The method to increase the thrust of high Mach number Scramjets

TL;DR

This paper analyzes the mechanisms behind engine unstart in high Mach scramjets, proposing a method to increase thrust by controlling shock wave velocities and fuel injection based on detonation theory.

Contribution

It introduces a theoretical approach using C-J detonation theory to enhance thrust in high Mach scramjets by managing shock wave velocities and fuel injection strategies.

Findings

Engine unstart is linked to shock/shock interactions and DDT processes.

Scramjets operate stably when isolator velocity exceeds C-J detonation velocity.

Injecting extra fuel can increase thrust if shock wave velocity remains below isolator velocity.

Abstract

The problem of engine unstart of scramjets has not been resolved. In this paper, the mechanism of engine unstart is discussed from the point of view of shock/shock interaction and deflagration-to-detonation transition. The shock/shock interaction leads to the nonlinear, transient and discontinuous process of the supersonic combustion flow field. This process is similar to the deflagration-to-detonation transition process. If the velocity of pre-combustion shock wave is faster than the velocity in the isolator, it will propagate upstream and cause the engine unstart. The C-J detonation velocity is defined as the stable operation boundary of scramjets, which is the maximum shock wave produced by combustion theoretically. The scramjets will work stable if the velocity in the isolator is faster than the corresponding C-J detonation velocity. The combustion characteristics and propulsive performance of scramjets is theoretically analyzed by using C-J detonation theory. For high Mach number scramjets, the velocity in the isolator is much faster than the C-J detonation velocity. Therefore, extra fuel and oxygen can be injected into the combustor to increase the thrust as long as the shock wave velocity driven by the combustion products is slower than the air velocity in the isolator. The theoretical results agree well with the existing experimental results, which can be used as a baseline for the development of scramjets.