Rib Vortex Breakdown in Transitional High-Speed Planar Wakes

Flame flashback from the combustion chamber to the premixing section is a major operability issue when using high H2 content fuels in lean premixed combustors. Depending on the flow-field in the combustor, flashback can be triggered by different mechanisms. In this work, three flashback mechanisms of H2/CH4 mixtures were visualized in an atmospheric variable swirl burner using high speed OH* chemiluminescence imaging. The H2 mole fraction of the tested fuel mixtures varied between 0.1 and 0.9. The flow-field in the combustor was varied by changing the swirl number from 0.0 to 0.66 and the total air mass-flow rate from 75 to 200 SLPM (standard liters per minute). The following three types of flashback mechanism were observed: Flashback caused by combustion induced vortex breakdown occurred at swirl numbers ≥ 0.53 for all of the tested fuel mixtures. Flashback in the boundary layer and flashback due to autoignition were observed at low swirl numbers and low total air mass-flow rates. The temporal and spatial propagation of the flame in the optical section of the premixing tube during flashback was studied and flashback speed for different mechanisms was estimated. The flame propagation speed during flashback was significantly different for the different mechanisms.

Download Full-text

On the Breakdown at High Incidences of the Leading Edge Vortices on Delta Wings

Journal of the Royal Aeronautical Society ◽

10.1017/s0368393100073314 ◽

1960 ◽

Vol 64 (596) ◽

pp. 491-493 ◽

Cited By ~ 19

Author(s):

B. J. Elle

Keyword(s):

High Speed ◽

Shaping Air Flow Characteristics of a High-Speed Rotary-Bell Sprayer for Automotive Painting Processes

Journal of Fluids Engineering ◽

10.1115/1.4030703 ◽

2015 ◽

Vol 137 (11) ◽

Cited By ~ 7

Author(s):

Ch. Stevenin ◽

Y. Béreaux ◽

J.-Y. Charmeau ◽

J. Balcaen

Keyword(s):

High Speed ◽

Vortex Breakdown ◽

Flow Behavior ◽

Air Flow ◽

Flow Characteristics ◽

Rotation Frequency ◽

Zone Length ◽

Swirling Jets ◽

Automotive Parts

Electrostatic rotary bell sprayers (ERBS) are widely used in automotive painting applications. These processes involve complex airflows to shape paint sprays and transport droplets toward automotive parts to be coated. Despite the importance of shaping airflow on global spray characteristics, a detailed characterization of this aerodynamic flow is still missing. For this purpose, an experimental study was conducted on the influence of some ERBS operating parameters on the development and characteristics of shaping airflow. Results show that, for low swirl numbers, the flow behavior is close to that of annular swirling jets and a good agreement is found between ERBS flow characteristics and data available in literature. When rotational speed of the bell cup is sufficiently fast, a change of regime is observed with a shift in the longitudinal flow development and an increase of recirculation zone length. This change of regime is attributed to vortex breakdown instability, known to occur when high swirl strength is beyond a critical value. Experimental results obtained in this study put forward a clear link between the shaping air flow rate and the rotation frequency on the aerodynamics and also provide valuable leads to design shaping air flow in modern ERBS.

Download Full-text

Flame and Flow Topologies in an Annular Swirling Flow

Volume 1B: Combustion, Fuels and Emissions ◽

10.1115/gt2013-96009 ◽

2013 ◽

Author(s):

I. Chterev ◽

C. W. Foley ◽

S. Kostka ◽

A. W. Caswell ◽

N. Jiang ◽

...

Keyword(s):

High Speed ◽

Vortex Breakdown ◽

Swirling Flow ◽

Flow Fields ◽

Shear Layers ◽

Flame Stabilization ◽

Preheat Temperature ◽

Breakdown Region ◽

Flame Shape ◽

Annular Swirling Flow

A variety of different flame configurations and heat release distributions, with their associated flow fields, can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle life, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper discusses the flame and flow topologies that exist in these flows. These results illustrate the importance of the sensitivity of flame configurations to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel air ratio) parameters. We particularly emphasize the centerbody shape as differentiating between two different families of flame shapes. Results are shown illustrating the time averaged and instantaneous flame shape and flow fields, using high speed PIV, OH-PLIF, and luminosity imaging.

Download Full-text

Flashback in a Swirl Burner With Cylindrical Premixing Zone

Volume 2: Coal, Biomass and Alternative Fuels; Combustion and Fuels; Oil and Gas Applications; Cycle Innovations ◽

10.1115/2001-gt-0054 ◽

2001 ◽

Cited By ~ 12

Author(s):

Jassin Fritz ◽

Martin Kröner ◽

Thomas Sattelmayer

Keyword(s):

Flame Propagation ◽

Gas Turbines ◽

High Speed ◽

Vortex Breakdown ◽

Swirling Flow ◽

Laser Doppler Anemometry ◽

Burning Velocity ◽

Turbulent Flame ◽

Measurement Techniques ◽

Mean Flow

Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NOx burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying High Speed Video recordings, the Laser Doppler Anemometry and the Laser Induced Fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechansim of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.

Download Full-text

Aerodynamic Analysis of a Supersonic Transport Aircraft at Landing Speed Conditions

Energies ◽

10.3390/en14206615 ◽

2021 ◽

Vol 14 (20) ◽

pp. 6615

Author(s):

Andrea Aprovitola ◽

Pasquale Emanuele Di Nuzzo ◽

Giuseppe Pezzella ◽

Antonio Viviani

Keyword(s):

High Speed ◽

Cfd Simulation ◽

Vortex Breakdown ◽

Delta Wing ◽

Research Effort ◽

Static Stability ◽

Commercial Aviation ◽

Supersonic Flight ◽

Supersonic Transport ◽

Wide Range

Supersonic flight for commercial aviation is gaining a renewed interest, especially for business aviation, which demands the reduction of flight times for transcontinental routes. So far, the promise of civil supersonic flight has only been afforded by the Concorde and Tupolev T-144 aircraft. However, little or nothing can be found about the aerodynamics of these aeroshapes, the knowledge of which is extremely interesting to obtain before the development of the next-generation high-speed aircraft. Therefore, the present research effort aimed at filling in the lack of data on a Concorde-like aeroshape by focusing on evaluating the aerodynamics of a complete aircraft configuration under low-speed conditions, close to those of the approach and landing phase. In this framework, the present paper focuses on the CFD study of the longitudinal aerodynamics of a Concorde-like, tailless, delta-ogee wing seamlessly integrated onto a Sears–Haack body fuselage, suitable for civil transportation. The drag polar at a Mach number equal to 0.24 at a 30 m altitude was computed for a wide range of angles of attack (0∘,60∘), with a steady RANS simulation to provide the feedback of the aerodynamic behaviour post breakdown, useful for a preliminary design. The vortex-lift contribution to the aerodynamic coefficients was accounted for in the longitudinal flight condition. The results were in agreement with the analytical theory of the delta-wing. Flowfield sensitivity to the angle of attack at near-stall and post-stall flight attitudes confirmed the literature results. Furthermore, the longitudinal static stability was addressed. The CFD simulation also evidenced a static instability condition arising for 15∘≤α≤20∘ due to vortex breakdown, which was accounted for.

Download Full-text

Effect of coherent structures on convective heat transfer in a swirling impinging jet

Journal of Physics Conference Series ◽

10.1088/1742-6596/2119/1/012014 ◽

2021 ◽

Vol 2119 (1) ◽

pp. 012014

Author(s):

A S Lobasov

Keyword(s):

Heat Transfer ◽

High Speed ◽

Vortex Breakdown ◽

Temperature Fluctuations ◽

Impinging Jet ◽

Time Resolved ◽

Impact Surface ◽

Helical Vortex ◽

Jet Nozzle ◽

The Impact

Abstract The present paper reports on the detailed investigation of the flow dynamics and unsteady heat transfer in an impinging jet in regimes with high swirl and vortex breakdown. A combination of the time-resolved stereoscopic PIV, time-resolved PLIF and high-speed IR-thermometry methods is used. Two cases of distances between the jet nozzle and impingement surface are considered, H = d and H = 2d. The Reynolds number is fixed as Re = 5000. The temperature distribution in the flow has a maximum on the jet axis near the surface in the region of the central recirculation zone. The data are processed using the POD method to extract coherent flow structures and quantify temperature fluctuations on the impact surface. The helical vortex structure in the case of H = d influences heat transfer between the swirling jet and the surface, the temperature fluctuations on the surface reach 0.05 degrees.

Download Full-text

Impact of a Centrebody on the Unsteady Flow Dynamics of A Swirl Nozzle: Intermittency of PVC Oscillations

10.1115/gt2021-58358 ◽

2021 ◽

Author(s):

Saarthak Gupta ◽

Santosh Hemchandra ◽

Masayasu Shimura ◽

Santosh Shanbhogue ◽

Ahmed Ghoniem

Keyword(s):

Nozzle Exit ◽

High Speed ◽

Vortex Breakdown ◽

Flow Dynamics ◽

Reacting Flow ◽

Exit Plane ◽

Swirl Combustor ◽

Time Resolved ◽

Precessing Vortex Core ◽

The Impact

Abstract The precessing vortex core (PVC) is a self-excited flow oscillation state occurring in swirl nozzles. This is caused by the presence of a marginally unstable hydrodynamic helical mode that induces precession of the vortex breakdown bubble (VBB) around the flow axis. The PVC can impact emissions and thermoacoustic stability characteristics of combustors in various ways, as several prior studies have shown. In this paper, we examine the impact of centrebody diameter (Dc) on the PVC in a non-reacting flow in a single nozzle swirl combustor. Time resolved high speed stereoscopic PIV (sPIV) measurements are performed for combinations of two swirl numbers, S = 0.67 and 1.17 and Dc = 9.5 mm, 4.73 mm and 0 (i.e. no centrebody). The bulk flow velocity at the nozzle exit plane is kept constant as Ub = 8 m/s for all cases (Re ∼ 20,000). The centrebody end face lies in the nozzle exit plane. A new modal decomposition technique based on wavelet filtering and proper orthogonal decomposition (POD) provides insight into flow dynamics in terms of global modes extracted from the data. The results show that without a centrebody, a coherent PVC is present in the flow as expected. The introduction of a centrebody makes the PVC oscillations intermittent. These results suggest two routes to intermittency as follows. For S = 0.67, the vortex breakdown bubble (VBB) and centrebody wake recirculation zone (CWRZ) regions are nominally distinct. Intermittent separation and merger due to turbulence result in PVC oscillations due to the de-stabilization of the hydrodynamic VBB precession mode of the flow. In the S = 1.17 case, the time averaged VBB position causes it to engulf the centrebody. In this case, the emergence of intermittent PVC oscillations is a result of the response of the flow to broadband stochastic forcing imposed on the time averaged vorticity field due to turbulence.

Download Full-text

Dynamic-Stability Characteristics of Premixed Methane Oxy-Combustion

Volume 2: Combustion, Fuels and Emissions, Parts A and B ◽

10.1115/gt2011-45753 ◽

2011 ◽

Cited By ~ 2

Author(s):

Andrew P. Shroll ◽

Santosh J. Shanbhogue ◽

Ahmed F. Ghoniem

Keyword(s):

Dynamic Stability ◽

High Speed ◽

Vortex Breakdown ◽

Burning Velocity ◽

Turbulent Flame ◽

Flame Temperature ◽

Wave Mode ◽

Oxygen Mixture ◽

Quarter Wave ◽

Fuel Mixtures

This work explores the dynamic stability characteristics of premixed CH4/O2/CO2 mixtures in a 50kW swirl stabilized combustor. In all cases, the methane-oxygen mixture is stoichiometric, with different fractions of carbon dioxide used to control the flame temperature (Tad). For the highest Tad’s, the combustor is unstable at the five-quarter wave mode. As the temperature is reduced, the combustor jumps to the three quarter mode and then to the quarter wave before eventually reaching blowoff. Similar to the case of CH4/air mixtures, the transition from one mode to another is predominantly a function of the Tad of the reactive mixture, despite significant differences in laminar burning velocity and/or strained flame consumption speed between air and oxy-fuel mixtures for a given Tad. High speed images support this finding by revealing similar vortex breakdown modes and thus similar turbulent flame geometries that change as a function of flame temperature.

Download Full-text

Dynamic-Stability Characteristics of Premixed Methane Oxy-Combustion

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4004737 ◽

2012 ◽

Vol 134 (5) ◽

Cited By ~ 20

Author(s):

Andrew P. Shroll ◽

Santosh J. Shanbhogue ◽

Ahmed F. Ghoniem

Keyword(s):

Natural Frequency ◽

Dynamic Stability ◽

High Speed ◽

Vortex Breakdown ◽

Burning Velocity ◽

Turbulent Flame ◽

Flame Temperature ◽

Low Frequency ◽

Oxygen Mixture ◽

Fuel Mixtures

This work explores the dynamic stability characteristics of premixed CH4/O2/CO2 mixtures in a 50 kW swirl stabilized combustor. In all cases, the methane-oxygen mixture is stoichiometric, with different dilution levels of carbon dioxide used to control the flame temperature (Tad). For the highest Tad’s, the combustor is unstable at the first harmonic of the combustor’s natural frequency. As the temperature is reduced, the combustor jumps to fundamental mode and then to a low-frequency mode whose value is well below the combustor’s natural frequency, before eventually reaching blowoff. Similar to the case of CH4/air mixtures, the transition from one mode to another is predominantly a function of the Tad of the reactive mixture, despite significant differences in laminar burning velocity and/or strained flame consumption speed between air and oxy-fuel mixtures for a given Tad. High speed images support this finding by revealing similar vortex breakdown modes and thus similar turbulent flame geometries that change as a function of flame temperature.

Download Full-text