Rotating Detonation Engines

Rotating Detonation Engines (RDEs) and Rotating Detonation Rocket Engines (RDREs) are novel types of propulsion. RDEs fall under Pressure Gain Combustion (PGC) systems, in which stagnation pressure can theoretically be higher leaving the system than entering. In practice, however, positive pressure gain has yet to be realized experimentally.

At BEFAST, we are investigating the operability and thermal stress limits of RDEs to help inform the state of the art and fill critical knowledge gaps.

RDE Background

An RDE is an inherently coupled system:

  1. fresh propellants are constantly fed to the combustor
  2. After ignition, a detonation wave begins to propagate circumferentially around the annular combustion chamber.
  3. The products continue through the combustion chamber before exiting through a nozzle.

Example Schematic of RDE Operation

RDEs are particularly sought after compared to other PGC options due to its continuous operation. Furthermore, RDEs are much more compact, allowing for a smaller engine size for higher energy output.

Operability

Operability, in the context of RDEs, is a broad term summarizing the flow physics and system operation of the combustor. The key operability point boils down to detonation versus deflagration, the latter dominating contemporary combustion applications such as gas turbines and rocket engines. Deflagrations feature an idealized constant pressure process, in which expansion of products dominates length of combustion and the flame front is fixed according to a subsonic flame speed that is balanced with incoming flow. Detonations feature a constant volume process. Most typical constant volume processes feature mechanical actuating components that manually fix volume; however, detonations have reactants pass through a shock, drastically raising static temperatures and pressures, and concentrating heat release at a short distance downstream of the shock, which supports the supersonically propagating shock and acoustically isolates the combustion, allowing it to continue as long as reactants are present upstream. 

Since combustion heat release is more or less instantaneous, this removes the expansion requirement; thus, decreasing combustor size, increasing thermal output (and therefore thermal efficiency of combustion) per unit length, and can result in an average chamber pressure larger than that of the supply pressure: pressure gain. Containing the detonation however is difficult to accomplish, as mentioned previously, due to the supersonic combustion front. RDEs achieve this containment by utilizing an annular combustion chamber, in which the combustion propagates azimuthally while reactants inject axially shortly before the chamber to mix inside the chamber. The topics mentioned bring up the core mechanisms of operability for an RDE: Geometry and injection.

Beginning with geometry, which is the primary mechanism that governs stability and performance, it is typically comprised of injection of propellants upstream and some sort of area increase downstream of injection. This is most typically shown in the backwards facing step (BFS) geometry shown in the image below.

The trade off integral to RDE operability arises from how the injection area and chamber area are related. The more ‘open’ a geometry is (injection area and chamber area are closer in size), the less loss is incurred over the backwards facing step, meaning the pressure recovered from a detonation is more likely to rise above the supply pressure. However, this results in significant backflow into the plenum, leading to combustion instabilities and possible termination of the detonation wave. The more ‘restricted’ geometry  (injection area decreases relative to chamber area), the more loss is incurred but the incoming propellants are less interrupted, resulting in continuous refueling typically seen in idealized geometries.

Turning to injection, it requires the reactants mix in the chamber but are separate before reaching the chamber to prevent flashback, this resulted in the most standard injection scheme for RDEs: jet in crossflow. This scheme allows for induced turbulence due to injection, creating local vortices that slow and mix propellants for the incoming wave. However, this scheme draws difficulty in fully mixing propellants, especially with smaller chambers, resulting in off-design equivalence ratio at combustion. Furthermore, when utilizing a typical BFS geometry, there is a recirculation region of hot products, thus creating a further trade off of increased residence time of the reactants in this region (introducing what is known as parasitic deflagration) and mixing.

This is the basis of our research, determining ways of improving injection while maintaining a geometry that favors improved performance and minimizing instability, then using these strategies to apply to the aforementioned applications of power generation and thrust capability.

Thermal Stress