Experimental and Numerical Studies of Diamond-Shaped Injector in a Supersonic Flow

Abstract

IMPROVING fuel–air mixing and flame holding are current research areas for combined-cycle engines for hypersonic propulsion [1–6]. The scramjet mode is particularly difficult because fuel and air residence times within the engine are of the order of milliseconds; during this time, the fuel must penetrate into and mix with the freestream air and then substantially react to completion. Ideally, penetration and then mixing of the fuel should be done with minimal pressure loss, and thus there is an incentive, especially for small-scale engines, to accomplish fuel injection through so-called nonintrusive injection ports, the most basic of which is the circular, flush-wall, normal injector. Furthermore, while penetration and mixing with the crossflow are typically of interest, other considerations may be important too, such as entrainment into a flameholding device. In the present study, the focus is on flush-wall injection through a diamond-shaped orifice [7,8]. Recently, Srinivasan and Bowersox [9,10] investigated with computational fluid dynamics (CFD) the possibility of tailoring the flow structure to enhance mixing and produce a stable vortex for gas-dynamically induced flame holding. The goal was to control the shape of the interaction barrel shock to produce a flow structure that resembled a blunt bodywith a truncated transverse plane at the trailing edge. A diamond-shaped port was found to produce the desired flow structure. Boundary-layer and injector fluid would then be entrained into this recirculation zone, termed the lateral counter-rotating vortex pair (LCVP), because the structure contains a vortex pair spanning the width of the barrel shock. To determine the robustness of the LCVP, parametric studies were performed for freestream Mach numbers from 2 to 5; it was found that by controlling the injector pressure, the LCVP could be created. The residence time within the LCVP was estimated to be an order of magnitude longer than the flow time through the solution domain, indicating that LCVP may be sufficient for flame holding under some circumstances. The objective for the present studywas to visualize the structure of the LCVP and assess reactivity within it. As with the previous study [11], the tools employed include planar laser-induced fluorescence (PLIF) of the hydroxyl (OH) and nitric oxide (NO) molecules and complementary 3-D nonreacting CFD simulations.