Homogeneous and Heterogeneous Combustion in Hydrogen-Fueled Catalytic Microreactors

Abstract

The hetero-/homogeneous combustion and interaction of hydrogen-fueled catalytic microreactors were investigated numerically. A two-dimensional CFD (computational fluid dynamics) model was developed, using elementary homogeneous and heterogeneous chemical reaction schemes, surface radiation heat transfer, heat conduction in the solid wall, and external heat losses. Computations were carried out to study the effects of the wall thermal conductivity, equivalence ratio, microreactor dimension, and inlet velocity on combustion characteristics, flame stability, and hetero-/homogeneous interaction. Despite the micro-scale, large transverse gradients in species mass fractions and temperature exist in the fluid and large axial gradients in temperature may exist in the walls. Wall thermal conductivity is crucial in determining the flame stability, as the walls transfer heat upstream for ignition of the cold incoming reactants but at the same time are responsible for heat losses. Combustible mixtures with compositions away from the stoichiometric point decrease the homogeneous chemistry contribution and the operating temperature. The microreactor dimension and inlet velocity have a strong effect on homogeneous flame stability. Smaller microreactors result in extinction because of the inhibition of homogeneous combustion induced by heterogeneous reaction; larger microreactors result in blowout due to the reduction of the heterogeneous contribution. Hetero-/homogeneous interaction maps were constructed in terms of microreactor dimension and inlet velocity.