PRESSURE HOT-WIRE AND LASER DOPPLER ANEMOMETER STUDIES OF FLAME ACCELERATION IN LONG TUBES

Results are presented from an experimental study of the propagation of premixed natural gas-air flames in a long rectangular duct closed at one end. Oscillations in pressure have been observed which arise from acoustic interactions between the flame and the open end. The measured frequencies and amplitudes are in good agreement with the values predicted using a theoretical model proposed by Jones [Proc. R. Soc. Lond. A 367:291 (1979)]. Flame front motion was monitored using photodiodes. These show that after an initial slow acceleration an oscillatory phase developed, corresponding to the observed acoustic interactions. This was followed by rapid flame acceleration during the final stages of propagation just prior to the flame front exiting the tube. Hot-wire and laser doppler anemometer determinations of the mean and turbulent flow field were also taken and the results compared. In general the laser doppler signals were consistently lower than the hot-wire values, and this is ascribed to the variations in the wire calibration due to temperature changes in the flow. In addition, and unlike the hot-wire, the laser technique continued to provide valid data as the reaction zone traversed the measuring point. From these velocity determinations it is concluded that, for the present experimental configuration, flame acceleration is not strongly dependent on the root mean square turbulent flow velocity. The onset of a final rapid acceleration phase is attributed to turbulence arising from interactions of the pressure waves propagating in the tube with the density discontinuity at the flame front. At later times, the relative importance of shear generated turbulence and that arising from the interaction of pressure and density fields is difficult to quantify.