Session: 04-14 Combustor Design II

Paper Number: 152950

Numerical Investigation of Vertical Spacing Effects on Flame Behaviour and NOx Emissions in Hydrogen Micromix Injector Pairs 

Authors: Guillermo González (Cranfield University), Gaurav Singh (Cranfield University), Vishal Sethi (Cranfield University), David Cadrecha (ITP Aero), Pedro Romero (ITP Aero).

Hydrogen is being recognised as a potential alternative to meet the ambitious CO2 emission reduction targets set by the aviation industry. Its wide flammability limits enable lean combustion and low thermal NOx emissions. However, hydrogen’s high diffusivity and burning velocity present significant challenges, such as heightened risks of auto-ignition and flashback. Micromix combustion addresses these challenges by using a jet-incrossflow configuration to generate multiple miniaturised diffusion flames which intensifies the fuel-air mixing, reducing thermal NOx, and mitigates flashback risk. However, micromix injectors are highly sensitive to variations in design parameters and operating conditions, which can significantly change flame behaviour and NOx emissions. Understanding the influence of geometrical design parameters is therefore critical. Earlier studies focused on characterising air/hydrogen mixtures, improving the predictive capability of numerical models, and an initial exploration of the design space. Key parameters, such as the Momentum Flux Ratio, which controls hydrogen penetration, and the air gate geometry, which governs hydrogen diffusion, have been identified.

The novelty of this research lies in exploring a key design parameter, the blockage ratio (BR), focusing specifically on the vertical separation between injectors. This parameter influences the structure of recirculation zones, directly affecting flame behaviour and NOx generation. To isolate the effect of flow blockage, the vertical spacing between injectors was adjusted by displacing the airgate, while maintaining constant energy density, overall injector dimensions, and momentum flux ratio. This investigation utilised RANS simulations with the FGM combustion model, following best practices established in previous research, and thermal NOx emissions were calculated as a post-processing step in the solver. The baseline geometry from the ENABLEH2 project and boundary conditions from the CFM LEAP-1A turbofan thrust class were used.

Results were analysed in two phases. First, the impact of geometric modifications on flow characteristics, including hydrogen penetration, the structure of recirculation zones, and flame behaviour, was examined. Second, the trends in NOx generation were studied to identify the dominant factors influencing NOx production. The contribution of this research lies in providing insights into how these design parameters affect flow structure and flame characteristics, and in offering design recommendations that could reduce NOx emissions by up to 15% compared to the reference geometry under various operating conditions.

Research has shown two dominant effects on NOx generation. Increasing the separation between injectors reduces flame interaction, lowering thermal NOx emissions. However, further increasing injector spacing leads to the intensification of recirculation zones, increasing residence times and thermal NOx emissions. The relative importance of these effects depends on the operating conditions: flame interaction is dominant at low equivalence ratios (<0.2), while the impact of the recirculation region becomes more significant at higher equivalence ratios. The optimal blockage ratio aims to balance these competing effects to minimise NOx emissions.

Keywords: Hydrogen, Micromix, NOx Formation, Combustion, Injector Design.

Presenting Author: Guillermo González López Cranfield University

Presenting Author Biography: My name is Guillermo González, and I was born in a small town in the province of Ávila, Spain. From a young age, I was fascinated by fluid mechanics and aerodynamics, which led me to pursue a degree in Aerospace Engineering. I studied at the Technical University of Madrid (UPM), where I completed my Final Degree Project at ITP Aero. In this project, I focused on validating a four-equation turbulence model, applying advanced knowledge of fluid dynamics.

After finishing my degree, I pursued a Master’s in Aerospace Engineering at UPM. In my second year, I had the opportunity to study at Cranfield University in the UK, as part of a double-degree program, where I completed the MSc in Thermal Power. My Master’s Thesis was a collaborative project between Cranfield and ITP Aero, focusing on exploring the design space of micromix injectors for hydrogen combustion, contributing to the development of clean technologies in the aerospace sector. The present paper is an extension of my Master’s Thesis, further developing the research carried out during that project.

Currently, I work at ITP Aero, where I continue to apply my expertise in aerospace engineering, contributing to the development of advanced propulsion technologies.

Authors:

Guillermo González López Cranfield University
Gaurav Singh Cranfield University
Vishal Sethi Cranfield University
David Cadrecha Robles ITP Aero
Pedro Romero Vega ITP Aero