Liquid jet injection into a supersonic crossflow: Influence of jet orifice shape on penetration and atomization

Abstract

Liquid jet injection into supersonic crossflow is one of the conceptually simple ways of fuel atomization in SCRAMJET engines. The success of these engines demands the efficient mixing of incoming supersonic air and the fuel injected since the residence time of air inside the supersonic combustor is extremely limited. In the present study, we experimentally investigate the mean and unsteady characteristics of jet penetration, bow shock position, plume area, and spray droplet size and velocities when a liquid jet is injected from a circular and an elliptical orifice into a supersonic crossflow. All the experiments are conducted using an open circuit supersonic blow-down wind tunnel with a freestream Mach number (M∞) of 2.5. The thesis mainly comprises of two parts. The first part of the study focuses on the investigations using a liquid jet from a circular orifice of diameter of 1 mm. The experiments are carried over different jet-to-crossflow momentum flux ratios, J (6.7 to 20.4). The pulsed laser shadowgraphy technique is employed to understand the jet penetration height and its spray morphology. The near-field unsteady interactions of the injected jet and the bow shock formed are investigated using high-speed shadowgraphy and unsteady pressure measurements in the injected liquid line. The results show that there is a substantial unsteadiness in the jet penetration height and shock motion with a strong correlation between them. Unsteady pressure measurements reveal that there exist significant self-generated mass flow fluctuations that occur in the injection line when the liquid jet is injected in the presence of supersonic crossflow. The frequency of these oscillations is seen to be close to a Strouhal number, St = fδ /U∞ ≈ 0.007, close to the low frequencies that are characteristic of Shock-Wave Boundary Layer Interactions (δ = boundary layer thickness). Later, Particle/droplet image Analysis (PDIA) along with Particle Tracking Velocimetry (PTV) is extensively used to understand the impact of these near-field unsteady interactions on the downstream spray characteristics like droplet count, droplet size, and its velocity. The results show that there is a substantial variation in the number of droplets, Sauter Mean Diameter (SMD), and droplet velocity with time. Sprays from elliptical orifices are known to exhibit better air entrainment and mixing characteristics from ambient studies. Hence, the second part of the thesis is dedicated to the investigations on the effect of the elliptical orifice orientation (area of jet interaction) with the incoming turbulent boundary layer and the crossflow. An elliptical orifice with an equivalent diameter of 1.2 mm is chosen for the study. The orientation of the elliptical orifice major axis parallel and perpendicular to the crossflow leads to the lower and higher frontal areas of the jet interaction, respectively. This is represented using the parameter aspect ratio (AR), defined as the ratio of the spanwise dimension to the streamwise dimension of the elliptical orifice. Near-field visualizations show that higher AR jets are prone to quicker bending and early breakup. Also, the penetration heights and scaling laws, shock strength, and unsteadiness are significantly altered by the AR of the injected jet. Planar Laser Mie Scattering (PLMS) is used in the cross-sectional view to record the jet cross-sectional details and entrainment characteristics for different ARs. Instantaneous images reveal that there are large variations in the spray plume area/entrainment between the instants viz. jet penetration height and shock motion. An important metric called spray plume area and its variation with the streamwise position, J, and AR are quantified. In addition to penetration height and spray plume area, a detailed analysis of downstream spray characteristics for different ARs is done. The results show that AR = 3.3 produces smaller droplets with a better gain in velocity.