Instantaneous velocity and temperature measurements in oscillating diffusion flames

Abstract

Measurements of instantaneous velocity, instantaneous temperature, and the corresponding mean and r. m. s. values, obtained in a range of diffusion flames, are presented. The velocity measurements were obtained with a laser anemometer and the temperature measurements with a thermocouple. The flames were formed by burning methane, town gas and hydrogen at the exit from burner tubes of external diameters 15.0, 9.2, 6.3 and 3.2 mm; the corresponding inside diameters were 13.0, 5.1, 5.1 and 2.5 mm. The mean velocity of the gas at exit from the tube ranged from 0.6 to 5.3 m/s. All flames exhibited discrete frequencies in the vicinity of 11 Hz. The instantaneous velocity and temperature signals were close to sinusoidal, except in the vicinity of the reaction zone where double frequencies and spiky signals were observed. The r. m. s. temperature distributions exhibited minima in the region of the maximum values of mean temperature; the r. m. s. velocity distributions were similar in form but the location of the minimum occurred downstream of the corresponding r. m. s. temperature minimum. The minimum in the r. m. s. velocity and temperature distributions were consistant with the observed frequency doubling and stemmed from the need for the frequency to increase to allow an increase in mean temperature in regions where the instantaneous temperature had attained its adiabatic flame value. A flame model is postulated and shown to represent the many observed features of the oscillating flames. It appears that the oscillations stem from aerodynamic instabilities associated with inflexion points in the local, instantaneous velocity distributions.