Propagation of symmetric and non-symmetric lean hydrogen flames in narrow channels: influence of heat losses

Abstract

In this paper we present results of direct numerical simulations of lean hydrogen/air flames freely propagating in a planar narrow channel with varying flow rate, using detailed chemistry and transport and including heat losses through the channel walls. Our simulations show that double solutions, symmetric and non symmetric, can coexist for a given set of parameters. The symmetric solutions are calculated imposing symmetric boundary conditions in the channel mid plane and when this restriction is relaxed non symmetric solutions can develop. This indicates that the symmetric solutions are unstable to non symmetric perturbations, as predicted before within the context of a thermo diffusive model and simplified chemistry. It is also found that for lean hydrogen/air mixtures an increase in heat losses leads to a discontinuity of the steady state response curve, with flames extinguishing inside a finite interval of the flow rate. Non symmetric flames burn more intensely and in consequence are much more robust to flame quenching by heat losses to the walls. The inclusion of the non symmetric solutions extends therefore the parametric range for which flames can propagate in the channel. This analysis seems to have received no attention in the literature, even if it can have important safety implications in micro scale combustion devices burning hydrogen in a lean premixed way.