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Research Papers

Effects of a Diverging Cup on Swirl Number, Flow Pattern, and Topology of Premixed Flames

[+] Author and Article Information
A. Degenève

Laboratoire EM2C,
CNRS, CentraleSupélec,
Université Paris-Saclay,
3, rue Joliot Curie,
Gif-sur-Yvette cedex 91192, France;
Air Liquide,
Centre de recherche Paris Saclay,
Chemin de la Porte des Loges,
B.P. 126,
Les Loges en Josas 78354, France

P. Jourdaine

Air Liquide,
Centre de recherche Paris Saclay,
Chemin de la Porte des Loges,
B.P. 126,
Les Loges en Josas 78354, France

C. Mirat, R. Vicquelin

Laboratoire EM2C,
CNRS, CentraleSupélec,
Université Paris-Saclay,
3, rue Joliot Curie,
Gif-sur-Yvette cedex 91192, France

J. Caudal

Air Liquide,
Centre de recherche Paris Saclay,
Chemin de la Porte des Loges,
B.P. 126,
Les Loges en Josas 78354, France

T. Schuller

Laboratoire EM2C,
CNRS, CentraleSupélec,
Université Paris-Saclay,
3, rue Joliot Curie,
Gif-sur-Yvette cedex 91192, France;
Institut de Mécanique des Fluides de Toulouse,
IMFT,
Université de Toulouse,
CNRS,
Toulouse 31400, France
e-mail: arthur.degeneve@centralesupelec.fr

Manuscript received July 12, 2018; final manuscript received August 28, 2018; published online October 22, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 141(3), 031022 (Oct 22, 2018) (10 pages) Paper No: GTP-18-1487; doi: 10.1115/1.4041518 History: Received July 12, 2018; Revised August 28, 2018

Impact of the diverging cup angle of a swirling injector on the flow pattern and stabilization of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experiments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments.

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References

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Figures

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Fig. 1

OXYTEC atmospheric test-rig

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Fig. 2

Sketch of the injector: (a) axial cut and (b) transverse cut through the swirler

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Fig. 3

OH* intensity distribution as a function the diverging cup angle α. Gray elements indicate solid components of the combustor. Dimensions are in millimeters: (a) α = 0 deg, (b) α = 5 deg, (c) α = 10 deg, (d) α = 30 deg, and (e) α = 45 deg.

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Fig. 4

Top: probability of presence of the flame front deduced from OH-PLIF measurements in an axial plane with the overlaid velocity field. The gray lines delineate the positions where the flame front is present 20% and 10% of the time. Bottom: velocity field colored by the velocity magnitude |u¯|=(u¯z2+u¯x2)1/2 obtained by PIV. The black contour delineates the position of the IRZ where the axial velocity u¯z is zero.

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Fig. 7

Laser Doppler velocimetry measurements of the cold swirling flow for α = 0, 5, 10, and 30 deg. S0 = 0.85, Re = 18,000: (a) mean axial velocity, (b) mean azimuthal velocity,(c) rms axial velocity fluctuation, and (d) rms azimuthal velocity fluctuation.

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Fig. 6

Internal recirculation zone leading edge position zSP/r0 in nonreacting (black squares) and reacting (empty diamonds) flow conditions, and flame leading edge position zf (black disks) as a function of the injector diffuser cup angle α

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Fig. 5

Angle β (black disks) of the swirling jet flow at the injector outlet and position of the IRZ leading edge stagnation point zSP/r0 measured (empty diamonds) and predicted by Eq. (18) (continuous line) as function of the diffuser cup angle α

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Fig. 8

Model for the theoretical analysis. Right: the swirl number increases in a converging nozzle because pΣ > p (CF < 0). Left: the swirl number decreases in a diffuser because pΣ < p (CF > 0) provided the pressure loss is not too large.

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Fig. 9

Swirl number ratio S2/S1 for different inlet pressure-less swirl number S1, with k = 0

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Fig. 10

Schematic of the swirled flow pattern depicted as a stagnation flow between the injector quarl and the IRZ

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