Analysis and Scalings of Blowoff Limits of 2D and Axisymmetric Bluff Body Stabilized Flames

2012 ◽  
Author(s):  
Christopher W. Foley ◽  
Jerry Seitzman ◽  
Tim Lieuwen
Keyword(s):  
Shear Layer ◽  
Layer Thickness ◽  
Bluff Body ◽  
Scale Dependence ◽  
Flame Stabilization ◽  
Length Scale ◽  
Measured Velocity ◽  
Shearing Flow ◽  
Flame Stretch ◽  
Geometric Length

This paper considers shear layer flame stabilization with a particular focus on velocity scaling of blowoff limits. Analysis of the expression for hydrodynamic flame stretch, κ, in a shear layer shows that it consists of two contributions, associated with normal and shearing flow strain. These two contributors lead to flame stretch scalings of SL/δ and U/L, respectively, where δ and L denote shear layer thickness and characteristic geometric length scale. These two flame stretch terms have different velocity and length scalings (roughly U1/2 and U1, respectively) and so different blowoff trends can be expected depending upon which term dominates. These scalings are used to interpret a variety of bluff body blowoff data in the literature by analyzing the velocity and length scale dependence of extinction stretch rates calculated at the measured blowoff conditions. We also show that the measured velocity sensitivities to chemical time at blowoff range from U−0.3 to U−1.6. A key point of this study is that blowoff boundaries do not necessarily follow a U−1 scaling suggested by classical Damköhler number scalings and that more work is needed to understand the controlling extinction processes of near-blowoff flames.

Density ratio effects on reacting bluff-body flow field characteristics

2012 ◽  
Vol 706 ◽  
pp. 219-250 ◽  
Author(s):  
Benjamin Emerson ◽  
Jacqueline O’Connor ◽  
Matthew Juniper ◽  
Tim Lieuwen
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Shear Layer ◽  
Parametric Excitation ◽  
Bluff Body ◽  
Density Ratio ◽  
Global Mode ◽  
Measured Velocity ◽  
Number Range ◽  
Flow Field Characteristics

AbstractThe wake characteristics of bluff-body-stabilized flames are a strong function of the density ratio across the flame and the relative offset between the flame and shear layer. This paper describes systematic experimental measurements and stability calculations of the dependence of the flow field characteristics and flame sheet dynamics upon flame density ratio,${\rho }_{u} / {\rho }_{b} $, over the Reynolds number range of 1000–3300. We show that two fundamentally different flame/flow behaviours are observed at high and low${\rho }_{u} / {\rho }_{b} $values: a stable, noise-driven fixed point and limit-cycle oscillations, respectively. These results are interpreted as a transition from convective to global instability and are captured well by stability calculations that used the measured velocity and density profiles as inputs. However, in this high-Reynolds-number flow, the measurements show that no abrupt bifurcation in flow/flame behaviour occurs at a given${\rho }_{u} / {\rho }_{b} $value. Rather, the flow field is highly intermittent in a transitional${\rho }_{u} / {\rho }_{b} $range, with the relative fraction of the two different flow/flame behaviours monotonically varying with${\rho }_{u} / {\rho }_{b} $. This intermittent behaviour is a result of parametric excitation of the global mode growth rate in the vicinity of a supercritical Hopf bifurcation. It is shown that this parametric excitation is due to random fluctuations in relative locations of the flame and shear layer.

Countercurrent shear layer control of flame stabilization

1997 ◽  
Author(s):  
E. Koc-Alkislar ◽  
L. Lourenco ◽  
A. Krothapalli ◽  
P. Strykowski ◽  
E. Koc-Alkislar ◽  
...  
Keyword(s):  
Shear Layer ◽  
Flame Stabilization ◽  
Countercurrent Shear ◽  
Layer Control

scholarly journals A frictional Cosserat model for the slow shearing of granular materials

2002 ◽  
Vol 457 ◽  
pp. 377-409 ◽  
Author(s):  
L. SRINIVASA MOHAN ◽  
K. KESAVA RAO ◽  
PRABHU R. NOTT
Keyword(s):  
Angular Velocity ◽  
Granular Material ◽  
Shear Layer ◽  
Granular Materials ◽  
Layer Thickness ◽  
Couette Flow ◽  
Velocity Fields ◽  
Cosserat Continuum ◽  
Cosserat Model ◽  
Cylindrical Couette Flow

A rigid-plastic Cosserat model for slow frictional flow of granular materials, proposed by us in an earlier paper, has been used to analyse plane and cylindrical Couette flow. In this model, the hydrodynamic fields of a classical continuum are supplemented by the couple stress and the intrinsic angular velocity fields. The balance of angular momentum, which is satisfied implicitly in a classical continuum, must be enforced in a Cosserat continuum. As a result, the stress tensor could be asymmetric, and the angular velocity of a material point may differ from half the local vorticity. An important consequence of treating the granular medium as a Cosserat continuum is that it incorporates a material length scale in the model, which is absent in frictional models based on a classical continuum. Further, the Cosserat model allows determination of the velocity fields uniquely in viscometric flows, in contrast to classical frictional models. Experiments on viscometric flows of dense, slowly deforming granular materials indicate that shear is confined to a narrow region, usually a few grain diameters thick, while the remaining material is largely undeformed. This feature is captured by the present model, and the velocity profile predicted for cylindrical Couette flow is in good agreement with reported data. When the walls of the Couette cell are smoother than the granular material, the model predicts that the shear layer thickness is independent of the Couette gap H when the latter is large compared to the grain diameter dp. When the walls are of the same roughness as the granular material, the model predicts that the shear layer thickness varies as (H/dp)1/3 (in the limit H/dp [Gt ] 1) for plane shear under gravity and cylindrical Couette flow.

A Study on Length Scale Effects on Indentation Delamination of Thin Films

2004 ◽  
Author(s):  
W. Li ◽  
S. Qu ◽  
T. Siegmund ◽  
Y. Huang
Keyword(s):  
Plastic Deformation ◽  
Strain Gradient ◽  
Cohesive Zone ◽  
Cohesive Zone Model ◽  
Scale Effects ◽  
Scale Dependence ◽  
Zone Model ◽  
Length Scale ◽  
Gradient Plasticity ◽  
Elastic Substrates

Simulations of indentation delamination of ductile films on elastic substrates are performed. A cohesive zone model accounts for initiation and growth of interface delaminations and a strain gradient plasticity framework for the length scale dependence of plastic deformation. With the cohesive zone model and the strain gradient formulation two length scales are introduced in to the analysis.

The structure of a highly decelerated axisymmetric turbulent boundary layer

2021 ◽  
Vol 929 ◽  
Author(s):  
N. Agastya Balantrapu ◽  
Christopher Hickling ◽  
W. Nathan Alexander ◽  
William Devenport
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Large Scale ◽  
Convection Velocity ◽  
Mean Flow ◽  
Mean Velocity ◽  
The Mean

Experiments were performed over a body of revolution at a length-based Reynolds number of 1.9 million. While the lateral curvature parameters are moderate ( $\delta /r_s < 2, r_s^+>500$ , where $\delta$ is the boundary layer thickness and r s is the radius of curvature), the pressure gradient is increasingly adverse ( $\beta _{C} \in [5 \text {--} 18]$ where $\beta_{C}$ is Clauser’s pressure gradient parameter), representative of vehicle-relevant conditions. The mean flow in the outer regions of this fully attached boundary layer displays some properties of a free-shear layer, with the mean-velocity and turbulence intensity profiles attaining self-similarity with the ‘embedded shear layer’ scaling (Schatzman & Thomas, J. Fluid Mech., vol. 815, 2017, pp. 592–642). Spectral analysis of the streamwise turbulence revealed that, as the mean flow decelerates, the large-scale motions energize across the boundary layer, growing proportionally with the boundary layer thickness. When scaled with the shear layer parameters, the distribution of the energy in the low-frequency region is approximately self-similar, emphasizing the role of the embedded shear layer in the large-scale motions. The correlation structure of the boundary layer is discussed at length to supply information towards the development of turbulence and aeroacoustic models. One major finding is that the estimation of integral turbulence length scales from single-point measurements, via Taylor's hypothesis, requires significant corrections to the convection velocity in the inner 50 % of the boundary layer. The apparent convection velocity (estimated from the ratio of integral length scale to the time scale), is approximately 40 % greater than the local mean velocity, suggesting the turbulence is convected much faster than previously thought. Closer to the wall even higher corrections are required.

Length-Scale Dependence of Hydration Free Energy: Effect of Solute Charge

2011 ◽  
Vol 145 (2) ◽  
pp. 253-264 ◽  
Author(s):  
Jihang Wang ◽  
Dusan Bratko ◽  
Alenka Luzar
Keyword(s):  
Free Energy ◽  
Scale Dependence ◽  
Length Scale ◽  
Hydration Free Energy ◽  
Energy Effect

scholarly journals Temperature and length scale dependence of solvophobic solvation in a single-site water-like liquid

2013 ◽  
Vol 138 (6) ◽  
pp. 064506 ◽  
Author(s):  
John R. Dowdle ◽  
Sergey V. Buldyrev ◽  
H. Eugene Stanley ◽  
Pablo G. Debenedetti ◽  
Peter J. Rossky
Keyword(s):  
Scale Dependence ◽  
Single Site ◽  
Length Scale ◽  
Site Water

Review and Analysis of Bluff Body Flame Stabilization Data

2008 ◽  
Author(s):  
Sajjad A. Husain ◽  
Ganesh Nair ◽  
Santosh Shanbhogue ◽  
Tim C. Lieuwen
Keyword(s):  
Activation Energy ◽  
Kinetic Modeling ◽  
First Principles ◽  
Bluff Body ◽  
Large Range ◽  
Flame Stabilization ◽  
Local Extinction ◽  
Data Set ◽  
Flame Sheet ◽  
Semi Empirical

This paper compiles and analyzes bluff body stabilized flame blowoff data from the literature. Many of these studies contain semi-empirical blowoff correlations that are, in essence, Damko¨hler number correlations of their data. This paper re-analyzes these data, utilizing various Damko¨hler number correlations based upon detailed kinetic modeling for determining chemical time scales. While the results from this compilation are similar to that deduced from many earlier studies, it demonstrates that a rather comprehensive data set taken over a large range of conditions can be correlated from “first-principles” based calculations that do not rely on empirical fits or adjustable constants (e.g., global activation energy or pressure exponents). The paper then discusses the implications of these results on understanding of blowoff. Near blowoff flames experience local extinction of the flame sheet, manifested as “holes” that form and convect downstream. However, local extinction is distinct from blowoff — in fact, under certain conditions the flame can apparently persist indefinitely with certain levels of local extinction. We hypothesize that simple Damko¨hler number correlations contain the essential physics describing this first stage of blowoff; i.e., they are correlations for the conditions where local extinction on the flame begins, but do not fundamentally describe the ultimate blowoff condition itself. However, such correlations are reasonably successful in correlating blowoff limits because the ultimate blowoff event appears to be correlated to some extent to the onset of this first stage.

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