Investigations of Sooting Laminar Coflow Diffusion Flames at Elevated Pressures

Abstract

Soot is a common byproduct of hydrocarbon based combustion systems. It poses a risk to human and environmental health, and can negatively or positively affect combustor performance. As a result, there is significant interest in understanding soot formation in order to better control it. More recently, the need to study soot formation in engine relevant conditions has become apparent. One engine relevant parameter that has had little focus is the ambient pressure. This body of work focuses on the formation of soot in elevated pressure environments, and a number of investigations are carried out with this purpose. Laminar coflow diffusion flames are used as steady, simple soot producers. First, a commonly studied flame configuration is further characterized. Coflow flames are frequently used for fundamental flame studies, particularly at elevated pressures. However, they are more susceptible to buoyancy induced instabilities at elevated pressures. The velocity of the coflow is known to have an effect on flame stability and soot formation, though these have not been characterized at elevated pressures. A series of flames are investigated covering a range of flowrates, pressures, and nozzle diameters. The stability limits of coflow flames in this range is investigated. Additionally, an alternative strategy for scaling these flames to elevated pressures is proposed. Finally, the effect of coflow rate on soot formation is evaluated. Identification of fundamental flames for coordinated research can facilitate our understanding of soot formation. The next study of this work focuses on adding soot concentration and particle size information to an existing fundamental flame dataset for the purpose of numerical model validation. Soot volume fraction and average particle diameters are successfully measured in nitrogen-diluted ethylene-air laminar coflow flames at pressures of 4, 8, 12, and 16 atm. An increase in particle size with pressure is found up to 12 atm, where particle sizes plateau. Particle size in the annulus is more sensitive to pressure. Next, the development of an alternative particle size measuring technique is studied. Time Resolved Laser Induced Incandescence (TiRe-LII) is a commonly used technique to measure soot concentrations and particle size at atmospheric pressure. However, Laser Induced Incandescence (LII) models suffer from an incomplete understanding of the effects of elevated pressures on the absorption, annealing, and cooling of soot. The present study focuses on what affect the laser temporal pulse shape and duration may have on particle sizing. TiRe-LII in flames at 1 and 15 bar is carried out, using laser pulses with tophat or Gaussian temporal profiles of varying duration. Mono-disperse equivalent primary particle diameters are calculated using the KAUST LII model. Little difference in particle sizing is found for different laser pulses. However, this data will be useful for validating the KAUST LII model when absorption and poly-dispersion are accounted for. In an effort to move one step closer to logistical fuel studies, the sooting tendencies of a number of liquid fuels are studied at pressures up to 10. Of parallel relevance, a sooting index for surrogate development is evaluated for elevated pressure applications. The Yield Sooting Index (YSI) methodology is applied to 11 normal, cyclic, and branched alkanes. When referencing to two n-alkane fuels, the YSI of n-alkanes determined at atmospheric pressures accurately reflects the relative sooting tendencies of these fuels at elevated pressures. The relative sooting tendencies of cyclo- and methyl-alkanes have a lower pressure sensitivity than n-alkanes.