A COMPARISON OF LUMPED AND DISTRIBUTED MODELS OF MONOLITH CATALYTIC COMBUSTORS

The purpose of the work has been the analysis of the adequacy of lumped models of catalytic combustors by comparison with the more detailed and numerically expensive distributed ones. The study has been performed for steady-state conditions, laminar flow, circular and square channel shape. The results have pointed out that lumped models generally fail in predicting the wail temperature profiles (i.e. the light-off position). However, for long enough monolith segments, gas exit temperatures predicted by one-dimensional models compare well with those provided by distributed models when local Nu(T) or, better, interpolation of 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 mass transfer coefficients. Simulation of segmented monoliths is more critical due to the presence of multiple inlet regions (i.e. multiple ignitions). Small differences on predicted gas exit temperatures have been obtained only for square channels, while for circular channels lumped models provide not conservative estimates (Delta T > 50 K). Finally, it is shown that lumped models may provide misleading predictions when simulating combustor monoliths where gas phase reactions occur to a significant extent.