Pressure and Flow Residence Time Effects on Soot Properties in Counterflow Non-Premixed Hydrocarbon-Air Flames

Numerous studies have reported the adverse health, environmental, and climatic effects of aerosol or soot particulate emissions from the combustion of hydrocarbon fuels in boilers, furnaces, gas turbines and other internal combustion engines. Considering the significant dependence that our modern society places on hydrocarbon fuels, it is of ethical interest to reduce or mitigate the resulting pollutants. With advanced laser based diagnostic techniques under development, the potential for future regulation on particulate emissions provides further motivation. While production of some pollutant species is well understood, knowledge of soot particulate nucleation and growth remains in its infancy. Precise synthesis of flame generated carbon nanoparticles may also prove useful as an industrial and technical commodity to increase efficiency and reduce cost for a variety of applications. One of the most elementary and important effects on soot formation and growth relevant to modern combustion engines is that of pressure. Utilizing the simple laminar, steady counterflow burner configuration, the goal of this work is to investigate the effect of elevated pressure on the soot nucleation, growth, and oxidation mechanisms of hydrocarbon combustion over a wide range of flow residence times.

An absolute irradiance calibrated two-color time resolved Laser Induced Incandescence (LII) technique was developed and utilized to collect quantitative soot incandescence data for determination of soot particle temperature, primary particle size, soot volume fraction, and number density. The approach requires a comprehensive LII nano-scale heat transfer model with coupled extinction and elastic light scattering submodels. Thermophoretic soot sampling and Transmission Electron Microscopy (TEM) analysis were conducted to provide further insight into soot structure and morphology. Particle Image Velocimetry (PIV) was also employed to quantify flow residence time. All sources of uncertainty from both measured experimental variables and assumed heat transfer model variables were considered and included in a detailed uncertainty analysis to determine total uncertainties on all LII extracted soot quantities. The uncertainty analysis of PIV measurements was also presented. The resulting total standard uncertainties of soot volume fraction, primary particle size, and soot number density are reported. Several improvements to the LII procedure are presented. The results indicate that uncertainty associated with the local gas temperature, soot absorption function, and thermal accommodation are the dominant variables dictating uncertainty on all LII extracted quantities. Experimental repeatability uncertainty was found significantly lower than the combined total standard measurement uncertainty for soot volume fraction and primary particle diameter indicating additional research into key heat transfer model parameters is still needed to reduce the overall uncertainty of LII measurements.

The results of TEM analysis on thermophoretically sampled soot are presented. An extensive analysis of number density, soot volume fraction, and primary particle size is conducted. Particle nucleation is identified as the driver of total soot loading in the counterflow configuration and exhibits a clear pressure and temperature dependency. The results indicate that increased combustion pressure should be targeted to reduce total soot loading while allowing for increased combustion temperatures before soot particle nucleation is initiated. From thermodynamic first principles, increased pressure and temperature are directly related to improved engine efficiency and energy density. The finding is encouraging for practical application in industry.