Mohammad Reza Kholghy | University of Toronto (original) (raw)

Papers by Mohammad Reza Kholghy

Different non-intrusive optical and intrusive non optical diagnostic methods are used to measure ... more Different non-intrusive optical and intrusive non optical diagnostic methods are used to measure flame and soot properties in a laminar coflow diffusion flame in order to compare and analyze the sensitivity of each technique to soot particles with different age and morphology. Flame temperature is measured using rapid thermocouple insertion (RTI) method and also by measuring soot spectral emissions (SSE). Soot volume fraction (f v) is measured quantitatively with laser extinction (LE), time resolved laser induced incandescent (TiRe-LII) and SSE methods and qualitatively from the transmission electron microscope (TEM) images of the thermophoretically sampled soot particles. Particle internal/aggregate nanostructure, and primary particle diameter are also analyzed based on TEM images from the sampled particles and TiRe-LII. It is shown that the optical methods are only sensitive to mature soot particles with solid appearance and cannot detect either temperature or f v in regions where liquid like nascent soot particles are dominant. f v measured by LE and TiRe-LII agree well while the values measured by SSE are lower. This discrepancy is attributed to the high sensitivity of f v measured by SSE to the measured temperature values. Temperature profiles measured by SSE are considerably higher than the values measured by RTI. It is shown that not considering the change of the surface emissivity of the thermocouple junction due to particle deposition for estimating radiation loss in regions where nascent or mature soot particles are dominant contributes to this discrepancy. Primary particle sizes measured based on TEM images and TiRe-LII agree reasonably well. Soot aggregate fractal dimension is shown to decrease as the soot particles age and become more mature.

The oxidation of soot, obtained from 1-decene and ethylene flames, in a mixture of ionized and mo... more The oxidation of soot, obtained from 1-decene and ethylene flames, in a mixture of ionized and molecular O 2 , was observed in real time by environmental transmission electron microscopy for the first time. The oxidation mode (surface vs. internal), and rate, was measured for individual primary particles, demonstrating that mature primary particles primarily oxidize through surface reactions. Further experiments with less mature soot particles showed oxygen permeation into the core of the primary particles, causing internal oxidation, as well as surface reactions, demonstrating a link between soot ageing and the oxidation mode. Aggregate structural changes and fragmentation throughout oxidation were also characterized; with surface reactions weakening the bridges between primary particles until the aggregate breaks up. In the last stages of aggregate oxidation, the primary particles were seen to lose their graphitic shell and spherical nature, with the remaining disordered carbon reforming into large amorphous masses before burning away. The role of ionized oxygen species on oxidation rates is also discussed, and showed a strong dependence on electron beam voltage.

A fully-coupled soot formation model is developed to predict the concentration, size, and aggrega... more A fully-coupled soot formation model is developed to predict the concentration, size, and aggregate structure of soot particles in the atmospheric pressure laminar coflow diffusion flames of a three-component surrogate for Jet A-1, a three-component surrogate for a Fischer-Tropsch Synthetic Paraffinic Kerosene (SPK), and n-decane. To model the chemical structure of the flames and soot precursor formation, a detailed chemical kinetic mechanism for fuel oxidation, with 2185 species and 8217 reactions, is reduced and combined with a Polycyclic Aromatic Hydrocarbon (PAH) formation and growth scheme. The mechanism is coupled to a highly detailed sectional particle dynamics model that predicts the volume fraction, structure, and size of soot particles by considering PAH-based nucleation, surface growth, PAH surface condensation, aggregation, surface oxidation, fragmentation, thermophoresis, and radiation. The simulation results are validated by comparing against experimental data measured for the flames of pre-vaporized fuels. The objectives of the present effort are to more accurately simulate the physical soot formation processes and to improve the predictions of our previously published jet fuel soot formation models, particularly for the size and aggregate structure of soot particles. To this end, the following improvements are considered: (1) addition of particle coalescence submodels to account for the loss of surface area, reduction of the number of primary particles, and increase of primary particle diameters upon collision, (2) consideration of a larger PAH molecule (benzopyrene instead of pyrene) for nucleation and surface growth to enhance the agreement between the soot model and the measured chemical composition of soot particles, and (3) implementation of a dimerization efficiency in the soot inception submodel to account for the collisions between PAH molecules that do not lead to dimerization. The results of two different particle coalescence submodels show that this process is too slow to account for the growth of primary particles, mainly because of the limited rate of particle collisions. Soot volume fraction predictions on the wings and at lower flame heights are considerably improved by using benzopyrene, due to the different distribution of the soot forming PAH molecule in the flame. The computed number of primary particles per aggregate and the diameters of primary particles agree very well with the experimentally measured values after implementing the dimerization efficiency for PAH collisions, because of the reduced rate of soot inception compared to growth by PAH condensation. Concentrations of major gaseous species and flame temperatures are also well predicted by the model. The underprediction of soot concentration on the flame centerline, observed in previous studies, still exists despite minor improvements.

An experimental study is performed to investigate the evolution of soot morphology in an atmosphe... more An experimental study is performed to investigate the evolution of soot morphology in an atmospheric pressure laminar coflow diffusion flame of a three-component surrogate for Jet A-1. The laser extinction measurement method and the rapid thermocouple insertion technique are used to obtain soot volume fraction profiles and temperature profiles, respectively. Thermophoretic sampling followed by transmission electron microscopy and atomic force microscopy is used to study the morphology of soot particles at different locations inside the flame. Soot formation on the centerline appears to be different from conventional models. Liquid-like particles, which are transparent at the wavelength of 623 nm, are formed and grow up to a volume equivalent diameter of d p = 60 nm at temperatures below T = 1500 K. When the temperature exceeds 1500 K, transition of the transparent particles to the mature agglomerated particles happens immediately, i.e. in less than 12 ms. The volume of the liquid-like particles just before the start of their transformation to solid is about five times larger than the volume of mature primary particles. This significant size difference suggests that a large liquid-like particle does not transform into a single primary particle. In addition, multiple dark nuclei are observed in the liquid-like particles prior to carbonization. The significant size discrepancy and the presence of multiple dark nuclei may indicate that primary particle formation and agglomeration on the centerline happen inside the liquid-like particles. In contrast to the centerline, on another streamline with a significantly different temperature history, soot particles form from relatively small liquid-like particles. These particles have the same size as mature primary particles. Carbonization happens early on the streamline. A single dark nucleus grows inside each liquid-like particle and primary particles agglomerate after carbonization is completed. Most of the currently used computational soot models consider a single evolution process for all of the streamlines inside the flame which may not be an accurate assumption. This study shows that soot evolution processes may be different across the flame and are a function of temperature and the concentration of specific species inside the flame.

The effects of the ester moiety on soot formation and species concentrations in a laminar coflow ... more The effects of the ester moiety on soot formation and species concentrations in a laminar coflow diffusion flame of a surrogate for a B100 biodiesel are investigated. The surrogate is a mixture of 50% n-decane/ 50% methyl-octanoate (molar) to represent methyl-oleate. The combustion chemistry and soot formation are solved using a mechanism with 288 species and 2073 reactions coupled with a sectional soot model, respectively. Soot volume fraction (f v ) and temperature profiles are compared to the experimentally measured values for this biodiesel surrogate. In addition, the effects of the ester moiety on soot formation and flame chemistry are studied by numerically comparing the biodiesel surrogate flame with a pure n-decane flame. The model predicts both temperature and f v profiles with a good accuracy. Some discrepancies for f v on the flame centerline are observed between the model and the experiments; it is suggested that these discrepancies are because the model and the experiment cannot distinguish nascent transparent soot from mature soot and because the mechanism under-predicts PAH formation rates. Both n-decane and B100 surrogate flames have similar f v and temperature profiles when both flames have the same energy input. This suggests that the ester moiety does not have a major impact on soot formation. In addition, early production of CO and higher concentrations of some oxygenated species such as formaldehyde are observed in the predicted concentration contour plots of the B100 surrogate flame when they are compared to the ndecane flame. Reaction pathway analysis reveals that the higher peak concentrations of formaldehyde and the early production of CO from CH 2 CO and CH 3 CO 2 that come directly from the ester moiety in the B100 surrogate are much more pronounced than other species in the B100 surrogate flame and are recognized as the main differentiating characteristics of the B100 surrogate flame from the n-decane flame.

The present numerical and experimental study aims at enhancing the quantitative accuracy of soot ... more The present numerical and experimental study aims at enhancing the quantitative accuracy of soot formation predictions in atmospheric pressure, laminar, sooting, coflow diffusion flames of jet fuel, improving upon a previously published study that used a polycyclic aromatic hydrocarbon (PAH) nucleation based soot model. That model used a conventional acetylene-based PAH growth reaction scheme to calculate PAH concentrations in a Jet A-1 surrogate flame. Its results were compared to the experimental data for a real Jet A-1 flame. In the central region of the flame, that model, similar to many soot models in the literature, underpredicted soot concentration by more than one order of magnitude. The following improvements are made in the present work: (1) flame temperature and soot volume fraction profiles are experimentally measured in a flame using surrogate Jet A-1 rather than real Jet A-1, so that more direct comparisons can be made, (2) a novel reaction scheme for PAH growth (mechanism 2 in the present work), with a more comprehensive set of pathways for aromatic ring formation and growth, is used to model soot formation, and (3) the empirical soot surface growth parameter, a, is updated. The simulation data, using mechanism 2, are compared to the measurement results and another set of computational data, using the less detailed acetylene-based mechanism (mechanism 1). It is shown that only mechanism 2 can predict the correct order of magnitude of the centerline soot concentrations, reproducing them within a factor of one to five. Soot particles are shown to be exposed to similar temperatures and acetylene concentrations with both mechanisms. Hence, this improvement on the centerline is because of the enhanced PAH growth model, which produces higher levels of pyrene and increases the soot particle nucleation rate.

An experimental study is conducted for the laminar, atmospheric pressure, sooting, coflow diffusi... more An experimental study is conducted for the laminar, atmospheric pressure, sooting, coflow diffusion flames of prevaporized Jet A-1 and four synthetic jet fuels to compare their sooting characteristics and flame structures. Soot volume fraction, species concentration, and temperature profiles are measured, using the laser extinction measurement method, gas chromatography, and fine wire thermocouples, respectively. To evaluate the sooting tendency of the fuels, their smoke point heights are measured using the ASTM D1322 method and compared. The synthetic jet fuels under investigation are (1) Fully Synthetic Jet Fuel (FSJF), which is a coal-to-liquids (CtL) kerosene plus some coal tar derived material from Sasol, (2) FischerÀTropsch Synthetic Paraffinic Kerosene (FT-SPK), which is a gas-to-liquids (GtL) product from Shell, (3) SPK plus naphthenic cut (50% by volume), and (4) SPK plus hexanol (20% by volume). The threshold sooting index (TSI) of the proposed surrogate mixtures for the fuels are calculated and compared to the TSI of the real fuels to evaluate the performance of the surrogates in predicting the sooting behavior of the actual fuels. The surrogate mixtures underpredict the sooting tendency of the Jet A-1, FSJF, and SPK + naphthenic cut, although they correctly capture the trend. It is shown that the soot concentration and the TSI values are strongly dependent on the aromatic content of the fuels. Fuels with the largest soot concentrations and sooting indexes, in decreasing order, are Jet A-1, FSJF, SPK + naphthenic cut, FT-SPK, and SPK + hexanol. There are minor differences between the species concentration and temperature profiles of the heavily sooting flames (Jet A-1, FSJF, and SPK + naphthenic cut) and the lightly sooting flames (FT-SPK and SPK + hexanol), caused by the lower contribution of aromatics to the formation of light aliphatic species and the higher radiative heat transfer by soot particles in the heavily sooting flames. It is demonstrated that the soot levels in flames are proportional to benzene concentrations but not to acetylene levels. Thus, only aromatic-based inception models can capture the differences in soot formation for jet fuels.

A novel model called Surface Shell Formation (SSF) is developed which predicts soot maturity base... more A novel model called Surface Shell Formation (SSF) is developed which predicts soot maturity based on the equilibrium nanostructure of poly aromatic hydrocarbons (PAHs) inside soot primary particles. The characteristic isotropic core-graphitic shell internal nanostructure of soot primary particles is used to distinguish nascent from mature soot primary particles. A new sectional soot model is developed to track particle Hydrogen to Carbon (H/C) ratio and the growth in the molecular weight of PAHs inside soot primary particles. An independent Arrhenius term describes particle dehydrogenation/carbonization. It is shown that soot maturity depends on both particle size and H/C ratio. Graphitic shell formation in mature soot particles is related to the surface PAHs that change configuration from edge on surface for nascent soot to face on surface for mature soot particles. The new model is validated against experimental data for laminar premixed, partially premixed and diffusion flames. The SSF model addresses the two major limitations of the current soot modeling approaches. First, it predicts H/C ratio of soot particles by considering soot carbonization. Second, it distinguishes nascent soot from mature soot primary particles based on the internal nanostructure of soot primary particles and the presence of the graphitic shell.

Structural effects of biodiesel in terms of unsaturation, i.e. C = C double bonds, its position a... more Structural effects of biodiesel in terms of unsaturation, i.e. C = C double bonds, its position and the presence of the ester moiety on soot formation are studied in this paper. Laminar coflow diffusion flames of 5-decene, 1-decene, n-decane, and a biodiesel surrogate consisting of a 50%/50% molar blend of n-decane and methyl-octanoate are selected for the investigation. It is observed that the presence of unsaturation promotes soot formation in the flames by increasing soot chemical surface growth species and soot inception species (aromatics) as it is evident from the larger primary particle size and number density of soot particles in the 1-decene, and 5-decene flames compared to the n-decane flame, respectively. More centrally located C = C double bond further speeds up soot inception as it promotes the formation of aromatic species. This is also evident from earlier detection of soot on the centerline and higher soot primary particle number density of the 5-decene flame compared to 1-decene and n-decane flames measured by the laser extinction and thermophoretic sampling methods, respectively. Effects of the ester moiety on soot formation are more on the concentrations of soot chemical surface growth species because the biodiesel surrogate flame has similar number density of primary particles compared to the n-decane flame but soot primary particles have smaller diameters.