Comparison of a Direct-Connect and Freejet Dual-Mode Scramjet

Abstract

Testing of identical dual-mode scramjet flowpath geometries in the freejet and direct-connect configurations was conducted in two separate facilities. These tests enabled a comparison study between the two configurations, which was directed toward the determination of the effects of inlet distortion and backpressure on the performance and operability of a dual-mode scramjet. Bulk flow conditions were matched between the two facilities at the isolator entrance plane to simulate Mach 4.8 flight, and a series of metrics were established to quantify similarities and differences. The effects of flowpath backpressure in the direct-connect case were seen to be isolated to regions close to the exhaust. An approximate 10% decrease in combustor pressure rise and consequently integrated pressure force were observed in the freejet configuration; shock train length, however, remained the same. Mode transition was delayed from an equivalence ratio of ∼0.5 in the direct-connect case to ∼0.7 in the freejet configuration. Further, ignition difficulties were experienced in the freejet tests which were not encountered in those of direct-connect tests, limiting the scope of comparable test points. This work represents the first attempt at a quantitative comparison in the literature of an identical direct-connect and freejet dual-mode scramjet. Nomenclature A = cross-sectional area D = isolator duct height F = integrated pressure force H = fuel injector ramp normal height L = shock train length M 0 = freestream Mach number M u = Mach number at shock train leading edge M c = Mach number at combustor entrance _ m TBIVmc = predicted freejet flowpath mass capture _ m TBIVnom = nominal freejet facility mass flow rate _ m test = as tested facility mass flow rate (direct connect or freejet) P s = static pressure P 0 = total pressure p d ∕p u = peak static pressure/static pressure at leading edge of shock train Re u = unit Reynolds number, m −1 Re θ = Reynolds number based on boundary-layer momentum thickness T s = static temperature T 0 = total temperature x = axial distance θ = boundary-layer momentum thickness at the leading edge of shock train ϕ = angle of flowpath divergence from the horizontal φ = fuel equivalence ratio