Microgravity experiments on flame spread of an n-decane droplet array in a high-pressure environment

The flame spread phenomena of an n-decane droplet array in the supercritical pressure range were experimentally investigated in microgravity. Experiments were conducted at pressures up to 5.0 MPa, which is over the critical pressure of n-decane. Observations of the flame-spread phenomenon were conducted using OH-radical emission, Schlieren, and back-lit images recorded by a high-speed video camera. The flame-spread rates were calculated on the basis of the time history of the OH-emission images. In microgravity, the flame-spread rate decreased with increasing pressure, had a minimum at a pressure around half of the critical pressure, and then increased again. It had a maximum at the pressure near the critical pressure and then decreased gradually with pressure. In normal gravity, the flame-spread rate monotonously decreased and there was a pressure limit beyond which flame spread did not occur. Around the critical pressure, a jet-like flow of fuel vapor from an unburned droplet heated by the flame of a burning droplet was observed. The fuel-vapor jet from the side opposite the heating region of the unburned droplet reached another adjoining unburned droplet and then flame propagated along the jet, leading to heating prior to ignition of the unburned droplet. The mechanism of the fuel-vapor jet was examined based on the shear flow near the droplet surface induced by Marangoni convection of the unburned droplet heated non-uniformly by the burning droplet. The droplet internal flow rate was measured in normal gravity and confirmed the existence of Marangoni convection. The internal flow rate increased with pressure and had a maximum near the critical pressure. It was expected that the mechanism responsible for the maximum flame-spread rate near the critical pressure was the enhanced heat and mass transfer caused by the fuel-vapor jet and flame propagation along that jet.