Initiation and propagation of laminar premixed cool flames

Cool flames, being essential features of chemical kinetics of large hydrocarbon fuels, are closely related to the negative temperature coefficient (NTC) phenomenon and engine knock. In this work, the coupling of cool flame chemistry and convective-diffusive transport is computationally and experimentally investigated. A 1-D planar premixed cool flame induced by a hot pocket is first simulated for DME/O₂/N₂ mixtures with detailed chemistry and transport, demonstrating the existence of a residence time window for quasi-steady propagation. Then with residence time limited by aerodynamic straining, a steady-state premixed cool flame is simulated in a counterflow of heated N₂ against a DME/O₂/N₂ mixture. It is found that with a high strain rate, corresponding to short residence time, low-temperature heat release is suppressed, resulting in a stretched low-temperature S-curve system response; and that with a sufficiently low strain rate, corresponding to long residence time, ignition induced by low-temperature chemistry would transition to a high-temperature, intensely burning flame. Consequently, a steady-state premixed cool flame exists only for residence time in a strain rate window. A symmetric counterflow configuration is then simulated to determine the cool flame temperature and flame speed at a fixed local strain rate, showing very different controlling chemistry and characteristics as compared to the normal laminar flames governed by high-temperature chemistry. In a companion experimental investigation, premixed cool flames in the counterflow were observed with a high-sensitivity CCD camera in the UV spectrum, with/without a bandpass filter corresponding to the characteristic wavelength of excited HCHO. The chemiluminescence from the cool flame is found to become more intense with increasing equivalence ratio, even for rich mixtures, while the position of the cool flame is insensitive to variation in the equivalence ratio at the same strain rate. These observations qualitatively agree with the numerical simulations, demonstrating the essential features of premixed cool flames.