In the present doctoral thesis, fundamental experimental and numerical studies are conducted for the laminar, atmospheric pressure, sooting, coflow diffusion flames of Jet A-1 and synthetic jet fuels. The first part of this thesis presents a comparative experimental study for Jet A-1, which is a widely used petroleum-based fuel, and four synthetically produced alternative jet fuels. The main goals of this part of the thesis are to compare the soot emission levels of the alternative fuels to those of a standard fuel, Jet A-1, and to determine the effect of fuel chemical composition on soot formation characteristics. To achieve these goals, experimental measurements are constructed and performed for flame temperature, soot concentration, soot particle size, and soot aggregate structure in the flames of pre-vaporized jet fuels. The results show that a considerable reduction in soot production, compared to the standard fuel, can be obtained by using synthetic fuels which will help in addressing future regulations. A strong correlation between the aromatic content of the fuels and the soot concentration levels in the flames is observed. The second part of this thesis presents the development and experimental validation of a fully-coupled soot formation model for laminar coflow jet fuel diffusion flames. The model is coupled to a detailed kinetic mechanism to predict the chemical structure of the flames and soot precursor concentrations. This model also provides information on size and morphology of soot particles. The flames of a three-component surrogate for Jet A-1, a three-component surrogate for a synthetic jet fuel, and pure n-decane are simulated using this model. Concentrations of major gaseous species and flame temperatures are well predicted by the model. Soot volume fractions are predicted reasonably well everywhere in the flame, except near the flame centerline where soot concentrations are underpredicted by a factor of up to five. There is an excellent agreement between the computed and measured data for the numbers of primary particles per aggregate and the diameters of primary particles. This model is an important stepping-stone in the drive to simulate industry-relevant and multi-dimensional flames of practical liquid fuels, with detailed chemistry and soot formation.

An experimental study was conducted using axisymmetric co-flow laminar diffusion flames of methane-air, methane-oxygen and ethylene-air to examine the effect of pressure on soot formation and the structure of the temperature field. A liquid fuel burner was designed and built to observe the sooting behavior of methanol-air and n-heptane-air laminar diffusion flames at elevated pressures up to 50 atm. A non-intrusive, line-of-sight spectral soot emission (SSE) diagnostic technique was used to determine the temperature and the soot volume fraction of methane-air flames up to 60 atm, methane-oxygen flames up to 90 atm and ethylene-air flames up to 35 atm. The physical flame structure of the methane-air and methane-oxygen diffusion flames were characterized over the pressure range of 10 to 100 atm and up to 35 atm for ethylene-air flames. The flame height, marked by the visible soot radiation emission, remained relatively constant for methane-air and ethylene-air flames over their respected pressure ranges, while the visible flame height for the methane-oxygen flames was reduced by over 50 % between 10 and 100 atm. During methane-air experiments, observations of anomalous occurrence of liquid material formation at 60 atm and above were recorded. The maximum conversion of the carbon in the fuel to soot exhibited a strong power-law dependence on pressure. At pressures 10 to 30 atm, the pressure exponent is approximately 0.73 for methane-air flames. At higher pressures, between 30 and 60 atm, the pressure exponent is approximately 0.33. The maximum fuel carbon conversion to soot is 12.6 % at 60 atm. For methane-oxygen flames, the pressure exponent is approximately 1.2 for pressures between 10 and 40 atm. At pressures between 50 and 70 atm, the pressure exponent is about -3.8 and approximately -12 for 70 to 90 atm. The maximum fuel carbon conversion to soot is 2 % at 40 atm. For ethylene-air flames, the pressure exponent is approximately 1.4 between 10 and 30 atm. The maximum carbon conversion to soot is approximately 6.5 % at 30 atm and remained constant at higher pressures.

In the present thesis, the sensitivity of flame propagation to the turbulent motion of burning gases is investigated. The long-standing issue of the 'bending effect' is focused upon, which refers to the experimentally-observed inhibition of flame propagation velocity at high intensities of turbulence. Plausible mechanisms for the bending effect are investigated by isolating systematically the effects of turbulence intensity. By providing a novel perspective on this topic, the thesis addresses the fundamental limits of turbulent burning. The investigation employs Direct Numerical Simulation (DNS), which enables the basic conditions of burning to be controlled directly. A parametric DNS dataset is designed and generated by increasing turbulence intensity over five separate simulations. Effects of turbulent motion are isolated in this manner, such that the bending effect is reproduced in the variation of flame propagation velocity recorded. Subsequently, the validity of Damköhler's hypotheses is investigated to ascertain the mechanism of bending. Analysis of the DNS dataset highlights the significance of kinematic flame response in determining turbulent flame propagation. Damköhler's first hypothesis is found to be valid throughout the dataset, suggesting that the bending effect may be a consequence of self-regulation of the flame surface. This contradicts the dominant belief that bending occurs as a result of flame surface disruption by the action of turbulence. Damköhler's second hypothesis is found to be valid in a relatively limited regime within the dataset, its validity governed by flame-induced effects on the prescribed turbulent flow field. Therefore, this thesis presents turbulent flame propagation and the bending effect as emergent from the dynamics of a flame surface that retains its internal thermo-chemical structure. Finally, experimental validation is sought for the proposed mechanisms of bending. Comparisons have been initiated with measurements in the Leeds explosion vessel, based on which the widely accepted mechanism of bending was hypothesized twenty-five years ago. Modifications to the DNS framework warranted by this comparison have aided the development of novel computationally-efficient algorithms. The ongoing work may yield insights into the key mechanism of the bending effect in turbulent flame propagation.

Soot formation is one of the most complex phenomena in combustion science and an understanding of the underlying physico-chemical mechanisms is important. This work adopted both experimental and numerical approaches to study soot formation in laminar counterfl ow diffusion flames. As polycyclic aromatic hydrocarbons (PAHs) are the precursors of soot particles, a detailed gas-phase chemical mechanism describing PAH growth upto coronene for fuels with 1 to 4 carbon atoms was validated against laminar premixed and counter- flow diffusion fl ames. Built upon this gas-phase mechanism, a soot model was then developed to describe soot inception and surface growth. This soot model was sub- sequently used to study fuel mixing effect on soot formation in counterfl ow diffusion flames. Simulation results showed that compared to the baseline case of the ethylene flame, the doping of 5% (by volume) propane or ethane in ethylene tends to increase the soot volume fraction and number density while keeping the average soot size almost unchanged. These results are in agreement with experimental observations. Laser light extinction/scattering as well as laser induced fluorescence techniques were used to study the effect of strain rate on soot and PAH formation in counterfl ow diffusion ames. The results showed that as strain rate increased both soot volume fraction and PAH concentrations decreased. The concentrations of larger PAH were more sensitive to strain rate compared to smaller ones. The effect of CO2 addition on soot formation was also studied using similar experimental techniques. Soot loading was reduced with CO2 dilution. Subsequent numerical modeling studies were able to reproduce the experimental trend. In addition, the chemical effect of CO2 addition was analyzed using numerical data. Critical conditions for the onset of soot were systematically studied in counterfl ow diffusion ames for various gaseous hydrocarbon fuels and at different strain rates. A sooting temperature index (STI) and a sooting sensitivity index (SSI) were proposed to present the sooting tendencies of different fuels and their sensitivities to strain rates.

Proceeding from the aim to reduce global pollution emissions from the continuous burning of hydrocarbons stimulated by increasing energy demand, more efficient and ultra-low emissions’ combustion concepts such as the homogenous charge compression ignition engines (HCCI) have been developed. These new engines rely on the low temperature chemistry (LTC) combustion concept. A detailed investigation of the properties of cool flames, governed by LTC, is essential for the design of these new engines. The primary goal of this work was to build a fundamental counterflow experiment for cool flames studies in a diffusive system, to better understand combustion in LTC engines. The project was intended to provide a basic understanding of the low-temperature reactivity and cool flames properties of butane isomers under atmospheric pressure conditions. This was achieved by establishing self-sustaining cool flames through a novel technique of ozone addition to an oxygen stream in a non-premixed counterflow model. The ignition and extinction limits of butane isomers’ cool flames have been investigated under a variety of strain rates. Results revealed that establishment of cool flames are favored at lower strain rates. Iso-butane was less reactive than n-butane by showing higher ignition and extinction limits. Ozone addition showed a significant influence on cool flame ignition and sustenance; it was found that increasing ozone concentration in the oxidizer stream dramatically increased the reactivity of both fuels. Results showed increased fuel reactivity as the temperature of the fuel stream outlet increased. 4 A numerical analysis was performed to simulate ignition and extinction of the cool flame in diffusive systems. The results revealed that ignition and extinction limits of cool flames are predominantly governed by LTC. The model qualitatively captured experimental trends for both fuels; however, it overpredicted both ignition and extinction limits under all strain rates and ozone concentrations. The discrepancies were within a factor of eight for the ignition limit and a factor of two for the extinction limit. Finally, sensitivity analyses were conducted to understand the reactions responsible for cool flames ignition. It was found that majority of the sensitive reactions are those that occur at low temperatures.

Halon 1301 (CF3Br), or bromotrifluoromethane, had been widely used as fire-extinguishing agent for many years before its production and consumption were severely regulated by the Montreal Protocol due to its hazardous depletion effect to the stratospheric ozone layer. It is therefore imperative to find an effective replacement fire-fighting agent before the mandated deadline of the complete phase out of CF3Br. Currently there are intensive efforts in searching for an environmentally acceptable fire suppression replacement. This, however, is hampered by a lack of fundamental understanding of how CF3Br suppresses the chemical reactions in a flame environment so effectively. Recent experimental evidence has shown that the addition of CF3Br significantly reduced the burning velocity of premixed H2/Air flames by depleting the important radical species that are important to sustain chemical reactions. Extending this finding to understand the suppression of more complicated diffusion flames and unsteady three dimension turbulent flames in the presence of Halon 1301, however, still faces enormous challenge because of the prohibitive requirement of the computational power. The present chemical reaction mechanism for even the simplest hydrocarbon fuel (CH4) combustion involves more than 300 elemental reactions and the addition of CF3Br adds approximately 70 more elemental reactions. This large number of reactions and the associated large number of reaction species, many of which still involve uncertain reaction coefficients and thermodynamics properties, present significant computing challenges for applications in multidimensional non-premixed flames that are often encountered in practice. Therefore, it is of interest to systematically reduce the full chemical mechanism to a few global reactions while still maintaining the accuracy of the original mechanism. The present research systematically reduced the complex H2/Air/CF3Br chemical reaction mechanism with 94 initial elemental reactions to 5 global reaction steps. The reduced mechanism results in dramatic savings in computer time and is capable of predicting the major species and important steady state species with high accuracy. Through detailed sensitivity and production rate analysis the present research was able to find the key elemental reactions that are responsible for the fire suppression behavior of CF3Br. Predicted maximum concentrations of H and OH were found to correlate closely with the existing laminar burning velocity data measured for the premixed H2/Air/CF3Br flames. Better agreement with the experimental data was found when two activation energies for the two most important elementary reactions from QRRK calculations were adopted. The reduced mechanism developed through this research can be used to assist in the calculation and the understanding of fire suppression of CF3Br for more practical multidimensional nonpremixed laminar and turbulent flames, and the effort in searching for other effective fire suppressing agents.

No description available.

<p> Lifted non-premixed turbulent jet flames in the Transverse Instability Combustor (TIC) have been analyzed using qualitative and quantitative methods. Lifted flames in the TIC have been observed to stabilize about zero to five injector exit diameters downstream of the dump plane into the chamber and exhibit pulsating, unsteady burning. Anchored flames immediately begin reacting in the injector recess and burn evenly in a uniform jet from the injector exit through the entire optically accessible region. Statistically significant, repeatable behavior lifted flames are observed. It is shown that the occurrence of lifted flames is most likely for an injector configuration with close wall-spacing, second greatest for a configuration with close middle-element spacing, and lowest for a configuration with even element-spacing. For all configurations, of those elements that have been observed to lift, the center element is most likely to lift while the second element from the wall was likely. Flames at the wall elements were never observed to lift. Evidence is shown to support that close injector element spacing and stronger transverse pressure waves aid lateral heat transfer which supports flame stability in the lifted position. It is hypothesized that the stability of lifted flames is influenced by neighboring ignition sources, often a neighboring anchored flame. It is also shown that instances of lifted flames increase with the root-mean-squared magnitude of pressure fluctuation about its mean (P’ RMS) up to a threshold, after which flames stabilize in the anchored recess position.</p> <p>Dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) analyses of CH* chemiluminescence data is performed. It is found that lateral ignition of the most upstream portion of lifted flames is dominated by the 1W mode. Furthermore, it is shown that low-frequency high energy modes with spatial layers resemble intensity-pulses, possibly attributable to ignition. These modes are trademarks of CH* chemiluminescent intensity data of lifted flames. It was also shown that the residence time in the chamber may be closely associated with those low-frequency modes around 200 Hz. DMD and POD were repeated for a downstream region on the center element, as well as a near-wall element, highlighting differences between the lifted flame dynamics in all three regions. </p> <p>It is shown that lifted flames are best characterized by their burning behavior and in rare cases may stabilize in the recess, while still being “lifted”. Furthermore, it is shown that flame position differentiation can extend into an initial period of highly stable combustor operation. Dynamic mode decomposition is explored as potential method to understand physical building blocks of proper orthogonal spatial layers. Non-visual indicators of lifted flames within the high-frequency (HF) pressure signal are sought to seek a method that allows for observation of lifted flames in optically inaccessible combustors, such as those in industry. Some attributes of power-spectral diagrams and cross-correlations of pressure signals are provided as potential indicators. </p>

Deep knowledge of combustion phenomena is of great scientific and technological interest. In fact, better design of combustion equipments (furnaces, boilers, engines, etc) can contribute both in the energy efficiency and in the reduction of pollutant formation. One of the limitations to design combustion equipments, or even predict simple flames, is the resolution of the mathematical formulation. Analytical solutions are not feasible, and recently numerical techniques have received enormous interest. Even though the ever-increasing computational capacity, the numerical resolution requires large computational resources due to the inherent complexity of the phenomenon (viz. multidimensional flames, finite rate kinetics, radiation in participating media, turbulence, etc). Thus, development of capable mathematical models reducing the complexity and the stiffness as well as efficient numerical techniques are of great interest.The main contribution of the thesis is the analysis and application of the laminar flamelet concept to the numerical simulation of both laminar and turbulent non-premixed flames. Assuming a one-dimensional behavior of combustion phenomena in the normal direction to the flame front, and considering an appropriate coordinates transformation, flamelet approaches reduce the complexity of the problem.The numerical methodology employed is based on the finite volume technique and a parallel multiblock algorithm is used obtaining an excellent parallel efficiency. A post-processing verification tool is applied to assess the quality of the numerical solutions.Before dealing with flamelet approaches, a co-flow partially premixed methane/air laminar flame is studied for different levels of partial premixing. A comprehensive study is performed considering different mathematical formulations based on the full resolution of the governing equations and their validation against experimental data from the literature. Special attention is paid to the prediction of pollutant formation.After the full resolution of the governing equations, the mathematical formulation of the flamelet equations and a deep study of the hypothesis assumed are presented. The non-premixed methane/air laminar flame is considered to apply the flamelet modelling approach, comparing the results with the simulations obtained with the full resolution of the governing equations. Steady flamelets show a proper performance to predict the main flame features when differential diffusion and radiation are neglected, while unsteady flamelets are more suitable to account for these effects as well as pollutant formation. Assumptions of the flamelet equations, the scalar dissipation rate modelling, and the evaluation of the Lagrangian flamelet time for unsteady flamelets are specially analysed. For the numerical simulation of turbulent flames, the mathematical formulation based on mass-weighted time-averaging techniques, using RANS EVM two-equation models is considered. The laminar flamelet concept with a presumed PDF is taken into account. An extended Eddy Dissipation Concept model is also applied for comparison purposes. A piloted non-premixed methane/air turbulent flame is studied comparing the numerical results with experimental data from the literature. A clear improvement in the prediction of slow processes is shown when the transient term in the flamelet equations is retained. Radiation is a key aspect to properly define the thermal field and, consequently, species such as nitrogen oxides. Finally, the consideration of the round-jet anomaly is of significant importance to estimate the flame front position.In conclusion, flamelet modelling simulations are revealed to be an accurate approach for the numerical simulation of laminar and turbulent non-premixed flames. Detailed chemistry can be taken into account and the stiffness of the chemistry term is solved in a pre-processing task. Pollutant formation can be predicted considering unsteady flamelets.

Syngas, a gaseous mixture of H2, CO and diluents such as N2, CO2, is a clean fuel generated via gasification of coal or biomass. Syngas produced via gasification typically has low calorific values due to very high dilution levels (~60% by volume). It has been recognized as an attractive energy source for stationary power generation applications. The present work focuses on experimental and numerical investigation of syngas-air counterflow diffusion flames with varying composition of syngas. Laser-based diagnostic techniques such as Particle Imaging Velocimetry, Rayleigh thermometry and Laser-induced fluorescence have been used to obtain non-intrusive measurements of local extinction strain rates, temperature, quantitative OH and NO concentrations, respectively, for three different compositions of syngas. Complementing the experiments, numerical simulations of the counterflow diffusion flame have been performed to assess the performance of five H2/CO chemical kinetic mechanisms from the literature. The first part of the work involved determination of local extinction strain rates for six H2 /CO mixtures, with H2:CO ratio varying from 1:4 to 1:1. The extinction strain rates were observed to increase from 600 sec-1 to 2400 sec-1 with increasing H2:CO ratio owing to higher diffusivity and reactivity of the H2 molecule. Numerical simulations showed few mechanisms predicting extinction conditions within 5% of the measurements for low H2:CO ratios, however, deviations of 25% were observed for higher H2 :CO ratios. Sensitivity analyses revealed that the chain branching reactions, H+O2 <=>O+OH, O+H2 <=>H+OH and the third body reaction H+O2 +M<=>HO2 +M are the key reactions affecting extinction limits for higher H2:CO mixtures. The second phase of work involved quantitative measurement of OH species concentration in the syngas-air diffusion flames at strain rates varying from 35 sec-1 to 1180 sec-1. Non-intrusive temperature measurements using Rayleigh thermometry were made in order to provide the temperature profile necessary for full quantification of the species concentrations. The [OH] is observed to show a non-monotonous trend with increasing strain rates which is attributed to the competition between the effect of increased concentrations of H2 and O2 in the reaction zone and declining flame temperatures on the overall reaction rate. Although the kinetic mechanisms successfully captured this trend, significant deviations were observed in predictions and measurements in flames with H2:CO ratios of 1:1 and 4:1, at strain rates greater than 800 sec-1 . The key reactions affecting [OH] under these conditions were found to be the same reactions identified earlier during extinction studies, thus implying a need for the refinement of their reaction-rate parameters. Significant disagreements were observed in the predictions made using the chemical kinetic mechanisms from the literature in flames with high H2 content and high strain rate. The final phase of work focused on measurement of nitric oxide (NO) species concentrations followed by a comparison with predictions using various mechanisms. NO levels as high as ~ 48 ppm were observed for flames with moderate to high H2 content and low strain rate. Quantitative reaction pathway diagrams (QRPDs) showed thermal-NO, NNH and prompt-NO pathways to be the major contributors to NO formation at low strain rates, while the NNH pathway was the dominant route for NO formation at high strain rates. The absence of an elaborate CH chemistry in some of the mechanisms has been identified as the reason for underprediction of [NO] in the low strain rate flames. Overall, the quantitative measurements reported in this work serve as a valuable reference for validation of H2/CO chemical kinetic mechanisms, and the detailed numerical studies while providing an insight to the H2:CO kinetics and reaction pathways, have identified key reactions that need further refinement.