A THEORETICAL MODEL FOR COMBUSTION REACTIONS INSIDE A REFRACTORY TUBE

A kinetic and thermal model has been developed for the evaporation and combustion of a single chain of uniformly sized droplets of hexane in a turbulent stream of air inside a refractory tube. A single global rate expression is utilized for the oxidation of hexane to CO and H₂0, but 44 reversible, free-radical mechanisms thereafter. Adiabatic flow is postulated for the post-flame zone and plug flow throughout. An integro-differential energy balance for the wall is avoided by the use of an experimental wall temperature profile for each set of conditions. The model is restricted to external conditions such that evaporation is completed prior to ignition. Calculated concentrations of NO are in good agreement with measured values for fuel-lean mixtures over a wide range of post-flame residence times. Calculated concentrations of CO are up to twice as high as experimental values but demonstrate the same dependence on residence time and equivalence ratio, suggesting that some of the individual rate constants are incorrect. The predictions for both NO and CO are a great improvement over those of prior, simplified models, The predictions of NO deviate increasingly from measured values as the equivalence ratio increases above unity, undoubtedly due to the inapplicability of the global model for the oxidation of rich mixtures of hexane.

The assumption of a pseudo steady state concentration was not found to be valid for any of the free radicals of the detailed model, thereby accounting for the inaccuracy of the various simplified models for the production of NO and the oxidation of CO.