4 and 356.0 Hz. This oscillation is independent of the supply system and is primarily attributed to flame oscillates within the outer recirculation zone.