Measurements of fluctuating temperature and velocity in a turbulent premixed flame

Abstract

An experimental study was carried out to elucidate the local structure of a turbulent premixed burner flame and to characterize quantitatively the wrinkled laminar flame. Detailed measurements were made of the mean temperature, rms of temperature fluctuation, probability density functions, mean velocity and turbulence intensity, using fine thermocouples and a laser anemometer. Eulerian spatial correlations of the temperature fluctuation were also obtained. As regards compensation for the thermal inertia of the thermocouple, it was found that the use of the time constant determined by the step response technique leads to unrealistic instantaneous temperature signals, and that plausible results are obtained using the time constant determined from the probability density functions. In the turbulent premixed burner flame, high amplitude temperature fluctuations (250°C–400°C) occur, being followed by small fluctuations in the burnt gas. High amplitude fluctuation is characterized by a relatively high frequency structure. It disturbs the flow and increases turbulence intensity. This is essentially different from low frequency turbulent mixing of the burnt gas and ambient air. Near the flame tip, the influence of mixing is found to extend to the flame due to the break up of the low frequency structure of the mixing zone, while it does not reach the flame near the burner exit. The temperature fluctuation is inhomogeneous and the microscale of the fluctuation increases gradually in the flame.