On scalar dissipation and partially premixed flame propagation

Watson, K.A.; Lyons, Kent; Donbar, Jeffrey M.; Carter, C.D.

doi:10.1080/00102200302393

Cited by 34 publications

(22 citation statements)

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“…Upatneiks et al (2004) further show that flame stabilization based on large eddy theory is also inconsistent with the experimental measurements. The experimental investigations of Starner et al (1997) and Watson et al (2003) show that the theory of flame quenching due to excessive scalar dissipation is inconsistent with experimental results. Muniz and Mungal (1997) have reported PIV measurements on lifted methane and ethylene jet flames in the Reynolds number range of 3800-22000.…”

Section: Introductionmentioning

confidence: 90%

Prediction of Flame Liftoff Height of Diffusion/Partially Premixed Jet Flames and Modeling of Mild Combustion Burners

Kumar

Paul

Mukunda

2007

Combustion Science and Technology

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In this article, a new flame extinction model based on the k=e turbulence time scale concept is proposed to predict the flame liftoff heights over a wide range of coflow temperature and O 2 mass fraction of the coflow. The flame is assumed to be quenched, when the fluid time scale is less than the chemical time scale (Da < 1). The chemical time scale is derived as a function of temperature, oxidizer mass fraction, fuel dilution, velocity of the jet and fuel type. The present extinction model has been tested for a variety of conditions: (a) ambient coflow conditions (1 atm and 300 K) for propane, methane and hydrogen jet flames, (b) highly preheated coflow, and (c) high temperature and low oxidizer concentration coflow. Predicted flame liftoff heights of jet diffusion and partially premixed flames are in excellent agreement with the experimental data for all the simulated conditions and fuels. It is observed that flame stabilization occurs at a point near the stoichiometric mixture fraction surface, where the local flow velocity is equal to the local flame propagation speed. The present method is used to determine the chemical time scale for the conditions existing in the mild= flameless combustion burners investigated by the authors earlier. This model has successfully predicted the initial premixing of the fuel with combustion products before the combustion reaction initiates. It has been inferred from these numerical simulations that fuel injection is followed by intense premixing with hot combustion products in the primary zone and combustion reaction follows further downstream. Reaction rate contours suggest that reaction takes place over a large volume and the magnitude of the combustion reaction is lower compared to the conventional combustion mode. The appearance of attached flames in the mild combustion burners at low thermal inputs is also predicted, which is due to lower average jet velocity and larger residence times in the near injection zone.

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Section: Introductionmentioning

confidence: 90%

Prediction of Flame Liftoff Height of Diffusion/Partially Premixed Jet Flames and Modeling of Mild Combustion Burners

Kumar

Paul

Mukunda

2007

Combustion Science and Technology

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“…Although the concentration gradient data in the cold flows of the present lifted flames are not available, the following estimation on the data of Watson et al (17) , who examined cold flows at the bases of lifted methane nonpremixed flames using a simultaneous CH-PLIF and Rayleigh scattering system, may be instructive. Their experimental conditions named cases 1 and 2 are close to the present conditions in terms of exit Reynolds number, but their fuel is methane and fuel nozzle diameter is 5 mm.…”

Section: Evaluation Of Scalar Dissipation Ratementioning

confidence: 99%

Nonpremixed Flamelet Statistics at Flame Base of Lifted Turbulent Jet Nonpremixed Flames

Noda

Mori

Hongo

et al. 2005

JSME international journal. Ser. B, Fluids and thermal engineer

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Nonpremixed flamelet statistics at the flame base of lifted turbulent nonpremixed flames are investigated experimentally using a planar temperature Rayleigh scattering method. A methane/hydrogen mixture is supplied from a tube of 3.2 mm I.D. into the surrounding air flow so as to form two lifted turbulent nonpremixed flames having exit velocities of 50 m/s (Re = 4 200) and 80 m/s (Re = 6 700), respectively. Temperature data are related to maximum temperature at several flame positions based on the instantaneous flame base tip as containing information on the reaction region of each nonpremixed flamelet at each position. The statistics in terms of maximum temperature, thermal dissipation rate, scalar dissipation rate, and flame brush are discussed with reference to the modeling of the flame base structure. The scalar dissipation rate has log-normal statistics and the quenching scalar dissipation rate is lower than the critical value predicted using the uniformly strained counter nonpremixed flame. These statistics are compared to those obtained using a model proposed by Müller et al.(1) , which combines the flamelet model and the scalar field variable to predict lifted turbulent nonpremixed flames. The comparison has shown that the model can qualitatively predict lifted turbulent nonpremixed flames, but modification is required in order to obtain more accurate quantitative prediction taking into account the edge flame structure, the statistics of scalar dissipation rate, and the statistics of flame brush.

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“…Since the blowout phenomenon happens typically in an abrupt and unpredictable manner, its transient characteristics are difficult to study experimentally. Additionally, the large width of the fuel jet, the small gradients in the scalar and velocity fields, and the relatively low values of fuel concentration make the situation, in many ways, more challenging to fully characterize than the situations described in the studies of Watson et al [2][3][4].…”

Section: Introductionmentioning

confidence: 80%

“…A diffusion flame has no burning velocity so it is the premixed flame front that is generally assumed to act as a stabilizing anchor. Many studies, like that of Muñiz and Mungal [1] and Watson et al [2][3][4], have investigated stable lifted flame reaction zone structures that settle at moderate downstream positions. If the reaction zone moves further downstream, it eventually enters a region that can no longer support combustion due to the low fuel concentration and all reaction abruptly ceases, a condition known as flame blowout (Kalghatgi [5], Pitts [6], Coats [7], Chao et al [8,9].…”

Section: Introductionmentioning

confidence: 99%

Observations on Jet-Flame Blowout

Moore

McCraw

Lyons

2008

International Journal of Reacting Systems

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The mechanisms that cause jet-flame blowout, particularly in the presence of air coflow, are not completely understood. This work examines the role of fuel velocity and air coflow in the blowout phenomenon by examining the transient behavior of the reaction zone at blowout. The results of video imaging of a lifted methane-air diffusion flame at near blowout conditions are presented. Two types of experiments are described. In the first investigation, a flame is established and stabilized at a known, predetermined downstream location with a constant coflow velocity, and then the fuel velocity is subsequently increased to cause blowout. In the other , an ignition source is used to maintain flame burning near blowout and the subsequent transient behavior to blowout upon removal of the ignition source is characterized. Data from both types of experiments are collected at various coflow and jet velocities. Images are used to ascertain the changes in the leading edge of the reaction zone prior to flame extinction that help to develop a physically-based model to describe jet-flame blowout. The data report that a consistent predictor of blowout is the prior disappearance of the axially oriented flame branch. This is witnessed despite a turbulent flames' inherent variable behavior. Interpretations are also made in the light of analytical mixture fraction expressions from the literature that support the notion that flame blowout occurs when the leading edge reaches the vicinity of the lean-limit contour, which coincides approximately with the conditions for loss of the axially oriented flame structure.

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On scalar dissipation and partially premixed flame propagation

Cited by 34 publications

References 0 publications

Prediction of Flame Liftoff Height of Diffusion/Partially Premixed Jet Flames and Modeling of Mild Combustion Burners

Prediction of Flame Liftoff Height of Diffusion/Partially Premixed Jet Flames and Modeling of Mild Combustion Burners

Nonpremixed Flamelet Statistics at Flame Base of Lifted Turbulent Jet Nonpremixed Flames

Observations on Jet-Flame Blowout

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