The Effect of Sound on Laminar Propane-Air Flames

Abstract

The application of sound of known intensity and a frequency of 12.7 kc to a laminar propane-air flame is found to produce a shift in the stability limits of the flame. The sound alters the flashback limit so that the flame becomes more stable and the blowoff limit so that the flame becomes less stable. It is shown that the normal burning velocity is not changed with the application of sound and that the flow-velocity profile in the tube is likewise unchanged. From the Lewis and von Elbe theory of flame stabilization it is then concluded that the “penetration distance” must be increased. The “penetration distances” are calculated from the flow-velocity distribution and the normal burning velocity and, near stoichiometric, are found to increase about 20 percent at the maximum sound intensity of 10 milliwatts/cm2. Sound at a frequency of 18.3 kc is found to affect the blow-off limit in a similar manner as the lower frequency but no appreciable effect is noted on the flashback limit. It is concluded that these results in combination with further experimental studies should yield useful information about the nature of the “penetration distance” and flame holding in general, but that sound at these low frequencies does not offer a useful tool for studying flame propagation. It is recommended that the effects of sound on practical burners be given careful study because the results presented indicate that the maximum quantity of energy that can be released in a given burner volume may be limited by the sound level in the combustion chamber.