Axial evolution of helical flame and flow disturbances in a transversely forced combustor

Abstract

This paper presents 5 kHz stereo PIV and OH PLIF measurements of transversely forced, triple nozzle swirl flames. This work is motivated by the problem of transverse instabilities in annular and can combustion systems. A key distinction of this problem from the axial instability problem is that the acoustic excitation field can differ substantially from one nozzle to the next; e.g., different nozzles can be located at either a pressure node or antinode in the case of a standing wave (where the pressure and transverse acoustic velocity field have an approximate 90 degree phase difference), or be excited by a traveling wave (where they are in phase). Prior modeling and experimental work has clearly shown that the dominant shear flow instabilities excited by these acoustics disturbances differ substantively as well. For example, axisymmetric jet modes are excited at pressure anti-nodes and helical modes at pressure nodes. These hydrodynamic disturbances then evolve axially at spatial growth rates controlled by the stability characteristics of the shear layer and nonlinear interactions with each other. This paper describes measurements in the r-z and r-θ planes to characterize the axial evolution of these hydrodynamic disturbances excited by transverse acoustic waves. It shows that different helical modes, whose relative amplitudes near the nozzle outlet can be predicted in a straightforward way, evolve in quite different manners and are strongly influenced by not only the nature of the excitation field, but the nominal hydrodynamic stability characteristics of the swirling shear flow as well. These observations are compared with the results of a linear hydrodynamic stability analysis, demonstrating good qualitative agreement and supporting the idea that linear mechanisms, as opposed to nonlinear interactions, control which helical modes dominate the flow near the nozzle.