Self-sustaining n-heptane cool diffusion flames activated by ozone

A novel method to establish self-sustaining cool diffusion flames with well-defined boundary conditions is experimentally demonstrated by adding ozone to the oxidizer stream in counterflow configuration. It is found that the atomic oxygen produced through the decomposition of ozone dramatically shortens the induction timescale of the low temperature chemistry, extending the flammable region of cool flames. Thus, it enables the establishment of self-sustaining cool flames at the pressure and timescales at which normal cool flames may not be observable. The present method, for the first time, provides an opportunity to study cool flame dynamics, structure, and chemistry simultaneously in well-known flame geometry. Extinction limits of n-heptane/oxygen cool diffusion flames are measured and a cool diffusion flame diagram is experimentally determined. Numerical simulations reveal that the extinction limits of cool diffusion flames are strongly governed by species transport and low temperature chemistry activated by ozone decomposition. The structure of cool diffusion flame is further investigated by measuring the temperature and species distributions with a micro-probe sampling technique. The kinetic model over-predicts the rate of n-heptane oxidation, the heat release rate, and the flame temperature. Measurements of intermediate species, such as CH₂O, acetaldehyde, C₂H₄, and CH₄, suggest that the model over-predicts the QOOH thermal decomposition reactions to form olefins, resulting in substantial over-estimation of C₂H₄, and CH₄ concentrations. The new method and data of the present study will contribute to promote understandings of cool flame chemistry.