Determination of laminar flame speed of methaneair flames at subatmospheric conditions using the cone method and ch* emission

Experimental measurements of laminar fl ame speed for premixed methane-air fl ames were carried out for different equivalence ratios at subatmospheric conditions, 852 mbar and 298 K. The fl ames were obtained using a rectangular port burner with a water cooler system necessary to maintain the temperature of the mixture constant. An ICCD camera was used to capture chemiluminescence emitted by OH-CH radicals present in the fl ame and thus defi ne the fl ame front. Laminar fl ame speed was calculated using the cone method and experimental results were compared with those reported by other authors and the numerical simulations made with the software CHEMKIN using the GRIMECH 3.0 mechanism. In this work it was found that decreasing the barometric pressure from 1013 mbar to 852 mbar generated an increase of 7% in the laminar fl ame speed

Premixing quality and flame stability: A theoretical and experimental study

Models for predicting flame ignition and blowout in a combustor primary zone are presented. A correlation for the blowoff velocity of premixed turbulent flames is developed using the basic quantities of turbulent flow, and the laminar flame speed. A statistical model employing a Monte Carlo calculation procedure is developed to account for nonuniformities in a combustor primary zone. An overall kinetic rate equation is used to describe the fuel oxidation process. The model is used to predict the lean ignition and blow out limits of premixed turbulent flames; the effects of mixture nonuniformity on the lean ignition limit are explored using an assumed distribution of fuel-air ratios. Data on the effects of variations in inlet temperature, reference velocity and mixture uniformity on the lean ignition and blowout limits of gaseous propane-air flames are presented

Scale Effect of Premixed Methane-Air Combustion in Confined Space Using LES Model

Gas explosion is the most hazardous incident occurring in underground airways. Computational Fluid Dynamics (CFD) techniques are sophisticated in simulating explosions in confined spaces; specifically, when testing large-scale gaseous explosions, such as methane explosions in underground mines. The dimensions of a confined space where explosions could occur vary significantly. Thus, the scale effect on explosion parameters is worth investigating. In this paper, the impact of scaling on explosion overpressures is investigated by employing two scaling factors: The Gas-fill Length Scaling Factor (FLSF) and the Hydraulic Diameter Scaling Factor (HDSF). The combinations of eight FLSFs and five HDSFs will cover a wide range of space dimensions where flammable gas could accumulate. Experiments were also conducted to evaluate the selected numerical models. The Large Eddy Simulation turbulence model was selected because it shows accuracy compared to the widely used Reynolds\u27 averaged models for the scenarios investigated in the experiments. Three major conclusions can be drawn: (1) The overpressure increases with both FLSF and HDSF within the deflagration regime; (2) In an explosion duct with a length to diameter ratio greater than 54, detonation is more likely to be triggered for a stoichiometric methane/air mixture; (3) Overpressure increases as an increment hydraulic diameter of a geometry within deflagration regime. A relative error of 7% is found when predicting blast peak overpressure for the base case compared to the experiment; a good agreement for the wave arrival time is also achieved

Numerische Modellierung des Wandrückschlags in vorgemischten Wasserstoff-Luft-Verbrennungssystemen

In the current work, the numerical prediction of confined and unconfined BLF limits of hydrogen-air flames is investigated by means of large eddy simulations with detailed chemical kinetics, a detailed diffusion model and the quasi-laminar combustion model. It is shown that the chosen modelling approach is capable of quantitatively reproducing experimental flashback limits. Furthermore, detailed insight into the flashback process is obtained from the numerical simulations.In dieser Arbeit wird die Vorhersagbarkeit von Wandrückschlaggrenzen mit Hilfe von numerischen Simulationen untersucht. Hierzu werden Large Eddy Simulationen mit detaillierter Chemie, einem detaillierten Diffusionsmodell und dem quasi-laminaren Verbrennungsmodell angewandt. Die Simulationen des eingeschlossenen und nicht eingeschlossenen Wandrückschlags zeigen, dass der gewählte Modellierungsansatz geeignet ist, um experimentelle Rückschlaggrenzen quantitativ wiederzugeben. Des Weiteren ermöglichen die Simulationen einen detaillierten Einblick in den Rückschlagvorgang und dessen Einflussgrößen

Ultrashort-Laser-Pulse Multi-Photon Excitation Schemes for Combustion Diagnostics

Laser-based diagnostic approaches using ultra-short femtosecond (fs) laser pulses is a promising method for investigating complex chemically reacting flow fields such as flames. Femtosecond pulses offer several advantages over traditionally used nanosecond (ns) and picosecond (ps) pulses because of their broad spectral bandwidth, high peak power, and high repetition rates. They can be especially advantageous for efficient multi-photon excitation schemes. During the past decade, fs laser diagnostic has been developed and demonstrated primarily in gaseous flames and canonical geometries; however, their implementation for studying more practically relevant harsh combustion environments has been limited. Thus, the objective of this thesis research is to investigate methodologies to extend the applications of ultrashort, fs-pulse-based laser-induced fluorescence (LIF) techniques to imaging studies in realistic flame environments. This thesis consists of four main research tasks; (i) understanding the role of H-atoms in sooting formation by implementing ts two-photon LIF (fs-2pLIF) imaging, (ii) explore the applicability of fs imaging schemes in practically relevant hardware containing thick optical windows by using fs 3-photon LIF (fs-3pLIF) detection of H-atoms, (iii) extension of fs LIF methods for simultaneous multi-species detection using a single laser pulse by demonstrating H-atom and OH radicals imaging, and (iv) increasing imaging dimensionality by developing efficient, high-energy tunable laser source via direct frequency conversion. Under the first task, the previously demonstrated H-atom 2pLIF scheme ( = 205 nm) in laminar methane/air flames was extended to the harsh environment of heavily sooting flames for the first time. The implementation of several signal interference mitigation strategies enabled analyzing the relevance of H-atom concentration on soot formation pathways at atmospheric pressure ethylene/air flames. The experiments were performed on a series of sooting flames ranging from lean ( = 0.8) to very rich ( = 3.0). These studies revealed an inverse dependence of soot volume fraction (fV) on the H-atom number density ([H]), and a strong dependence of [H] and fV on flame temperature. In combustion test hardware such as optically accessible internal combustion (IC) engines and gas turbines test rigs, reactions take place at elevated pressures (10–50 bar) and hence involve test sections with thick optical windows. It has been realized that 205-nm deep UV (DUV) pulses can be problematic in cases with thick transmissive optics resulting in high absorption losses and other adverse nonlinear effects. Therefore, for detecting H atoms, red-shifting the excitation wavelength to 307-nm pulses and using 3pLIF excitation scheme can be beneficial in such situations. Under the second task, a detailed 2pLIF vs 3pLIF comparison was performed to characterize different levels of photolytic production, photoionization, and stimulated emission interferences. To the best of our knowledge, the H-atom 3pLIF scheme using = 307.7-nm photons was realized for the first time during this work. The implementation of the above 3pLIF scheme also enabled the extension of this diagnostic approach for simultaneous imaging of multiple species using a single excitation laser pulse. Under the third task, simultaneous detection of H-atom (via 3pLIF) and OH (via single-photon LIF) was achieved using the excitation wavelength of λ = 307.7 nm. This simultaneous detection provides an insight for understanding the effects of light species transport and preferential diffusion of H by revealing their spatial location with respect to the reaction zone marked by the OH radicals. The simultaneous H/OH detection is also important in understanding complex flame phenomena such as local extinction and reignition in turbulent flames and as soot formation pathways. In the last part of this thesis research, a more efficient wavelength generation scheme for acquiring high-energy tunable fs laser pulses was developed to generate high-energy UV pulsed for fs LIF imaging applications with increased dimensionality. The availability of high pulse energy is important for diagnosis in practical combustion systems to account for numerous transmission losses as well as increase the size of the field of view to discern turbulent structures. The high-efficiency third-harmonic generation (THG) scheme was implemented to generate tunable UV pulses near = 283 nm. The THG scheme was implemented for high-fidelity single-laser-shot planer LIF (PLIF) imaging of OH at 1-kHz data rate to study the flame structure and reaction zone in turbulent non-premixed flames using a high-speed imaging system. The above developments and the associated results discussed in this thesis is a significant step forward in implementing ultrashort pulse laser imaging techniques of intermediate chemical species for practically relevant combustion studies

Development and application of a laminar coflow burner for combustion studies at high pressure

In the present thesis, it is attempted to build a bridge between the most simplified systems that have already been studied in literature (laminar flame burners, in which simple fuels such as methane, ethane or ethylene are burnt at atmospheric pressure) and the much more complex and demanding environment that is found in practical combustion engines. This is accomplished by designing and constructing a high pressure vessel and laminar burner (HPVB) integrated with an evaporation system. The capability of the HPVB setup of burning vaporized liquid fuels in laminar diffusion and partially premixed flames brings the important opportunity to isolate more easily the impact of fuel chemistry on the combustion behavior of relevant fuels and biofuels. This experimental setup is particularly designed to offer optical accessibility for laser diagnostic techniques, also allowing their assessment and development at elevated pressures. Background information on the type of burners mostly used for combustion studies in laminar flames is given in chapter 2 with focus on measurements in a high pressure environment. A detailed description of the design capabilities of the HPVB setup and the flame stability issues encountered are also presented and discussed. The theory behind the laser diagnostic techniques applied in this work are reviewed in chapter 3. The chapters 4 to 6 present the measurements carried out to characterize a large range of flames from gaseous and vaporized liquid fuels using laser diagnostics. In chapter 7, the heat flux method for laminar burning velocity measurements is presented and a feasibility study is performed to extend the applicability of the method to higher pressures. This is accomplished by integrating the heat flux burner in the high pressure vessel of the HPVB setup. Chapter 8 summarizes the conclusions and provide recommendations

Effects of Equivalence Ratio and Iodine Number on NOx Emissions from the Flames of Biofuels and Hydrocarbons

Increased energy consumption in the United States has led to a demand for the development of new bio-derived fuels. As biofuels are used more frequently in diesel and gasoline engines it has become increasingly important to test the emissions resulting from the combustion of these fuels. This study was motivated by the need to test these fuels, predict their combustion and pollutant formation potential when used in engines, and provide quick feedback to fuel researchers on the combustion characteristics.This dissertation presents a technique that characterizes the combustion properties of liquid fuels based on the chemistry of the fuel alone. This includes the development of the method for the rapid characterization of combustion properties, such as emission index and flame radiation. Using this method provides a way of predicting the combustion behavior of the fuels without the use of an engine for existing hydrocarbon fuels and newly developed fuels such as biodiesel. This technique in comparison to engine testing studies requires only small amounts of fuel, time, and provides a method to compare fuels on a normalized basis.Increased pollutant emission NO was observed when burning biodiesel when compared to petroleum based diesel. This same trend has also been documented for various diesel engine studies; however, reasons for this increase have not been determined. Using the developed experimental method the cause of the formation of NO in diesel and biodiesel fuels was then studied. Equivalence ratio and iodine number were varied and their effect on the formation of NO studied for five different fuels: canola methyl ester, soy methyl ester, diesel, methyl stearate, and normal dodecane fuels

Spacecraft Fire Safety 1956 to 1999: An Annotated Bibliography

Knowledge of fire safety in spacecraft has resulted from over 50 years of investigation and experience in space flight. Current practices and procedures for the operation of the Space Transportation System (STS) shuttle and the International Space Station (ISS) have been developed from this expertise, much of which has been documented in various reports. Extending manned space exploration from low Earth orbit to lunar or Martian habitats and beyond will require continued research in microgravity combustion and fire protection in low gravity. This descriptive bibliography has been produced to document and summarize significant work in the area of spacecraft fire safety that was published between 1956 and July 1999. Although some important work published in the late 1990s may be missing, these citations as well as work since 2000 can generally be found in Web-based resources that are easily accessed and searched. In addition to the citation, each reference includes a short description of the contents and conclusions of the article. The bibliography contains over 800 citations that are cross-referenced both by topic and the authors and editors. There is a DVD that accompanies this bibliography (available by request from the Center for Aerospace Information) containing the full-text articles of selected citations as well as an electronic version of this report that has these citations as active links to their corresponding full-text article

Modeling and Numerical Simulations of Two-Phase Ignition in Gas Turbine

In order to meet the new international environmental regulations while maintaining a strong economic competitiveness, innovative technologies of aeronautical combustion chambers are developed. These technologies must guarantee fast relight in case of extinction, which is one of the most critical and complex aspects of engine design. Control of this phase involves a thorough understanding of the physical phenomena involved. In this thesis the full two-phase ignition sequence of an aeronautical engine has been studied, from the breakdown of the spark plug to thepropagation of the flame in the complete engine. For this purpose, Large-Eddy Simulations (LES) using a detailed description of the liquid phase (Euler-Lagrange formalism) and of the combustion process (Analytically Reduced Chemistry) were performed. The results also led to the development of a simplified model for the prediction of ignition probability map, which is particularly useful for the design of combustion chambers