Near Field Structure Of Acoustic Excited Jet Diffusion Flames

Author : A. Cabelli
Publisher :
ISBN 13 :
Total Pages : pages
Book Rating : 4.:/5 (681 download)

DOWNLOAD NOW!

Download or read book Fluid Dynamic Structures in Diffusion Flames written by A. Cabelli and published by . This book was released on 1987 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Experiments were carried out in a combustion facility which allowed optical and other measurements to be made on a diffusion flame formed by a jet of fuel gas and a co-flowing jet of air. Acoustic perturbations could be imposed to the flow of fuel with a loudspeaker mounted at the bottom of a settling chamber in the fuel line. A two mirror schlieren system was used to image the gas flames and a colour filter was placed at the focus of the second mirror to produce coloured schlieren images when a continuous white light source was used. Two types of coherent structures were observed, located concentrically along the axis of the flame. The outer structures, found outside the luminous part of the flame, outlined the boundary between products of combustion and the cold airstream, while the inner structures, located well inside the luminous flame, marked the interface between the cold fuel and the reaction zone. The structures were periodic and their frequencies identified by spectral analysis of optical and pressure signals. The signal from a pressure probe placed in the flame and from a phototransistor focused at the corresponding position in the schlieren image, correlated well. Over the range of Reynolds numbers tested, the outer structures exhibited a dominant frequency of approximately 13 Hz whereas the faster inner structures occurred at approximately 256 Hz. Acoustic excitation at these frequencies made the corresponding structures more coherent and influenced the development of the instability close to the nozzle exit. The response of the experimental equipment to band limited white noise, with the jet not ignited, revealed spectral peaks at the frequency of the inner structures which confirm the nexus between the flow and the acoustic field (even without external excitation). The wavelength of these structures (and their convection velocity) did not vary over the range of Reynolds numbers considered, at a given distance from the nozzle exit, but increased slightly with axial position. When the flame was acoustically forced at a frequency different from the preferred frequency, their wavelength remained unchanged but the velocity changed to accommodate the change in frequency.