Flame structure and soot and carbon monoxide (CO) formation were studied in laminar co-flowing co-annular inverse diffusion flames (IDFs) in normal and microgravity. An IDF is a non-premixed flame that consists of an inner air flow surrounded by a fuel flow. Soot formation is important to understand because soot particles are a health concern and have a strong influence on flame radiation, while CO formation is important because of the role it plays in fire-related deaths. Soot formation in normal diffusion flames (NDFs) is difficult to study because soot forms in the center of the flame and is oxidized so that soot cannot be sampled easily. Soot formation can be studied more easily in IDFs because soot forms on the fuel side of the reaction zone, is convected away from the reaction zone without oxidizing, and cools quickly. Therefore, in IDFs, newly formed soot is easier to sample than in NDFs. Soot formation was studied in microgravity IDFs because buoyancy-induced vortices that affect soot formation are eliminated in the absence of gravity. Flame structure of IDFs must be understood because it determines where soot and CO are formed and how they evolve.
IDFs are also useful for studying soot and CO formation in underventilated fires because both IDFs and underventilated fires have insufficient oxygen for complete combustion. Therefore, they both emit unoxidized soot, CO, and other products of incomplete combustion. Soot emission from underventilated fires also contributes to rapid fire spread due to increased radiation heat transfer.
Laser diagnostics, including planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF) and planar laser-induced incandescence of soot particles (soot PLII), thermocouple probing, and video imaging, were used to study the flame structure and distributions of temperature, soot and PAH in ethylene and methane IDFs. Comparison of OH PLIF with visible images and thermocouple temperature measurements in an ethylene IDF showed that peak OH PLIF occurred on the air side very close to the reaction zone and coincided with peak temperature. Reaction zone height determined by OH PLIF was found to be less than visible flame height because the reaction zone was masked by luminous soot in an annular region around and above the reaction zone. Reaction zone heights increased with air flow rate and were predicted well using a modified version of Roper’s analysis for co-flowing co-annular NDFs.
Soot PLII was observed in ethylene IDFs on the fuel side very close to the reaction zone. Comparisons of radial profiles of soot PLII to radiation-corrected thermocouple temperature measurements in an ethylene IDF showed that, close to the reaction zone, peak soot PLII coincided with a temperature of about 1300 K. This temperature is known from the literature to be the minimum critical temperature for soot formation. Radially integrated soot PLII increased with axial position until a position downstream of the flame height, leveling off to a maximum constant value, in agreement with the trend reported in a previously published IDF soot extinction study. The fact that the soot PLII continued to increase downstream of the flame tip suggests that soot inception ceases above the reaction zone and that soot growth can occur at temperatures below the critical inception temperature, and consequently that IDFs may be used to isolate soot growth. The maximum radially integrated soot PLII increased linearly with increasing air flow rate, indicating that bulk soot production rate is proportional to near-reaction zone residence time.
In both methane and ethylene IDFs, PAH PLIF was observed on the fuel side of the reaction zone, outside the soot zone but close to it. The proximity of PAH to the soot layer suggests it may be a constituent in soot formation, in agreement with the literature. The intensity of PAH PLIF was an order of magnitude greater in ethylene IDFs than in methane IDFs. Radially integrated PAH PLIF increased monotonically with axial position for both fuels. PAH PLIF and predicted streamlines from computer modeling indicated that buoyancy-induced vortices occurred above the reaction zone where PAH and soot were present. Comparisons of PAH PLIF to computer modeling of an ethylene IDF showed that PAH PLIF followed predicted streamlines, temperature isotherms, and contours of constant mixture fraction.
Microgravity IDFs did not flicker, were more luminous, were slightly longer, were more rounded, and radiated more energy than IDFs in normal gravity. Soot, PAH, and other condensed materials collected from post combustion gases of IDFs had a greater fraction of elemental carbon in microgravity than in normal gravity. The elemental carbon fraction increased as air flow rate increased, and was also greater in ethylene IDFs than methane IDFs for both gravity conditions, which indicates that IDFs produce carbonized soot as well as soot precursor particles and that microgravity IDFs produce more carbonaceous soot than normal gravity NDFs. In both gravity conditions, IDFs emitted CO as a major component of the exhaust stream. Emission of CO was slightly greater in microgravity than in normal gravity and increased with air flow rate, which suggests that CO formation depends on residence time. Buoyancy-induced vortices that affected soot and PAH formation in normal gravity were eliminated in microgravity.