Laminar flame speed of a novel liquid bio-fuel has been determined experimentally using a closed spherical combustion vessel of 29 L equipped with two pairs of fused silica windows for optical access at atmospheric pressure and elevated temperature conditions. Schlieren technique was used to visualize and record the temporal evolution of the outwardly spherical flame front, and an in-house developed Matlab code was employed to process the flame front images and calculate its area by applying several image processing techniques. The test conditions consisted of varying the fuel-air mixture equivalence ratio at atmospheric standard pressure and different initial temperatures. Validation of the present method was achieved by measuring and comparing the flame speed of methane/air and n-heptane/air mixture with their published counterparts. Experimental results revealed comparable laminar flame speed of the novel liquid biofuel (1, 3- dimethoxyoctane) to heavy liquid hydrocarbons such as n-heptane and isooctane, especially at stoichiometric and fuel rich conditions. Additionally, the flammability limits of this novel fuel showed similarities with those of gaseous hydrocarbons fuels (e.g. methane, ethane) but higher than those of liquid hydrocarbons (e.g. diesel, gasoline). / February 2016

Gasoline is a complex fuel containing hundreds of species, and, therefore, it is quite difficult to model all components present in gasoline. Alternatively, researchers tend to employ simpler surrogates that mimic targeted physical and chemical properties of gasoline. Two properties of gasoline, i.e., autoignition and laminar flame speed, play key role in the overall performance of spark-ignition and modern engines. For fuel-engine optimization, it is very important to have simple models which can accurately predict autoignition and laminar flame speed of gasoline. In this work, universal laminar flame speed correlation is proposed for typical gasolines. This correlation is based on Monte-Carlo simulations of randomly generated mixtures comprising of 21 gasoline-relevant molecules. Laminar flame speed of each molecule is numerically computed over a wide range of thermodynamic conditions using detailed chemical kinetic models, while flame speed of each mixture is estimated using a mixing rule. The proposed universal correlation is validated against experimentally-measured laminar flame speed of various gasoline fuels.

Natural gas is the primary fuel used in industrial gas turbines for power generation. Hydrocarbon blends of methane, ethane, and propane make up a large portion of natural gas and it has been shown that dimethyl ether can be used as a supplement or in its pure form for gas turbine combustion. Because of this, a fundamental understanding of the physical characteristics such as the laminar flame speed is necessary, especially at elevated pressures to have the most relevance to the gas turbine industry. This thesis discusses the equations governing premixed laminar flames, historical methods used to measure the laminar flame speed, the experimental device used in this study, the procedure for converting the measured data into the flame speed, the results of the measurements, and a discussion of the results. The results presented in this thesis include the flame speeds for binary blends of methane, ethane, propane, and dimethyl ether performed at elevated pressures, up to 10-atm initial pressure, using a spherically expanding flame in a constant-volume vessel. Also included in this thesis is a comparison between the experimental measurements and four chemical kinetic models. The C4 mechanism, developed in part through collaboration between the National University of Ireland Galway and Texas A&M, was improved using the data presented herein, showing good agreement for all cases. The effect of blending ethane, propane, and dimethyl ether with methane in binary form is emphasized in this study, with the resulting Markstein length, Lewis number (Le), and flame stability characterized and discussed. It was noticed in this study, as well as in other studies, that the critical radius of the flame typically decreased as the Le decreased, and that the critical radius of the flame increased as the Le increased. Also, a rigorous uncertainty analysis has been performed, showing a range of 0.3 cm/s to 3.5 cm/s depending on equivalence ratio and initial pressure.

Biomass is among the primary alternative energy sources that supplements the fossil fuels to meet today’s energy demand. Gasification is an efficient and environmental friendly technology for converting the energy content in the biomass into a combustible gas mixture, which can be used in various applications. The composition of this gas mixture varies greatly depending on the gasification agent, gasifier design and its operation parameters and can be classified as low and medium LHV gasified biomass. The wide range of possible gas composition between each of these classes and even within each class itself can be a challenge in the combustion for heat and/or power production. The difficulty is primarily associated with the range in the combustion properties that may affect the stability and the emission levels. Therefore, this thesis is intended to provide data of combustion properties for improving the operation or design of atmospheric combustion devices operated with such gas mixtures. The first part of this thesis presents a series of experimental work on combustion of low LHV gasified biomass (a simulated gas mixture of CO/H2/CH4/CO2/N2) with variation in the content of H2O and tar compound (simulated by C6H6). The laminar flame speed, lean blowoff limit and emission levels of low LHV gasified biomass based on the premixed combustion concept are reported in paper I and III. The results show that the presence of H2O and C6H6 in gasified biomass can give positive effects on these combustion parameters (laminar flame speed, lean blowoff limit and emission levels), but also that there are limits for these effects. Addition of a low percentage of H2O in the gasified biomass resulted in almost constant laminar flame speed and combustion temperature of the gas mixture, while its NOx emission and blowoff temperature were decreased. The opposite condition was found when H2O content was further increased. The blowoff limit was shifted to richer fuel equivalence ratio as H2O increased. A temperature limit was observed where CO emission could be maintained at low concentration. With C6H6 addition, the laminar flame speed first decreased, achieved a minimum value, and then increased with further addition of C6H6. The combustion temperature and NOx emission were increased, CO emission was reduced, and blowoff occurs at slightly higher equivalence ratio and temperature when C6H6 content is increased. The comparison with natural gas (simulated by CH4) is also made as can be found in paper I and II. Lower laminar flame speed, combustion temperature, slightly higher CO emission, lower NOx emission and leaner blowoff limit were obtained for low LHV gas mixture in comparison to natural gas. In the second part of the thesis, the focus is put on the combustion of a wide range of gasified biomass types, ranging from low to medium LHV gas mixture (paper IV). The correlation between laminar flame speed or lean blowoff limit and the composition of various gas mixtures was investigated (paper IV). It was found that H2 and content of diluents have higher influence on the laminar flame speed of the gas mixture compared to its CO and hydrocarbon contents. For lean blowoff limit, the diluents have the greatest impact followed by H2 and CO. The mathematical correlations derived from the study can be used to for models of these two combustion parameters for a wide range of gasified biomass fuel compositions. / <p>QC 20130411</p>

Predictive modeling of pre-chamber combustion engines relies primarily on the correct prediction of laminar and turbulent flame speeds. While the latter has been rigorously derived, the former correlations are mostly semi-empirical and valid for a limited range of operating conditions. The current work aims at highlighting the fundamental significance of correct laminar flame speed prediction on numerical modeling of ultralean prechamber engine combustion. Gulder's empirical correlation for laminar flame speed was chosen for the current work. It was modified for ranges beyond what it was originally derived for. It was initially observed that the numerical results that utilize Gulder's correlation for the laminar flame speed underperform compared to the one computed from the skeletal GRI3.0 by Lu and Law. In all cases, Peters' turbulent flame speed correlation was used, which evidences that any potential difference comes from the laminar flame speed. Using Lu and Law's chemical mechanism as a reference for laminar flame speed calculations, the values of the empirical constants α, η, and ξ in Gulder's correlation were optimized to yield more accurate flame speeds at ultralean engine conditions. The updated Gulder's correlation for methane was implemented in CONVERGE, a three-dimensional computational fluid dynamics (CFD) solver, and validated against the experimental engine results from KAUST. The flame topology was also explored to correlate the observed behaviors in the pressure predictions among all tested cases. Finally, the Borghi-Peters diagram provides insightful information on combustion regimes encountered in pre-chamber combustion engines.

The thesis comprises effects of electric fields on a fundamental study of spheri­cal premixed flame propagation.Outwardly-propagating spherical laminar premixed flames have been investigated in a constant volume combustion vessel by applying au uni-directional electric potential.Direct photography and schlieren techniques have been adopted and captured images were analyzed through image processing. Unstretched laminar burning velocities under the influence of electric fields and their associated Markstein length scales have been determined from outwardly prop­agating spherical flame at a constant pressure. Methane and propane fuels have been tested to assess the effect of electric fields on the differential diffusion of the two fuels.The effects of varying equivalence ratios and applied voltages have been in­vestigated, while the frequency of AC was fixed at 1 KHz. Directional propagating characteristics were analyzed to identify the electric filed effect. The flame morphology varied appreciably under the influence of electric fields which in turn affected the burning rate of mixtures.The flame front was found to propagate much faster toward to the electrode at which the electric fields were supplied while the flame speeds in the other direction were minimally influenced. When the voltage was above 7 KV the combustion is markedly enhanced in the downward direction since intense turbulence is generated and as a result the mixing process or rather the heat and mass transfer within the flame front will be enhanced.The com­bustion pressure for the cases with electric fields increased rapidly during the initial stage of combustion and was relatively higher since the flame front was lengthened in the downward direction.

The arising environmental concerns make emission reduction from combustion devices one of the greatest challenges of the century. Modern dry low-NOx emission combustion systems often operate under lean premixed turbulent conditions. In order to design and operate these systems efficiently, it is necessary to have a thorough understanding of combustion process in these devices. In premixed combustion, flame speed determines the conversion rate of fuel. The flame speed under highly turbulent conditions is defined as turbulent flame speed. Turbulent flame speed depends on laminar flame speed, which is a property of the combustible mixture. The goal of this thesis is to estimate laminar flame speed and NOx emissions under certain conditions for specific industrial gas turbines. For this purpose, an in-house one-dimensional code, GENE-AC, is used. At first, a data validation is performed in order to select an optimized chemical reaction mechanism which can be used safely with the fuels of interest in gas turbines. Results show that GRI-Mech 3.0 performs well in most cases. This mechanism is selected for further simulations. Secondly, laminar flame speed is calculated using GRI-Mech 3.0 at SGT-800 conditions. Results show that at gas turbine conditions, increasing ambient temperature and fuel to air ratio enhances flame speed, mainly due to faster reaction rates. Moreover, laminar flame speed is highly affected by fuel composition. In particular, adding hydrogen to a fuel changes chemical processes significantly, because hydrogen is relatively light and highly diffusive. Calculations are conducted over a range of equivalence ratios and hydrogen fractions in methane at atmospheric as well as gas turbine operating conditions. Results reveal some trends for changes in laminar flame speed, depending on hydrogen content in the mixture. The final part of the thesis involves the development of a reactor network model for the SGT-700 combustor in order to predict NOx emissions. The network model is built in GENE-AC based on results from available computational fluid dynamics (CFD) simulations of the combustor. The model is developed for full load conditions with variable pilot fuel ratios. The NOx emissions are predicted using GRI-Mech 3.0 mechanism. A parametric study shows the dependency of NOx emissions on equivalence ratio and residence time. For SGT-700 running on natural gas, NOx emissions are fitted to measurement data by tuning equivalence ratio and residence time. The model is then tested for a range of ambient temperatures and fuel compositions. It is found that, although the model can correctly predict the trends of ambient temperature and fuel effects on NOx emissions, these effects are to some extent over-estimated. Using future engine tests and amending calibration can improve the results.

The focus of the present work is on the empirical characterization and modeling of ignition trends of ternary blends of three distinct hydrocarbon classes, namely a methyl ester (C9H18O2), a linear alkane (n-C9H20), and a cycloalkane (MCH). Numerous surrogate biofuel formulations have been proposed in the literature, yet specific blending of these species has not been studied. Moreover, the effects of blending biofuel compounds with conventional hydrocarbons are not widely studied and a further point is the lack of studies paying specific attention to the effects of fuel variation within a given blended biofuel. To this end, a statistical Design of Experiments L9 array, comprised of 4 parameters (%MO, %MCH, pressure, and equivalence ratio) with 3 levels of variation, constructed in order to systematically study the effects of relative fuel concentrations within the ternary blend enabled variations in fuel concentration for methyl octanoate and MCH of 10% - 30% and 20% - 40%, respectively. Variation in pressure of 1 atm, 5 atm, and 10 atm and in equivalence ratio of 0.5, 1.0, and 2.0 were used, respectively. The fuel-volume percentage of n-nonane varied from 30% - 70%. In total, 10 ternary blends were studied. Ignition delay times for the ternary blends and for the three constituents were obtained by monitoring excited-state OH or CH transitions, A2Epsilon+ -> X2Pi or A2Delta -> X2Pi, respectively, behind reflected shock waves using a heated shock tube facility. Dilute conditions of 99% Ar (vol.) were maintained in all shock tube experiments with the exception of a separate series of n-nonane and MCH experiments under stoichiometric conditions which used 4% oxygen (corresponding to ~ 95% Ar dilution). Temperatures behind reflected shock waves were varied over the range 1243 < T (K) < 1672. From over 450 shock tube experiments, empirical ignition delay time correlations were constructed for all three pure fuels and a master correlation equation for the blended fuels. Ignition experiments conducted on the pure fuels at 1.5 atm indicated the following ignition delay time order, from shortest to longest: methyl octanoate < n-nonane < MCH. With increased pressure to 10 atm (nominal) the order remained, in general, consistent. Under fuel-lean conditions, ignition trends between methyl octanoate and n-nonane exhibited overlap at temperatures below 1350 K, below which the trends diverged with methyl octanoate having shorter ignition delay times. Similar behavior was observed under fuel-rich conditions, yet with the overlap occurring above 1450 K. Stoichiometric ignition trends did not display overlapping behavior under either 1.5 atm or 10 atm pressure. Laminar flame speed measurements were performed at 1 atm and an initial temperature of 443 K on the pure fuel constituents. Additional flame speed measurements of MCH were conducted at 403 K to compare with literature values and were shown to agree strongly with experiments conducted in a constant-volume apparatus. The experiments conducted herein, for the first time, measure laminar flame speeds methyl octanoate. A detailed chemical kinetics mechanism was compiled from three independent, well-validated models for the constituent fuels, where the sub-mechanisms for methyl octanoate and MCH were extracted for integration into a base n-nonane model. The compiled mechanism in the present study (4785 reactions and 1082 species) enables modeling of oxidation processes of the ternary fuel blends of interest. Calculations were performed using the compiled model relative to the base models to assess the impact of utilizing different base chemistry sets. In general, results were reproduced well relative to base models for both n-nonane and MCH, however results for methyl octanoate from both the compiled model and the base model are in disagreement with the results measured herein. Ignition delay times of the fuel blends are well-predicted for several conditions, specifically for blends at lean/high-pressure and stoichiometric/high-pressure conditions, however are not accurately modeled at fuel-rich, high-pressure conditions.

Research with heterogeneous mixtures involving solid particulate in closed, constant-volume bombs is typically limited by the powder dispersion technique. This work details the development of an experimental apparatus that promotes ideal conditions, namely a quiescent atmosphere and uniform particle distribution, for measuring laminar, heterogeneous flame propagation. In this thesis, two methods of dispersing particles are investigated. In the first, heterogeneous mixtures are made in a secondary vessel that is connected to the main experiment. Particles are dispersed into the secondary vessel by adapting a piston-driven particle injector, which has been shown to produce uniform particle distributions. The heterogeneous mixture is then transferred to the main bomb facility and ignited after laminar conditions are achieved. In the second method of dispersion, particles are directly injected into the main experimental facility using a strong blast of compressed air. As with the first approach, enough time is given (~4 minutes) for the mixture to become quiescent before ignition occurs. An extinction diagnostic is also applied to the secondary mixing vessel as well as the primary experimental facility (for both dispersion methods) to provide a qualitative understanding of the dispersion technique. To perform this diagnostic a 632.8-nm, 5-mW Helium-Neon (HeNe) laser was employed. Aluminum nano-particles with an average diameter of 100 nm were used in this study. It was found that for typical dust loadings produced with both dispersion techniques, a pure dust-air system would not ignite due to the current spark ignition system. Thus, a hybrid mixture of Al/CH4/O2/N2 was employed to achieve the project goal of demonstrating a system for controlled laminar flame speed measurements in aerosol mixtures. With the hybrid mixture, the combustion characteristics were studied both with and without the presence of nano-Al particles. Based on the experimental results, the simplicity of the "direct-injection" methodology compared to that of the "side-vessel" is desirable and will be further investigated as a viable alternative, or improvement, to the side-vessel technology.

Iso-octane, n-heptane, ethanol and their blends were tested in a constant volume combustion chamber to measure laminar burning velocities. The experimental apparatus was modified from the previous version to an automatically-controlled system. Accuracy and speed of data acquisition were improved by this modification. The laminar burning velocity analysis code was also improved for minimized error and fast calculation. A large database of laminar burning velocities at elevated temperatures and pressures was established using this improved experimental apparatus and analysis code. From this large database of laminar burning velocities, laminar flame speeds were extracted. Laminar flame speeds of iso-octane, n-heptane and blends were investigated and analysed to derive new correlations to predict laminar flame speeds of any blending ratio. Ethanol and ethanol blends with iso-octane and/or n-heptane were also examined to see the role of ethanol in the blends. Generally, the results for iso-octane and n-heptane agree with published data. Additionally, blends of iso-octane and n-heptane exhibited flame speeds that followed linear blending relationships. A new flame speed model was successfully applied to these fuels. Ethanol and ethanol blends with iso-octane and/or n-heptane exhibited a strongly non-linear blending relationship and the new flame speed model was not applied to these fuels. It was shown that the addition of ethanol into iso-octane and/or n-heptane accelerated the flame speeds. / text