scholarly journals Pressure gradient tailoring effects on the mechanisms of bluff-body flame extinction

2020 ◽  
Vol 215 ◽  
pp. 224-237 ◽  
Author(s):  
Anthony J. Morales ◽  
Jonathan Reyes ◽  
Peter H. Joo ◽  
Isaac Boxx ◽  
Kareem A. Ahmed
Keyword(s):  
Pressure Gradient ◽  
Bluff Body ◽  
Flame Extinction

Development of a bluff body wake under the combined influence of curvature and pressure gradient

1995 ◽  
Vol 18 (5) ◽  
pp. 311-318 ◽  
Author(s):  
E. G. Tulapurkara ◽  
V. Ramjee ◽  
Jacob George
Keyword(s):  
Pressure Gradient ◽  
Bluff Body ◽  
Bluff Body Wake

The laminar separation of an incompressible fluid streaming past a smooth surface

1977 ◽  
Vol 356 (1687) ◽  
pp. 443-463 ◽  
Keyword(s):  
Pressure Gradient ◽  
Incompressible Fluid ◽  
Smooth Surface ◽  
Bluff Body ◽  
Stokes Equations ◽  
Adverse Pressure Gradient ◽  
Separation Point ◽  
Large Reynolds Number ◽  
Laminar Separation ◽  
Boundary Layer Problem

Sychev’s (1972) proposal, that in general the laminar separation and breakaway of an incompressible fluid streaming past a smooth surface (e. g. on a bluff body) takes place through a triple-deck structure around the separation point, is examined numerically in this paper. The proposed pattern for large Reynolds number ( Re ) flows is based on a modification of the classical Kirchhoff (1869) free streamline theory, in which a slight adverse pressure gradient is provoked in the inviscid motion immediately ahead of the breakaway. This pressure gradient is just enough to generate a triple-deck development closer to the separation point. The major task then is to decide whether or not a solution of the basic triple-deck problem exists, and is regular at separation, and if it is unique. The numerical in-vestigation, an iterative calculation of the relevant boundary layer problem, together with the potential flow relation between the unknown pressure and displacement, points fairly firmly to both the existence and uniqueness of a solution. Thus, for the bluff body problem when Re ≫1, the triple-deck determines exactly how far the separation point lies from the position implied by inviscid (Kirchhoff) theory. Comparisons with separating incompressible fluid motions determined numerically from the Navier-Stokes equations and measured experimentally give some support overall to the triple-deck description. For the flow past a circular cylinder the agreement in the variation of pressure and skin friction near separation is in general very encouraging, for Reynolds numbers as low as 30.

Effect of Fuel Dilution on the Stability Characteristics of Syngas Diffusion Flames

2008 ◽  
Author(s):  
Kejin Mo ◽  
Yongsheng Zhang ◽  
Zhedian Zhang ◽  
Yue Wang ◽  
Yunhan Xiao ◽  
...  
Keyword(s):  
Bluff Body ◽  
Diffusion Flames ◽  
Maximum Flow ◽  
Operating Conditions ◽  
Flame Stability ◽  
Oh Radical ◽  
Flame Extinction ◽  
Reaction Zones ◽  
Fuel Dilution ◽  
Dilution Effects

In order to investigate the effects of fuel dilution on flame stability characteristics, open syngas diffusion flames are established and H2O, N2 and CO2 are employed respectively as diluents. The burner configuration used in this study consists of a bluff body with a central jet flow of the fuel and a surrounding coflow of the air. The syngas is composed of 50% of H2 and 50% of CO (by volume). The experiments are conducted at 1 atmospheric pressure, and the temperatures of the fuel and the air are kept constant at about 400 K. The results show that the flame tapers inward and becomes more cylindrical in the shape as after the dilution, the flame becomes unstable due to dilution effects. It has been found that there is a maximum flow rate of diluents responsible for the flame extinction. Among these three dilutions, H2O diluted flames exhibit a highest stability, while CO2 diluted flames have the lowest one due to its large specific heat. Planar Laser-Induced Fluorescence (PLIF) measurements of the OH radical are applied to study the behavior of the OH radical in the flames. The results show that as the diluents introduced into the flame increases, the overall OH mole fraction significantly decreases, and the flame width also decreases. The structures of flame bases are also studied to obtain a better understanding of fuel dilution effects on the flame stability. The radial stabilization distance is decreased and the local flame extinctions in the reaction zones are found as dilution increases. For operating conditions close to the flame extinction limit, the flame reaction zones in the flame bases take on a more intermittent, shredded appearance.

scholarly journals Experimental and Theoretical Investigation of the Flashback of a Swirling, Bluff-Body Stabilised, Premixed Flame

2015 ◽  
Vol 229 (5) ◽  
Author(s):  
Nader Karimi ◽  
Christoph Heeger ◽  
Loizos Christodoulou ◽  
Andreas Dreizler
Keyword(s):  
Pressure Gradient ◽  
Flame Propagation ◽  
Bluff Body ◽  
Static Pressure ◽  
Adverse Pressure Gradient ◽  
Premixed Flame ◽  
Turbulent Premixed Flame ◽  
Physical Mechanisms ◽  
Gauge Pressure ◽  
Flame Flashback

AbstractFlashback of an open turbulent, premixed flame in a swirl burner with central bluff-body is considered. The aim is to obtain further understanding of the physical mechanisms responsible for the upstream flame propagation. Previous studies on the same configuration hypothesised that there is an adverse pressure gradient in the direction of flame propagation. In this paper this is further investigated experimentally and theoretically. Static gauge pressure is measured on the surface of the bluff-body during flame flashback. Simultaneously, flame luminosity is imaged at 5 kHz. The results indicate that the static pressure rises downstream of the propagating reactive front. This is, then, discussed in the context of the theory of vortex bursting. An existing theory of flame propagation in the core flow is extended to a configuration similar to that investigated experimentally. The theory, although highly simplified, explains the generation of adverse pressure gradient across the flame and is qualitatively consistent with the experiment.

Examination of Predictive Flame Extinction Boundaries for Bluff Body Stabilized Flames at Elevated Temperature and Pressure Conditions

2020 ◽  
Vol 142 (10) ◽  
Author(s):  
Candy Hernandez ◽  
Vincent McDonell
Keyword(s):  
Bluff Body ◽  
Elevated Temperatures ◽  
Premixed Combustion ◽  
Flame Stability ◽  
Flame Extinction ◽  
Stability Studies ◽  
Fuel Type ◽  
Wide Range ◽  
Hydrogen Mixtures ◽  
Temperature And Pressure

Abstract A correlation for the prediction of flame lean extinction limits for premixed combustion systems by Sullivan-Lewis and McDonell is examined. The correlation was developed with the data collected from Sullivan-Lewis' experiments of methane and hydrogen mixtures at elevated temperatures and pressures, similar to gas turbine conditions. Recent flame stability studies have since appeared in literature and has allowed for inspection of the validity of the predictive flame extinction boundary correlation with new data. The blow-off boundary correlation is also examined with previous flame blow off studies with significant variances of parameters compared to the original study used to build the correlation. The data used for comparison differ with fuel type, equivalence ratios, pressures, temperatures, turbulence intensities, and flameholder geometries. The analysis concludes that the predictive flame extinction correlation developed is able to accurately predict a wide range of extinction conditions reported in the literature. However, it is observed that the correlation is not able to fully capture the behavior of the data for conditions in which turbulence intensities are above 5%.

Forcing symmetry exchanges and flow reversals in turbulent wakes

2017 ◽  
Vol 829 ◽  
Author(s):  
Diogo Barros ◽  
Jacques Borée ◽  
Olivier Cadot ◽  
Andreas Spohn ◽  
Bernd R. Noack
Keyword(s):  
Pressure Gradient ◽  
Bluff Body ◽  
Separated Flow ◽  
High Sensitivity ◽  
Normal Pressure ◽  
Bluff Bodies ◽  
Turbulent Wakes ◽  
Symmetry Properties ◽  
And Control ◽  
Flow Symmetry

Turbulent wakes past bluff bodies commonly present asymmetric flow states reminiscent of bifurcations in the laminar regime. Understanding the sensitivity of these states to flow forcing is crucial to the modelling and control of flow symmetry properties. In this study, the near wake of a rectangular bluff body in proximity to a wall is disturbed by the use of passive devices located between the model and the wall, upstream of the massive flow separation occurring at the blunt trailing edges. Due to the proximity to the boundary, the wake initially presents wall-normal asymmetry and a negative wall-normal pressure gradient along the base. The application of disturbances with variable size, however, sets flow symmetry along the wall-normal plane, leading to the intermittent spanwise wake reversals reported recently in the literature. A further increase in the size of perturbation suppresses wake switching, and wall-normal asymmetry is recovered, but with a positive wall-normal pressure gradient. The dynamical features of this bifurcation scenario can be retrieved using two coupled symmetry-breaking models for spanwise and wall-normal pressure gradients. This confirms the high sensitivity of the separated flow to external perturbations. More importantly, the results unify observations of the bluff-body wake topologies covered in previous investigations.

Mechanisms of flame extinction and lean blowout of bluff body stabilized flames

2019 ◽  
Vol 203 ◽  
pp. 31-45 ◽  
Author(s):  
Anthony J. Morales ◽  
Ian M. Lasky ◽  
Marissa K. Geikie ◽  
Christian A. Engelmann ◽  
Kareem A. Ahmed
Keyword(s):  
Bluff Body ◽  
Flame Extinction ◽  
Lean Blowout ◽  
Stabilized Flames
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