Experimental Investigation of Inlet-Combustor Isolators for a Dual-Mode Scramjet at a Mach Number of 4

Experimental studies were conducted in the cold-flow Mach 4 Blowdown Facility (M4BDF) at the Langley Research Center to parametrically investigate inlet-isolator performance in an airframe-integrated ramjet/scramjet engine. The inlet-isolator test data presented herein result from both variations in geometry (isolator length and rearward-facing step height) and flow-field properties (boundary-layer thickness and oblique-glancing sidewall shock interaction). These data from the coupling of the inlet and isolator provide a portion of the parametric database required in a cycle deck to predict inlet-isolator performance over the ramjet envelope for the design of a hypersonic vehicle. In order to generate such a database, a generic, two-dimensional, planar inlet-isolator-diffuser model was designed and fabricated to replicate the lines typical of a dual-mode scramjet integrated with a hypersonic vehicle (i.e., a design typical of a flight engine). A large and flight-realistic parametric variation of test data was obtained by providing several interchangeable, rotating cowls of different lengths and also planar isolator sections of different lengths. The combination of inlet cowls and isolator sections resulted in a total of 250 geometric configurations. The length of the isolator varied from 2.7 to 16.7 inlet throat heights by combining sections of different lengths. Rearward-facing steps were also introduced in the isolator to simulate fuel injector locations that are typically used when the isolator section serves as a combustor for supersonic combustion ramjet (scramjet) operation. Each inlet-isolator geometry was also tested with and without a horizontal forebody plate to alter the thickness of the turbulent boundary layer approaching the inlet. The simulation of combustion pressure rise (to study inlet-combustor isolation) during the ramjet operational mode was accomplished by back pressuring the model flow path by using a variable-area throttling mechanism. This mechanism, when attached to the aft end of the isolator-diffuser model, was designed to throttle the flow gradually via a movable flap pivoting about a hinge near the throttling device exit. For each geometry tested, back pressuring was increased gradually by closing the throttling mechanism until the inlet was forced to unstart. Model instrumentation included 110 wall static pressure orifices mounted flush on the inlet ramp, sidewalls, cowl, isolator, and throttling mechanism sections. Each data cycle, which represents the pressure distribution throughout the model at a given time, was recorded via an electronic-sensing pressure system that sampled data at 1 Hz. The results reveal that the performance of each isola-tor is dependent not only on inlet geometry and forebody …