An experimental study on flame propagation in lean fuel droplet-vapor-air mixtures by using microgravity conditions

A basic study of spray combustion has been made with a rapid expansion apparatus that can produce monodispersed fuel droplet clouds. The rapid expansion apparatus, which is essentially Wilson's cloud chamber, generates droplet clouds by reducing the pressure of saturated fuel vapor air mixtures. For droplet clouds more than 15 μm in mean droplet diameter, experiments were performed under microgravity conditions to prevent droplets from falling down by gravity. The mean diameter of droplet clouds was obtained from the laser lights scattered by droplets. Ethanol was used as a fuel. Pressure and total equivalence ratio were set 0.2 MPa and 0.8. The mean droplet diameter d m and the ratio of the liquid fuel mass to the total fuel mass were varied in the range of 8–24 μm and 0%–45%, respectively. Almost monodispersed and mono-sized droplet clouds were generated and ignited successfully under normal and microgravity conditions. Two types of flame propagation were observed. For the mixtures less than about 20 μm in mean droplet diameter, flames propagate in a smooth spherical shape like premixed gas flames. For the mixtures more than about 20 μm in mean droplet diameter, flames propagate in a rough spherical shape after the flames grow up to about 20 mm in diameter. The flame speed of the fuel droplet-vapor-air mixtures decreases with the increase in the mean droplet diameter of the mixtures. In the mixtures of d m