Closed-Loop Control of a Dual-Mode Scramjet with Fuel Injection Distribution

This study demonstrates closed-loop control of the shock train leading-edge location in a dual-mode scramjet combustor using fuel-injection distribution as the control input. Experiments were performed in a direct-connect, cavity-stabilized combustor with distributed ethylene injection.

In the first part, wall-pressure measurements, high-speed CH* chemiluminescence, and high-speed shadowgraph imaging were used to examine the effects of fuel distribution on flame stabilization, thermal choking, and shock train dynamics. At sufficiently high equivalence ratios, 37% downstream injection produced a stable jet-wake flame, whereas upstream-only injection at similar conditions produced an oscillating flame between the cavity shear layer and fuel jet wake. A thermal choking regime map showed that sufficiently increasing the downstream injection fraction can shift the combustor from thermally choked to unchoked. Shadowgraph images showed shock structures in the combustion region in both thermally choked and unchoked cases. Shock train locations for thermally choked cases, obtained using a pressure-standard-deviation criterion, shifted upstream with increasing equivalence ratio or decreasing downstream injection fraction.

In the second part, wall-pressure measurements were used to estimate the shock train location, and a proportional-integral controller adjusted the downstream injection fraction in real time. Quasi-steady experiments showed that the shock train response to fuel redistribution is nonlinear and depends on the operating condition, with reduced control authority near the thermal choking boundary. Open-loop step response experiments were used to identify low-order dynamic models for controller design. Closed-loop tests demonstrated commanded shock train motion and suppression of upstream shock train displacement caused by an increase in equivalence ratio. These results show the potential of distributed fuel injection as an active control method for shock train positioning and unstart prevention in dual-mode scramjet engines.

This work is towards an M.Sc. degree under the supervision of Assoc. Prof. Dan Michaels, The Stephen B. Klein Faculty of Aerospace Engineering, Technion.