Numerical simulations of axisymmetric reactive jets with one-step Arrhenius kineticsare used to investigate the problem of deflagration initiation in a premixed fuel–airmixture by the sudden discharge of a hot jet of its adiabatic reaction products. For themoderately large values of the jet Reynolds number considered in the computations,chemical reaction is seen to occur initially in the thin mixing layer that separates the hotproducts from the cold reactants. This mixing layer is wrapped around by the startingvortex, thereby enhancingmixing at the jet head, which is followed by an annular mixinglayer that trails behind, connecting the leading vortex with the orifice rim. A successfuldeflagration is seen to develop for values of the orifice radius larger than a criticalvalue aϲ in the order of the flame thickness of the planar deflagration δL. Introductionof appropriate scales provides the dimensionless formulation of the problem, withflame initiation characterised in terms of a critical Damk¨ohler number ∆ϲ = (aϲ/δL)²,whose parametric dependence is investigated. The numerical computations reveal that,while the jet Reynolds number exerts a limited influence on the criticality conditions,the effect of the reactant diffusivity on ignition is much more pronounced, with thevalue of ∆ϲ increasing significantly with increasing Lewis numbers Le. The reactantdiffusivity affects also the way ignition takes place, so that for reactants with Le ≳ 1 theflame develops as a result of ignition in the annular mixing layer surrounding the developingjet stem, whereas for highly diffusive reactants with Lewis numbers sufficientlysmaller than unity combustion is initiated in the mixed core formed around the startingvortex. The analysis provides increased understanding of deflagration initiation processes,including the effects of differential diffusion, and points to the need for furtherinvestigations incorporating detailed chemistry models for specific fuel–air mixtures.
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