Soot formation in laminar diffusion flames at elevated temperatures

Abstract

The line-of-sight soot volume fractions and soot surface temperatures were measured as a function of axial position in overventilated axisymmetric laminar diffusion flames. The measurements were made at various temperatures of the reactants to study the influence of flame temperature on soot formation, and the temperature sensitivity of different fuels. The reactant temperatures were varied from 300 to 680 K. Soot volume fractions were measured by extinction, and soot particle temperatures were determined from the measurements of flame extinction and two wavelength brightness temperatures. In the first set of experiments, fuel flow rates for a given fuel were kept constant at all reactant temperatures. In the second set, the fuel flow rates were adjusted to obtain smoke point flames at all reactant temperatures. In both sets of experiments extensive data were obtained with ethylene, propylene, and isooctane. Propane data from our previous study are included in the assessment of effects of temperature on soot formation. Soot volume fraction and soot formation rate of olefins (propylene and ethylene) show a lower sensitivity to flame temperature than alkanes (propane and isooctane). Changes in maximum soot volume fractions and hence soot formation rates result from changes in the flame temperature, but these observed changes in soot volume fractions are not as large as those measured when adding an inert diluent to the fuel to alter the flame temperature. For this reason, the data obtained previously by other investigators to examine the temperature effects on soot information, through the nitrogen dilution of the fuel, should be reevaluated.