Temperature and velocity measurements of an acoustically excited jet diffusion flame

An experimental study is carried out to investigate the temperature and velocity distributions, and vortex interactions in an internal acoustically excited diffusion jet flame. One dimensional laser-Doppler velocimetry (LDV), and Rayleigh scattering are used. Titanium tetrachloride and laser sheet illumination are used for flow visualization. Jet diameter is 6.35 mm. Reynolds number for the steady cold flow is 1,300. Fuel mixture is methane (88.3%, weight) and hydrogen. The evolution of the large and small scale vortices in the flame structure are captured by a standard camcorder.;Acoustic effects, frequency and amplitude, on flame structure are studied. The temperature and velocity data are acquired at the point where the excitation signal passes its positive peak value. The evolution process of the induced small scale vortices is visible through the formation of titanium dioxide in the fuel/flame interface. The flow structures are observed on a monitor, and can be transferred onto VHS tapes simultaneously for future color hardcopy reproductions. This flow visualization method yields high quality pictures of the flame structure and is highly compatible with the conventional high speed film. The excitation parameters in this part of the study are from 60 to 245 Hz, and 0.5 to 10 Vpp. The fuel is resonant at subharmonic frequencies of 120 and 269 Hz. The induced vortices are strongly affected by the excitation frequency, and only quantitatively by the amplitude. However, the buoyancy induced large scale vortices formation frequency is slightly changed from 13.5 Hz in a non-excited flame to 13.75 Hz in an excited flame.;LDV and Rayleigh scattering techniques are applied to higher excitation frequencies flames, i.e., 200 Hz or higher. The studied flame will not respond to excitation frequency higher than 320 Hz. Under excitation, fuel/flame interface becomes curved which provides more surface area for the fuel-air diffusion, and small scale vortices are also induced. These vortices the fuel/air mixing and thus enhanced the combustion. The volume of the flames are smaller under excitation, and results in reduction in combustor size. Local fuel temperatures are higher in excited flames.;The above mentioned acoustic effects on flames will reduce combustion emission and pollution.