Spray breakup and structure of spray flames for low-volatility wet fuels

Abstract

Studies of high-water-content fuels (a.k.a., wet fuels) have demonstrated that, under proper conditions, stable combustion can be achieved at very high water concentrations. Stable spray flames of wet fuels have been attained with fuel/water mixtures having stoichiometric adiabatic flame temperatures as low as 251°C. In this study, we investigate low-volatility wet fuels, using glycerol as the fuel and ethanol as a stabilization additive. This study expands on previous work by determining the minimum amount of ethanol that needs to be added to a glycerol/water mixture to produce a stable flame and by investigating the spray dynamics and structure for these fuels, to delineate the mechanism of ignition and to understand how ethanol alters the vaporization behavior, droplet breakup, and spray dynamics. Detailed 2-D velocity, Sauter mean diameter (SMD), 2-D flux, and number concentration measurements were performed with a Phase Doppler Particle Analyzer (PDPA) in sprays of three fuel/water mixtures: (a) 30% glycerol/70% water, (b) 30% glycerol/10% ethanol/60% water, and (c) the same mixture as (b) but in a combusting spray. All percentages are by weight. Results show that the addition of ethanol to the glycerol/water mixture turns the hollow-cone spray pattern into a narrow full-cone pattern, leading to recirculation of fine droplets in the region just downstream of the nozzle, which is essential to ignition. The high concentration of fine droplets, along with the high vapor pressure and high activity coefficient of ethanol, lead to extremely rapid vaporization of ethanol in the inner recirculation zone. The combustion of the ethanol raises the temperature in this region, while the swirling flow brings heat upstream towards the nozzle, further enhancing stability. These results explain why the addition of 10% ethanol can lead to robust flames of glycerol/water mixtures that might not be expected to yield stable combustion.