Mixing Measurements in a Supersonic Expansion-Ramp Combustor

Abstract

This paper reports results on molecular mixing for injection via an expansion-ramp into a supersonic freestream with M 1 = 1.5. This geometry produces a compressible turbulent shear layer between an upper, high-speed "air" stream and a lower, low-speed "fuel" stream, injected through an expansion-ramp at α = 30 • to the high-speed freestream. Mass injection is chosen to force the shear layer to attach to the lower guide wall. This results in part of the flow being directed upstream, forming a recirculation zone. Employing the hypergolic hydrogen-fluorine chemical reaction and pairs of "flip" experiments, molecular mixing is quantified by measuring the resulting temperature rise. Initial experiments established the fast-chemistry limit for this flow in terms of a Damköhler number (Da). For Da ≥ 1.4, molecularly mixed fluid effectively reacts to completion. Parameters varied in these experiments were the measurement station location, the injection velocity of the (lower) "fuel" stream, the stoichiometry for the flip experiments, and the density ratio of the fuel and air streams. As expected, mixing increases with increasing distance from the injection surface. The mixed fluid fraction increases by 12% when changing the fuel-to-air stream density ratio from 1 to 0.2. Comparisons with measurements at subsonic (high-speed) "air" stream velocities show that the trend of decreasing mixing with increasing speed documented in free-shear layer flows is also encountered in these flows. The current geometry produces higher mixing levels than do free shear layers.

Molecular Mixing and Flowfield Measurements in a Recirculating Shear Flow. Part II: Supersonic Flow

“…Data from the subsonic/transonic experiments in a similar geometry are also included [24]. There are two significant differences between the subsonic/transonic and the supersonic experiments: a) the test section height h (h = 8.38 cm in present study vs. 9.78 cm), which tends to increase δ m / h, and b) the distance L to the measurement station (L = 46.7 cm in present study vs. 38.1 cm), which tends to increase δ m /δ t [30]. Overall, the observed trend of decreasing mixing with increasing convective Mach number persists to the supersonic-flow regime [17].…”

“…In such an event, the mixing time scale would be less than estimated, leading to a lower Damköhler number. However, the balloon reactor model gives a conservative estimate for the chemical time scale, as it was formulated for a free shear layer and does not account for the benefits of the recirculation zone that makes hot products and radicals available for entrainment, increasing the overall kinetic rate [30]. Therefore, there is some inherent uncertainty in the calculation of the Damköhler number.…”

“…The presence of the rake support body can potentially introduce a minor systematic error in the temperature measurements, since in the recirculating region, the probes are in the support structure's wake. The stoichiometric adiabatic flame temperature rise, T f , for the low-φ experiment was approximately twice that of the high-φ experiment, to match the flow in the two cases [30].…”

“…Beyond convergence of the normalized temperature-rise profiles, further increases in kinetic rate do not increase overall chemical product formation and mixing-limited reacting flow is established. Figure 6 presents the normalized temperature rise results, T/ T f , as a function of the normalized distance from the test section midpoint, y/h, for M 1 = 2.5, while the equivalent figure for M 1 = 1.5 can be found in [30]. Considering the 2% and 4% reactant concentration cases, there is a significant difference in the location of the maximum flame temperature.…”

“…If Z is the mole mixture fraction of high-speed fluid, a stoichiometric mixture ratio of φ = 4, or Z = 0.8, implies that four parts (moles) of upper-stream fluid must mix per part of lower-stream fluid for complete consumption of all reactants. Details of the flipexperiment technique can be found in the literature [1,22,24,30]. From this analysis, the probability of mixed fluid at a location in the flow is given by [8],…”