Thesis (Ph. D.)--University of Wisconsin--Madison, 1948. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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Modern spark ignition engines are downsized and boosted to meet stringent emission standards and growing customer demands on performance and fuel economy. They operate under high intake pressures and close to their limits to engine knock. As the intake pressure is increased knock becomes the major barrier that prevents further improvement on downsized boosted spark ignition engines. It is generally accepted that knock is caused by end gas autoignition ahead of the propagating flame. The propagating flame front has been identified as one of the most influential factors that promote the occurrence of autoignition. Systematic understanding and numerical relation between the propagating flame front and the occurrence of knock are still lacking. Additionally, knock mitigation strategy that minimizes compromise on engine performance needs further researching. Therefore the objectives of the current research consist of two steps: 1). study of turbulent flame propagation in both naturally aspirated SI engine. 2) study of the relationship between flame propagation and the occurrence of engine knock for downsized and boosted SI engine. The aim of the current research is, firstly, to find out how turbulent flames propagate in naturally aspirated and boosted S.I. engines, and their interaction with the occurrence of knock; secondly, to develop a mitigation method that depresses knock intensity at higher intake pressure. Autoignition of hydrocarbon fuels as used in spark ignition engines is a complex chemical process involving large numbers of intermediate species and elementary reactions. Chemical kinetics models have been widely used to study combustion and autoignition of hydrocarbon fuels. Zero-dimensional multi-zone models provide an optimal compromise between computational accuracy and costs for engine simulation. Integration of reduced chemical kinetics model and zero-dimensional three-zone engine model is potentially a effective and efficient method to investigate the physical, chemical, thermodynamic and fluid dynamic processes involved in in-cylinder turbulence flame propagation and knock. The major contributions of the current work are made to new knowledge of quantitative relations between intake pressure, turbulent flame speed, and knock onset timing and intensity. Additionally, contributions have also been made to the development of a knock mitigation strategy that effectively depresses knock intensity under higher intake pressure while minimizes the compromise on cylinder pressure, which can be directive to future engine design.

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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.

The work presented in this thesis is an investigation of the dynamics of unconfined hydrogen-air flames in the presence of buoyant effects and the determination of an ignition criterion for flame propagation between adjacent pockets of reactive gas separated by air. The experimental work was conducted using the soap bubble technique and visualized with high speed schlieren or large scale shadowgraph systems. A study was first conducted to determine the most suitable soap solution additive among glycerol, guar and polyethylene oxide for conducting the experiments, isolating guar as the best candidate. The soap solution was then used to study the dynamics of flames in single or multiple soap bubbles filled with reactive mixtures of different compositions. The soap bubble method was also further improved by designing a soap dispenser that can maintain a bubble indefinitely and a method to burst the soap solution prior to an experiment using timed heated wires. In the experiments with single bubbles, it was found that for sufficiently lean hydrogen-air mixtures, buoyancy effects become important at small scales. The critical radius of hemispherical flames that will rise due to buoyancy was measured and estimated using a model comparing the characteristic burning speed and the rise speed of the flame kernel. Excellent agreement was found between the model predictions and the measured critical flame radii. The experiments with multiple bubbles provided the scaling rules for flame transition between neighboring pockets of hemispherical or spherical shape separated by an inert gas. The test results demonstrated that the separation distance between the bubbles is mainly determined by the expansion ratio when the buoyancy effects are negligible, corresponding to near stoichiometric mixtures. For leaner mixtures with stronger buoyant effects, the critical separation distance was no longer governed by the expansion ratio alone, as buoyancy forces render the flame propagation across the inert gas more difficult. Visualization of the ignition dynamics confirmed that buoyancy forces tend to accelerate the first kernel up before ignition of the second kernel can be achieved.

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.

Accurate flash point and flammability limit data are needed to design safe chemical processes. Unfortunately, improper data storage and reporting policies that disregard the temperature dependence of the flammability limit and the fundamental relationship between the flash point and the lower flammability limit have resulted in compilations filled with erroneous values. To establish a database of consistent flammability data, critical analysis of reported data, experimental investigation of the temperature dependence of the lower flammability limit, and theoretical and empirical exploration of the relationship between flash points and temperature limits are undertaken. Lower flammability limit measurements in a 12-L ASHRAE style apparatus were performed at temperatures between 300 K and 500 K. Analysis of these measurements showed that the adiabatic flame temperature at the lower flammability limit is not constant as previously thought, rather decreases with increasing temperature. Consequently the well-known modified Burgess-Wheeler law underestimates the effect of initial temperature on the lower flammability limit. Flash point and lower temperature limit measurements indicate that the flash point is greater than the lower temperature limit, the difference increasing with increasing lower temperature limit. Flash point values determined in a Pensky-Martens apparatus typically exceed values determined using a small-scale apparatus above 350 K. Data stored in the DIPPR® 801 database and more than 3600 points found in the literature were critically reviewed and the most probable value recommended, creating a database of consistent flammability data. This dataset was then used to develop a method of estimating the lower flammability limit, including dependence on initial temperature, and the upper flammability limit. Three methods of estimating the flash point, with one based entirely on structural contributions, were also developed. The proposed lower flammability limit and flash point methods appear to predict close to, if not within, experimental error.

La phase d’allumage est une composante critique à prendre en compte lors de la conception et du dimensionnement d’une chambre de combustion aéronautique, en particulier lorsque de nouvelles technologies ou architectures sont envisagées dans l’objectif de réduire les émissions de polluants causées par la combustion de carburants d’origine fossile. Il est donc primordial d’atteindre une compréhension détaillée du processus complexe qu’est l’allumage dans des conditions réalistes afin d’être en mesure de choisir les meilleures géométries qui assurent un fonctionnement fiable, stable et sûr des moteurs tout au long de leur cycle de vie. Des simulations aux grandes échelles de l’allumage circulaire d’une chambre de combustion annulaire avec injection de carburant liquide sont réalisés pour trois points de fonctionnement et comparées avec les données expérimentales en termes de structure de flamme et de délai d’allumage. Une analyse détaillée des trois séquences d’allumage numériques permet d’identifier certains aspects clés de la propagation de la flamme dans le mélange froid diphasique. Enfin, les pertes thermiques aux parois sont prises en compte, dans l’objectif d’évaluer la capacité de la simulation à retrouver la forte chute de la vitesse de propagation observée expérimentalement lorsque les parois sont à température ambiante. / Ignition is one of the critical issues that arise in the design and dimensioning of aeronautic combustors, in particular when new technologies are envisioned to reduce the amount of pollutants generated by the combustion of fossil fuels. It is therefore important to achieve a detailed understanding of this complex process in realistic conditions in order to enable informed design choices leading to reliable, stable and safe operation of the engines.Large eddy simulations of the light-round with two phase injection are carried out for three operating conditions and compared to experimental data in terms of flame structure and global duration. The liquid phase is described with a mono-disperse Eulerian approach.A detailed analysis of the three numerical light-round sequences allows to identify some key aspects of the flame propagation in the two-phase mixture. Interactions between the flame, the flow field and the liquid sprays create heterogeneities in the liquid repartition and wakes on the downstream side of the swirling jets formed by the injectors, with notable effects on the motion of the leading point and on the absolute flame velocity.Finally, heat losses at the walls are accounted for during the light-round in order to assess the simulation's ability to retrieve the marked slowdown of the flame propagation observed experimentally when the quartz walls are at ambient temperature.