Session: 04-41 Combustion Experiments V

Paper Number: 152481

Experimental Investigation of Strut Injectors for Hydrogen/Air Combustion at Atmospheric Pressure in Non-Reactive and Reactive Conditions 

To tackle the current environmental crisis related to global warming, alternative fuels such as green hydrogen need to be considered to successfully decarbonize the aviation sector. Until now, the transition from carbon-based fuels to hydrogen represents a technological challenge for obtaining high performances in terms of operability. Indeed, the thermo-chemical properties of H₂, including molecular diffusivity, flame velocity and temperature, differ significantly from kerosene fuel one’s. In addition, the use of gaseous H₂ will require modifications to the architecture of combustion systems, and in particular to the injection systems that currently operate with liquid fuels. As H₂ and air will be injected separately for safety reasons and to prevent any flashback due to the high velocity of premixed flames, an emergence of high-temperature diffusion flames is promoted. This configuration can result in a flame occurring in proximity to the injector, increasing the thermal constraint on the injector wall. In addition, high temperatures can lead to increased NOx emissions via the thermal pathway of the Zeldovich reaction mechanism. These combined effects highlight the necessity of developing new injection technologies, designed to deliver an efficient H₂/air mixture close to the injector outlet, thus favoring the formation of premixed flames associated with lower flame temperatures.

Recently, the micro-mixing technology has been demonstrated as a promising technology for H₂/air combustion. The fuel is injected via miniaturized injectors positioned perpendicularly to an air crossflow stream. Such configuration improves the mixing performances and produces multiple short-size flames anchoring downstream of the lip of the injector. The NO_x production can be reduced due to the enhanced mixing and the reduced residence time of gases at high temperature. However, this injection geometry leads to flame-wall interactions acting to stabilize the flame, but consequently to an increase in the thermal stress on the injection system.

Alternatively, the technology of strut injectors can provide a mixing process that eliminates the thermal load. This concept was developed in the early 2000s for supersonic combustion where fast mixing was required because of the short residence time of the fresh fuel/air gases inside the combustion chamber. Essentially, a strut injector is composed of a symmetrical leading edge, a parallel central body and a trailing edge equipped with a pattern of alternating ramps at fixed angles. The mainstream air that flows around the strut is guided through the ramps towards small fuel orifices that are uniformly distributed along the span of the injector body. Several configurations were developed and studied for H₂/air supersonic combustion [1]. However, this injection technology has never been considered for subsonic conditions representative of commercial aircraft combustion chambers.

The aim of this study is to explore the potential of strut injectors in the production of stabilized H₂/air flames with minimal wall thermal load and weak NOx emissions. First, the performance of the fuel/air mixing is experimentally studied under non-reactive conditions using the Planar Laser-induced Fluorescence (PLIF) optical diagnostic on acetone. Stereo particle-image-velocimetry (PIV) is implemented to measure the 3D velocity distributions at the injector outlet and to identify the aerodynamical mechanisms behind the various mixing modes observed. Second, strut injectors are tested under reactive conditions at atmospheric pressure. Stability combustion diagrams reveal large flammability limits demonstrating a wide operability range. The influence of the injector geometry on the flame properties is then studied. The effects on the flame stabilization and flame structure are characterized by PLIF on the OH molecule. Finally, the pollutant emissions are investigated by performing global NOx measurements using a sampling probe analyzer located at the outlet of the combustion chamber. Experiments performed in equivalence ratio lower than 0.2 and in flow velocity regimes similar to those encountered in aeronautical combustors have resulted in short-size flames stabilized at the injector outlet, while emitting low levels of NOx. These promising results confirm the high potential of these innovative strut injectors for their implementation in future aeronautical combustors operating with pure hydrogen.

Presenting Author: Vincent Gope CORIA - CNRS UMR 6614

Presenting Author Biography: I am a third-year PhD student at CORIA working on the development of injection technologies for hydrogen/air combustion.

Authors:

Vincent Gope CORIA - CNRS UMR 6614
Alexis Vandel CORIA – CNRS UMR 6614
Benjamin Quevreux CORIA – CNRS UMR 6614
Maxim Kuvshinov CORIA – CNRS UMR 6614
Pradip Xavier CORIA – CNRS UMR 6614
Gilles Cabot CORIA – CNRS UMR 6614
Frédéric Grisch CORIA – CNRS UMR 6614