Aero Engine Concepts Beyond 2030: Part 3 — Experimental Demonstration of Technological Feasibility

Pursuing the targets of the Flightpath 2050 program, i.e. a reduction of 75% in CO₂ and 90% in NO_x emissions, the aviation industry must look beyond the incremental improvement of the conventional gas turbine cycle and consider completely new propulsion concepts. Therefore, a consortium of industrial, academic and national research institutions was formed to investigate and demonstrate the feasibility of two promising aircraft propulsion concepts up to technology readiness level of four (TRL4).

The exhaust heat generated steam injection engine concept brings the technology of steam injection, reliably and successfully used in many ground-based applications, into the air. The innovation lies in the concept-specific ability to continuously condense and recover the injected steam from the exhaust gases over a large part of the entire flight envelope. Through the use of the gas turbine exhaust heat the recovered and subsequently pressurized liquid water is again evaporated and injected into the combustion chamber as steam. Thus, the new steam injection concept engine implements a quasi-closed water circuit in the open gas turbine cycle. Thereby it does not only increase the overall system efficiency significantly but also reduces CO₂ and almost all NO_x emissions, as well as particular matters and condensation trails.

The composite cycle engine is composed of two different and individually driven combustion engines, i.e. a gas turbine and a piston engine, which are not mechanically connected but operate with the same working fluid. Here, several free-piston motors replace the high pressure core of the gas turbine. Through the piston engine’s conceptual ability to work at much higher temperatures and pressures, the overall system’s efficiency can significantly be increased.

Preliminary conceptual design and process simulations of both concepts have been performed in order to illustrate the projected efficiency improvement and to identify and discuss challenges regarding their realization.

The focus of this paper is on describing the step-by-step procedure in the cooperative project to demonstrating the proof of concept of both innovative concepts. Starting from, firstly, numerical simulations on propulsion, aircraft system and test bench level, secondly, preliminary tests and fundamental experiments will be carried out in this project to lay the basis for, thirdly, a system demonstration at laboratory level. Thus, allowing for a final statement on the attainability of the requirements for future aircraft propulsion systems.

The involved consortium partners will initially present methods and tools to numerically investigate fundamental effects specific for both concepts, e.g. inflight water condensation, wet combustion or coupling of continuous and intermittent flow. This allows to explore not only possible aircraft applications and integration aspects but also to design the test benches for the subsequent experiments.

Those will fundamentally investigate the condenser, the droplet separator and the combustion chamber handling under very high water vapor concentrations as the key technologies of the steam-injecting and water-recovering cycle concept. In case of the composite cycle concept, the operational behavior as well as emission production, in particular NO_x, of the highly boosted free-piston system will be assessed on a specific test bench yet to be developed.

The knowledge gained from the numerical studies as well as the conduced experiments will flow into the conceptual design of the laboratory demonstrator. Here, a Rolls Royce Allison M250 series turboshaft engine will serve as basis. In the underlying paper, the approach to modify the M250 towards demonstrating both concepts including the installation of necessary instrumentation will be discussed in detail. An overview is given how the test rig will be extended with the relevant components of both new concepts in order to map their performance and specific characteristics. The demonstration and the resulting validated design tools are expected to mark a major step towards maturity of the technologies and a breakthrough in the pursuit of efficient and clean aircraft propulsion.