Dynamics of FREI with/without cool flame interaction

Abstract

The interactions between cool flames and flames with repetitive extinction and ignition (FREI) of stoichiometric n-heptane/air mixture were studied using a micro flow reactor with a controlled temperature profile from 373 to 1300 K. Two different flame dynamics with and without cool flames were observed in reactors with inner diameters dinner of 1 and 2 mm. Cool flames and FREI are spatially separated at dinner= 1 mm, whereas interactions between cool flames and FREI are observed at dinner= 2 mm. At dinner= 1 mm, the brightness intensity from cool flames depends on the inlet velocity (uinlet). Approximately above uinlet= 10 cm/s, the brightness intensity from cool flames decreases with increasing inlet velocity, despite a large amount of mixture input. This is because before low temperature ignition occurs under higher inlet velocity conditions, the mixture archives temperature where negative temperature coefficient is dominant. Reaction front propagation speed of FREI decreases monotonically due to heat loss because the extinction points of FREI are located in higher temperatures than the cool flame region. At dinner= 2 mm, the acceleration of the reaction front in the cool flame region is confirmed experimentally, as predicted in our previous two-dimensional numerical simulations. Additionally, the instantaneous reaction front speed after autoignition is analyzed at dinner= 1 mm. The instantaneous reaction front speed decreases as the time from extinction to ignition tex_ig becomes longer because a moderate mixing zone of reactants and products is formed.