Mixing Effect by Tone-Excitation in Round Jet Diffusion Flame

An experimental investigation has been conducted with the objective of studying the mixing mechanism near the nozzle exit in a tone-excited jet diffusion flame. The fuel jet was pulsed by means of a loudspeaker-driven cavity. The excitation frequencies were chosen for the two cases of the non-resonant and resonant frequency identified as a fuel tube resonance due to acoustic excitation. The effect of tone-excitations on mixing pattern near the nozzle exit and flame was visualized using various techniques, including schlieren photograph and laser light scattering photograph from TiO2 seed particles. In order to clarify the details of the flame feature observed by visualization methods, hotwire measurements have been made. Excitation at the resonant frequency makes strong mixing near the nozzle. In this case, the fuel jet flow in the vicinity of nozzle exit breaks up into disturbed fuel parcels. This phenomena affects greatly the combustion characteristics of the tone excited jet and presumably occurs by flow separation from the wall inside the fuel nozzle. As a result, in the resonant frequency the flame length reduces greatly

가진된 비 예혼합 화염의 혼합기구에 관한 실험적 연구

학위논문(박사) - 한국과학기술원 : 기계공학과, 1992.2, [ xiv, 101 p. ]An experimental investigation has been made with the objective of studying mixing mechanism near the nozzle exit in a tone excited non-premixed jet flame. Excitation of the jet is done by introducing acoustic disturbances produced by a loudspeaker mounted coaxially beneath the fuel nozzle. The excitation frequencies are chosen to the two cases of the resonant and non-resonant frequency identified as a pipe resonance due to acoustic excitation. The effect of different sinusoidal excitation conditions on mixing pattern near the nozzle exit and flame is visualized using various techniques, including schlieren photograph and laser light scattering photograph from $TiO_2$ seed particles. The effect of the excitation frequency and the excitation amplitude on the appearance of the flame are discussed. In order to clarify the details of the flame feature observed by visualization methods, phase-averaged velocity measurements are carried out. Velocity data near the exit and just upstream of the jet are presented. Excitation at the resonant frequency makes "a collapsable mixing" near the nozzle even for a laminar jet. In this case, the fuel jet flow in the vicinity of nozzle exit breaks up into disturbed fluid parcels. Results are presented at two conditions for which added frequencies are of the resonant case and the non-resonant one, and analyzed to identify the interplay between the flame length and the mixing pattern just outside the nozzle exit. From the visualizations, the behavior of jet flame being excited by the resonant frequency of experimental apparatus is greatly different from that of the non-resonant frequency. It is seen that the effect of the resonant frequencies significantly influences the flame length. When the jet is excited at the fundamental resonant frequency and its harmonic frequencies, much variation of the velocity has occurred all over the nozzle and vortex with large strength exist. It is also found that for some phase of excitation, the jet...한국과학기술원 : 기계공학과

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