Cool diffusion flames of butane isomers activated by ozone in the counterflow

Abstract

Ignition in low temperature combustion engines is governed by a coupling between lowtemperature oxidation kinetics and diffusive transport. Therefore, a detailed understanding of the coupled effects of heat release, low-temperature oxidation chemistry, and molecular transport in cool flames is imperative to the advancement of new combustion concepts. This study provides an understanding of the low temperature cool flame behavior of butane isomers in the counterflow configuration through the addition of ozone. The initiation and extinction limits of butane isomers' cool flames have been investigated under a variety of strain rates. Results revealed that, with ozone addition, establishment of butane cool diffusion flames was successful at low and moderate strain rates. Iso-butane has lower reactivity than n-butane, as shown by higher fuel mole fractions needed for cool flame initiation and lower extinction strain rate limits. Ozone addition showed a significant influence on the initiation and sustenance of cool diffusion flames; as ozone-less cool diffusion flame of butane isomers could not be established even at high fuel mole fractions. The structure of a stable n-butane cool diffusion flame was qualitatively examined using a time of flight mass spectrometer. Numerical simulations were performed using a detailed chemical kinetic model and molecular transport to simulate the extinction limits of the cool diffusion flames of the tested fuels. The model qualitatively captured experimental trends for both fuels and ozone levels, but over-predicted extinction limits of the flames. Reactions involving low-temperature species predominantly govern extinction limits of cool flames. The simulations were used to understand the effects of methyl branching on the behavior of n-butane and iso-butane cool diffusion flames.

Combustion in the future: The importance of chemistry

“…Ju et al [181] have recently reviewed the importance of cool flame phenomena for ignition, flame extinction, and knock, and the possibility to study LT chemistry under flame conditions. Ozone addition has been demonstrated as being valuable [182] , [183] , [184] , [185] , [186] , [187] , [188] , [189] , albeit not necessary [190] , to enhance or control the reactivity in cool flames. Recent work includes premixed [191] and non-premixed [183 , 184 , 186 , 189 , 190] , laminar and turbulent regimes [192] as well as engine combustion systems [185] .…”

Effects of Carbon Chain Length on N-Alkane Counterflow Cool Flames: A Kinetic Analysis

“…Previous studies mainly focused on the low-temperature combustion behavior of small molecular hydrocarbon fuels, such as butane [14][15][16][17], hexane [18,19] and dimethyl ether [7,[20][21][22]; experimental and numerical studies on their flame structure, initiation and extinction limits were carried out. However, the relevant studies on larger hydrocarbon fuels account for a large fraction of practical transportation fuels are relatively limited.…”

Kinetic Study of Two-Stage Low-Temperature Heat Release in iso-Octane Autoignition

“…Recognizing the important, initiating role of the first ignition delay, studies have been conducted on its characteristics in various combustion and flame phenomena. These include, for example, measurement of the propagation speed and structure of n -heptane cool flames, the cool flame behavior of the butane isomers through ozone addition, the three-stage cool flame burning of n -alkane droplets at elevated pressures, the derivation of an explicit expression of the ignition delay, the structure and propagation of the associated premixed flames and detonations, and the limit phenomena of flammability and the ignition–extinction S-curve . Reference provides an overview of the recent progress on cool flames.…”