Exergy analysis of hypersonic propulsion systems: Performance comparison of two different scramjet configurations at cruise conditions

An exergy analysis of an advanced hypersonic vehicle, a scramjet, is presented and discussed with a twofold scope. First, to perfect the exergy approach to the design and optimization of aerospace propulsion systems: the exergy flow diagram can provide aircraft engineers and system designers with additional insight on the avoidable and unavoidable systemic losses, thus allowing for effective design improvements. Second, to explore limits and merits of two different fuelling solutions for a scramjet-powered aircraft. Two configurations are critically compared: one with a direct H2 injection and one with an on-board kerosene reformer. The present study treats the scramjet-propelled plane as a Large Complex Energy System (“LCES”), and applies system balances (mass, energy, exergy) to calculate the relevant losses. The exergy analysis confirms that the introduction of an on-board reformer is advantageous from the point of view of the thrust efficiency (with a gain of 3 percentage points with respect to the H2-fuelled engine) and, more importantly, from the point of view of a more correct use of the available resources (the fuel in the tanks). Another advantage of the on-board reforming is that the higher value of the volumetric-specific impulse allows for reducing the fuel tank size. All calculations have been performed with CAMEL®, a modular simulator for energy conversion processes conceived and developed in the last decade by the Authors’ group at the Mechanical and Aeronautical Engineering Department of the University of Roma 1 “La Sapienza”. Some additional component models have been studied and implemented, and a specific tool dedicated to the analysis of propulsion systems has been created and integrated in the simulation package.