A COMPARISON OF LUMPED AND DISTRIBUTED MODELS OF MONOLITH CATALYTIC COMBUSTORS

The purpose of the work has been the analysis of the adequacy of lumpe d models of catalytic combustors by comparison with the more detailed and numerically expensive distributed ones. The study has been perform ed for steady-state conditions, laminar flow, circular and square chan nel shape. The results have pointed out that lumped models generally f ail in predicting the wail temperature profiles (i.e. the light-off po sition). However, for long enough monolith segments, gas exit temperat ures predicted by one-dimensional models compare well with those provi ded by distributed models when local Nu(T) or, better, interpolation o f local Nu(T) and Nu(H)(Nu(H2) for square channels) from solutions of the Graetz-Nusselt problem are used for the estimation of heat and mas s transfer coefficients. Simulation of segmented monoliths is more cri tical due to the presence of multiple inlet regions (i.e. multiple ign itions). Small differences on predicted gas exit temperatures have bee n obtained only for square channels, while for circular channels lumpe d models provide not conservative estimates (Delta T > 50 K). Finally, it is shown that lumped models may provide misleading predictions whe n simulating combustor monoliths where gas phase reactions occur to a significant extent.