Burning velocity of premixed turbulent flames in the weakly wrinkled regime
Turbulent burning velocities have been measured for methane/air and ethylene/air planar flames stabilised in a wide-angled conical diffuser where the flow is decelerated axially. Novel instrumentation, involving a rotating drum device, has been developed to measure the instantaneous flame height, by utilising the UV emission from the excited OH radical in the flame. Six horizontal slits in the drum allow the UV radiation from the flame to fall periodically on the photodiode. Secondary flow in a high-speed wall jet is used to generate a uniform primary flow across the diffuser. The cold flow parameters are measured at different axial and radial positions inside the diffuser using a hot wire anemometer. The effect of imposed acoustic velocity oscillations on the turbulent burning velocity is also investigated. Speakers are placed upstream to force the oscillations. The instantaneous flame lift-off height, with and without external forcing, is measured using the rotating drum. A high-speed camera is also used to capture the flame images, with and without external forcing. For the non-excited condition, the turbulent burning velocity is assumed equal to the mean cold flow velocity at the height corresponding to the average flame lift-off measured using the drum. This measured turbulent burning velocity do not agree with correlations from the literature for u'/Sl <1. In this regime flames are affected by gas expansion and the growth of the Darrieus-Landau instability. For the excited condition, the flame lift-height at each phase angle in a cycle is tracked using the rotating drum. The ensemble averaged flame lift-off height shows sinusoidal movement similar to the imposed acoustic velocity, but lags the acoustic velocity by a certain phase, which depends upon the excitation frequency. The mean turbulent burning velocities are suppressed but the magnitude of the transfer function is non-zero at low Strouhal number and changes sharply at high Strouhal number.