Effects of multi-component diffusion and heat release on laminar diffusion flame liftoff

Numerical simulations were conducted of the liftoff and stabilization phenomena of laminar jet diffusion flames of inert-diluted C3H8 and CH4 fuels. Both non-reacting and reacting jets were investigated, including multi-component diffusivities and heat release effects (buoyancy and gas expansion). The role of Schmidt number for non-reacting jets was investigated, with no conclusive Schmidt number criterion for liftoff previously arrived at in similarity solutions. The cold-flow simulation for He-diluted CH4 fuel does not predict flame liftoff; however, adding heat release reaction lead to the prediction of liftoff, which is consistent with experimental observations. Including reaction was also found to improve liftoff height prediction for C3H8 flames, with the flame base location differing from that in the similarity solution – the intersection of the stoichiometric and iso-velocity (equal to 1-D flame speed) is not necessary for flame stabilization (and thus liftoff). Possible mechanisms other than that proposed for similarity solution may better help to explain the stabilization and liftoff phenomena.