Isobutane cool flames in a CSTR: The behavior dependence on temperature and residence time

Abstract

Hydrocarbon combustion has been recently recognized to evolve through oscillatory reaction under certain conditions of temperature, pressure, fuel to oxygen ratio, and residence time. The reported results are a part of a long-term study aiming to assess the parametric dependence of the oscillatory behavior. The work is concerned with the isobutane behavior dependence on temperature and residence time. The reaction occurs in a continuous stirred tank reactor in the gas phase at atmospheric pressure, unit molar feeding ratio, in the temperature range 553–733 K and in the residence time range 7.5–20 s. The isobutane oxidation evolves through five different reaction modes. Different oscillatory phenomena have been detected, including purely periodic cool flames and damped cool flames. The analysis of the results confirms the thermokinetic nature of the observed oscillations. The heat release rate is experimentally evaluated. The form of the resulting curves explains the observed hysteresis. The negative temperature coefficient of the reaction rate is shown to be closely associated with the damped cool flames. A critical diagram in the temperature-residence time plane is presented. Isobutane presents a smaller sensitivity to residence time than other investigated hydrocarbon systems. The analysis on the temperature-residence time plane shows that the isobutane system is physicochemically bistable, i.e., two different steady states are possible in the system at the same system temperature.