Structure and Soot Properties of Nonbuoyant Ethylene/Air Laminar Jet Diffusion Flames

Abstract

Observations of the structure and soot properties of round, soot-emitting, nonbuoyant, laminar jet diffusion flames are described, based on long-duration (175-230 s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia. Experimental conditions included ethylene-fueled flames burning in still air at nominal pressures of 50 and 100 kPa and an ambient temperature of 300 K with luminous flame lengths of 49-64 mm. Measurements included luminous flame shapes using color video imaging, soot concentration (volume fraction) distributions using deconvoluted laser extinction imaging, soot temperature distributions using deconvoluted multiline emission imaging, gas temperature distributions at fuel-lean (plume) conditions using thermocouple probes, soot structure distributions using thermophoretic sampling and analysis by transmission electron microscopy (TEM) and flame radiation using a radiometer. After an initial 20s flame stabilization period (caused by effects of ignitor disturbances, fuel flow rate adjustments and transient development of flame structure), the flames reached steady-state conditions aside from slow (quasisteady) changes due to pressure increases and ambient oxygen consumption within the test chamber caused by combustion. The present flames were larger, and emitted soot more readily, than comparable flames observed during ground-based microgravity experiments due to closer approach to truly steady conditions resulting from the longer test times and the reduced gravitational disturbances of the space-based experiments. Increasing the pressure from 50 to 100 kPa for soot-emitting flames of similar length caused maximum soot volume fractions to increase from 2 to 32 ppm and average primary soot particle diameters to increase from 24 to 40 nm, showing that soot emissions are the result of the relative rates of soot formation and oxidation and do not correlate closely with peak soot concentrations and primary particle sizes within the flames. In addition, comparable sootemitting buoyant laminar diffusion flames at normal gravity and 100 kPa have significantly smaller maximum primary soot particles (32 nm diameter implying roughly 50 percent less mass) than the nonbuoyant flames. It was also found that the tipopening phenomena associated with nonbuoyant sootemitting flames is caused by extinction of the flame near its tip due to radiative heat losses, which means that emissions of unburned fuel are associated with emissions of soot in the present nonbuoyant flames. Finally, soot production properties (characterized by maximum soot concentrations) are similar for various paths through the 50 kPa flame where effects of radiative extinction and soot particle thermophoresis are small, suggesting potential for a simple state relationship between soot concentrations and mixing level (mixture fraction) at flame conditions representative of many practical applications. This behavior follows because flame residence times are relatively independent of path for nonbuoyant laminar jet diffusion flames, and may help to explain the universality of many properties of soot emitted from practical flames (which generally are relatively nonbuoyant).