Strain Characteristics Near the Flame Attachment Point in a Swirling Flow

In the present work, a direct numerical simulation (DNS) of an experimental high Karlovitz number (Ka) CH 4 /air piloted premixed flame was analysed to study the inner structure and the stabilisation mechanism of the turbulent flame. A reduced chemical mechanism for premixed CH 4 /air combustion with NO x based on GRI-Mech3.0 was used, including 268 elementary reactions and 28 transported species. The evolution of the stretch factor, I 0 , indicates that the burning rate per unit flame surface area is considerably reduced in the near field and exhibits a minimum at x/D = 8. Downstream, the burning rate gradually increases. The stretch factor is different between different species, suggesting the quenching of some reactions but not others. Comparison between the turbulent flame and strained laminar flames indicates that certain aspects of the mean flame structure can be represented surprisingly well by flamelets if changes in boundary conditions are accounted for and the strain rate of the mean flow is employed; however, the thickening of the flame due to turbulence is not captured. The spatial development of displacement speeds is studied at higher Ka than previous DNS. In contrast to almost all previous studies, the mean displacement speed conditioned on the flame front is negative in the near field, and the dominant contribution to the displacement speed is normal diffusion with the reaction contribution being secondary. Further downstream, reaction overtakes normal diffusion, contributing to a positive displacement speed. The negative displacement speed in the near field implies that the flame front situates itself in the pilot region where the inner structure of the turbulent flame is affected significantly, and the flame stabilises in balance with the inward flow. Notably, in the upstream region of the turbulent flame, the main reaction contributing to the production of OH, H+O 2 O+OH (R35), is weak. Moreover, oxidation reactions, H 2 +OHH+H 2 O (R79) and CO+OHCO 2 +H (R94), are influenced by H 2 O and CO 2 from the pilot and are completely quenched. Hence, the entire radical pool of OH, H and O is affected. However, the fuel consumption layer remains comparably active and generates heat, mainly via the reaction CH 4 +OHCH 3 +H 2 O (R93).

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“…In particular, it is known that a t is sensitive to the alignment of the flame with respect to the flow [55,56].…”

Section: Turbulent and Laminar Flame Structuresmentioning

confidence: 99%

A direct numerical simulation study of flame structure and stabilization of an experimental high Ka CH4/air premixed jet flame

Wang

Hawkes

Chen

2017

Combustion and Flame

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“…These flame front structures form a wide angle relative to the incoming flow due to the effect of the high swirl intensity and seem to occasionally penetrate from the periphery toward the base of the flame within the recirculation region. The overall disposition of the flame front under stratification seems to differ both in terms of length, placement, and anchoring behaviour from both nonpremixed [16,19] and fully premixed axisymmetric [4,7,8] counterpart topologies. The transient OH * chemiluminescent images for the square verify the two-tongued flame front placement previously implied by the time-mean experimental and simulation results ( reaction within the formation region, which is in stark contrast to the reported behavior [6,12] of ultralean and limiting slender body stabilization of fully premixed flames.…”

Section: Journal Of Combustionmentioning

confidence: 96%

“…Evidently due to the complexity of the above described phenomena, better knowledge of the details of the turbulent transport and reaction processes over a range of operating conditions from lean to ultra-lean and approaching blowoff (LBO) is important, if a full exploitation or even extension of the operational margin is to be realised (e.g., [4,[6][7][8]). A range of systematic experiments have been performed to study the structure of such flame configurations (e.g., [4][5][6][7][8]) along with a number of supporting simulations that employ various levels of complexity for the turbulence-chemistry treatment and usually involve adaptive local gridding within a time-dependent procedure (e.g., [9]). Phenomenological analyses (e.g., [2,6,8]) and global correlations (e.g., [3,10]) are also utilized to complement the overall development effort and relieve the burden and cost involved in the multiscalar approaches.…”

Section: Introductionmentioning

confidence: 99%

“…A range of systematic experiments have been performed to study the structure of such flame configurations (e.g., [4][5][6][7][8]) along with a number of supporting simulations that employ various levels of complexity for the turbulence-chemistry treatment and usually involve adaptive local gridding within a time-dependent procedure (e.g., [9]). Phenomenological analyses (e.g., [2,6,8]) and global correlations (e.g., [3,10]) are also utilized to complement the overall development effort and relieve the burden and cost involved in the multiscalar approaches.…”

Section: Introductionmentioning

confidence: 99%

“…These experimental and computational works have so far produced detailed descriptions of the ultra-lean behaviour in traditional stabilizers such as fully premixed axisymmetric (e.g., [3,4,7,8]) and slender two-dimensional (e.g., [6,11]) bluff-body arrangements. However, stratified bluff-body flames operated at ultra-lean and near blow off conditions have not been investigated as extensively.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

A Comparison of the Characteristics of Planar and Axisymmetric Bluff-Body Combustors Operated under Stratified Inlet Mixture Conditions

Paterakis

Souflas

Dogkas

et al. 2013

Journal of Combustion

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The work presents comparisons of the flame stabilization characteristics of axisymmetric disk and 2D slender bluff-body burner configurations, operating with inlet mixture stratification, under ultralean conditions. A double cavity propane air premixer formed along three concentric disks, supplied with a radial equivalence ratio gradient the afterbody disk recirculation, where the first flame configuration is stabilized. Planar fuel injection along the center plane of theleading faceof a slender square cylinder against the approach cross-flow results in a stratified flame configuration stabilized alongside the wake formation region in the second setup. Measurements of velocities, temperatures,OH∗andCH∗chemiluminescence, local extinction criteria, and large-eddy simulations are employed to examine a range of ultralean and close to extinction flame conditions. The variations of the reacting front disposition within these diverse reacting wake topologies, the effect of the successive suppression of heat release on the near flame region characteristics, and the reemergence of large-scale vortical activity on approach to lean blowoff (LBO) are investigated. The cross-correlation of the performance of these two popular flame holders that are at the opposite ends of current applications might offer helpful insights into more effective control measures for expanding the operational margin of a wider range of stabilization configurations.

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