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Combustion Powered Actuation (COMPACT)

Chemically-based actuation presents an attractive alternative to conventional actuation techniques for the generation of high-impulse actuation jets for high-speed flow control.  COMPACT (Combustion Powered Actuation) exploits the chemical energy of gaseous fuel/oxidizer mixture to create a high pressure burst and subsequent high momentum jet of exhaust products.  The basic element of the system may be regarded as, essentially, a fluidic amplifier where fuel and oxidizer having comparatively low momentum fill a small (O~1 cm3) combustion chamber bounded by an orifice plate (Fig. 1).  A spark (or other ignition source) ignites the mixture, creating a high pressure burst within the combustor and a subsequent jet emanating from one or more exhaust orifices.  At the scales envisioned, the entire combustion process is complete over a period of milliseconds and the pressure within the chamber drops again to a baseline level below the supply pressure, at which time flow of fresh reactants into the chamber resumes.  The flow of fuel and oxidizer into the chamber is typically regulated by passive fluidic elements which exploit the pressure rise within the chamber to shut off the inlet flow, obviating the need for mechanical valving at the actuator.  The COMPACT actuator may ultimately be considered a specialized variation on pulsed blowing schemes, where the pulsation is created chemically rather than mechanically, thus providing a no-moving-parts approach to pulsing the jet rather than relying on infrastructure intensive mechanical valves to open and close a high pressure line at each actuation point.  This also minimizes the need for high pressure plumbing to all actuators (as the jet momentum is provided by the chemical reaction rather than a high pressure source) and can provide a faster pressure rise time than many valves.

A representative actuator burst is shown in Fig. 2 which includes the pressure-time history within a 1 cm3 combustor and a sequence of corresponding phase-locked Schlieren flow images of the ejected jet at the exhaust orifice. The images are recorded at t = 0.44, 0.70, 1.2, 2, 3, and 4.8 ms following the spark trigger (using a 125 ms shutter speed) and the streamwise field of view is approximately 25 orifice diameters (d = 1.3 mm). Following the spark ignition (t = 0), there is a sharp rise in the chamber pressure with a peak normalized pressure (Pr, defined as the ratio of the chamber pressure to atmospheric pressure) of approximately 2.8 at t = 0.7 ms. A jet emanates from the exhaust orifice as soon as the pressure in the chamber begins to rise, with flow in the far field appearing to be turbulent as is evidenced by the presence of small-scale motions. The strength of the jet increases with the chamber pressure and, near the peak pressure level, shock cells are detected in the flow within 5 orifice diameters (6 mm) of the exhaust (Pr ≥ 1.89 required to generate sonic orifice velocities). The pressure subsequently decays and, at t = 2.7 ms, reduces to atmospheric levels, at which point a jet no longer emanates from the exhaust orifice although its earlier flow is visible in the far field. After a delay of 1.7 milliseconds, a small vortex ring appears at the orifice which is followed by a low-velocity steady jet, indicating the resumption of flow of fuel and oxidizer into the chamber and the displacement of remnant exhaust gases.

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