Comparison of Air-Breathing Engines with Slow and Detonation Combustion

Abstract

The ramjets of different schemes with slow and detonation combustion are compared. Steady and unsteady processes in these engines are described by simple models of gasdynamics and thermodynamics, detonation waves, air deceleration in air intakes, and combustion product acceleration in supersonic sections of nozzles. Within the framework of these models, at a fixed adiabatic exponent the characteristics of any engine depend on two parameters, namely, the flight Mach number and the dimensionless combustion heat of the combustible mixture. The comparison performed for all actual values of these parameters, together with an analysis of thermodynamic cycles and one-dimensional time-dependent calculations (for the engines with combustion in traveling detonation waves), confirmed the importance of taking the unsteady processes in combustors into account. The comparison made in this study is actual, due to frequent claims about a possible considerable increase in the thrust characteristics on replacement of ramjets with slow combustion under a constant pressure by engines with combustion in pulsed or rotating detonation waves (pulse-detonation engines (PDE) or rotating detonation engines (RDE)). Usually, these assertions are made on the basis of the comparison of the thermal efficiencies and specific thrusts and impulses calculated according to these values. In the case of unsteady flow in the combustor, the recalculation of the thrusts and impulses according to the thermal efficiency overestimates their values. The validity of this statement for multichambered PDEs is confirmed by time-dependent calculations. In the case of instantaneous opening and closing of the entrance into the detonation chambers and an instantaneous, without energy expenditures, detonation wave initiation, the PDE thrust is less than the ramjet thrust, starting from small supersonic flight Mach numbers. Analogous calculations for the RDEs are unjustified due to the passage into a rotating noninertial coordinate system.