Pressure effects on combustion of methanol and methanol/dodecanol single droplets and droplet pairs in microgravity

This paper presents the results of an experimental investigation on the com bustion of single droplets and two-droplet arrays of pure methanol and meth anol/dodecanol mixtures in air under microgravity conditions. The initial d roplet diameters d(0), were nominally 0.9 mm. The independent experimental variables were the ambient pressure (0.1-9.0 MPa), fuel mixture ratio (meth anol/dodecanol: 100/0-15/85), and interdroplet separation distance l (l/d(0 ) = 2.3-8.0). For pure methanol, the results show that the droplet lifetime decreases with increasing interdroplet separation distances at low pressur es. At higher pressures (3.0 MPa and above) the droplet lifetime was indepe ndent of separation distance. The flame extinguished at a finite droplet si ze only for pure methanol at 0.1 MPa, in qualitative agreement with theoret ical predictions. The extinction droplet diameter was nearly independent of the droplet spacing. Methanol/dodecanol-mixture droplets exhibited microex plosion for both single droplets and droplet arrays. The paper presents map s of the disruption regime for both single droplets and droplet pairs, The difference between the disruptive behavior of single droplets and droplet p airs is explained by differences in liquid-phase circulation induced by the gas-phase asymmetry of the droplet pair. The paper also presents results o f the dependence of the onset of disruption (in terms of both volume and ti me) on the pressure and initial fuel mixture ratio. (C) 2000 by The Combust ion Institute.