V-Shaped and M-Shaped Premixed Flame Response to Acoustic Excitations

An experimental study was conducted on the response of V-shaped and M-shaped premixed flames to external acoustic excitation. Low emission premixed combustors with anchored V-flames are susceptible to combustion instabilities. V-flames attach to an anchoring rod at the centre of the burner، whereas M-flames attach to the anchoring rod as well as the burner rim. This is in contrast to conical flames which are anchored only on the burner rim. As a consequence، V- and M-shaped flames are more sensitive to external flow perturbations. In the present study، a mixture of propane-air was used. The mixture equivalence ratio was changed to determine the stability range of V- and M-shaped premixed flames. The results indicated that M-shaped flames have a narrower stability range compared to that of V- flames. This limited the equivalence ratios range for acoustic excitation experiments. In order to study the flame response to external acoustic excitations، a loud speaker was placed upstream of the flame to generate acoustic waves in a certain range of frequencies. Pressure fluctuations caused by the acoustically excited flame were measured by a microphone placed downstream of the burner. Flame imaging was done with a CCD high speed camera during acoustic excitations. These images were used for flame response determination. Image processing was accomplished using the MATLAB software to obtain the amplitude and phase of the flame response. Variations of the flame shape and the intensity of light emitted by the flame were examined during a period of excitation for various values of equivalence ratios and flow rates. The results indicated that the vortex shedding and roll-up due to the velocity perturbations play an important role on the flame response. The phase response of flames evolves quasi-linearly with excitation frequency، indicating a certain time delay for the fluctuations to reach the flame surface. Also، in a certain range of excitation frequency، the flame gain response exhibits more sensitivity to variation of the equivalence ratio. Observations of the V- flame behavior during a period of excitation have led to determination of the effects of nonlinear parameters on the flame response.