Coherent structures in combustion

Coats, C.M.

doi:10.1016/s0360-1285(96)00011-1

Cited by 159 publications

(62 citation statements)

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“…Interestingly, these flow conditions are similar to those found in the flames under investigation [31].…”

Section: Combustion Modellingsupporting

confidence: 79%

“…Photographs of flame appearance are in terms of its luminosity, which is caused by excited species and soot [31]. Flame images are widely used to measure flame length which can give an indication of the size of the high-temperature zone, in particular for blue flames such as those of methane, and the residence time available for pollutant formation within the flame [41].…”

Section: Flame Appearancementioning

confidence: 99%

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CFD predictions of wake-stabilised jet flames in a cross-flow

Lawal¹,

Fairweather²,

Gogolek³

et al. 2013

Energy

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This study describes an investigation into predicting the major flow properties in wake-stabilised jet flames in a cross flow of air using first-and second-order turbulence models, applied within a Reynolds-averaged Navier-Stokes (RANS) modelling framework. Standard and re-normalisation group (RNG) versions of the k-turbulence model were employed at the first-order level and the results compared with a secondmoment closure, or Reynolds stress model (RSM). The combustion process was modelled using the laminar flamelet approach together with a thermal radiation model using the discrete ordinate method. The ability of the various turbulence models to reproduce experimentally established flame appearance, profiles of velocity and turbulence intensity, as well as the combustion efficiency of such flames is reported. The results show that all the turbulence models predict similar velocity profiles over the majority of the flow domain considered, except in the wake region, where the predictions of the RSM and RNG k-models are in closer agreement with experimental data. In contrast, the standard k-model over-predicts the peak turbulence intensity. Also, it is found that the RSM provides superior predictions of the planar recirculation and flame zones attached to the release pipe in the wake region. 1.IntroductionA large quantity of oil field associated gas, and waste hydrocarbons from the process industry, is flared globally each year. With the predicted increase in oil production in the next decades, a significant increase in flaring is expected [1,2]. However, flare operations can result in the formation of pollutants such as unburned hydrocarbons, CO, NO x and noise and the effect of a cross-flow of wind can be significant. In order to minimise these effects, a detailed understanding of the flow features and structures of these non-premixed jet flames in a cross-flow (JFICF) is necessary. Depending on the value of the jetto-cross-flow momentum flux ratio, R= j u j 2 / cf u cf 2 , where j , cf and u j , u cf are the density and velocity of the fuel jet and the cross-flow, respectively, the flow features that arise in the JFICF have an important influence on the flame's combustion characteristics.For flares operating at low values of R, typically R ≤ 1.0, the flame is either seated on the flare stack or stabilised in its wake [3]. The latter case is also known as a wake-stabilised flare which belongs to a flow regime where the flame is severely bent-over by the cross-flow and has high turbulence intensities in the near-field [4]. This category of flare is commonly found in offshore crude oil production facilities.The three main flow features in the wake-stabilised flare include: a counter-rotating vortex pair (CVP)[5], coherent structures around the upper surface of the jet [3] and the planar recirculation zone in the wake of the stack [6]. The latter zone is part of the low-pressure region where a secondary flame is attached to the release pipe, as shown in Fig. 1, and the vortex dynamics and mixing in this ...

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“…Interestingly, these flow conditions are similar to those found in the flames under investigation [31].…”

Section: Combustion Modellingsupporting

confidence: 79%

Section: Flame Appearancementioning

confidence: 99%

CFD predictions of wake-stabilised jet flames in a cross-flow

Lawal¹,

Fairweather²,

Gogolek³

et al. 2013

Energy

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“…Another reason might be the effects of gravity in the situation in which a fluid is stratified vertically. The buoyancy force would then destabilize the flame if the direction of the flow is such as to place the low-density combustion products below the high density un-burnt mixtures [31]. The central fuel jet has a lower density than that of the fuel-air mixture in the annular flow which would lead to the buoyancy-driven instabilities.…”

Section: Flame Structuresmentioning

confidence: 99%

Experimental Study on Bluff-Body Stabilized Premixed Flame with a Central Air/Fuel Jet

Tong

Chen

et al. 2017

Energies

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Bluff-body flame holders are commonly employed in many industrial applications. A bluff-body is usually adopted to enhance the downstream mixing of the combustion products and the fresh fuel-air mixtures, thus to improve the flame stability and to control the combustion process. In the present paper, flames stabilized by a conical-shape bluff-body flame holder with a central air/fuel jet were studied. Effects of both a central air jet and a central fuel jet on the structures and lean blowout limits of the premixed annular flames, and on the temperature on the upper surface of the bluff-body were investigated and presented. It was revealed that a central jet led to a considerable reduction of the temperature on the upper surface of the bluff-body. It was proposed to be caused by the alternation of flow structures (in the case with a central air jet) altogether with the flame lifting from the burner (in the case with a central fuel jet). Thus, it might be used to solve the problem of the bluff-body with high heat loads in practical applications. The flame stability characteristics, for example the unstable flame dynamics and the lean blowout limits, varied with the injection of an air or fuel jet through the central pipe. Different blowout behaviors, being with or without the occurrence of flame split and flashing, caused by a central air jet were presented in the paper. In addition, when a small amount of central fuel jet (i.e., U f /U a = 0.045) was injected into the flow fields, an unsteady circular motion of the flame tip along the outer edge of the bluff-body was observed as well. Whereas, with an increase in the amount of the central fuel jet, the flame detached from the outer edge of the bluff-body and then became much more unstable. With a central air or fuel jet injecting into the flow field, premixed flames stabilized by the bluff-body became more unstable and easier to blowout.

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“…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]. The term blowout seems more physically descriptive than the sometimes used blowoff since the global reaction zone does not seem to blow off the downstream end of the jet, but rather, to locally cease (Liñán and Williams [10]).…”

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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Coherent structures in combustion

Cited by 159 publications

References 0 publications

CFD predictions of wake-stabilised jet flames in a cross-flow

CFD predictions of wake-stabilised jet flames in a cross-flow

Experimental Study on Bluff-Body Stabilized Premixed Flame with a Central Air/Fuel Jet

Observations on Jet-Flame Blowout

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