Characterisation of non-premixed, swirl-stabilised, wet hydrogen/air flame using large eddy simulation

Abstract

The quest for decarbonisation of industry calls for phasing out fossil fuels and in some cases replacing them with green hydrogen in line with the Global Sustainable Goals. To that end novel hydrogen burners will be developed. In this work, the non-premixed combustion of steam-diluted hydrogen combustion is studied numerically in a swirl-stabilised combustor. The flame shape and stabilisation is discussed along with the effects of flow-field, stratification and temperature distribution on the flame. The characteristics of the swirl-stabilised flame is illustrated using several time-averaged and instantaneous contours of parameters including heat-release rate, local equivalence ratio and temperature. The effect of mixing and temperature distribution in the non-premixed configuration is studied quantitatively, subsequently identifying several flame regions. The combustion modes in these regions are also reported. Importantly, neither local hotspots nor flame flashback is observed in this steam-diluted hydrogen combustion.While a typical short hydrogen flame is observed near the centreline of the combustor, a weak V-shaped flame tail also co-exists. The heat-release rate in the flame tail is about two orders of magnitude lower than the main flame. Examining the flame structure using a normalised flame index showed that the combustion in the main flame is premixed-like while that in the main flame is non-premixed. The effect of the flow-field on the transport of fuel and air seems to influence this observation in the flame tail.The high heat-release rate regions of the flame comprised lean (ϕ<1), moderately hot (T = 800 to 1400 K) mixture. The flame tail region resulted from lean, hot (T >1500 K) mixtures. A flame region with rich mixture (ϕ = 1 to 2), featuring moderate heat-release rate, was found to be trapped between an axial fuel jet and the central recirculation region. All these flame regions had flameless combustion as the dominant mode of combustion. In particular, the flame tail region burned completely in the flameless mode. The lean and dominantly flameless combustion in the main flame regions are likely to lead to lower NOx.