Mixing Lengths of Reacting and Nonreacting Coaxial Injectors in a Laboratory Rocket Combustor

Abstract

Stoichiometric mixing lengths are obtained for coaxial jets with and without combustion in a rocket fuel injector configuration. With a center jet of oxidizer (oxygen or air) and a surrounding annular jet of hydrogen these flames are relatively short resulting in the mixing primarily occurring in the near field. This produces a different scaling than the far field analysis of a turbulent jet flame, where a fuel jet is injected into a coflow of oxidizer. Stoichiometric mixing lengths (LS), defined as the distance along the centerline where the stoichiometric condition occurs, were measured using Planar Laser Induced Fluorescence (PLIF). Acetone seeded into the center jet along with quantitative acetone PLIF allowed the direct measurement of the average and instantaneous mixture fraction fields for a range of velocity and density ratios. For hydrogen-oxygen and hydrogen-air coaxial jet flames, LS was measured from the OH radical field obtained using OH PLIF. Due to the inverse natural of these flames and since all cases were run fuel rich, OH forms thin layers near the stoichiometric contour. Using strained laminar flame calculations from Chemkin and correcting for absorption and quenching effects, the stoichiometric value of the OH signal was related to the peak signal. In nonreacting cases the use of a nondimensional momentum ratio collapses the nonreacting coaxial jet data. To account for the effect of heat release in reacting cases the equivalence principle of Tacina and Dahm is utilized to produce an equivalent outer gas density to create a new effective momentum ratio. This method is found to slightly under predict the effect of heat release for both hydrogen-oxygen and hydrogen-air turbulent coaxial jet flames.

Numerical Simulations of the Flame of a Single Coaxial Injector

“…The mixing of gases injected by a shearcoaxial injector was examined by Schumaker [6] in a combustion chamber at the University of Michigan. The attention was turned to the length of the inner core in dependency of velocity and density ratios.…”

“…The experimental data used here were obtained with the use of planar laser induced fluorescence (PLIF). The experiments in [6,7] show more information about processes in coaxial jets than the "Mascotte" test case [5]; however, they were carried out at injection temperature of 300 K which is limiting the validity range of the tested CFD model.…”

“…In the current work we have used experimental data by Schumaker [6] and by Vaidyanathan [7] for the validation of our numerical model. The experimental data used here were obtained with the use of planar laser induced fluorescence (PLIF).…”

Discontinuous Galerkin Computation of Gaseous Mixture Coaxial Jets

“…In addition to this, the impact of coaxial jets fluid dynamics is crucial not only on the performances of rockets and combustion devices, but also in non-reactive components such as injectors and exhausts of turbofan engines. Studies on the behaviour of coaxial jets date back to the seventies with the pioneering experimental investigations presented in [7,8,9]; recent works [10,11] confirm the existence of different flow configurations depending on the velocity and density ratios between inner and outer jets, as well as on geometrical details of the apparatus. The two main distinctive features are the "wake-like" and "jet-like" turbulent shear structures that, roughly speaking, develop respectively when the jets velocity ratio is near unity or far away from this value.…”

Geometric analysis of turbulent macrostructure in jets laid on flat surfaces for turbulence intensity calculation

“…Based on different visual researches [11][12][13][14][15] and many other, a simplified approach is offered for jet flows and other flows with large-scale vorticity, which simplifies such flows as a group of adjoining round largescale vortices (puffs). It can give averaged parameters of the flows based on geometrical and kinematic analysis only avoiding integration in the most of cases.…”