Fundamental studies of aerosol combustion
The combustion of clouds of fuel droplets is of great importance in many industrial applications, such as gasoline and diesel engines, gas turbines and furnaces. Here, efficient combustion has to be combined with minimum noxious emissions. Aerosols also might produce a particularly hazardous explosion risk. To optimise their performance a fundamental understanding of the complex processes in aerosol combustion systems is necessary. A fundamental study of aerosol combustion has been conducted to quantify the parameters of importance. For this, a novel aerosol combustion apparatus was developed, that offers a well controlled environment with respect to aerosol properties, temperature, pressure and turbulence. Aerosols were generated using the Wilson cloud chamber principle of expansion cooling, which produces a homogeneously distributed, near monodisperse droplets cloud. Drop sizes of 10 to 30μm, pressures between 100 and 360kPa and temperatures of 263 to 292K were used. Laminar mixtures between the overall equivalence ratios of 0.8 and 1.2 were studied. A considerable burning velocity enhancement of up to 420% was observed. This enhancement was shown to be a function of drop size and liquid fraction. From the study, it was concluded that burning velocity enhancement probably is caused by the increase in surface area due to wrinkling, caused by the development of instabilities. At low temperature (<275K) the formation and destruction of wrinkles and cells was random. At higher temperatures (>290K) cell formation and division was progressive and traceable, like that observed in gaseous flames. Cellular acceleration at these temperatures was similar to that of gaseous flames. Stretch appeared to have a damping effect on the instabilities, caused by the aerosol. Oscillating flames were observed for some experimental conditions and these also showed enhanced flame speeds. These oscillations were possibly caused by aerodynamic interaction between droplets and gas motion ahead of the flame. Also Stretch and radiation probably influenced these oscillations. Inert glass particles in a gaseous fuel-air mixture had no effect on flame speed or structure. However, water aerosols caused significant burning velocity enhancement (50%). These findings contradict the hypotheses that fuel rich pockets, flame propagation through "easy-toburn" regions or a "grid-effect" trigger instabilities in aerosols. Comparison with a linear stability analysis of heat loss from the flame (Greenberg et al.,1998), yielded good qualitative agreement with the data of the present work.