The effective operation of high-speed air-breathing engines (ram and scram-jets) over a wide range of flow parameters (velocity and altitude) is one of the most technically difficult challenges in design of hypersonic vehicles. The promising approach is a scram-jet having flexible gas dynamic configurations that meets a number of extremely challenging technical issues: materials, gears, etc. Commonly discussed trade-off assessment consists of a fixed geometrical configuration based on some ‘characteristic’ Mach number of operation and a control system augmenting a poorer performance for lower and higher values of Mach number, especially for ram/scram transition mode.
To improve the overall capability of a scram-jet with fixed duct geometry at variable conditions, some methods could be applied such as a staged fuel injection, additional mechanical flame holding, or electrical discharge as an ignitor and flame holder. The main advantage of plasma application for control of fuel ignition/combustion is due to non-equilibrium, non-uniform and transient nature of electrical discharges, which deliver a synergy with thermal effects (heating) owing to essentially nonlinear behavior of induction time versus gas temperature. Those properties may also be of critical importance under non-premixed flow conditions, enhancing air–fuel mixing in compressible flows.
Three main ideas underlie the concept of plasma-assisted combustion:
- The gas heating/non-equilibrium excitation by the discharge.
- Fuel–air mixing enhancement owing to gas dynamic instability generation.
- Control of flow structure in the vicinity of the reaction zone.