Aero Engine Concepts Beyond 2030: Part 1 — The Steam Injecting and Recovering Aero Engine
Recognizing the attention currently devoted to the environmental impact of aviation, this two-part publication introduces two new aircraft propulsion concepts for the timeframe beyond 2030. Part one focuses on the exhaust heat generated steam injection engine concept featuring in-flight condensation and recovery of the used water. In the second part, the free-piston composite cycle engine concept is presented. Based on consistent thermodynamic descriptions, preliminary designs and initial performance studies, the concepts potentials are analyzed and subsequently discussed. A third but separate publication, building upon those two concepts, presents the procedure for demonstrating their feasibility with numerical simulation and test bench experiments up to a technology readiness level of four.
The development of both concepts is an effort to drastically reduce climate-impacting emissions from aviation by 2050. The conventional gas turbine engine used in today's civil aviation has undergone enormous developments since its invention. At present, it seems possible that the Joule-Brayton cycle-based gas turbine will meet the 2030 emission reduction targets through continuous but increasingly expensive technological improvements. However, the reduction of emissions beyond this point requires novel concepts to overcome the approaching physical limits of the known gas turbine cycle.
The idea of exhaust heat generated steam injection into gas turbines for power augmentation and efficiency improvement is not new. Steam injection was already used by the inventors of the gas turbine and has found a multitude of applications until today. In 1976 Cheng proposed a gas turbine cycle in which the heat of the exhaust gas is used to produce steam in a heat recovery steam generator (HRSG). It is a combination of the open Joule-Brayton gas turbine cycle and the closed Clausius-Rankine cycle.
Liquid water is pressurized by a pump and then evaporated by the exhaust heat of the gas turbine. Subsequently, the steam is injected ahead of the turbine, e.g. into the combustion chamber. The water is finally recovered from the exhaust steam-gas mixture after being cooled in the HRSG and a subsequent condenser.
Due to the higher heat capacity of steam compared to that of an air-gas-mixture, the turbine can extract more work from the same mass flow. In addition, the power required to pump the (incompressible) water is considerably lower than to compress the same amount of air. Furthermore, the injection of steam into the combustion process improves the local heat distribution and, thus, is an accepted means to reduce NOx emissions. And even in variable load scenarios, steam-injected gas turbines are known to be still advantageous.
Nevertheless, all gas turbines with steam injection known today are used exclusively in ground-based power plants. Although several power plants use steam-injected aircraft derivative gas turbines, neither an existing steam-injecting and water-recovering aircraft propulsion system is known nor are there any published scientific studies or concept ideas to bring the technology into the air. In particular, the size of the systems for condensation, recovery and purification of the required water may have been considered too large and heavy for flight operations, in the past.
The proposed exhaust heat generated steam injection engine concept features a novel condensation and water recovery system which allows for a continuous operation over a large part of the entire flight envelope. Thus, the presented concept implements a quasi-closed water circuit in the open aero gas turbine cycle. Thereby it does not only increase the overall system efficiency significantly but also reduces CO2 and almost all NOx emissions, as well as particulate matters and condensation trails.
Aero Engine Concepts Beyond 2030: Part 1 — The Steam Injecting and Recovering Aero Engine
Category
Technical Paper Publication
Description
Session: 05-05 Cycles for Propulsion II
ASME Paper Number: GT2020-15391
Start Time: September 22, 2020, 12:45 PM
Presenting Author: Dr.-Ing. Oliver Schmitz
Authors: Oliver Schmitz MTU Aero Engines AG
Hermann Klingels MTU Aero Engines AG
Petra Kufner MTU Aero Engines AG