Flame Lifting

Debido a la diversificación de las fuentes de combustible gaseoso, la posibilidad de predecir la intercambiabilidad de un gas combustible por otro gas o una mezcla de gases en una aplicación de combustión, ha generado un creciente interés en el desarrollo de métodos cualitativos y cuantitativos que permitan estimar el cambio relativo del comportamiento de la combustión, luego de sustituir el gas combustible para el cual se ajustó originalmente el quemador. Entre los métodos principales se encuentran Índices Simples y Múltiples, además de las aproximaciones de Gráficas, más ampliamente aceptadas en Europa. Entre los índices más importantes desarrollados para evaluar el desempeño de un gas, se encuentran los índices de puntas amarillas, retrollama, desprendimiento y combustión incompleta. En este artículo se hace una revisión de los principales métodos de intercambiabilidad utilizados en la actualidad y se comparan los índices utilizados. También, se presentan las ventajas de uno de los índices simples más utilizados, denominado Índice de Wobbe, y el índice múltiples de AGA y Weaver. Al final se describe la relación de la altura del cono azul con la presión atmosférica, y se explica la importancia de este parámetro como criterio para la determinación de la intercambiabilidad de dos mezclas de gases. Además, se definen otras propiedades de combustión que se sugiere deban entrar en la caracterización de la intercambiabilidad de dos combustibles gaseosos, especialmente el gas natural y el gas de síntesis.

Atomization, vaporization, turbulent mixing, and the chemical kinetics of a liquid fuel impact the combustion efficiency, stability, and emissions within IC engines. When fuels with varying physical properties are used in the same fuel injection system, they can exhibit very different atomization, represented by droplet size (SMD) and spray droplet distribution (SDD), which can then potentially influence the flame stability. In the current paper, the effect of the atomization process on flame stability is investigated by using a single nozzle size with three different hydrocarbon fuels, i.e., n-heptane, n-dodecane, and toluene. This study also investigates the flame stability for the same fuels using a unique approach in which nozzle sizes are selectively varied for each fuel to control the atomization process and minimize differences in the spray characteristics. The results show that the atomization process plays a major role in the flame liftoff heights and blowout limits. n-Dodecane, which produces the largest droplets and exhibits the lowest volatility, experiences highest flame liftoff heights and the greatest resistance to flame blowout compared to the other tested fuels. When the atomization process is controlled, more similar liftoff heights and blowout limits for all fuels were observed. The differences in liftoff height was shown to be a result of two-phase (spray) flame speed (i.e., pre-vaporized fuel reactivity (SL), and time scales associated with droplet evaporation). Differences in blowout was shown to be largely tied to fuel volatility, droplet size (i.e., the liquid loading into the flame), and the heat of combustion.

Carbon dioxide emissions are considered one of the most important factors that cause global warming. For this reason, it is suggested to burn fuels using an oxidizer mixture consisting of oxygen and carbon dioxide rather than the conventional burning in air. This combustion technique ensures that the resulting products consist mainly of carbon dioxide and water vapor which facilitates capturing and sequestration of the carbon dioxide gas and thus eliminates its release into the atmosphere. One of the main challenges
that faces this technique is the need to ensure the flame stability in a wide range of operating conditions because of  he adverse effects of adding carbon dioxide to the oxidizer mixture on chemical kinetics. This study experimentally examines some of the conditions that must be met to ensure stable flame operation when burning compressed natural gas in a controlled mixture of oxygen and carbon dioxide stabilized over a perforated-plate burner at different equivalence ratios and at different oxygen fractions. Two sets of experiments are carried-out in this study. In the first set, the study identifies the range of equivalence ratios at constant oxygen fractions necessary for stable flame operation. In the second set, the study identifies the range of oxygen fraction at constant equivalence ratios necessary for stable flame operation. The study also documents the visual flame length and color and identifies the extinction mechanism outside the flammability limits for both sets. The first set of experiments reveals that, in the range of Reynolds number considered and at an oxygen fraction of  6%, stable flame operation is observed with flammability limits that are approximately 80% of the corresponding air-fuel combustion under the same Reynolds number. This ratio decreases as the oxygen fraction decreases. The visual flame length is longer than that of air-fuel combustion by approximately 15%. Extinction occurs by blow-off outside the upper and lower flammability limits. The second set of experiments reveals that for the burner and the range of  Reynolds numbers considered, operation below an oxygen fraction of 29% is not possible at all equivalence ratios considered (namely 0.7, 0.85, 1 and 1.15) while the upper flammability limit is a function of the equivalence ratio. Extinction occurs by blow-off below the lower flammability limit, and by flash-back above the upper flammability limit.
Keywords: Oxy-fuel combustion, CNG/O2/CO2 combustion, flammability limit, extinction mechanism, visual flame appearance and perforated-plate burner.

Liquid fuels have different physical and chemical properties which can affect the atomization, evaporation and mixing processes inside heat engines, including turbine and reciprocating internal combustion engines. In this work, the relationship between the fuel’s properties and its ignition/flame stability is investigated. A spray burner was designed and manufactured to simulate the mixing dynamics experienced in a turbine engine combustor capable of delivering high fuel and air co-flow rates. The flame behavior and stability have been studied by measuring the flame liftoff heights and blowout limits for different single component fuels at different fuel and air flow conditions. In addition, a Fuel Ignition Tester (FIT) was used to investigate the impact of volatility and local enrichment on ignition delay. Results indicate a strong influence of fuel volatility and atomization on the flame stability. The less volatile/heavier fuels have higher liftoff heights, while the fuels with smaller droplet sizes and higher volatility blowout easier than less volatile fuels and those which atomize into larger droplets. In the FIT, it was noticed that the local equivalence ratio defined by the fuels volatility has more effect on the ignition delay than the global air fuel ratio. As a result, ndodecane ignites faster than n-heptane, despite the having a lower measured global equivalence ratio.