Session: 03-03 Hydrogen Applications 1

Paper Number: 128859

128859 - Numerical Investigation on the Geometrical Scaling of Hydrogen Micromix Injectors 

Hydrogen (H₂) micromix combustion is a promising technology to reduce the environmental impact of gas turbines by delivering carbon-free and ultra-low-NOx combustion. The miniaturised and intensified mixing produced by sub-millimeter jets in crossflow generates micro-sized flames with low residence time, significantly suppressing the production of thermal NOx with reduced auto-ignition and flashback risks associated with pre-mixing.

For future H₂-fuelled aircraft, more advanced fuel systems need to be developed to transfer and heat the H₂ from liquid state in the tanks (for reducing tank volume) to gaseous state before combustion (for better engine thermal efficiency). Within the scope of the Rolls-Royce UK-led, UK-ATI HYEST project, an H₂ pre-conditioning system is being designed to facilitate this. It comprises an H₂-fuelled pre-burner to deliver high temperature to a downstream heat exchanger to heat the H₂ before it is introduced to the main engine combustor.

A key criterion for the pre-burner is outlet flow uniformity (temperature and velocity) for the inlet of the heat exchanger. However reasonable pressure loss and low NOx emissions are also important criteria. A micromix combustor may be ideal for this application, due to the compact flame size and better outlet flow uniformity. Previous studies have indicated that a larger number of smaller diameter (~0.3mm) H₂ injectors are more beneficial for reducing NOx and providing a more uniform outlet profile. However, this may present manufacturing challenges for full-scale combustion systems (both for a pre-burner as well as a main combustor). The novelty of the research presented in this paper is a design space exploration of H₂ micromix injectors for an H₂-fuelled pre-burner to assess the impact of H₂ injection diameter on performance (i.e., outlet flow distribution and NOx emissions). H₂ injection diameters ranging from 0.3mm to 1.0mm were investigated. Design parameters were scaled up in 2D, to match the momentum flux ratio and energy density requirements.

The first part of the study focused on determining the non-dimensional flow characteristics of the flow at various operating conditions, i.e., Reynolds number and Damköhler number. This was to ensure that the flow and flame regime is not significantly altered for the scaled versions of the injector, hence the expected physics are consistent, and the numerical model used is applicable over the range of operation.

The second part involved the CFD simulations of injectors with different H₂ injection diameters, by scaling air gate cross sectional area and injector spacing. Non-reacting simulations were performed to quantify the quality of H₂-air mixing with increased jet size, followed by reacting flow simulations to characterise and compare the flame position, length, and interactions. NOx emissions, downstream temperature distribution and pressure loss were also evaluated. The numerical simulations also comprised a parametric study of the influence of other injector design parameters including H₂ and air off-set distance.    

This study has provided better insights into the 3D flow characteristics, which may not be explained by the 2D scaling of jet and air flow dimensions. In addition, the study also showed a broader design space than those identified with 0.3mm H₂ injection diameter, which can be further exploited to optimise between NOx and practicality rather than simply scaling up H₂ and air passages.

The outcomes of this research are being used to inform the design of the H₂ pre-burner system for the HYEST project.

Presenting Author: Xiaoxiao Sun Cranfield University

Presenting Author Biography: Dr Xiaoxiao Sun is a Lecturer on Low Emissions Gas Turbine Combustion in the Propulsion Engineering Centre.

Authors:

Xiaoxiao Sun Cranfield University
Mohamed Morsy Cranfield University
Charith Wijesinghe Cranfield University
Gaurav Singh Cranfield University
Vishal Sethi Cranfield University
John Rimmer Rolls-Royce plc
Kenneth Young Rolls-Royce plc