The interaction of turbulence and pressure gradients in a baffle-stabilized premixed flame

Heitor, M. V.; Taylor, A. M. K. P.; Whitelaw, J. H.

doi:10.1017/s0022112087002143

Cited by 49 publications

(10 citation statements)

References 48 publications

(61 reference statements)

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“…This observation is consistent with an earlier investigation using laser-Doppler anemometry in a similar burner. 26 The negative V z disappears and there is a strong reduction in the shear ‫ץ‬V z / ‫ץ‬x in the presence of a flame. The dilation of the eddies has also been observed for bluff body stabilized turbulent nonpremixed flames.…”

Section: Resultsmentioning

confidence: 99%

“…These results are consistent with previous studies. 26,47 The rms of velocity fluctuations within the flame brush is observed to be larger than the corresponding cold flow values because of the intermittency in the flame front location. The length and time scales of the turbulence are increased by the heat release as one would expect.…”

Section: Discussionmentioning

confidence: 97%

“…1 and it has been employed in earlier studies. [23][24][25] This burner is similar to that used by Heitor et al 26 but the flow and flame conditions investigated here are different. The burner was made up of a circular duct, 300 mm long with an inner diameter of 35 mm.…”

Section: A Burner and Flame Characteristicsmentioning

confidence: 99%

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Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

et al. 2008

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Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influenced by the heat release. The strong acceleration across the mean flame and the radial outward shift of the stagnation plane of the recirculating eddy yield negative radial velocities which are absent in the corresponding cold flow cases. The spatial intermittency of the flame front leads to an increase in the turbulent kinetic energy. Although a decrease in the mean and rms values of the strain rate tensor e ij components is observed for the reacting case as one would expect, the local flow acceleration across the flame front leads to a substantial increase in the skewness and the kurtosis of the probability density functions ͑PDFs͒ of e ij components. The turbulence-scalar interaction is studied by analyzing the orientation of the flame front normal with the eigenvectors of e ij . The PDFs of this orientation clearly show that the normals have an increased tendency to align with the extensive strain rate, which implies that the scalar gradients are destroyed by the turbulence as the scalar isosurfaces are pulled apart. This result questions the validity of passive scalar turbulence physics commonly used for premixed flame modeling. However, the influence of Lewis number on this alignment behavior is not clear at this time.

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 97%

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Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

et al. 2008

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“…However, we have already shown in figure 7 that this law fails to predict the scalar turbulent fluxes ρv c because they can be counter-gradient for high values of the expansion factor. Moreover, the external pressure gradient that may be imposed to the flame brush may also have a strong influence on the scalar turbulent fluxes (see for instance Shepherd et al 1982;Heitor et al 1987) leading also to the failure of the gradient law.…”

Section: Scalar Turbulent Fluxesmentioning

confidence: 99%

Direct and indirect thermal expansion effects in turbulent premixed flames

2011

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The thermal expansion induced by the exothermic chemical reactions taking place in a turbulent reactive flow affects the velocity field so strongly that the large-scale velocity fluctuations as well as the small-scale velocity gradients can be governed by chemistry rather than by turbulence. Moreover, thermal expansion is well known to be responsible for counter-gradient turbulent diffusion and flame-generated turbulence phenomena. In the present study, by making use of an original splitting procedure applied to the velocity field, we establish the occurrence of two distinct thermal expansion effects in the flamelet regime of turbulent premixed combustion. The first is referred to as the direct thermal expansion effect. It is associated with a local flamelet crossing contribution as previously considered in early analyses of turbulent transport in premixed flames. The second, denoted herein as the indirect thermal expansion effect, is an outcome of the turbulent wrinkling processes that increases the flame surface area. Based on a splitting procedure applied to the velocity field, the respective influences of the two effects are identified and analysed. Furthermore, the theoretical analysis shows that the thermal expansion induced through the local flames can be treated separately in the usual continuity and momentum equations. This description of the turbulent reactive velocity field, leads also to relate all of the usual turbulent quantities to the reactive scalar field. Finally, algebraic closures for the turbulent transport terms of mass and momentum are proposed and successfully validated through comparison with direct numerical simulation data.

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“…Further implications of the present results on modeling approaches can be obtained through analogy with the resuits of Heitor et al (1987). Those authors studied the interaction between turbulence and pressure gradients in the near-wake region of premixed CH 4 flames stabilized on a disk baffle.…”

Section: Implications For Turbulent Flame Modelingmentioning

confidence: 91%

Nonpremixed bluff-body burner flow and flame imaging study

Namazian

Kelly

Schefer

et al. 1989

Experiments in Fluids

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A nonpremixed bluff-body burner flow and flame have been studied using planar flow visualization and species concentration imaging techniques. The burner consists of a central jet of CH 4 in a cylindrical bluff-body and an outer coflowing-air stream. Planar flow visualization, using Mie scattering from seed particles added to the fuel jet, Raman scattering from CH 4 and laser-induced fluorescence of CH combined with Raman scattering of CH 4 provided information on turbulent flow, mixing and combustion. The CH 4 imaging system utilized two cameras, which enhanced the dynamic range of the diagnostic system by a factor of l0 over a single-camera system. It was observed that the fuel jet stagnated on the axis due to interaction with the high velocity air flow. The flow and mixing were found to have significant coherent and noncoherent, large-scale, time-varying structures. The detailed CH 4 Raman and CH fluorescence measurements of an air-dominated bluff-body flame revealed that the stagnation zone governs mixing and flame stability. Through large-scale mixing, the stagnated jet feeds the recirculation zone and also creates a favorable condition to stabilize the flame detached from the bluff-body. The instantaneous flame zone, as defined by CH, was found to be narrow and concentrated in an envelope around the stagnation zone. This narrow flame characteristic is consistent with that observed for jet flames. Although the internal structure of the flame envelops have not yet been defined, these results suggest that this bluff-body flame can be modeled by a "flame sheet" type approach, where the reaction front is captured by the large-scale structures. This should simplify the development of modeling approaches for these flows since molecular mixing and chemical reaction, which occur within the "flame sheet", can be separated from the large-scale mixing process.

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The interaction of turbulence and pressure gradients in a baffle-stabilized premixed flame

Cited by 49 publications

References 48 publications

Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

Direct and indirect thermal expansion effects in turbulent premixed flames

Nonpremixed bluff-body burner flow and flame imaging study

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