Detonation waves create a rise in pressure during combustion. Consequently, the pulsed detonation engine (PDE) must use a system of valves that close off the combustor while the wave is formed and then open for refueling. Mechanical valves can be used for cycle frequencies of about 100 Hz. The rotating detonation wave engine (RDE) is another concept that instead uses a detonation wave rotating tangentially around an annular combustor with axial fuel injection and exhaust flow. The operating frequency is the annulus circumference divided by the detonation wave speed, which can range from 1–10 kHz: too fast for mechanical valves. One of the main challenges with the RDE concept is creating an injector design that can rapidly refuel the annulus between wave fronts without experiencing a backflow condition as the wave passes over the orifice.

The image to the left is from an RDE test at UTA. The ignition system was designed to initiate the wave to rotate in one direction to start the engine and produce axial exhaust. In this study, a valve with no moving parts was tested to understand how it interacts with the detonation wave pressure and if it could be used to rapidly refuel an RDE combustor.

The valve contains an orifice, plenum cavity, and back wall leading to a fuel line fitting connection. The orifice is mounted flush to the combustor surface as shown in the figure below, and the diameter is variable. The fuel reaches a steady state pressure in the cavity during injection, but it is shut off as the detonation wave passes by. An incident shock followed by a contact surface between the fresh fuel and burnt gas enters the cavity. We believe the geometry and operating pressure can be designed so the contact surface is quickly expelled from the cavity so refueling can begin.

Two valves were mounted on a 3 meter long linear detonation tube. To understand behavior and confirm the theory of operation, parametric studies were conducted with fuel, cavity pressure, and orifice size with pressure transducers measuring the internal cavity dynamics. Timing the experiment was critical since the cavity pressure must be at a steady state before the detonation wave arrives.

The two graphs below show the pressures in the cavity and the main tube as the detonation wave passes by. If the plenum pressure is too low or the orifice is too large, the valve does not react well to the detonation wave. However, the other graph shows a successful test where steady state refueling is reached during the overpressure time of the detonation wave.

Since two pressure transducers were mounted in the cavities, the wave dynamics could be tracked. The x–t diagrams below below show the differences in behavior as the cavity pressure is raised. If it is too low, multiple reflections occur and the contact surface remains in the cavity, likely creating a situation where the burnt gas mixes with the fresh reactants. If the pressure is high enough, only one reflection is tracked and the contact surface is rapidly pushed back out of the valve.

A graphical method was used to define the ‘interruption time’, the time starting with the detonation wave blockage and ending when steady state injection was reached again. Below, these time values were collected and non-dimensionalized along with a pressure ratio to show that the valve behavior is dependent upon only two parameters. Extrapolating on these results, the interruption time of the injectors on an RDE scales with the operating frequency and ratio of the detonation pressure peak to the steady state cavity pressure.