Abstract
A liftedH2/N2turbulent jet flame issuing into a vitiated coflow is investigated using the conditional moment closure. The conditional velocity (CV) and the conditional scalar dissipation rate (CSDR) submodels are chosen such that they are fully consistent with the moments of the presumedβprobability density function (PDF). The CV is modelled using the PDF-gradient diffusion model. Two CSDR submodels based on the double integration of the homogeneous and inhomogeneous mixture fraction PDF transport equations are implemented. The effect of CSDR modelling is investigated over a range of coflow temperatures (Tc) and the stabilisation mechanism is determined from the analysis of the transport budgets and the history of radical build-up ahead of the stabilisation height. For allTc, the balance between chemistry, axial convection, and micromixing, and the absence of axial diffusion upstream of the stabilisation height indicate that the flame is stabilized by autoignition. This conclusion is confirmed from the rapid build-up ofHO2ahead ofH,O, andOH. The inhomogeneous CSDR modelling yields higher dissipation levels at the most reactive mixture fraction, which results in longer ignition delays and larger liftoff heights. The effect of the spurious sources arising from homogeneous modelling is found to be small but nonnegligible, mostly notably within the flame zone.
Highlights
The investigation of stabilisation mechanisms in lifted flames is an active area in combustion research
When Tc is decreased to 1030 K, the predictions remain in good agreement with the experiments up to x/d = 10 and the two conditional scalar dissipation rate (CSDR)
The influence of the CSDR modelling becomes more apparent at these axial locations
Summary
The investigation of stabilisation mechanisms in lifted flames is an active area in combustion research. Vanquickenborne and van Tiggelen [1] suggest the premixed flame propagation stabilisation theory. Their experimental findings indicate that the fuel and the oxidiser are premixed ahead of the base of lifted diffusion flames and that stabilisation takes place at stoichiometric locations where the local mean velocity and the turbulent burning velocity of a stoichiometric premixed flame are equal. Cabra et al [3] conduct experimental measurements for a lifted turbulent H2/N2 jet flame issuing into a coaxial vitiated coflow consisting of the hot combustion products of a lean premixed H2/air flame. The coflow conditions allow for the possibility of stabilisation by means of autoignition. Premixed flame propagation remains another possible stabilisation mechanism
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