Flame acceleration and deflagration to detonation transition in a microchannel with catalytic nickel walls

The characteristic behavior of the wall exerts a strong influence on the flame acceleration (FA) and deflagration-to-detonation transition (DDT) processes in microchannels [Ramachandran et al., “A numerical investigation of deflagration propagation and transition to detonation in a microchannel with detailed chemistry: Effects of thermal boundary conditions and vitiation,” Phys. Fluids 35, 076104 (2023)]. In this work, motivated by the catalytic microcombustors in realistic industrial settings, we study the influence of catalytic nickel walls on the FA & DDT processes. Highly resolved numerical simulations (spanning 10–20 grid points across the flame thickness) are performed, employing a 9-species 21-reaction combustion mechanism for H2-combustion by Li et al. [“An updated comprehensive kinetic model of hydrogen combustion,” Int. J. Chem. Kinet. 36, 566–575 (2004)] for the gas-phase chemistry and a 5-species 12-reaction submechanism derived from a methanation microkinetic mechanism by Schmider et al. [“Reaction kinetics of CO and CO2 methanation over nickel,” Ind. Eng. Chem. Res. 60, 5792–5805 (2021)] for the catalytic surface chemistry. Stoichiometric H2/air with and without 25% (by mole) of H2O dilution/vitiation are investigated. The simulations demonstrate that catalytic walls enhance flame propagation in the vitiated mixture (which exhibits lower flame speeds) by providing additional radical production and heat release at the surface. As a result, the traditionally observed parabolic-like flame front profile in microchannels inverts due to preferential propagation of the flame along the wall. In contrast, the unvitiated mixture exhibits rapid flame acceleration, and the influence of catalytic walls is found to be minimal. These observations are due to the fact that the residence time available for coupling the heterogeneous wall chemistry with the gas-phase combustion is smaller at higher flame speeds (in unvitiated mixtures).