Numerical Simulations of Vortex Breakdown in Swirling Jets and Diffusion Flames PDF Download
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Author: Benjamin Keeton
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
In combustion applications such as gas turbines, swirling jets are used to generate a vortex breakdown recirculation region that serves as a non-invasive flame stabilizer. The present work performs direct numerical simulations to study the effects of vortex breakdown on the structure and stabilization of laminar low-Mach-number gaseous diffusion flames. Vortex breakdown transitions are first studied for heated/cooled variable-density, non-reacting jets in solid body rotation issuing into an unconfined ambient atmosphere. For increasing values of the swirl number S, two vortex breakdown modes are observed, the bubble and the cone, and the associated transitions S[superscript]*[subscript]B and S[superscript]*[subscript]C are determined for different values of the ambient-to-jet temperature ratio [lambda]. Both axisymmetric and three-dimensional simulations show decreasing values of S[superscript]*[subscript]B with increasing lambda, while critical swirl numbers for the transition to the cone S[superscript]*[subscript]C, remain relatively constant for a fixed effective Reynolds number. The first study is then extended to evaluate the same transitions in axisymmetric methane-air flames in the Burke-Schumann limit of infinitely fast chemistry. Transitions S[superscript]*[subscript]B are relatively unaffected by fuel-feed dilution, and result in jet-like flames. For moderate values of dilution, further increase in S to S[superscript]*[subscript]C generates a steady conical breakdown, with the flame sheet again passing around the recirculating fuel and products. Extreme dilution, on the other hand, generates an enlarged cone that recirculates the ambient air, stabilizing the flame near the jet inlet. Effects of bubble vortex breakdown are then explored for finite-rate chemistry flames in an axisymmetric concentric swirling jet configuration, for which a central fuel jet is surrounded by a swirling co-annular stream of air. Liftoff and blow-off are analyzed by systematically varying the two relevant parameters, the swirl number S and the Damköhler number, D[subscript]N. For sufficiently low values of D[subscript]N, and large values of S, flames lift off the injector, and thermal expansion at the base of the triple flame redirects the flow radially inward, promoting the formation of a small recirculation zone. Axisymmetric and three-dimensional simulations of the isothermal flow are used to analyze the mechanism for the onset of the bubble, and identify post-breakdown flow structure for larger values of the Reynolds number.
Author: Benjamin Keeton
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
In combustion applications such as gas turbines, swirling jets are used to generate a vortex breakdown recirculation region that serves as a non-invasive flame stabilizer. The present work performs direct numerical simulations to study the effects of vortex breakdown on the structure and stabilization of laminar low-Mach-number gaseous diffusion flames. Vortex breakdown transitions are first studied for heated/cooled variable-density, non-reacting jets in solid body rotation issuing into an unconfined ambient atmosphere. For increasing values of the swirl number S, two vortex breakdown modes are observed, the bubble and the cone, and the associated transitions S[superscript]*[subscript]B and S[superscript]*[subscript]C are determined for different values of the ambient-to-jet temperature ratio [lambda]. Both axisymmetric and three-dimensional simulations show decreasing values of S[superscript]*[subscript]B with increasing lambda, while critical swirl numbers for the transition to the cone S[superscript]*[subscript]C, remain relatively constant for a fixed effective Reynolds number. The first study is then extended to evaluate the same transitions in axisymmetric methane-air flames in the Burke-Schumann limit of infinitely fast chemistry. Transitions S[superscript]*[subscript]B are relatively unaffected by fuel-feed dilution, and result in jet-like flames. For moderate values of dilution, further increase in S to S[superscript]*[subscript]C generates a steady conical breakdown, with the flame sheet again passing around the recirculating fuel and products. Extreme dilution, on the other hand, generates an enlarged cone that recirculates the ambient air, stabilizing the flame near the jet inlet. Effects of bubble vortex breakdown are then explored for finite-rate chemistry flames in an axisymmetric concentric swirling jet configuration, for which a central fuel jet is surrounded by a swirling co-annular stream of air. Liftoff and blow-off are analyzed by systematically varying the two relevant parameters, the swirl number S and the Damköhler number, D[subscript]N. For sufficiently low values of D[subscript]N, and large values of S, flames lift off the injector, and thermal expansion at the base of the triple flame redirects the flow radially inward, promoting the formation of a small recirculation zone. Axisymmetric and three-dimensional simulations of the isothermal flow are used to analyze the mechanism for the onset of the bubble, and identify post-breakdown flow structure for larger values of the Reynolds number.
Author: Michael Rudolf Ruith
Publisher:
ISBN:
Category :
Languages : en
Pages : 374
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Book Description
Author: Thierry Baritaud
Publisher: Editions TECHNIP
ISBN: 9782710806981
Category : Science
Languages : en
Pages : 328
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Book Description
Contents: Description of accurate boundary conditions for the simulation of reactive flows. Parallel direct numerical simulation of turbulent reactive flow. Flame-wall interaction and heat flux modelling in turbulent channel flow. A numerical study of laminar flame wall interaction with detailed chemistry: wall temperature effects. Modeling and simulation of turbulent flame kernel evolution. Experimental and theoretical analysis of flame surface density modelling for premixed turbulent combustion. Gradient and counter-gradient transport in turbulent premixed flames. Direct numerical simulation of turbulent flames with complex chemical kinetics. Effects of curvature and unsteadiness in diffusion flames. Implications for turbulent diffusion combustion. Numerical simulations of autoignition in turbulent mixing flows. Stabilization processes of diffusion flames. References.
Author: S. A. Berger
Publisher:
ISBN:
Category :
Languages : en
Pages : 32
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Book Description
The sensitivity of the onset and the location of vortex breakdowns in concentrated vortex cores, and the pronounced tendency of the breakdowns to migrate upstream have been characteristic observations of experimental investigations; they have also been features of numerical simulations and led to questions about the validity of these simulations. This behavior seems to be inconsistent with the strong time-like axial evolution of the flow, as expressed explicitly, for example, by the quasi-cylindrical approximate equations for this flow. An order-of-magnitude analysis of the equations of motion near breakdown leads to a modified set of governing equations, analysis of which demonstrates that the interplay between radial inertial, pressure, and viscous forces gives an elliptic character to these concentrated swirling flows. Analytical, asymptotic, and numerical solutions of a simplified non-linear equation are presented; these qualitatively exhibit the features of vortex onset and location noted above.
Author: José L. Stevens
Publisher:
ISBN:
Category : Aerodynamics
Languages : en
Pages : 142
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Book Description
Author: Daniel Thomas Martin
Publisher:
ISBN:
Category : Breakdown (Electricity)
Languages : en
Pages : 62
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Author: Justin D. Weiler
Publisher:
ISBN:
Category : Combustion
Languages : en
Pages :
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Book Description
The interactions between laminar premixed flames and counter-rotating vortex pairs in natural and synthetic gas mixtures have been computationally investigated through the use of Direct Numerical Simulations and parallel processing. Using a computational model for premixed combustion, laminar flames are simulated for single- and two-component fuel mixtures of methane, carbon monoxide, and hydrogen. These laminar flames are forced to interact with superimposed laminar vortex pairs, which mimic the effects of a pulsed, two-dimensional slot-injection. The premixed flames are parameterized by their unstretched laminar flame speed, heat release, and flame thickness. The simulated vortices are of a fixed size (relative to the flame thickness) and are parameterized, solely, by their rotational velocity (relative to the flame speed). Strain rate and surface curvature measurements are made along the stretched flame surfaces to study the effects of additive syngas species (CO and H2) on lean methane-air flames. For flames that share the same unstretched laminar flame speed, heat release, and flame thickness, it is observed that the effects of carbon monoxide on methane-air mixtures are essentially negigible while the effects of hydrogen are quite substantial. The dynamics of stretched CH4/Air and CH4/CO/Air flames are nearly identical to one another for interactions with both strong and weak vortices. However, the CH4/H2/Air flames demonstrate a remarkable tendency toward surface area growth. Over comparable interaction periods, the flame surface area produced during interactions with CH4/H2/Air flames was found to be more than double that of the pure CH4/Air flames. Despite several obvious differences, all of the interactions revealed the same basic phenomena, including vortex breakdown and flame pinch-off (i.e. pocket formation). In general, the strain rate and surface curvature magnitudes were found to be lower for the CH4/H2/Air flames, and comparable between CH4/Air and CH4/CO/Air flames. Rates of flame stretching are not explicitely determined, but are, instead, addressed through observation of their individual components. Two different models are used to determine local displacement speed values. A discrepancy between practical and theoretical definitions of the displacement speed is evident based on the instantaneous results for CH4/Air and CH4/H2/Air flames interacting with weak and strong vortices.
Author: Toufik Boushaki
Publisher: BoD – Books on Demand
ISBN: 1838807438
Category : Science
Languages : en
Pages : 120
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Book Description
Swirl flows are used in a wide range of industrial applications. In non-reacting cases, examples of applications include vortex amplifiers and reactors, heat exchangers, jet pumps, and cyclone separators. In reacting cases, swirlers are widely used in combustion systems, such as gas turbines, industrial furnaces, boilers, gasoline and diesel engines, and many other practical heating devices. Effects of using swirl on flow and combustion are significant and varied, and concern, for example, aerodynamics, mixing, flame stability, intensity of combustion, and pollutant emissions. The purpose of this book is to present recent research efforts to understand and characterize swirling flows of different types and in different applications. These include gaseous, liquid, and solid fuels in order to enhance combustion systems and their energy efficiency. Swirl flows are very complex and the studies proposed in this project are based on different means, including theoretical calculations, numerical modeling, and experimental measurements.
Author: Michele G. Macaraeg
Publisher:
ISBN:
Category :
Languages : en
Pages : 40
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Book Description
Author: Fenando F. Grinstein
Publisher: Cambridge University Press
ISBN: 1107137047
Category : Science
Languages : en
Pages : 481
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Book Description
Reviews our current understanding of the subject. For graduate students and researchers in computational fluid dynamics and turbulence.
