Characterization of Unsteady Combustion Phenomena in a University Scale Rocket Combustor

Abstract

We investigate the steady and unsteady flame properties in a moderate pressure, single shear coaxial injector operated with gaseous oxygen/hydrogen and how they relate to the underlying fluid dynamic properties of the flowfield. Under a certain range of conditions, we observe strong modifications of the flame structure as a result of a low-amplitude thermoacoustic instability that develops naturally under fuel-lean conditions. In particular, we observe a periodic evolution of the flame front where ejection events are followed by near-complete extinction and re-ignition. To link the onset of the thermoacoustic instability under fueled (reacting) conditions to the fluid dynamics of the system, we conduct corresponding mixing studies of variable density, non-reacting shear coaxial jets. We particularly investigate the onset and properties of self-excited hydrodynamic instabilities that are observed at high inner-to-outer jet momentum flux ratios. We thus attempt to link them to the unsteady behavior of the corresponding reacting cases. Nomenclature S Density ratio R Velocity ratio Re Reynolds number At Atwood number M Mach number φ Equivalence Ratio J Momentum flux ratio θ Momentum thickness ρ Density v Velocity D Diameter Subscript i Inner stream o Outer stream j Jet exit ∞ Ambient fluid surrounding jets

Variation of Dominant Instability Mode with Fuel Configuration in an Inverted Coaxial Injector

“…Table 1 summarizes the low pressure cases for both configurations up to the stability limit. In one of our previous works concerning non-reacting mixing experiments, 16 we found that the (inner-to-outer) momentum flux ratio J = SR 2 was an important parameter in delineating the steady and unsteady cases. R = v i /v o and S = ρ i /ρ o are the (inner-to-outer) velocity and density ratios respectively.…”

Investigation of Instability Mechanisms in a Laboratory Scale GH2/GO2 Combustor

“…In an attempt to link the observed behavior to possible causes (e.g., local fluctuations of fuel/oxidizer, equivalence ratio, etc), we have considered the properties, and in particular the mixing field, of variable-density, turbulent non-reacting coaxial jets. The results of this part of the study are described more in detail in our companion work; 22 here we report only the main result. From mixture measurements of the flowfield generated by coaxial injection of different molecular weight (i.e., different initial density) fluids we identify the formation of what are referred to as "side jets" under certain coaxial injection configurations.…”

“…More details on the experimental configuration use for the non-reacting, variable-density experiments is documented in the companion work. 22 For all cases in the present study, the inner tube has an inner diameter D i of 3.66 mm and an outer diameter of 4.76 mm, and the annulus has a diameter D o of 6.73 mm.…”

“…A more comprehensive description of the non-reacting cases is presented in our companion work. 22 For the reacting cases, the run time of the rocket is limited to 12 seconds to avoid overheating beyond its maximum tolerable temperature. Non-reacting cases are not limited by rocket heating and data is collected over a period of 30 seconds.…”