Experimental study of premixed flames in a Taylor-Couette combustor

Turbulent-flame speeds in methane-air mixtures were measured in a Taylor-Couette (TC) combustor with counter-rotating cylinders, used to generate turbulence that is nearly homogeneous and isotropic over many integral length and time scales. Laser Doppler velocimetry (LDV) was used to measure turbulence intensities in the axial and circumferential directions and their variations with cylinder rotational rates. The results show a linear relation between turbulence intensities and average cylinder surface speed. Flame-speed measurements indicate a decreasing sensitivity of the turbulent-flame speed to increases in turbulence intensity to occur beyond turbulence intensities of approximately 2.5 laminar-flame speed. This is possibly due to a transition to a non-flamelet combustion regime where flame propagation is influenced by both small-scale flame-structure modification and large-scale flame-front wrinkling. In the following phase of this study, influences of acoustic instabilities on laminar and turbulent premixed flames were studied in the TC combustor for downward flame propagation. For laminar flames, an initially wrinkled annular flame propagating downward, away from the open end of the combustor, is found to undergo first deceleration and planarization and then acceleration and enhanced flame wrinkling, while intensities of axial and circumferential velocity fluctuations upstream of the flame, initially negligible, are strongly amplified. It is determined that primary and secondary laminar-flame instabilities associated with the coupling between acoustic waves and the rate of chemical heat release govern the evolution of both the flame-speed and the upstream flow field. For turbulent flames, it is found that the existence of pre-ignition turbulence in the combustor (generated by the rotation of the cylinders) does not eliminate acoustic instabilities; however as the level of pre-ignition turbulence is increased, the influence of the secondary acoustic instability on the turbulent-flame speed becomes insignificant.