Three-dimensional direct numerical simulations of turbulent fuel-lean H2/air hetero-/homogeneous combustion over Pt with detailed chemistry

Abstract

Three-dimensional direct numerical simulations (DNS) with detailed heterogeneous and homogeneous chemistry and transport were carried out to investigate the turbulent combustion of fuel-lean hydrogen/air mixtures (equivalence ratios φ=0.16 and 0.18) in a platinum-coated channel with prescribed wall temperatures of 1250K and 1270K and an incoming bulk Reynolds number of 5700. Over the channel domain where only catalytic reactions were present, temporally and spatially segregated islands with increased concentration of the limiting hydrogen reactant formed on the walls, a manifestation of finite-rate surface chemistry effects. The formation of such islands correlated with high streamwise vorticity, which was in turn responsible for an increased turbulent mass transport towards the catalyst. Fluctuations of either gaseous or surface species were significant (up to 34%) at the upstream channel locations and dropped farther downstream due to the flow laminarization induced by the heat transfer from the hot walls and the growth of the turbulent boundary layer. Such strong fluctuations exemplified the necessity of appropriate catalytic chemistry turbulence closures similar to those used for gas-phase reactions. The homogeneous ignition location exhibited streamwise fluctuations with a standard deviation of three channel half-heights. Downstream of homogeneous ignition, gas-phase combustion was concentrated in elongated streamwise stripes confined close to the walls. Hydrogen was incompletely converted within the gaseous combustion zones, and the leaking fuel reacted on the catalytic walls leading to combined hetero-/homogeneous combustion over the entire post-ignition domain. Local extinction of gaseous combustion was finally observed in spatially isolated regions characterized by high streamwise vorticity. The increased turbulent mass transport towards the walls was responsible for the local flame extinction, which in turn led to an increased catalytic conversion at the extinguished flame locations.