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This thesis describes an experimental investigation of the flame structure close to the extinction and the blow-off events of non-premixed and spray flames stabilized on an axisymmetric bluff body in a confined swirl configuration. The comparison of flames of different canonical types in the same basic aerodynamic field allows insights on the relative blow-off behaviour. The first part of the thesis describes several velocity measurements in non-reacting and reacting flows. The main usefulness of this data is to provide the aerodynamic flow pattern and some discussion on the velocity field and the related recirculation zones. The velocity and turbulence information obtained are particularly useful for providing data, which is crucial for validation of computational models. The second part describes an experimental investigation of non-premixed stable flames very close to the blow-off condition. The measurements included visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, which allowed a quantification of the average duration of the blow-off transient. OH-PLIF images at 5 kHz for flames far from and close to extinction showed that the non-premixed flame intermittently lifts-off the bluff body, with increasing probability as the fuel velocity increases. The flame sheet shows evidence of localised extinctions, which are more pronounced as approaching blow-off. The measurements include blow-off limits and their attempted correlation. It was found that a correlation based on a Damkohler number does a reasonable job at collapsing the dataset. The final part examines the blow-off behaviour of swirling spray flames for two different fuels: n-heptane and n-decane. The measurements include blow-off limits and their att~mpted correlation, visualisation of the blow-off transient with 5 kHz OH* chemiluminescence, and the quantification of the average duration of the blow-off transient. It was found that the average duration of the blow-off event is in order of the tens of ms for both spray flames (10-16 ms). The blow-off event is therefore a relatively slow process for the spray ~ames using n-heptane and decane fuels. This suggests that control measures, such as fast fuel injection, coupled with appropriate detection, such as with chemiluminescence monitoring, may have a reasonable chance of success in keeping the flame alight very close to the blow-off limit. These results, together with those obtained for the non-premixed gaseous case form a wide body of experimental data available for the validation of turbulent flame models. The quantification of some properties during the blow-off transient can assist studies of extinction based on large-eddy simulation that have a promise of capturing combustion transients.

Heat transfer in circulating fluidized beds was studied in both a 7.3 m high, 152 x 152 mm square, pilot-scale combustor and a 9.3 m high, 152 mm ID transparent cold model unit. Results were obtained for particles of mean size 171-299 µm at superficial gas velocities from 4 to 9.5 m/s and for solids circulation rates up to 70 kg/m².s. For the combustor, results obtained by using membrane walls and a vertical tube as heat transfer surfaces show a strong influence of the cross-sectional area-averaged suspension density on time-averaged, length-averaged suspension-to-surface heat transfer coefficient. The influence of superficial gas velocity is found to be small. Radiation becomes significant at suspension temperatures higher than 400 C and at low suspension densities. Heat transfer coefficients were also found to vary with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter, allowing seemingly discrepant published results to be reconciled. For the cold model unit, sudden and dramatic peaks in instantaneous heat transfer coefficients were measured using an instantaneous heat transfer probe. Simultaneous heat transfer and capacitance measurements suggest that these peaks are caused by the arrivals of particle strands at the heat transfer surface. Two-probe heat transfer measurements suggest the existence of a characteristic residence length for the strands at the wall in this column. A proposed heat transfer model, based on an overall core-annulus flow structure in the riser, and periodic formation, movement along the wall, and disintegration of strands in the annulus, gives reasonable agreement with a wide range of published data. It accounts successfully for the effects of heat transfer surface length and particle sizes. However, the effect of the heat transfer surface configuration on the flow pattern of particles must also be taken into account to give improved agreement with experimental data. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

A two part study was undertaken to explain the performance of cyclones operated in circulating fluidized bed combustion (CFBC) systems. In the first part, collection efficiency tests were performed on a one-ninth scale polyacrylic cyclone model of the industrial scale cyclone at the 22 MWe CFBC facility at Chatham, New Brunswick. Emphasis was placed on scale-up considerations, loading effects, inlet geometry effects, and flow visualization trials. Experiments were performed at room temperature with inlet velocities between 3.7 and 5.5 m/s, solids loading between 0.05 and 7.5 mass solids/mass air with two different solids systems. There was disappointing agreement between the results from the Chatham unit, scaled according to Stokes Number scaling, and the findings obtained from the cold model unit. There was a minimum in the particle collection efficiency for particles of diameter 2.5 to 3.0 µm, apparently associated with agglomeration effects in the cyclone. Particle collection efficiency was found to increase with increased particle loading for the conditions studied. Changes in the inlet geometry gave inconclusive results. The experimental results were limited by problems associated with feeding and recycling the fines solids system used. In the second part radial gas concentration profiles of a secondary cyclone serving the UBC pilot scale Circulating Fluidized Bed Combustor were performed at temperatures of about 870 ℃. Concentrations of O₂ , CO₂ , NO[formula omitted] , CH₄ , CO and SO₂ were measured. An increase in [CO], and to a lesser extent [CO₂], was measured near the cyclone wall. There appeared to be little radial variation in the concentration of other species. Further work is required to allow the cold model to operate continuously, with particles which can be fed more freely, and to obtain radial gas concentration profiles within the primary cyclone of the UBC CFBC system. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate

Biomass fuel is considered a promising substitute for traditional fossil fuels. Amid a great variety of methods for converting the energy in biomass fuel into usable energy, direct combustion is still the dominant technology employed by industry. Because biomass fuel possess a much wider range of physical and chemical properties than fossil fuel, its combustion behavior is similarly diverse (and typically differs from fossil fuel), with a similar range seen in emissions characteristics. To address the variability the fuel stream imposes on the system, this work endeavors to use numerical modeling to investigate biomass combustion in a stoker boiler to provide physically insightful details while requiring minimal time and effort relative to the traditional experimental approach. In the first part of this work, a comprehensive model was developed to investigate the co-firing of different kinds of biomass including oat hulls, wood chips and natural gas with coal in a stoker boiler located in the Power Plant of the University of Iowa. Later, this model is employed in the optimization of the air supply system and plans for efficiently injecting light weight biomass, such as oat hulls, into the stoker boiler. The other key problem is in NOx prediction and reduction for the stoker boiler. This was by combining a standard CFD model describing the turbulent dynamics and combustion with several sub-models specifically developed for this study to model the fuel bed, fuel particle movement, and fuel gasification. To verify and baseline these sub-models, a series of experiments are performed, including a temperature measurement campaign for coal combustion in the boiler, a chemical lab analysis of oat hull chemical characteristics, an experiment measuring oat hull particles' physical properties, and a high-heating-rate gasification test of oat hulls. In particular, the stoker boiler temperature measurements are unique in the number of points measured and the range of firing conditions. The simulation showed that for the co-firing of oat hulls with coal, the flame temperature decreased with increasing oat hull fraction. The oat hull particles follow the flow and burn in suspension due to their light weight. The simulation showed that increasing injection velocity could slightly reduce the peak temperature and thereby reduce NOx levels. It was also observed that there is a critical velocity above which the trend of decreasing CO2 is reversed. The co-firing of other types of biomass such as wood chips and natural gas in the stoker boiler were also studied. The result of co-firing wood chips shows that adding wood chips decreases the flame. The flame zone is also shortened when compared to pure coal, primarily resulting from a higher oxygen environment above the coal bed due to the high oxygen content of the wood chips. Co-firing natural gas with coal resulted in the high temperature zone shifting from the back wall closer to the front wall, significantly reducing the overall flame length. The level of predicted NOx agreed very well with the experimental data. The simulations showed that injecting Urea with the secondary air system on the front wall can greatly reduce the NOx level inside the boiler for minimal cost and effort.

Detailed chemical kinetic models are important to the understanding and prediction of combustion properties. Better estimations require an accurate description of thermochemistry and kinetic rate parameters. This study identifies important reaction pathways at the low temperature chemistry of branched conventional and alternative fuels. Rate constants and branching ratios for important reactions are provided and important phenomena are investigated. The thermochemistry and kinetics of the 2-methylhexane model, an important component in gasoline surrogate, is updated using recent group values and rate rules from the literature. New reactions, such as hydroperoxyalkylperoxy (OOQOOH) alternative isomerization, are also added to the model. The results show that both conventional and alternative isomerization of OOQOOH radicals significantly affect the model reactivity. The kinetics of a biofuel; iso-butanol, is also investigated in this study to understand alcohol combustion chemistry and identify sensitive reactions that require more attention. The results indicate that iso-butanol is sensitive to the chain propagation reaction of α-RO2 radical and the water elimination of γ-QOOH. Because both reactions decrease model reactivity, accurate rate constants are needed to correctly determine fuel reactivity. In light of the above mentioned kinetic modeling studies, high levels computational chemistry calculations were performed to provide site-specific rates rules for OOQOOH conventional isomerization considering all possible reaction sites. This is also one of the first studies to investigate the effect of chirality on calculated rate constants. Results indicate that chirality is important when two chiral centers exist in the reactant. OOQOOH alternative isomerization rate constants are usually assigned in analogy to the isomerization of an alkylperoxy (RO2) radical which may introduce some uncertainty. To test the validity of using analogous rates, this study calculates the rate constants for selected alternative isomerization reactions. The effect of intramolecular hydrogen bonding in the calculated energies and rate constants for different reaction pathways is investigated. The result shows that alternative isomerization is a competing pathway only when it proceeds via a less strained transition state relative to the conventional isomerization transition state. A detailed analysis of the hydrogen bonding effect helped to identify cases where assigning rates in analogy may not be valid.

The impact of fuel drops on the walls of combustion chambers is unavoidable in direct-injection automotive engines. These drop-solid interactions can lead to splashing of the lubrication oil, its dilution or removal, which can damage the piston or the liner from dewetting. This can also cause irregular and inferior combustion or soot formation. Understanding the drop-splashing dynamics is therefore important, especially as modern IC engines are being down-sized to achieve higher thermal efficiency. Typical cylinders of IC engines contain metal liners on their walls, which have fine azimuthal grooves to support the lubricating oil as the piston moves inside the cylinder. In this thesis we study how these grooves affect the deposition or splashing of impacting diesel drops, while the solid surface is kept dry without the lubricating oil. For these experiments we use sections of actual cylinder liners and apply high-speed video imaging to capture the details of the drop impacts. The first set of experiments used normal impacts on horizontal substrates. These experiments include a range of drop sizes and impact velocities, to identify impact conditions in Reynolds and Weber number space where the transition from deposition to splashing occurs. We also study the maximum radial spreading factor of the impact lamella, finding about 8% larger spreading along the grooves than perpendicular to them. In the second set of experiments we look at the impact on inclined substrates, where the inclination angle is between 30o–60o. This produces strong asymmetry in the maximum spreading, with the tangential velocity governing the maximum radial motion. The inclined impacts change the splashing threshold, requiring larger impact velocities for splashing. The splashing threshold deviates quantitatively from earlier theories, but shows the same qualitative trends. Furthermore, a new splashing mechanism is observed, where the impact forms a prominent ejecta crown from the downstream edge. This crown ruptures first from the grooves at the sides and subsequently the capillarity detaches the downstream levitated liquid sheet from the substrate generating a myriad of splashed droplets. Preliminary observations with impacts on wet substrates show much stronger crown-formation from the lubricating oil film, with potential for dewetting.

The objective of this thesis is to explore the properties of supercritical carbon dioxide (CO2). In addition, the feasibility of building a small-scale low cost system will be explained. A supercritical fluid is a fluid which exhibits properties between liquid and gas with liquid like densities and viscosities similar to a gas. Since the discovery of supercritical fluids in 1822, the use of supercritical fluids, specifically supercritical CO2, has grown in popularity. The application of supercritical CO2 has continued to grow in industrial applications since the 1970’s. Supercritical CO2’s has many beneficial properties as a “green” solvent. Supercritical CO2 as a solvent is able to be implemented in a wide range of applications from aerospace, microchip manufacturing, food production, biomedical, pharmaceutical, dry-cleaning, and many more. This thesis project included designing, building and testing a supercritical CO2 extraction apparatus that examines the use of supercritical CO2 as a solvent in the extraction process of decaffeinating coffee. Due to the fact that supercritical CO2 requires high pressure operating conditions, the apparatus design is important not only for function but also for safety. In the description portion of this paper, design considerations related to each component’s function and their specific roles in the overall system are clearly stated. Furthermore, the build process is outlined along with the overall step-by-step operation of the apparatus. Different methods of data measurements are taken while the system is running, in order to interpret the apparatus’ overall functionality. Through the exploration of this experimental data, the results were compared between different operating parameters. In order to determine the feasibility of the supercritical apparatus, the devise was tested by applying the supercritical CO2 as a solvent for the extraction of caffeine from coffee beans. Analysis of the analytical data recorded from experimental testing confirms that the apparatus produced supercritical CO2. After testing specific operating conditions, it is proven that the supercritical CO2 is able to function as a “green” solvent in this small-scale system. The experimental results from these analytical runs are compared with theoretical maximums in order to determine the efficiency of the devise. Lastly, the paper presents an overview including lessons learned from the design process and from the information gathered. Data from experimental testing is interpreted and the system design is reevaluated with suggestions for future improvements.