Dual-Mode Scramjet Flameholding Operability Measurements

TLDR

Measurements were conducted in a direct‑connect combustor that simulates cavity flameholding in a hydrocarbon‑fuelled dual‑mode scramjet, where upstream shock trains significantly affect inlet flow and flameholding limits. A mechanical throttle at the downstream end supplied back‑pressurization to generate a shock train and decouple flameholder operation from heat‑release effects, while flameholding limits were determined by ramping inlet air temperature down until blowout, using a vitiated‑air heater, Mach 2.2 and 3.3 inlet nozzles, a 0.65‑in‑deep cavity, and ethylene or heated JP‑7 fuel. The baseline mean blowout temperature was 1502 °R, most sensitive to fuel injection location and flow rates, relatively insensitive to inlet Mach number and operating pressure, and unsteady flame structures were observed moving laterally and upstream of the flameholder.

Abstract

Measurements were made in a direct-connect combustor facility designed to simulate cavity flameholding in a hydrocarbon-fueled dual-mode scramjet combustor where the presence of a shock train upstream of the flameholder has a significant impact on the inlet flow to the combustor and on flameholding limits. A mechanical throttle was installed in the downstream end of the test rig to provide the backpressurization needed to form the shock train and to decouple the operation of the flameholder from the backpressure formed by heat release and thermal choking, as it would be in a flight engine. The flameholding limits were measured by ramping inlet air temperature down until blowout was observed. The test facility used a vitiated air heater, Mach 2.2 and 3.3 inlet nozzles, a 0.65–in.-deep cavity, and ethylene and heated JP-7 fuel. A Mean blowout temperature of 1502°R was measured at the baseline condition which used a Mach 2.2 inlet, a cavity pressure of 21 psia, and ethylene fuel. The blowout temperature was found to be most sensitive to fuel injection location and fuel flow rates, and relatively insensitive to inlet Mach number and operating pressure. Video imaging showed unsteady flame structures with significant movements laterally and upstream of the flameholder.