Direct numerical simulation and modelling of thermodiffusively-unstable lean premixed hydrogen flames and high-pressure micromix combustors

Abstract

Hydrogen combustion is of great significance in energy and propulsion systems, particularly in air-breathing rocket engines where efficient and stable combustion is crucial. This thesis presents several comprehensive studies on direct numerical simulations (dns) of hydrogen flames, specifically thermodiffusively-unstable lean premixed hydrogen flames and hydrogen combustion in high-pressure micromix combustors. The first part of the thesis investigates thermodiffusively unstable conditions in a twodimensional laminar configuration. It is shown that these flames are well-characterised by a parameter ω2 derived from linear stability analysis. Empirical relations are derived for flame thinning and acceleration, where alternate forms are required on either side of the most-unstable-surface present in ω2 in the pressure-temperature-equivalence ratio space. The subsequent part extends the study to three-dimensional laminar and turbulent configurations. The three-dimensional setting exacerbates thermodiffusive instability resulting in increased model constants compared to the previous part. Turbulence is found to further intensify the thermodiffusive response, and this effect is modelled using Karlovitz number as an additional parameter. A Markstein number model, solely relying on curvature, is identified. The flame surface is categorised based on principal curvature values, and the zonal distribution is found to vary with pressure in the laminar flame but not turbulent flame after appropriate normalisation. The final part of the thesis focuses on flame structure and stabilisation mechanisms in a micromix combustor. Analysis of time-averaged data identifies the lift-off height and presence of recirculation zones. Analysis of instantaneous data reveals mixture inhomoii geneity through equivalence ratio distribution and a premixed combustion regime through the flame index. Comparisons between flame and turbulent characteristics suggest a wide range of flame-turbulence interactions particularly at high pressure. Finally, it is demonstrated that flame stabilisation is achieved through ignition events in the shear layer resulting from the recirculation and entrainment of hot products and turbulent mixing of fuel