The Frequency Response of Counterflow Diffusin Flames

laser diagnostics;combustion;unsteady

Welle, Eric James ; Dr. Clement Kleinstreuer, Committee Member,Dr. Richard D. Gould, Committee Member,Dr. William L. Roberts, Committee Chair,Dr. H. Christopher Frey, Committee Member,Welle, Eric James ; Dr. Clement Kleinstreuer ; Committee Member ; Dr. Richard D. Gould ; Committee Member ; Dr. William L. Roberts ; Committee Chair ; Dr. H. Christopher Frey ; Committee Member

University:North Carolina State University

关键词: laser diagnostics;    combustion;    unsteady;   

Others  :  https://repository.lib.ncsu.edu/bitstream/handle/1840.16/5905/etd.pdf?sequence=1&isAllowed=y

美国|英语

来源: null

【 摘 要 】

Most practical combustion processes rely on turbulent diffusion flames due to their higher heat release rates when compared to laminar diffusion flames.The higher heat release rates occur because of the increased mixing that results from the flame's interaction with the turbulent flow field.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.To test the response, the reaction zone of a propane-air counter-flow diffusion flame was subjected to time varying flow fields using speakers.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.In diffusion flames, the limiting step for the transport of reactants to the flame front is a diffusion process.The time associated with the increasing phase difference is likely a result of the time necessary for reactants to travel through the diffusion layer.As the forcing frequency is increased, the time rate of change of reactants delivered to the edge of the diffusive zone increases; however, a finite amount of time is still necessary for the reactants to diffuse to the flame front.As the forcing frequency increases, this diffusion time becomes larger relative to the cycle time of the oscillation, which in turn shows up as an increasing phase difference.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.