Laminar burning speed is a thermophysical property of a combustible mixture. It is a measure of the rate of energy released during combustion in quiescent gas mixtures and incorporates the effects of overall reaction rates, energy (heat) of combustion and energy and mass transport rates. There are several experimental techniques to measure laminar burning speed and they can be broadly categorized into two general categories of stationary flames methods and those that are based on propagating flames. Investigation of spherical flame propagation in constant volume vessels is recognized to be one of the most accurate approaches for laminar burning speed measurement and flame structure study. In this thesis flame structure, laminar burning speed and onset of autoignition are studied for different premixed combustible mixtures including n-decane, jet-fuels, and Hydrofluorocarbon (HFC) refrigerants in air at high temperatures and pressures over a wide range of fuel-air equivalence ratios. The experimental facilities consist of two spherical and cylindrical vessels. The spherical vessel is used to collect pressure data to measure the burning speed and cylindrical vessel is used to take pictures of flame propagation with a high speed CMOS camera located in a shadowgraph system. A thermodynamic model is employed that assumes unburned gases compress isentropically and that burned gases are in local thermodynamic equilibrium. Burning speed is derived from the time rate change of mass fraction of burned gases. The major advantages of this method are that it circumvents the need for any extrapolation due to having low stretch rates and that many data points can be collected along an isentrope in a single experiment. Flame structures are studied to determine the cell formation conditions. Critical pressures at which the flame becomes cellular are identified and the effects of important parameters on cell formation are studied. Autoignition experiments are carried out for JP-8 fuels with high initial pressures in the spherical chamber. Autoignition occurs at specific temperature and pressure during the compression of unburned gas due to flame propagation.

Research studies into understanding the fundamental thermodynamic properties of Syngas are extremely relevant in engineering combustion modelling for stationary turbine based power plants and for IC engines. The models rely heavily on accurate experimental results and of critical importance is the data for: chemical kinetic rates, mass burning rate and auto-ignition conditions. To produce some of this necessary data, an experimental facility capable of measuring the pressure rise from spherically expanding flames was developed. The first core component of the facility includes a spherical combustion vessel that enables the measurement of the pressure rise from a combustion process, at high initial temperature and pressure. The second core component of the facility, which includes a lower pressure cylindrical combustion vessel, with optically clear sides, enables the direct measurement of laminar flame speed as well as the visualization of expanding spherical flames for the study of flame structures. An improved model to complement the experimental measurements has been developed. Using the thermodynamic model, the mass burning rate and laminar burning speed can be calculated from the pressure data measured using one of the two constant volume combustion vessels. The model is used to calculate the mass fraction of the burned gas by simultaneously solving the conservation of mass and energy equations coupled with equilibrium concentrations calculation of the combustion products using STANJAN. For smooth non-cellular flames laminar burning speed can be also calculated. The facility and model have been used and improved by many of the researchers at Northeastern University with results published in a variety of technical journals, and thus expanding the measured range of laminar flame speed. This thesis will describe in detail the experimental apparatus and report the laminar burning speed and mass burning rate for Syngas-Air and Syngas-O2-He at high temperature and pressure as well as auto-ignition characteristics of n-Heptane and GTL (S8), which is a synthetic surrogate for aviation fuel.

Flame speed data reported in most literature are acquired in conventional apparatus such as the spherical combusion bomb and counter flow burner, and are limited to atmospheric pressure and ambient or slightly elevated unburnt temperatures. As such, these data bear little relevance to internal combustion engines and gas turbines, which operate under typical pressures of 10-50 bar and unburnt temperature up to 900K or higher. These elevated temperatures and pressures not only modify dominant flame chemistry, but more importantly, they inevitably facilitate pre-ignition reactions and hence can change the upstream thermodynamic and chemical conditions of a regular hot flame leading to modified flame properties. This study focuses on how auto-ignition chemistry affects flame propagation, especially in the negative-temperature coefficient (NTC) regime, where dimethyl ether (DME), n-heptane and iso-octane are chosen for study as typical fuels exhibiting low temperature chemistry (LTC). The structure of this thesis consists of the introduction of the combustion, the governing equations in thermodynamics and chemical reactions as well as the general structure of the flame. Then, the typicl experimental configuration exploited in the measurement of laminar flame speed is introduced, which is followed by the manifestation of the low temperature chemistry and the gap between the reality and the experimental understandings. Finally, the simulation results of laminar flame speed at constant pressure condition and HCCI engine condition are presented and discussed respectively. The computation of laminar flame speed of lean and stoichiometric mixtures of fuel/air was performed at different ignition reaction progress, by selecting the thermal chemical states corresponding to different residence times during auto-ignition as the flame upstream condition. Using scaling and budget analysis, it is shown that a well-defined flame speed for such a partially reactive mixture in the classical diffusion-reaction limit could still be feasible in the appropriate computational domain, especially with a sufficiently reduced induction length. The comparison of flame speed against different types of progress variables indicates a nearly linear relationship between the flame speed and progress variables based on the fuel mass fraction and temperature. The overall effect of the cool-flame reformation has been studied by comparing the flame speed of the initial mixture and that of the instantaneous mixture under the same thermodynamic conditions. It is found that the enhanced propagation is shown to be largely a thermodynamic effect, while chemistry nevertheless plays an overall retarding role. Sensitivity analysis has been performed to identify the key species which most influence flame propagation at different reaction progress. A general scheme of simplified mixture was constructed to describe flame propagation in a partially reactive mixture, for both lean and stoichiometric, as well as high pressures conditions. The findings and general simplified mixture scheme are validated in HCCI engine conditions.

With increased interest in reducing emissions, the axial (sequential) stage combustion concept for gas turbine combustors and high exhaust gas recirculation rates for internal combustion engines are gaining in popularity. Despite the air-quality benefits of these technologies, introduction of inert combustion residuals into a combustion media affects the flame reactivity and stability. This dissertation examines the dilution effect on laminar flame characteristics of iso-octane/air, high/low research octane number gasoline/air, and methane/air mixtures through both experiments and numerical simulations.Spherically expanding flames under constant pressure are employed in an optically accessible constant volume combustion chamber to measure fundamental characteristics of premixed flames. Spherically expanding flames are severely affected by flame stretch in the early stage of combustion and therefore stretch models are of great importance in determining the uncertainty of experimental laminar flame speeds and burned gas Markstein lengths. In order to prevent the existing large scatter in experimental data of these two fundamental flame parameters, the effect of the lower radius limit for the flame speed calculation on extrapolation results of the stretch models is investigated in Chapter 3. Results show that there is a critical lower radius limit, where all laminar flame speed and burned gas Markstein length values obtained by the extrapolation of the stretch models converge to the same laminar flame speed and burned gas Markstein length.Chapter 4 presents the exhaust gas recirculation effect on CO2-diluted iso-octane/air and high/low research octane number gasoline/air mixtures at 1 bar and 373-473 K. The results of the measurements reveal that flame speeds of commercial gasolines do not vary significantly with the research octane number whereas the iso-octane flame speeds are consistently slower than those of gasoline. Numerical analyses are used to determine the dilution, thermal-diffusion, and chemical effects of the CO2 dilution on the flame speed, stretch, and stability.Over the years, many studies have investigated diluted methane flame characteristics with one of the main exhaust gases or a mixture of two. Chapter 5 experimentally and computationally shows that real combustion residuals cannot be accurately represented with only one or two of the main exhaust gases, as the thermodynamic properties and chemical reactivities of the combustion residuals are very distinctive and vary with temperature, pressure, and equivalence and dilution ratios. In Chapter 5, laminar burning velocity and burned gas Markstein length correlations are developed from the methane/air flame measurements at 1-5 bar, 373-473 K, and with 0-15% dilution. The physical and chemical aspects of changes in the laminar burning velocity and flame front stability due to changes in temperature, pressure, and equivalence and dilution ratios are discussed in detail.

This thesis presents the results of an experimental study to determine laminar flame speeds using the spherical flame method. An experimental combustion chamber, based on the constant-volume bomb method, was designed, built, and instrumented to conduct these experiments. Premixed Ethylene/air mixtures at a pressure of 2 atm, temperature of 298± 5K and equivalence ratios ranging from 0.8 to 1.5 were ignited and using a high speed video Schlieren system images were taken to measure the laminar flame speed in the expanding spherical flame front. The results were compared against published data for ethylene/air mixtures which yielded agreement within 5%. An attempt was made to measure the laminar flame speed for F-76 at a pressure of 5 atm and temperature of 500K; however, premixed conditions were unable to be met due to auto-ignition and vapor characteristics of F-76. Suggestions for future work provide a potential solution and improvement to the current design.