Combustion of Submillimeter Heptane/Methanol and Heptane/Ethanol Droplets in Reduced Gravity

Reduced-gravity experiments were performed on combustion of droplets composed of n-heptane mixed with methanol or ethanol. The initial alcohol mass fraction in a droplet was 0% (pure heptane) or 5%. The experiments were performed at 0.1？MPa and 25°C with air or with ambients of oxygen and helium with oxygen mole fractions of 0.3 or 0.4. Initial droplet diameters were in the range 0.67？mm to 0.92？mm. After considering measurement uncertainties, burning rates decreased appreciably as the initial droplet diameter increased for combustion in air but not for combustion in the oxygen/helium environments. It was also found that addition of either methanol or ethanol did not influence burning rates appreciably and that burning rates were larger for the oxygen/helium environments than for air if initial droplet diameter dependences were accounted for. 1. Introduction Because global petroleum reserves are limited, there is a need to develop alternative fuels. Alcohols are considered to be promising for this purpose. For example, blends of methanol and ethanol with hydrocarbon fuels have been suggested as a possible approach to extending the availability of liquid petroleum-based fuels and also for reducing soot emissions. Blending of ethanol with hydrocarbons is presently employed in practice, for example, with oxygenated gasoline. It is thus of interest to study combustion droplets composed of blends of hydrocarbons with alcohols. There are numerous experimental methods available for studying the combustion characteristics of fuels. These methods range from experiments in practical devices such as engines, which can be difficult to control and interpret at a detailed level, to fundamental experiments that allow one to focus on specific details of the combustion under well-controlled conditions. In the present case, we are interested in combustion of liquid fuels. Because liquid fuels are typically burned as sprays, many researchers have focused on studying the behaviors of individual droplets, which is the methodology we pursue here. In particular, we employ reduced gravity so that the complicating effects of buoyancy are strongly reduced. We also study droplets in the submillimeter size range, which can be observed in detail. It is much more difficult to acquire useful data for the smaller droplets (O (10–100？ m)) that are generally present in practical sprays. The study of single droplet combustion is useful on both scientific and practical grounds. Studying individual droplets may enable the development of droplet submodels for computational models of sprays