Direct numerical simulations of turbulent catalytic and gas-phase combustion of H2/air over Pt at practically-relevant Reynolds numbers

Abstract

Three-dimensional direct numerical simulations of turbulent catalytic and gas-phase H2/air combustion at a fuel-lean equivalence ratio φ=0.18 were performed in platinum-coated planar channels at two industrially-relevant flow conditions (inlet friction Reynolds numbers Reτ= 182 and 385) using detailed hetero-/homogeneous chemical reaction mechanisms. The preferential diffusion of hydrogen and oxygen, which was responsible for creating significantly higher surface equivalence ratios φw compared to the bulk gas-phase φ, was appreciably suppressed by turbulence at Reτ= 385. The higher turbulence intensity at this Reτ resulted in larger near-wall hydrogen excess that in turn yielded shorter homogeneous ignition distances compared to the lower Reτ case. Gas-phase ignition proceeded from isolated ignition kernels that subsequently formed axially elongated flames confined close to the catalytic walls. The coupling of catalytic and gas-phase chemistry inhibited homogeneous ignition, since at the vicinity of the ignition kernels the OH, H and O radical fluxes to the underlying catalytic wall were net-adsorptive and furthermore hydrogen was depleted by the catalytic reactions. The flame topology included alternating vigorously-burning and extinguished elongated streamwise stripes at Reτ=182 or islands at Reτ=385. The extinguished gas-phase reaction zones at Reτ=385 were characterized by underlying intense catalytic reaction rates. The flame topology and spatiotemporal correlation of the isolated burning and extinguished gaseous zones indicated that significant surface temperature non-uniformities could be obtained in practical catalytic reactors.