Countercurrent shear layer control of flame stabilization

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical toward the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence of CH radicals (CH-PLIF) measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.

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Shear Layer Flame Stabilization Sensitivities in a Swirling Flow

Volume 4: Heat Transfer, Parts A and B ◽

10.1115/gt2012-68513 ◽

2012 ◽

Cited By ~ 4

Author(s):

I. Chterev ◽

C. W. Foley ◽

D. R. Noble ◽

B. A. Ochs ◽

J. M. Seitzman ◽

...

Keyword(s):

Heat Transfer ◽

Shear Layer ◽

Vortex Breakdown ◽

Swirling Flow ◽

Shear Layers ◽

Flame Stabilization ◽

Preheat Temperature ◽

Swirl Number ◽

Breakdown Region ◽

Detailed Kinetics

A variety of different flame configurations and heat release distributions can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper focuses on the sensitivities of the outer shear layer stabilization point to bulkhead temperature, flow velocity, swirl number, preheat temperature, and fuel/air ratio. It also characterizes the hysteresis that is present in conditions where the outer shear layer locally re-attaches and blows off. The sensitivities to bulkhead temperature, preheat temperature and fuel/air ratio follow the expected trends. Moreover, the strong bulkhead temperature sensitivities show that computations must include heat transfer to combustor hardware in order to capture flame stabilization correctly. The preheat temperature and fuel/air ratio sensitivities are captured with detailed kinetics calculations of the extinction stretch rate of the mixture. Somewhat counter intuitively, there is little variation in transition conditions with swirl number for the Sm∼0.6 and 0.8 swirlers analyzed here. Finally, velocity sensitivities are in many cases much weaker than what would be predicted assuming that the fluid mechanic straining time scales as 1/u.

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On the Mechanisms Affecting Fluidic Vectoring Using Suction

Journal of Fluids Engineering ◽

10.1115/1.2375125 ◽

2006 ◽

Vol 129 (1) ◽

pp. 91-99 ◽

Cited By ~ 3

Author(s):

R. D. Gillgrist ◽

D. J. Forliti ◽

P. J. Strykowski

Keyword(s):

Shear Layer ◽

Secondary Flow ◽

Mass Flow ◽

Pressure Field ◽

Countercurrent Flow ◽

Primary Flow ◽

Countercurrent Shear ◽

Low Pressures ◽

Direction Opposite ◽

Subsonic Jet

Suction was applied asymmetrically to the exhaust of a rectangular subsonic jet creating a pressure field capable of vectoring the primary flow at angles up to 15deg. The suction simultaneously creates low pressures near the jet exhaust and conditions capable of drawing a secondary flow along the jet shear layer in the direction opposite to the primary jet. This countercurrent shear layer is affected both by the magnitude of the suction source as well as the proximity of an adjacent surface onto which the pressure forces act to achieve vectoring. This confined countercurrent flow gives rise to elevated turbulence levels in the jet shear layer as well as considerable increases in the gradients of the turbulent stresses. The turbulent stresses are responsible for producing a pressure field conducive for vectoring the jet at considerably reduced levels of secondary mass flow than would be possible in their absence.

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Enhancing Combustion in a Dump Combustor Using Countercurrent Shear: Part 2 — Heat Release Rate Measurements and Geometry Effects

Heat Transfer, Part A ◽

10.1115/imece2005-81274 ◽

2005 ◽

Cited By ~ 1

Author(s):

Alison A. Behrens ◽

Matthew J. Anderson ◽

Paul J. Strykowski ◽

David J. Forliti

Keyword(s):

Heat Release ◽

Shear Layer ◽

Recirculation Zone ◽

Rectangular Channel ◽

Sudden Expansion ◽

Flame Surface ◽

Release Rates ◽

Primary Flow ◽

Dump Combustor ◽

Countercurrent Shear

Research to advance our understanding of the countercurrent shear flow has been conducted, with particular emphasis on those characteristics of countercurrent shear that are beneficial for combustion applications. Studies carried out in a backward-facing step combustor burning prevaporized JP10-air mixtures, have examined the implementation of counterflow as a means to enhance turbulent burning velocities, with the overall objective of increasing volumetric heat release rates and thereby create a more compact combustion zone. The dump combustor is characterized by a nominally two-dimensional primary flow mixture of prevaporized fuel and air, entering a rectangular channel before encountering a 2:1 single-sided step expansion. Flow separation over the sudden expansion and the resulting recirculation set up a countercurrent shear layer downstream of the dump plane and a low velocity zone conducive to flame anchoring. Combustion control strategies aim to increase turbulent kinetic energy and flame three-dimensionality in an effort to increase flame surface area and thus burning rates. A secondary flow is created via suction at the dump plane as a fluidic control mechanism to enhance the naturally occurring countercurrent shear layer. Counterflow is shown to elevate turbulence levels and volumetric heat release rates downstream of the step in the base geometry while concomitantly reducing the scale of the recirculation zone[1]. Modifications to the rearward-facing step geometry are investigated using Particle Image Velocimetry (PIV) under isothermal flow conditions in an effort to extend the near field interaction between the recirculation zone and the incoming primary flow, thus exploiting the benefits of counterflow as seen in the base geometry. Using chemiluminescence, relative heat release rates are shown to increase by 90% with a counterflow level of 6% of the primary flow by mass in the base geometry, and a 150% increase with a counterflow level of 2.4% in the modified step geometry.

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