Velocity measurements in an unsteady counterflow diffusion flame using laser Doppler velocimetry

Local strain rates in an unsteady counterflow diffusion flame were experimentally measured as a function of initial strain rate, oscillation frequency, and amplitude of the imposed fluctuation for both strong and weak hydrocarbon flames. The strain rate was determined by curve fitting the measured axial velocity profile just before the preheat zone and finding the curve fit's maximum derivative. The phase relationship between speaker deflection and velocity fluctuation was quantified. The velocity profile was measured assuming both constant and variable phase angle to quantify the effect of the phase angle dependence on spatial location in the flow field. At the phase angle corresponding to maximum velocity, the strain rate was measured at oscillation amplitudes near the flame extinction limit for propane/air and both diluted and undiluted methane/air flames. At one frequency, the strain rate was measured at four different phase angles within the velocity oscillation. Flow reversal occurred in the strongly burning propane and methane flames at very low applied voltages. However, the weakly burning diluted methane flame extinguished before flow reversal occurred.