Turbulence effects on the early flame development of propane-air mixtures.

Abstract

The effect of turbulence on the initial flame kernel growth in a bomb is studied at the various turbulent conditions and near minimum ignition energy. Turbulence is generated by four fans. The temporal flame kernel growth in a propane-air mixture is photographed with a high speed laser schlieren system and a 35mm high speed camera. The experimental conditions are the fixed gap size of 1 mm, two different equivalence ratios of 0.7 and 0.8, and three different fan speeds of 1000, 2000 and 3000 RPM. The current and voltage waveforms of the spark are simultaneously measured for the computation of temporal spark power input. The mean convection velocity near the spark plug gap in a running engine is obtained by measuring the secondary current and voltage signals during the discharge of a conventional spark plug. A simple model is employed in which the spark path is modeled as a rectangular U-shape. The mean convection velocity calculated by the model needed calibration with a bench test which used a hot wire anemometer. This technique has a weak correlation at low velocities of 1-2 m/s, but correlates well at velocities up to 15 m/s. Based on some intuitive and multi-scale dimensional arguments, a relation between the Taylor and integral scales of turbulent velocity fluctuations in a fan stirred combustion bomb is established depending on the electrical power drawn by the fans. The proportionality constant is determined by a modest experimental program relying on hot wire measurements. Turbulence enhances the flame kernel growth more significantly for the lower equivalence ratio of 0.7 than that of 0.8. The minimum ignition energy depends on the equivalence ratio more strongly than the turbulence level. The flame kernel reaches the planar laminar adiabatic flame speed at earlier times as the equivalence ratio increases. A model is proposed from thermodynamic considerations. Initial conditions are obtained from the diffusion model for stagnant cases. Flame kernel growth between spark discharge and fully-developed turbulent flame is predicted by including the effect of the Taylor length scale and temperature. The model predicts flame kernel growth reasonably well with the measured spark energy input and turbulent scales.