The first goal of this thesis is to develop and validate a modeling tool into which fundamental combustion chemistry and aerosol dynamics theory are implemented for investigating soot formation/oxidation in multi-dimensional laminar coflow diffusion flames taking into account soot polydispersity and fractal-like aggregate structure. The second goal is to use the tool to study soot aggregate formation/oxidation in experimentally studied laminar coflow diffusion flames to advance the understanding of soot aggregate formation/oxidation mechanism. The first part of the thesis deals with the large CPU time problem when detailed models are coupled together. Using the domain decomposition method, a high performance parallel flame code is successfully developed. An advanced sectional aerosol dynamics model which can model fractal-like aggregate structure is successfully implemented into the parallel flame code. The performance of the parallel code is demonstrated through its application to the modeling of soot formation/oxidation in a laminar coflow CH4/air diffusion flame. The parallel efficiency reaches as high as 83%. The second part of the thesis numerically explores soot aggregate formation in a laminar coflow C2H4/air diffusion flame using detailed PAH-based combustion chemistry and a PAH-based soot formation/oxidation model. Compared to the measured data, flame temperature, axial velocity, C2H2 and OH concentrations, soot volume fraction, the average diameter and the number density of primary particles are reasonably well predicted. However, it is very challenging to predict effectively the average degree of particle aggregation. To do so, particle-particle and fluid-particle interactions that may cause non-unitary soot coagulation efficiency need to be considered. The original coagulation model is enhanced in this thesis to accommodate soot coagulation efficiency. Different types of soot coagulation efficiency are numerically investigated. It is found that a simple adjustment of soot coagulation efficiency from 100% to 20% provides good predictions on soot aggregate structure as well as flame properties. In the third part of the thesis, the effects of oxidation-driven soot aggregate fragmentation on aggregate structure and soot oxidation rate are studied. Three fragmentation models with different fragmentation patterns are developed and implemented into the sectional aerosol dynamics model. The implementation of oxidation-driven aggregate fragmentation significantly improves the prediction of soot aggregate structure in the soot oxidation region.

The current study represents the first application of commercial laser-induced incandescence (LII) instrumentation at the University of Toronto Combustion Research Laboratory, for the characterization of soot in atmospheric laminar co-flow diffusion flames. An experimental apparatus was designed to accommodate the optical diagnostic, and to provide the means to probe various regions of the flames. An experiment with a well-characterized non-smoking ethylene-air diffusion flame was carried out to validate the performance of the LII instrument. Three measurement heights were analyzed; those at 40, 50, and 60 mm above the fuel exit. The soot volume fraction results were found to be in good agreement with those from the literature. The highest value was found to be 8.3 ppm at a height of 40 mm. While the instrumentation could report primary particle diameters, it was determined from the validation trial that the results were still premature. Further work is needed to validate the results of the instrument, especially for the particle size data.

This work investigates the extinction limits of laminar diffusion counterflow flames for various gaseous (methane, syngas, biogas) fuels using a high flow rate counterflow burner designed and built for this work. Equal momenta of the fuel and oxidizer streams were not maintained to provide data to check the fidelity of the numerical schemes and their chemical mechanisms at "non-standard" conditions. Strain rate values at extinction were obtained as a function of fuel mole fraction. Preliminary work with the new burner found that the methane extinction limit results were consistent with results from literature. The results provide insight into the extinction limit conditions of the aforementioned fuels. The strain rate was found to increase with increasing fuel mole fraction. Extinction limit results indicated that fuels with the highest concentration of hydrogen have the greatest extinction limit, which is believed to be attributed to the high diffusivity and reactivity of hydrogen.

The current study represents the first application of commercial laser-induced incandescence (LII) instrumentation at the University of Toronto Combustion Research Laboratory, for the characterization of soot in atmospheric laminar co-flow diffusion flames. An experimental apparatus was designed to accommodate the optical diagnostic, and to provide the means to probe various regions of the flames. An experiment with a well-characterized non-smoking ethylene-air diffusion flame was carried out to validate the performance of the LII instrument. Three measurement heights were analyzed; those at 40, 50, and 60 mm above the fuel exit. The soot volume fraction results were found to be in good agreement with those from the literature. The highest value was found to be 8.3 ppm at a height of 40 mm. While the instrumentation could report primary particle diameters, it was determined from the validation trial that the results were still premature. Further work is needed to validate the results of the instrument, especially for the particle size data.

This work investigates the extinction limits of laminar diffusion counterflow flames for various gaseous (methane, syngas, biogas) fuels using a high flow rate counterflow burner designed and built for this work. Equal momenta of the fuel and oxidizer streams were not maintained to provide data to check the fidelity of the numerical schemes and their chemical mechanisms at "non-standard" conditions. Strain rate values at extinction were obtained as a function of fuel mole fraction. Preliminary work with the new burner found that the methane extinction limit results were consistent with results from literature. The results provide insight into the extinction limit conditions of the aforementioned fuels. The strain rate was found to increase with increasing fuel mole fraction. Extinction limit results indicated that fuels with the highest concentration of hydrogen have the greatest extinction limit, which is believed to be attributed to the high diffusivity and reactivity of hydrogen.

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Fully understanding soot formation in flames is critical to the development of practical combustion devices, which typically operate at high pressures, and fire suppression systems in space. Flames display significant changes under microgravity and high-pressure conditions as compared to normal-gravity flames at atmospheric pressure, but the exact causes of these changes are not well-characterized. As such, the effects of gravity and pressure on the stability characteristics and sooting behavior of laminar coflow diffusion flames were investigated. To study these effects, a new highly-scalable combustion modelling tool was developed specifically for use on large multi-processor computer architectures. The tool is capable of capturing complex processes such as detailed chemistry, molecular transport, radiation, and soot formation/destruction in laminar diffusion flames. The proposed algorithm represents the current state of the art in combustion modelling, making use of a second-order accurate finite-volume scheme and a parallel adaptive mesh refinement algorithm on body-fitted, multi-block meshes. An acetylene-based, semi-empirical model was used to predict the nucleation, growth, and oxidation of soot particles. Reasonable agreement with experimental measurements for different fuels and pressures was obtained for predictions of flame height, temperature and soot volume fraction. Overall, the algorithm displayed excellent strong scaling performance by achieving a parallel efficiency of 70% on 384 processors. The effects of pressure and gravity were studied for flames of two different fuels: ethylene-air flames between pressures of 0.5–5 atm and methane-air flames between 1–60 atm. Based on the numerical predictions, zero-gravity flames had lower temperatures, broader soot-containing zones, and higher soot concentrations than normal-gravity flames at the same pressure. Buoyant forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures lengthened the zero-gravity flames. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure which weakened as pressure was increased. This dependence decayed at a faster rate in zero gravity when pressure was increased beyond 1–10 atm.

The work presented in this thesis is the result of the KTH CICERO project “Dynamic Engine Performance” in which the main objective was to develop simple models foremission formation. The demand for such models is increasing, mainly due to the tightening emission legislation for diesel engines which has lead to more complex engines and thereby more laborious development and calibration processes. Simple emission models can be a valuable tool during the development phase, e.g. when used with models for gas exchange - and after-treatment systems, and for precalibration of the engine control settings. Since engines in automotive application typically work under dynamic load, the main prerequisites were that the models should be comprehensive enough to handle the extreme conditions that can occur in engines during load transients but still simple enough to be used for calibration. Two main approaches have been used; one where the combustion and emission formation processes were modeled from the flame front and downstream using equilibrium chemistry. In the other approach, the entire mixing/entrainment process was modeled and emission formation was modeled with kinetic chemistry. Both approaches were found to meet the requirements but had different advantages; the first, simpler approach had shorter calculation time while the latter was more comprehensive and required less tuning. The latter also resulted in a model for heat release rate which can be useful as a stand-alone model and allows the emission models to be used for untested conditions. Another objective in this project was to identify techniques/instruments that can be used for emission measurements during transient operation since these are essential for understanding of emission formation in these conditions and as validation data for the emission models. / QC 20110502