Single and twin liquid jet injection in crossflow: Influence of Mach number and jet spacing

Abstract

Supersonic air-breathing engine development towards hypersonic vehicles is challenging due to flow complexities arising from compressibility effects. In these engines, the incoming crossflow air enters the combustor at supersonic speeds, which significantly limits the time scales for the fuel-air mixing. The transverse injection of liquid fuel jets into the crossflow is one of the fuel injection techniques under these conditions. The liquid jet atomization and spray formation are strongly governed by various flow processes seen in the interaction between the jet and the supersonic crossflow, namely the jet penetration in the transverse direction, the dynamics of the bow shock formed upstream of the jet, the interaction between the bow shock with the crossflow wall boundary layer, and flow unsteadiness. In this thesis, we experimentally investigate the flow and atomization characteristics of transversely injected twin liquid jets in a supersonic crossflow of air at a free stream Mach number, M∞=2.5. The thesis work is presented in four parts, with the focus being a comprehensive understanding of the twin liquid jet spray interaction with the supersonic crossflow. The first part of the thesis investigates a single liquid jet interaction with crossflow over a wide range of Mach (M∞)/Weber (We∞) numbers. The influence of M∞/We∞ on the flow dynamics, liquid jet breakup morphologies, and subsequent spray formation downstream of the jet is studied as M∞ is varied from 0.2 to 2.5 with the corresponding We∞ ranging from 61 to 2066. The second part of the thesis investigates the breakup and atomization of an elevated liquid jet into a supersonic crossflow at M∞=2.5. In this case, the jet is elevated beyond the crossflow wall boundary layer to minimize the boundary layer’s influence on the jet. The twin liquid jet injection into a supersonic crossflow of M∞=2.5 is investigated in the third and fourth parts of the thesis. This study addresses the practical significance of distributing a single jet (SJ) injection mass flow rate into two identical twin liquid jets to enhance the spray characteristics, while maintaining the injection area to be the same between the SJ and twin-jet cases. The jet-to-jet separation distance in the streamwise (sx) and spanwise (sz) directions is varied. The streamwise and spanwise twin-jet spray characteristics are measured, and the influence of sx and sz on the overall spray performance is discussed. The first part of the thesis investigates a single liquid jet interaction with the crossflow over a wide range of M∞, from 0.2 to 2.5, at a constant momentum flux ratio of J = 9.7. With the increase in M∞, the jet breakup morphology transitions from bag breakup at low M∞=0.2 to catastrophic breakup at high M∞=2.5. The observed characteristics of bag breakup at M∞=0.2 are similar to that seen with the bag breakup of a single drop interaction with crossflow reported in the literature. As M∞ increases from 0.2 to 0.5, the bag breakup morphology is modified into ligament breakup of the jet with the formation of long thin ligaments with smooth windward surface along the jet. Further increase of M∞ from 0.5 to 0.7 resulted in the transition of ligament breakup into a shear-induced breakup with small-scale wave structures on the windward edge of the jet. A catastrophic breakup is observed for the jet interaction with the supersonic crossflow at M∞=2.5 with regular detachment of the liquid clumps from the jet windward surface. The instability wavelength on the jet windward surface, driven by Rayleigh-Taylor instability, decreases with increase in M∞, indicating an enhanced jet breakup at higher M∞. The primary breakup of the jet, influenced by M∞, leaves its imprint on the characteristics of the droplets formed downstream. At low M∞, the bag forms and ruptures close to the tunnel wall, and it moves along the transverse direction as further fragmentation occurs. The droplet size measurements along the jet transverse direction at x/D = 30 show the presence of fine droplets (10 μm-100 μm) close to the wall location, moderate size droplets(80 μm-240 μm) at the spray core, and larger diameter droplets (200 μm-400 μm) at the spray edge resulting from different bag breakup modes, namely bag film breakup, rim breakup, and node breakup, respectively. The variation of Sauter mean diameter (SMD), measured at downstream locations, shows a decreasing trend with increasing M∞ due to the enhanced breakup. For instance, at a constant J, the SMD of the spray decreases from 120 μm to 20 μm as M∞ increases from 0.2 to 2.5. Further, at a constant J, the measured jet penetration height decreases with M∞, and we find that the inclusion of wall boundary layer thickness leads to consistent jet penetration height scaling across different M∞. The unsteady spray plume behaviour resulting from the wall boundary layer fluctuations is discussed with the help of measurements obtained using Particle Image Velocimetry (PIV). In the second part of the thesis, the results on the characteristics of an elevated transversely injected liquid jet into the supersonic crossflow are presented. The discharging orifice is positioned at transverse height, Hp ~ 3δ, with δ being the crossflow wall boundary layer thickness. This method to minimize the crossflow boundary layer interaction by elevating the jet leads to the formation of an additional bow shock corresponding to the elevation object blockage to the oncoming supersonic crossflow. Though the primary direct interaction of the jet with the boundary layer is minimized by elevating the jet, the secondary effect of the additional bow shock leads to lower fluctuations of the elevated jet in response to the boundary layer fluctuations. For a given $J$, the elevated jet exhibits 30% reduced jet penetration height and 45% lower unsteadiness compared to the flush jets, and the measured SMD is decreased by 31%, from 18 μm for the flush jets to 12.5 μm for the elevated jet, demonstrating improved atomization. The third part of the thesis deals with the twin liquid jet injection into supersonic crossflow. In this case, the studies are performed for a wide range of streamwise spacing between the jets (sx/D = 5 to 18), and it is observed that the streamwise spacing influences the twin jet spray characteristics significantly. The streamwise twin liquid jet’s penetration height is found to be higher compared to that of the equivalent SJ due to the jet shielding by the upstream jet (UJ) on the downstream jet (DJ) from the incoming crossflow. The UJ’s shielding ability varies with sx /D as the UJ progresses into the atomization process before it encounters the DJ. The shadowgraph visualizations of the twin-jet near field interactions revealed three jet interaction regimes as sx /D is varied from 5 to 18: (i) shielding with jet impingement (sx/D < 7), (ii) shielding without jet impingement (7< sx/D≤10), and (iii) no shielding and no jet impingement (sx /D> 10). At sx/D = 10, the spray plume exhibits highest penetration with least unsteadiness (5.5% of the mean penetration height), lower shock-induced pressure loss with losses reducing to 17.4% compared to 31.3% recorded for the equivalent SJ, and smallest mean droplet size (7 μm-30 μm). Thus, sx/D = 10 is identified as an optimal streamwise distance between the liquid jets, with this corresponding to the shielding without jet impingement regime. Overall, the streamwise twinjet injection strategy shows better spray characteristics with minimum crossflow stagnation pressure losses compared to the equivalent SJ spray characteristics, which indicates significant benefit of distributing the SJ injection mass flow rate into twin jets. The final part of the thesis deals with spanwise twinjet interaction with a supersonic crossflow. Smaller jet spacing (sz/D= 5) results in strong interactions between jets and bow shocks, leading to corrugated shock structures. Intermediate spacing (sz/D = 10) develops scalloped bow shock structures due to the merging of bow shocks. Larger jet spacing (sz/D= 15) reduces these interactions, with individual bow shocks forming for each jet as if each jet is injected as an individual SJ. The measurements of spray plume characteristics are performed at z = 0 plane, which is the central plane between the spanwise spaced jets (z= 0). The results show that the penetration height trends vary with J, with sz/D = 10 achieving the highest jet penetration at higher J, and the spray unsteadiness also being lowest for sz/D = 10. The droplet characteristics are estimated along the wall normal direction (y direction) at the z = 0 plane and x = 85D, which shows a decrease in the droplet diameter range with an increase in sz/D. Apart from the z = 0 plane, droplet measurements are also performed along the spanwise (z) direction. Drop size measurements along z showed that the smallest SMD is measured at sz/D = 10, with diameters ranging from 7 μm to 30 μm, as the interactions between the jets are strong with formation of beneficial scalloped shock structures. The droplet characteristics at z = 0 plane are very different compared to that at other z locations along the spanwise direction. The thesis demonstrates the significant influence of crossflow Mach number, wall boundary layer interactions, and twin jet spacing on the liquid jet atomization and downstream spray plume characteristics. Quantitative measurements of jet penetration height, interfacial wavelength, jet breakup regimes, spray droplet size, and their unsteadiness are documented. The reported findings provide insights into optimizing liquid jet injection strategies for supersonic crossflow applications, with implications for propulsion, combustion, and other high-speed flow systems.