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.

An experimental study was conducted to better understand the mixing properties of coaxial jets as several parameters were systematically varied, including the velocity ratio, density ratio, and the Reynolds number. Diameters of the inner and outer jet were also varied. Coaxial jets are commonly used to mix fluids due to the simplicity of their geometry and the rapid mixing that they provide. A measure of the overall mixing efficiency is the stoichiometric mixing length (Ls), which is the distance along the jet centerline where the two fluids have mixed to some desired concentration, which was selected to be the stoichiometric concentration for H2/O2 and CH4/O2 in this case. For 56 cases, the profiles of mean mixture fraction, rms mixture fraction fluctuations (unmixedness), and Ls were measured using acetone planar laser induced fluorescence diagnostics. Results were compared to three mixing models. The entrainment model of Villermaux and Rehab showed good agreement with the data, indicating that the proper non-dimensional scaling parameter is the momentum flux ratio M. The work extends the existing database of coaxial jet scalar mixing properties because it considers the specific regime of large values of both the velocity ratio and the inverse density ratio, which is the regime in which rocket injectors operate. Also the work focuses on the mixing up to Ls where previous work focused on the mixing up to the end of the inner core. The Reynolds numbers achieved for a number of cases were considerably larger than previous gas mixing studies, which insures that the jet exit boundary conditions are fully turbulent.

An image processing technique is developed to automatically determine both average and instantaneous detonation wave properties within a rotating detonation rocket engine (RDRE) using high-speed imaging. This method entails segmenting the imaged RDRE annulus into 200 azimuthal bins and tracking integrated pixel intensity in each bin. By combining individual pixel intensity temporal histories across the azimuthal bins, this provides what is termed a detonation surface that visualizes the propagation of the individual detonation fronts azimuthally around the annulus. Average detonation modal properties including wave speed Ūwv, operational frequency fdet, and the number of waves m are determined automatically through a two-dimensional Fourier analysis of the detonation surface data. Also, instantaneous wave speeds Uwv for each individual detonation are determined by taking the numerical derivative of each waves’ angular position temporal history from the detonation surface. This provides useful insight into wave-to-wave variability for an operating condition, as well as denoting modal transitions and mode stability. For the flow conditions investigated, the number of waves ranges from 2 to 14, with Ūwv varying between 900 and 1700 m/s, corresponding to 33%–71% of the ideal Chapman-Jouguet detonation speed; these modes exhibit an operational frequency of 20–45 kHz, with an average of 40 kHz. Overall, these measurements advance the understanding of RDRE’s and may lead to performance gains above those achievable from constant pressure engines.

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