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 combustion of single droplets and two-droplet arrays of pure methanol and methanol/dodecanol mixtures in air under microgravity conditions. The initial droplet diameters, d₀, were nominally 0.9 mm. The independent experimental variables were the ambient pressure (0.1-9.0 MPa), fuel mixture ratio (methanol/dodecanol: 100/0-15/85), and interdroplet separation distance l (l/d₀ = 2.3-8.0). For pure methanol, the results show that the droplet lifetime decreases with increasing interdroplet separation distances at low pressures. At higher pressures (3.0 MPa and above) the droplet lifetime was independent of separation distance. The flame extinguished at a finite droplet size only for pure methanol at 0.1 MPa, in qualitative agreement with theoretical predictions. The extinction droplet diameter was nearly independent of the droplet spacing. Methanol/dodecanol-mixture droplets exhibited microexplosion for both single droplets and droplet arrays. The paper presents maps of the disruption regime for both single droplets and droplet pairs. The difference between the disruptive behavior of single droplets and droplet pairs is explained by differences in liquid-phase circulation induced by the gas-phase asymmetry of the droplet pair. The paper also presents results of the dependence of the onset of disruption (in terms of both volume and time) on the pressure and initial fuel mixture ratio.