Author: Carl Fredrik Stein
Publisher:
ISBN:
Category :
Languages : en
Pages : 66

Get Book Here

Book Description

Author: Benjamin Keeton
Publisher:
ISBN:
Category :
Languages : en
Pages : 0

Get Book Here

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: Aran Fitzgerald
Publisher:
ISBN:
Category : Axial flow
Languages : en
Pages : 196

Get Book Here

Book Description

Author: Sean Clees
Publisher:
ISBN:
Category :
Languages : en
Pages :

Get Book Here

Book Description
Swirling jets are commonly used in combustion applications to stabilize flames and improve emissions. Thus, their dynamics play an important role in combustor design. Despite the prevalence of swirling flows in industrial applications that involve highly turbulent flow fields, the majority of experimental and numerical studies consider only laminar conditions. In this study, the dynamics of the vortex core are investigated in a swirling, turbulent jet at swirl numbers in the range of the critical swirl number for vortex breakdown. Vortex breakdown, a bifurcation in the structure of a swirling jet, results in the establishment of a stagnation point and recirculation region along the centerline of the jet. To study these dynamics, dynamic mode decomposition, an order-reduction technique used to extract coherent structures from flow data, is implemented. Investigation of time-averaged velocity fields and profiles leads to the identification of three flow regimes: pre-breakdown, near-breakdown, and post-breakdown. Velocity fields in these regimes are further analyzed using dynamic mode decomposition, Rankine-vortex fitting, and proper orthogonal decomposition to characterize jet dynamics with a particular focus on the development of the recirculation region characteristic of vortex breakdown. A precessing vortex core is also identified in the post-breakdown regime and its behavior is discussed.

Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 173

Get Book Here

Book Description

Author: Ing Kiet Francis Toh
Publisher:
ISBN:
Category : Jets
Languages : en
Pages : 247

Get Book Here

Book Description
The mean flow properties and the instantaneous flow structures of the jets are then investigated. The introduction and increase of swirl result in a higher jet growth, decay and turbulent intensity, the formation of an off-axis axial velocity maximum and the occurrence of vortex breakdown. The swirl-induced jet growth enhancement can be categorized into three regimes: a low swirl regime in which there is a limited enhancement, a moderate swirl regime in which the enhancement scales with swirl, and a high swirl regime in which vortex breakdown dominates the process. The jet growth behavior in the high swirl regime is found to be dependent on the structure of the jet and the vortex breakdown configuration. A significant change in the flow structures and certain mean flow properties including the centerline and local maximum axial velocity decay are observed in the low and the moderate swirl regimes. These properties appear to be less sensitive to swirl in the high swirl regime after the occurrence of vortex breakdown. The effects of Reynolds number on swirling jets may not be identical to that on a non-swirling jet. The statistical characteristics measured show that low velocity occasions in the vicinity of the jet centerline start to appear at a sufficiently high degree of swirl. These occasions increase with swirl until the eventual occurrence of vortex breakdown. The statistical characteristics of jets that have undergone vortex breakdown are very similar, as they are dominated by the vortex breakdown phenomenon. The mean flow and the statistical measurements also reveal the presence of an unstable vortex breakdown that is difficult to detect using flow visualization and instantanous velocity field measurements.

Author: Ismet Gursul
Publisher:
ISBN:
Category :
Languages : en
Pages :

Get Book Here

Book Description

Author: Michael Rudolf Ruith
Publisher:
ISBN:
Category :
Languages : en
Pages : 374

Get Book Here

Book Description

Author: Tobias Luginsland
Publisher:
ISBN:
Category :
Languages : en
Pages :

Get Book Here

Book Description

Author: Sudev Das
Publisher: Springer Nature
ISBN: 981995990X
Category : Technology & Engineering
Languages : en
Pages : 840

Get Book Here

Book Description
This book comprises the proceedings of the 1st International Conference on Fluid, Thermal and Energy Systems. The contents of this book focus on phase change heat transfer, advanced energy systems, separated flows, turbulence and multi-phase modeling, computational fluid flow and heat transfer, thermal energy storage systems, integrated energy systems, nuclear thermal hydraulics, heat transfer in nanofluids, etc. This book serves as a useful reference to researchers, academicians, and students interested in the broad field of thermo-fluid science and engineering.