Liquid jet injection in supersonic crossflow: Self-induced jet oscillations and its effects

Abstract

Liquid jet injection into supersonic crossflow is one of the conceptually simple ways of fuel atomization in Scramjet engines. In the present work, we experimentally investigate the flow and spray formed due to the injection of a liquid jet in supersonic crossflow with free stream Mach number of M∞ = 2.5. The main focus of the work is on the unsteady aspects of the flow and spray, which are found to be self-induced and occur naturally in the flow. High speed shadowgraphy shows large variations in the windward spray edge and penetration with time. Unsteady pressure measurements in the liquid injection line show that in the presence of a supersonic crossflow, there exist significant self-generated mass flow fluctuations that occur in the injection line. The frequency of these oscillations is seen to be close to a Strouhal number, St = fδ /U∞ ≈ 0.007, close to the typical low frequencies seen in Shock-Wave Boundary Layer Interaction studies (δ = boundary layer thickness). The variations in mass flow rate within the injection line leads to significant fluctuations of the spray penetration and shocks. In particular, the variations in spray penetration with time is observed to lead to significant variations in both the droplet numbers and size at a given location in the generated spray, which can have significant implications for applications where such configurations are used.