scholarly journals A scaling law for the recirculation zone length behind a bluff body in reacting flows

2019 ◽  
Vol 875 ◽  
pp. 699-724 ◽  
Author(s):  
James C. Massey ◽  
Ivan Langella ◽  
Nedunchezhian Swaminathan
Keyword(s):  
Heat Release ◽  
Turbulence Intensity ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Scaling Law ◽  
Reacting Flows ◽  
The Body ◽  
Turbulent Shear ◽  
Zone Length ◽  
Isothermal Flows

The recirculation zone length behind a bluff body is influenced by the turbulence intensity at the base of the body in isothermal flows and also the heat release and its interaction with turbulence in reacting flows. This relationship is observed to be nonlinear and is controlled by the balance of forces acting on the recirculation zone, which arise from the pressure and turbulence fields. The pressure force is directly influenced by the volumetric expansion resulting from the heat release, whereas the change in the turbulent shear force depends on the nonlinear interaction between turbulence and combustion. This behaviour is elucidated through a control volume analysis. A scaling relation for the recirculation zone length is deduced to relate the turbulence intensity and the amount of heat release. This relation is verified using the large eddy simulation data from 20 computations of isothermal flows and premixed flames that are stabilised behind the bluff body. The application of this scaling to flames in an open environment and behind a backward facing step is also explored. The observations and results are explained on a physical basis.

Towards Modeling Lean Blow Out in Gas Turbine Flameholder Applications

2004 ◽  
Author(s):  
Won-Wook Kim ◽  
Jeffrey J. Lienau ◽  
Paul R. Van Slooten ◽  
Meredith B. Colket ◽  
Robert E. Malecki ◽  
...  
Keyword(s):  
Equivalence Ratio ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Reacting Flows ◽  
Experimental Measurements ◽  
Turbulent Reacting Flows ◽  
Zone Length ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Lean Limit

The objective of this study was to assess the accuracy of the Large-Eddy Simulation (LES) methodology, with a simple combustion closure based on equilibrium chemistry, for simulating turbulent reacting flows behind a bluff body flameholder. Specifically, the variation in recirculation zone length with change in equivalence ratio was calculated and compared to experimental measurements. It was found that the present LES modeling approach can reproduce this variation accurately. However, it understated the recirculation zone length at the stoichiometric condition. The approach was assessed at the lean blow out (LBO) condition to evaluate its behavior at the lean limit and to analyze the physics of combustion instability.

Towards Modeling Lean Blow Out in Gas Turbine Flameholder Applications

2004 ◽  
Vol 128 (1) ◽  
pp. 40-48 ◽  
Author(s):  
Won-Wook Kim ◽  
Jeffrey J. Lienau ◽  
Paul R. Van Slooten ◽  
Meredith B. Colket ◽  
Robert E. Malecki ◽  
...  
Keyword(s):  
Equivalence Ratio ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Reacting Flows ◽  
Experimental Measurements ◽  
Turbulent Reacting Flows ◽  
Zone Length ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Lean Limit

The objective of this study was to assess the accuracy of the large-eddy simulation (LES) methodology, with a simple combustion closure based on equilibrium chemistry, for simulating turbulent reacting flows behind a bluff body flameholder. Specifically, the variation in recirculation zone length with change in equivalence ratio was calculated and compared to experimental measurements. It was found that the present LES modeling approach can reproduce this variation accurately. However, it understated the recirculation zone length at the stoichiometric condition. The approach was assessed at the lean blow out condition to evaluate its behavior at the lean limit and to analyze the physics of combustion instability.

Numerical and Experimental Evaluation of Lift Forces in Flows Around Surface-Mounted Bluff Bodies

2002 ◽  
Author(s):  
T. Stengel ◽  
F. Ebert ◽  
M. Fallen
Keyword(s):  
Velocity Field ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Separation Zone ◽  
Laser Doppler Anemometry ◽  
Turbulence Models ◽  
Reynolds Numbers ◽  
The Body ◽  
Bluff Bodies ◽  
Large Eddy

The flow around a surface-mounted bluff body with cuboid shape is investigated. Therefore, the velocity field including the distribution of the turbulent kinetic energy is computed and compared with experimental Laser Doppler Anemometry data. Several different turbulence models, namely the standard k-ε model, the Wolfshtein two-layer k-ε model and a Large-Eddy approach are validated. Since the Large-Eddy model remains the only model representing the flow accurate, it is chosen for further investigations. The pressure distribution on the body and on the carrying surface around the body is analysed. The lift coefficients are computed for Reynolds numbers, ranging from 1.1 × 104 up to 4.4 × 104. The lengths of the separation zone above and the recirculation zone downstream the body are evaluated.

Design and Experimentation of Simulated Combustor Model for Aircraft Afterburner Applications

2016 ◽  
Vol 33 (2) ◽  
Author(s):  
K. Kirubhakaran ◽  
K. M. Parammasivam
Keyword(s):  
Experimental Study ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Dynamic Pressure ◽  
Pressure Measurements ◽  
Zone Length ◽  
Stabilized Flames ◽  
Dynamic Pressure Measurements

AbstractAn experimental study of the recirculation zone and mixing lengths for bluff-body stabilized flames is conducted at non-reactive conditions. This paper reports the prediction of recirculation zone length from dynamic pressure measurements. The auxiliary turbulence created from the wall of the combustor is also studied and maintained to levels as low as 5%. The experiments are conducted by varying the velocity from 5 m/s to 8 m/s for V-Gutters bluff-body with induced angles of 60, 90 and 120

scholarly journals Scalar Measurements in Bluff Body Stabilized Flames Using Cars Diagnostics

1991 ◽  
Author(s):  
J. C. Pan ◽  
M. D. Vangsness ◽  
S. P. Heneghan ◽  
D. R. Ballal
Keyword(s):  
Turbulence Intensity ◽  
Large Scale ◽  
Bluff Body ◽  
Recirculation Zone ◽  
Turbulent Flame ◽  
Flame Structure ◽  
Gradient Diffusion ◽  
Stirred Reactor ◽  
Flame Region ◽  
Layer Region

Measurements of mean and rms temperature fluctuations were performed in confined turbulent premixed methane-air flames, stabilized on a conical flameholder. A CARS system was used for these measurements. These tests employed flameholders of different blockage ratios (13% and 25%), and mixtures with different equivalence ratios (0.56, 0.65, 0.8, and 0.9) and approach turbulence intensity (2%, 17%, and 22%). It was found that the recirculation zone closely resembles a perfectly well-stirred reactor. Blockage ratio, equivalence ratio, or approach turbulence intensity did not alter the scalar field. The turbulent flame structure enveloping the recirculation zone comprises: (i) an ignition/thin flame region in the vicinity of the flameholder base, (ii) a reacting shear layer region of large-scale coherent structures, and (iii) a thick flame region where entrainment is the dominant mechanism. Finally, analysis suggests that the scalar gradient-diffusion relationship is valid and areas of non-gradient diffusion, if any, are probably small.

Effects of heat release on turbulent shear flows. Part 1. A general equivalence principle for non-buoyant flows and its application to turbulent jet flames

2000 ◽  
Vol 415 ◽  
pp. 23-44 ◽  
Author(s):  
KATHLEEN M. TACINA ◽  
WERNER J. A. DAHM
Keyword(s):  
Heat Release ◽  
Equivalence Principle ◽  
Scaling Laws ◽  
Shear Flows ◽  
Exothermic Reaction ◽  
Turbulent Jets ◽  
Reacting Flows ◽  
Turbulent Jet ◽  
Turbulent Shear ◽  
Reacting Flow

We address the differences observed due to heat release between reacting and non-reacting versions of otherwise identical shear flows under conditions for which buoyancy effects are negligible. The differences considered here result from density changes produced by exothermic reaction, and are shown to be similar to those produced by free-stream density differences in non-reacting flows. The piecewise linear variations of temperature with mole fraction allow the density changes due to exothermic reaction to be related to an equivalent non-reacting flow, in which the temperature of one of the fluids is raised to an effective value determined by the peak temperature and overall stoichiometry. This leads to a general equivalence principle by which the scaling laws for non-reacting flows can be extended to predict effects of heat release by exothermic reaction. This equivalence principle is then applied to axisymmetric turbulent jets, where it leads to a generalized momentum diameter d+ in which the scaling laws for burning and non-burning jets become identical – it effectively extends the classical momentum diameter d* of Thring & Newby (1953) and Ricou & Spalding (1961) to exothermic reacting flows. The resulting predicted effects of heat release, in both the near and far fields, show good agreement with experimental data from momentum-dominated turbulent jet diffusion flames. The equivalence principle is then applied to planar turbulent jets, for which it also accurately predicts the observed effects of combustion heat release.

Numerical study of the flow over the modified simple frigate shape

2020 ◽  
pp. 095441002097775
Author(s):  
Tong Li ◽  
Yibin Wang ◽  
Ning Zhao
Keyword(s):  
Bluff Body ◽  
Recirculation Zone ◽  
Numerical Study ◽  
Reference Value ◽  
Flow Fields ◽  
Navier Stokes ◽  
Detached Eddy Simulation ◽  
Eddy Simulation ◽  
Delayed Detached Eddy Simulation ◽  
Simulation Results

The simple frigate shape (SFS) as defined by The Technical Co-operative Program (TTCP), is a simplified model of the frigate, which helps to investigate the basic flow fields of a frigate. In this paper, the flow fields of the different modified SFS models, consisting of a bluff body superstructure and the deck, were numerically studied. A parametric study was conducted by varying both the superstructure length L and width B to investigate the recirculation zone behind the hangar. The size and the position of the recirculation zones were compared between different models. The numerical simulation results show that the size and the location of the recirculation zone are significantly affected by the superstructure length and width. The results obtained by Reynolds-averaged Navier-Stokes method were also compared well with both the time averaged Improved Delayed Detached-Eddy Simulation results and the experimental data. In addition, by varying the model size and inflow velocity, various flow fields were numerically studied, which indicated that the changing of Reynolds number has tiny effect on the variation of the dimensionless size of the recirculation zone. The results in this study have certain reference value for the design of the frigate superstructure.

scholarly journals Experimental Study on Backflow Patterns Induced by a Bilateral Groin Pair with Different Spacing

2021 ◽  
Vol 11 (4) ◽  
pp. 1486
Author(s):  
Cuiping Kuang ◽  
Yuhua Zheng ◽  
Jie Gu ◽  
Qingping Zou ◽  
Xuejian Han
Keyword(s):  
Cross Sections ◽  
Large Scale ◽  
Recirculation Zone ◽  
Flow Direction ◽  
Wake Flow ◽  
Zone Length ◽  
Array Configuration ◽  
Contraction Ratio ◽  
River Training ◽  
Spanwise Velocity

Groins are one of the popular manmade structures to modify the hydraulic flow and sediment response in river training. The spacing between groins is a critical consideration to balance the channel-depth and the cost of construction, which is generally determined by the backflow formed downstream from groins. A series of experiments were conducted using Particle Image Velocimetry (PIV) to observe the influence of groin spacing on the backflow pattern of two bilateral groins. The spacing between groins has significant effect on the behavior of the large-scale recirculation cell behind groins. The magnitude of the wake flow induced by a groin was similar to that induced by another groin on the other side, but the flow direction is opposite. The spanwise velocity near the groin tip dictates the recirculation zone width behind the groins due to the strong links between the spanwise velocity and the contraction ratio of channel cross-sections between groins. Based on previous studies and present experimental results, quantitative empirical relationships are proposed to calculate the recirculation zone length behind groins alternately placed at different spacing along riverbanks. This study provides better understanding and a robust formula to assess the backflow extent of alternate groins and identify the optimum groins array configuration.

Flow recirculation zone length and shear rate are differentially affected by stenosis severity in human coronary arteries

2013 ◽  
Vol 304 (4) ◽  
pp. H559-H566 ◽  
Author(s):  
Ashkan Javadzadegan ◽  
Andy S. C. Yong ◽  
Michael Chang ◽  
Austin C. C. Ng ◽  
John Yiannikas ◽  
...  
Keyword(s):  
Shear Rate ◽  
Coronary Arteries ◽  
Recirculation Zone ◽  
Fluid Dynamic ◽  
Zone Length ◽  
Flow Recirculation ◽  
Stenosis Severity ◽  
Recirculation Zones ◽  
The Relationship

Flow recirculation zones and shear rate are associated with distinct pathogenic biological pathways relevant to thrombosis and atherogenesis. The interaction between stenosis severity and lesion eccentricity in determining the length of flow recirculation zones and peak shear rate in human coronary arteries in vivo is unclear. Computational fluid dynamic simulations were performed under resting and hyperemic conditions on computer-generated models and three-dimensional (3-D) reconstructions of coronary arteriograms of 25 patients. Boundary conditions for 3-D reconstructions simulations were obtained by direct measurements using a pressure-temperature sensor guidewire. In the computer-generated models, stenosis severity and lesion eccentricity were strongly associated with recirculation zone length and maximum shear rate. In the 3-D reconstructions, eccentricity increased recirculation zone length and shear rate when lesions of the same stenosis severity were compared. However, across the whole population of coronary lesions, eccentricity did not correlate with recirculation zone length or shear rate ( P = not signficant for both), whereas stenosis severity correlated strongly with both parameters ( r = 0.97, P < 0.001, and r = 0.96, P < 0.001, respectively). Nonlinear regression analyses demonstrated that the relationship between stenosis severity and peak shear was exponential, whereas the relationship between stenosis severity and recirculation zone length was sigmoidal, with an apparent threshold effect, demonstrating a steep increase in recirculation zone length between 40% and 60% diameter stenosis. Increasing stenosis severity and lesion eccentricity can both increase flow recirculation and shear rate in human coronary arteries. Flow recirculation is much more sensitive to mild changes in the severity of intermediate stenoses than is peak shear.

Modeling and Experimental Validations of a Piezoelectric Energy Harvester Under Combined Galloping and Base Excitations

2014 ◽  
Author(s):  
Amin Bibo ◽  
Abdessattar Abdelkefi ◽  
Mohammed F. Daqaq
Keyword(s):  
Bluff Body ◽  
Energy Harvester ◽  
Constitutive Relations ◽  
Excitation Frequency ◽  
Primary Resonance ◽  
Beam Theory ◽  
Excitation Amplitude ◽  
The Body ◽  
Base Excitation ◽  
Piezoelectric Energy

This paper develops an experimentally validated model of a piezoelectric energy harvester under combined aeroelastic-galloping and base excitations. To that end, an energy harvester consisting of a thin piezoelectric cantilever beam subjected to vibratory base excitation is considered. To permit galloping excitation, a bluff body is rigidly attached at the free end such that a net aerodynamic lift is generated as the incoming airflow separates on both sides of the body giving rise to limit cycle oscillations when the flow velocity exceeds a critical value. A nonlinear electromechanical distributed-parameter model of the harvester under the combined excitation is derived using the energy approach and by adopting the nonlinear Euler-Bernoulli beam theory, linear constitutive relations for the piezoelectric transduction, and the quasi-steady assumption for the aerodynamic loading. The partial differential equations of the system are discretized and a reduced-order-model is obtained. The mathematical model is validated by conducting a series of experiments with different loading conditions represented by wind speed, base excitation amplitude, and excitation frequency around the primary resonance.

Sign in / Sign up

Export Citation Format

Share Document