Effects of Injector Geometry on Cryogenic Shear Coaxial Jets at Supercritical Pressures

Abstract

For shear coaxial injectors, which are widely used in liquid oxygen-gaseous hydrogen rocket engines, the key parameters that characterize injector geometry are the recess length, taper angle, and wall thickness of the liquid oxygen post, which is the inner tube of the coaxial injector. In the present study, the effects of the recessed length and taper angle of the liquid oxygen post on cryogenic coaxial jets at supercritical pressures were experimentally investigated. The large-scale structure and mixing enhancement of cryogenic nitrogen and gaseous helium coaxial jets were evaluated using a high-speed backlighting technique. Instantaneous backlit images revealed that the dark core, which is believed to be dense nitrogen, exhibited sinusoidal forms induced by the presence of the outer helium jets. However, the sinusoidal oscillations were less influenced by the injector geometry. The mixing enhancement of coaxial jets was evaluated with respect to the dark-core length and the jet spread angle. The recessed liquid oxygen post enhanced the mixing of cryogenic nitrogen when the recess length was sufficiently large, which suggests that a recess of a certain length is required to improve the mixing of cryogenic coaxial jets at supercritical pressures. Because the decrease in the dark-core length with the recessed length was consistent with the increase in the jet spread angle at the injector exit, the mixing enhancement caused by the recess can be regarded as a result of an improvement in the mixing between the inner and outer jets in the recessed region. The effects of the liquid oxygen post exit taper on the mixing enhancement were more significant than those of the recess. The decrease in the dark-core length with the inner-toouter momentum flux ratio (J number) is consistent with the relevant empirical correlation at supercritical pressures. Therefore, the mixing enhancement caused by the liquid oxygen post exit taper can be considered the result of an increase in the J number, rather than the result of giving azimuthal velocity to the cryogenic nitrogen jet at the liquid oxygen post exit. NomenclatureDe = outer diameter of the liquid oxygen post Di = inner diameter of the liquid oxygen post Do = inner diameter of the outer tube h = wall thickness of the liquid oxygen post L recess = recess length of the liquid oxygen post L taper = tapered length of the liquid oxygen post V = velocity θ taper = exit-taper angle of the liquid oxygen post ρ = density

Experimental study on spray characteristics of RP-3 kerosene under the near critical condition with different injection conditions

“…Figure 6 shows the relationship between the self-excited oscillation frequency of the jet and the injection conditions. It can be found that the self-excited oscillation frequency of the jet increases significantly as the distance of the nozzle from the cyclone increases, which is consistent with the phenomenon found in coaxial nozzles [10]. As the injection pressure increases, the momentum difference between the rotating gas and kerosene decreases, and the oil-gas interaction weakens, making the jet more stable.…”

Experimental Investigation of Self-Excited Combustion Instabilities in a LOX/LNG Rocket Combustor

“…In general, J is believed to represent the forces that govern the primary breakup of the LOX jet in shear coaxial injectors [23][24][25]. Higher values of J indicate enhanced the mixing processes between oxidizer and fuel [26].…”

Experimental Investigation of Flame Anchoring Behavior in a LOX/LNG Rocket Combustor

“…The momentum flux ratio (J-number) plays an important role in models describing the forces which govern the primary breakup of the LOX jet in shear coaxial injectors [18][19][20]. Higher values of J enhance the mixing processes between oxidizer and fuel [21]. Thus, the influence of the momentum flux ratio, calculated according to Equation ( 5), on the flame anchoring mechanism will be investigated.…”