James D. Kribs

The effects of collector surface area with electrostatic flows resulting from multiple corona discharges

To initiate an ionic induced flow within atmospheric air, the high voltages are applied to the atmosphere by the coronas, creating an ionic flow to a grounded collector or ring. In experiments with a focus on the viability of applying the ionic wind as a cooling mechanism, using a shallow ring as the grounded collector for the ionized air, it was found that there is a relationship between the size of the grounded ring and the velocity of the flow caused by the ionization from the electrodes, maintaining voltage as a constant, a smaller ring provides higher efficiencies, while larger rings provide higher flow rates at larger dynamic pressures. Further research is being conducted on the influence of multiple corona discharges on the velocity and the effective static pressure of ionic flows in air as well as combustible flows

Effects of Diluents on Lifted Turbulent Methane and Ethylene Jet Flames

The effects of diluents on the liftoff of turbulent, partially premixed methane and ethylene jet flames for potential impact in industrial burner operation for multifuel operation have been investigated. Both fuel jets were diluted with nitrogen and argon in separate experiments, and the flame liftoff heights were compared for a variety of flow conditions. Methane flames have been shown to liftoff at lower jet velocities and reach blowout conditions much more rapidly than ethylene flames. Diluting ethylene and methane jets with nitrogen and argon, independently, resulted in varying trends for each fuel. At low dilution levels (∼5% by mole fraction), methane flames were lifted to similar heights, regardless of the diluent type; however, at higher dilution levels (∼10% by mole fraction) the argon diluent produced a flame which stabilized farther downstream. Ethylene jet flames proved to vary less in liftoff heights with respect to diluent type. Significant soot reduction with dilution is witnessed for both ethylene and methane flames, in that flame luminosity alteration occurs at the flame base at increasing levels of argon and nitrogen dilution. The increasing dilution levels also decreased the liftoff velocity of the fuel. Analysis showed little variance among liftoff heights in ethylene flames for the various inert diluents, while methane flames proved to be more sensitive to diluent type. This sensitivity is attributed to the more narrow limits of flammability of methane in comparison to ethylene, as well as the much higher flame speed of ethylene flames.

Experimental Observations of Nitrogen Diluted Ethylene and Methane Jet Flames

While the liftoff mechanisms of nitrogen-diluted methane jet flames have been well documented, higher order fuels, such as ethylene, have not been studied as extensively with regards to flame stabilization and behavior. Higher order fuels generally burn more intensely, and thus produce much different stabilization patterns than those of simple hydrocarbon fuels, such as methane. The purpose of this study was to observe the effects of nitrogen dilution on ethylene combustion and compare to that witnessed in typical methane jet flames; specifically, the influence on the liftoff height, blowout, and flame chemiluminescence. Liftoff and blowout velocities were compared for various mixtures of ethylene without nitrogen. It was observed that the reason behind the varying stabilization patterns is due to the higher thermal diffusivity of ethylene as well the higher flame speeds that are characterized in the combustion of ethylene. Using a sequence of images from each mixture, the flame liftoff heights were recorded. Due to the strong chemiluminescence of ethylene flames, little fluctuation between liftoff parameters was observed, with respect the velocity; however, there was a significant effect on the liftoff height, with respect to dilution. Blowout for fuel mixtures was much more difficult to achieve due to the higher thermal diffusivity of ethylene, meaning the flame would stabilize at positions much farther downstream than those of simple hydrocarbon fuels.Copyright

Effects of Hydrogen Enrichment on the Reattachment and Hysteresis of Lifted Methane Flames

The purpose of this study is to observe the effects of hydrogen enrichment on the stability of lifted, partially premixed, methane flames. Due to the relatively large burning velocity of hydrogen-air flames when compared to that of typical hydrocarbon-air flames, hydrogen enriched hydrocarbon flames are able to create stable lifted flames at higher velocities. In order to assess the impact of hydrogen enrichment, a selection of studies in lifted and attached flames were initiated. Experiments were performed that focused on the amount of hydrogen needed to reattach a stable, lifted methane jet flame above the nozzle. Although high fuel velocities strain the flame and cause it to stabilize away from the nozzle, the high burning velocity of hydrogen is clearly a dominant factor, where as the lifted position of the flame increased, the amount of hydrogen needed to reattach the flame increased at the same rate. In addition, it was observed that as the amount of hydrogen in the central jet increased, the change in flame liftoff height increased and hysteresis became more pronounced. It was found that the hysteresis regime, where the flame could either be stabilized at the nozzle or in air, shifted considerably due to the presence of a small amount of hydrogen in the fuel stream. The effects of the hydrogen enrichment, however small the amount of hydrogen compared to the overall jet velocity, was the major factor in the flame stabilization, even showing discernible effects on the flame structure.Copyright

Assessment of Stabilization Mechanisms of Confined, Turbulent, Lifted Jet Flames: Effects of Ambient Coflow

The aim of this investigation is to determine the effects of confinement on the stabilization of turbulent, lifted methane (CH4) jet flames. A confinement cylinder (stainless steel) separates the coflow from the ambient air and restricts excess room air from being entrained into the combustion chamber, and thus produces varying stabilization patterns. The experiments were executed using fully confined, semi-confined, and unconfined conditions, as well as by varying fuel flow rate and coflow velocity (ambient air flowing in the same direction as the fuel jet). Methane flames experience liftoff and blowout at well-known conditions for unconfined jets, however, it was determined that with semi-confined conditions the flame does not experience blowout. Instead of the conventional unconfined stabilization patterns, an intense, intermittent behavior of the flame was observed. This sporadic behavior of the flame, while under semi-confinement, was determined to be a result from the restricted oxidizer access as well as the asymmetrical boundary layer that forms due to the viewing window. While under full confinement the flame behaved in a similar method as while under no confinement (full ambient air access). The stable nature of the flame while fully confined lacked the expected change in leading edge fluctuations that normally occur in turbulent jet flames. These behaviors address the combustion chemistry (lack of oxygen), turbulent mixing, and heat release that combine to produce the observed phenomena.Copyright

Stability and Blowout Behavior of Jet Flames in Oblique Air Flows

The stability limits of a jet flame can play an important role in the design of burners and combustors. This study details an experiment conducted to determine the liftoff and blowout velocities of oblique-angle methane jet flames under various air coflow velocities. A nozzle was mounted on a telescoping boom to allow for an adjustable burner angle relative to a vertical coflow. Twenty-four flow configurations were established using six burner nozzle angles and four coflow velocities. Measurements of the fuel supply velocity during liftoff and blowout were compared against two parameters: nozzle angle and coflow velocity. The resulting correlations indicated that flames at more oblique angles have a greater upper stability limit and were more resistant to changes in coflow velocity. This behavior occurs due to a lower effective coflow velocity at angles more oblique to the coflow direction. Additionally, stability limits were determined for flames in crossflow and mild counterflow configurations, and a relationship between the liftoff and blowout velocities was observed. For flames in crossflow and counterflow, the stability limits are higher. Further studies may include more angle and coflow combinations, as well as the effect of diluents or different fuel types.

Nitrogen Diluted Jet Flames in the Presence of Coflowing Air

The purpose of this study is to observe methane jet flames under varying levels of nitrogen dilution and coflowing air. The jet flames were examined in order to determine the conditions for which liftoff and blowout occur under conditions that strain the flame. Methane flow rates were varied, corresponding to intermediate lifted positions to blowout. A sequence of images were taken at each level of dilution and coflow, and were used to determine the lowest radial and axial position of the flammability limit. These flammability regions were compared to the lean flammability limit. It was observed that flame shape and liftoff were considerably more influenced by the effects of the coflowing air compared to the presence of the diluents, and that flames under coflow lost the trailing diffusion flame earlier, which has been shown to be a marker for flame blowout.Copyright