Combustion of single droplets and droplet pairs in a vibrating field under microgravity

Abstract

This paper presents results of an experimental investigation on acoustic effects on combustion of single droplets and droplet pairs in microgravity. The ambient gas was air at atmospheric temperature and pressure, with octane as the fuel. A loudspeaker at the bottom of the chamber produced the acoustic field. Experimental results of single droplets showed that at low frequency and small to moderate acoustic intensities the evaporation rate increases, and the burning rate constant is nearly proportional to the product of frequency, f, and square of displacement, Xa2, fXa2. At higher acoustic intensities, the burning rate constant either remains constant or decreases, and in some cases, flame extinction occurs at a finite droplet diameter. The burning rate constant for a droplet pair is consistently lower than that for a single droplet. At lower frequencies, the burning rate constant reaches a maximum at an intermediate acoustic intensity. At higher frequencies, the burning rate constant increases monotonically with increasing acoustic intensity. The flame size decreases as a result of interactions, as does the critical spacing that indicates a merged flame around the droplet pair versus individual flames surrounding the droplets. The results also show that interactions stabilize the flame, in that droplet pairs burn to completion under conditions in which the flame surrounding a single droplet extinguishes at a finite droplet diameter.