The frequency response of counterflow diffusion flames

Most practical combustion processes rely on turbulent diffusion flames due to their higher heat release rates when compared to laminar diffusion flames. Unfortunately, the turbulent combustion process is very complex; therefore, simplified models have been constructed. Flamelet Theory is a method that characterizes turbulent diffusion flames as a collection of strained, laminar, one-dimensional flamelets. One caveat of this model is the flamelets are assumed to respond quasi-steadily to the applied flow field. The focus of my research has been to elucidate the influence of a time varying flow field on the combustion process and soot formation.; To test the response, counter-flow diffusion flames were subjected to time varying flow fields. The results of the experiments illustrate a reaction zone that responds quasi-steadily at forcing frequencies up to 50 Hz. Above this threshold, significant departures occur from steady flame behavior. At elevated frequencies, conditions were found where the reaction zone temperature was found to be in phase with the strain rate, signaling a significant deviation from the quasi-steady state assumption.; Another aspect of this research was to determine the effects of the transient flow field on soot formation. The formation of soot is of great concern as it is considered to be highly carcinogenic. Unlike other flame parameters such as flame temperature and thickness that responded quasi-steadily at low forcing frequencies, the soot volume fraction showed significant deviations from steady flame behavior at lower frequencies. At higher forcing frequencies, it was found that the soot field asymptotes to a steady structure. The cause of the low frequency response is a result of the long time scales associated with soot production.; The results from this work help illuminate the fundamental physics that governs a flame's response to a time varying flow field. It has shown that significant errors can occur when following the quasi-steady state assumption of the traditional Flamelet Theory. It was also shown that even moderately forced flames exhibit a dramatically different sooting structure when compared to steady flames.