Search

Experimental observations of two-stage autoignition dynamics of fiber-supported normal dodecane droplets in air under normal gravity are presented for a range of pressures and temperatures. High-sp...

This paper describes reduced-gravity experiments on the combustion of droplets composed of metha-nol/dodecanol and ethanol/dodecanol mixtures. The experiments used the NASA Glenn Research Center 2·2 s drop tower as well as facilities in Spacelab during the USML-2 flight. In the drop-tower experiments, initial droplet diameters ranged from 0·8 mm to 1.2 mm, initial dodecanol mass fractions, Y, (in both mixtures) were 0·1, 0·25 and 0·5, and the ambient gas mixture was either an 02/He mixture (with a molar O2 concentration of 0·5) or atmospheric air. Most drop-tower experiments were at 0·1 MPa ambient pressure, although two tests were at 0·04 MPa. In the Spacelab experiments, the methanol/dodecanol droplets burned in ambient shuttle air (0·21 oxygen mole fraction) at 0·1 MPa, the initial droplet sizes were in the range 4·2 mm to 5·4 mm, and the initial values of Y were 0·2 and 0·4. Burning rates varied with experimental conditions, and disruptive burning was very frequent. One of the largest methanol/dodecan...

This paper addresses the behavior of a candle flame in a long-duration, quiescent microgravity environment both on the space Shuttle and the Mir Orbiting Station. On the Shuttle, the flames became dim blue after an initial transient where there was significant yellow (presumably soot) in the flame. The flame lifetimes were typically less than 60 seconds. The safety-mandated candlebox that contained the candle flame inhibited oxygen transport to the flame and thus limited the flame lifetime. The flames on the Mir were similar, except that the yellow luminosity persisted longer into the flame lifetime because of a higher initial oxygen concentration, The Mir flames burned for as long as 45 minutes. The difference in the flame lifetime between the Shuttle and Mir flames was primarily the redesigned candlebox that did not inhibit oxygen transport to the flame. In both environments, the flame intensity and the height-to-width ratio gradually decreased as the ambient oxygen content in the sealed chamber slowly decreased. Both sets of experiments showed spontaneous, axisymmetric flame oscillations just prior to extinction. The paper also presents a numerical model of a candle flame. The formulation is two-dimensional and time-dependent in the gas phase with constant specific heats, thermal conductivity and Lewis number (although different species can have different Lewis numbers), one-step finite-rate kinetics, and gas-phase radiative losses from CO2 and H2O. The treatment of the liquid/wick phase assumes that the, fuel evaporates from a constant diameter sphere connected to an inert cone. The model predicts a steady flame with a shape and size quantitatively similar to the Shuttle and Mir flames. The computation predicts that the flame size will increase slightly with increasing ambient oxygen mole fraction. The model also predicts pre-extinction flame oscillations if the rate of decrease in ambient oxygen is small enough, such as that which would occur for a flame burning in a sealed ambient.

The objective of this research is to improve understanding of the combustion of binary fuel mixtures in the vicinity of the critical point. Fiber-supported droplets of mixtures of w-heptane and n-hexadecane, initially 1 mm in diameter, were burned in room-temperature air at pressures from 1 M Pa to 6 M Pa under free-fall microgravity conditions. For most mixtures the total burning time was observed to achieve a minimum value at pressures well above the critical pressure of either of the pure fuels. This behavior is explained in terms of critical mixing conditions of a ternary system consisting of the two fuels and nitrogen. The importance of inert-gas dissolution in the liquid fuel near the critical point is thereby re-emphasized, and nonmonotonic dependence of dissolution on initial fuel composition is demonstrated. The results provide information that can be used to estimate high-pressure burning rates of fuel mixtures.

Recent experimental work in the area of spark ignition of hydrocarbon fuel sprays has demonstrated that it is impossible to define a unique minimum ignition energy. Instead there exists a range of energies over which ignition becomes more or less probable. In connection with recent experimental and modelling work studying the spark ignition of a monodisperse fuel spray, a model has been developed to predict the ignition frequency based on the variation of the spark energy and the instantaneous spark gap equivalence ratio. Random normal distributions of these two parameters were generated with a Monte Carlo routine and used in conjunction with a Characteristic Time Model for ignition. The resulting ignition frequency simulations were nearly identical to the experimentally observed values, supporting the hypothesis that the probabilistic nature of spray ignition is the result of variations in the energy levels of individual sparks and the random distribution of droplets in the fuel spray.