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.