Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A26: Reactive Flows I: Turbulent Combustion Experiments |
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Chair: Tim Lieuwen, Georgia Institute of Technology Room: 31B |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A26.00001: Dynamics of a Variable Density Ratio, Reacting Jet Issuing into a Vitiated Crossflow Benjamin Wilde, Jerry Seitzman, Tim Lieuwen This work reports recent experimental characterization of a reacting jet issuing into a turbulent vitiated crossflow. Previous studies on unforced, non-reacting jets in crossflow showed that the flow transitions from global to convective instability with increasing jet-to-crossflow momentum flux ratio, J, whose value is a function of the jet-to-crossflow density ratio, $\rho _{j}$/$\rho _{\infty }$. This work utilizes a new facility designed to study both of these stability boundaries in a reacting configuration, where the densities of the inflow jet, approaching crossflow, and flame can be systematically varied. The jet, consisting of varying mixtures of CH$_{4}$, H$_{2}$, N$_{2}$, and He, enters the test section through a flush-mounted contoured nozzle. The density ratio spans from 0.4 to 1.0 as a function of the jet constituent concentrations. The vitiated crossflow temperature, T$_{\infty }$, varies from 1000 to 1600K, and the J range is 2 to 25. High-frame-rate imaging and PIV measurements show evidence of narrowband, self-excited fluctuations. Cross correlations computed from the windward and leeward flame edge motion show a tendency towards classical wake-like sinuous motion at lower J and jet-like varicose motion at higher J. Varying crossflow temperature alters the flame stabilization location but not the spectral content of the flow. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A26.00002: Simultaneous krypton PLIF, LII and PIV measurements in a sooting non-premixed jet flame Oliver Buxton, Ross Burns, Noel Clemens Simultaneous krypton planar laser induced fluorescence (PLIF), laser induced incandescence (LII) and stereoscopic PIV measurements are made in a sooting jet flame for the first time. Krypton is seeded into the jet stream to provide a conserved scalar marker from which mixture fraction can be inferred. 2\% krypton is seeded into a non-premixed ethylene/nitrogen jet flame with a cold jet Reynolds number based on bulk velocity of 5100. An electronic transition is accessed through a two photon excitation using an incident 214.7 nm light sheet, producing fluorescence at 760 nm. This fluorescence is spectrally isolated from the flame's bulk luminescence and PAH fluorescence. The fluorescence signal can then be converted to mixture fraction by measuring the relative krypton quenching rates of the various species present in the flame core and making use of a flamelet-based chemical state relationship. This technique is used simultaneously with LII, which will provide the soot volume fraction, and stereoscopic PIV which will provide all three velocity components in a plane. It will thus be possible to observe the influence of the velocity field on the formation and transport of soot, and the evolution of mixture fraction, and its effect on soot formation, within the sooting jet flame. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A26.00003: Experimental investigation of self-turbulent flames Christophe Almarcha, Joel Quinard When propagating downwards, premixed flames undergo hydrodynamic instabilities. The resulting dynamics exhibits multiple corrugations of the light emitting reaction zone. By changing the reactive mixture composition or the shape of the propagation volume, the characteristic lengths of perturbation are changed. We present here the experimental study of propane-air and methane-air flames propagating in vertical circular tubes and in vertically oriented Hele-Shaw cells. This last configuration allows comparison with two dimensional numerical models. The thermo acoustic instability, usually acting when flames propagate in confined volume, is damped thanks to an acoustic absorber, allowing the study of wide flames at the meter scale. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A26.00004: Turbulent Premixed Combustion in V-flames: Statistics of Flame Front Position Sina Kheirkhah, \"{O}mer G\"{u}lder Flame front characteristics of turbulent premixed V-flames were experimentally investigated using Mie scattering and particle-image-velocimetry techniques. Experiments were performed at three mean bulk flow velocities of 4, 6.2, and 8.6 m/s along with three fuel-air equivalence ratios of 0.7, 0.8, and 0.9. Effects of the vertical distance from the flame-holder, mean bulk flow velocity, and fuel-air equivalence ratio on statistics of the flame front position were studied. Results show that, mean and RMS of distance between the flame front and the vertical axis increase with increasing the vertical distance from the flame-holder. At a fixed vertical distance above the flame-holder, mean and RMS of the distance between the flame front and the vertical axis decrease with increasing the mean bulk flow velocity; however, these statistics increase with increasing the fuel-air equivalence ratio. Results show that probability-density-function of the distance between the flame front and the vertical axis features a bell-shaped distribution. Power spectral analysis of the flame front position shows that, for all experimental conditions tested, the averaged and normalized power-spectrum-densities of the flame front position collapse and show a power-law relation with the wave number. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A26.00005: Vortex Breakdown in a Swirl-Stabilized Combustor Zvi Rusak, Cou Umeh, Ephraim Gutmark Results of lean premixed reacting flow tests in a swirl-stabilized combustor show the complex interaction between the flame and the vortex breakdown (VB) zone and oscillations in the position of both. PIV measurements give the detailed velocity field, from which the swirl ratio for a given swirler is computed. Position and size of the VB zone are also determined at various equivalence ratios. Simultaneous OH chemiluminescence snapshots identify the location of the flame. For the given setup with a fixed swirler, upstream pressure and mass flux, it is found that the VB zone occurs near the expansion plane in the nonreacting cold flow case and is pushed downstream when flow is preheated. For low equivalence ratio ($\Phi$) near the flammability limit ($0.48$ to $0.52$), the VB is anchored at the expansion plane and the flame oscillates inside it. At higher $\Phi$ ($0.55$ to $0.65$), the VB zone apex and flame front are close to each other and oscillate together near the expansion plane. At even higher $\Phi$ ($>0.7$), the flame is anchored at the expansion corners while the VB zone oscillates downstream of it. A theoretical discussion that is based on the compressible flow vorticity transport equation sheds light on the various mechanisms that govern VB position in reacting flows. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A26.00006: High speed OH-PLIF measurement of self-excited circumferential instabilities in an annular combustion chamber Nicholas Worth, James Dawson Self-excited thermo-acoustic instabilities are a significant issue in the development of lean burn gas turbine combustors. Such instabilities arise through coupling of the unsteady heat release and acoustic waves, which can propagate both longitudinally and circumferentially in annular combustor geometries. Although a large number of studies have investigated longitudinal fluctuations in single axisymmetric flames, it is currently uncertain whether these results can be used to emulate circumferential oscillations in annular geometry. Therefore, the aim of the current project is to investigate the flame dynamics in an annular model gas turbine combustor during self-excited circumferential oscillations. Pressure measurements are used to characterise the circumferential oscillations, with high-speed OH chemiluminescence and OH-PLIF used to capture the flame dynamics. The flame structure and dynamics are significantly affected by both the proximity of neighbouring flames and the excitation mode; with different responses observed for small and large separation distances, and standing and spinning modes. These observations indicate that results from single flame investigations may only be representative of self-excited flames in annular geometry under in a limited set of conditions. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A26.00007: Turbulent Flame Speed Scaling for Positive Markstein Number Expanding Flames in Near Isotropic Turbulence Swetaprovo Chaudhuri, Fujia Wu, Chung Law In this work we clarify the role of Markstein diffusivity on turbulent flame speed and it's scaling, from analysis and experimental measurements on constant-pressure expanding flames propagating in near isotropic turbulence. For all C0-C4 hydrocarbon-air mixtures presented in this work and recently published C8 data from Leeds, the normalized turbulent flame speed data of individual mixtures approximately follows the recent theoretical and experimental $Re_{T,f}^{0.5}$ scaling, where the average radius is the length scale and thermal diffusivity is the transport property. We observe that for a constant $Re_{T,f}^{0.5}$, the normalized turbulent flame speed decreases with increasing Mk. This could be explained by considering Markstein diffusivity as the large wavenumber, flame surface fluctuation dissipation mechanism. As originally suggested by the theory, replacing thermal diffusivity with Markstein diffusivity in the turbulence Reynolds number definition above, the present and Leeds dataset could be scaled by the new $Re_{T,f}^{0.5}$ irrespective of the fuel considered, equivalence ratio, pressure and turbulence intensity for positive Mk flames. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A26.00008: Oscillatory Flame Response in Acoustically Driven Fuel Droplet Combustion Cristhian Sevilla, Ayaboe Edoh, Jeffrey Wegener, Aaron Sung, Kelvin Chen, Brett Lopez, Owen Smith, Ann Karagozian This experimental study focuses on combustion of liquid fuel droplets during exposure to external acoustic disturbances generated as standing waves within a closed acoustic waveguide. Both visible imaging as well as phase-locked OH* chemiluminescence imaging are used to quantify flame motion and response during such excitation. Acoustic perturbations create both a mean flame deflection as well as flame front oscillations in time that can be dependent on the droplet's location relative to the pressure node (PN) or pressure antinode (PAN) in the waveguide. A range of acoustic forcing frequencies and droplet locations can be used to investigate flame movement. Phase-locked OH* chemiluminescence imaging reveals not only a deflected flame which oscillates in position relative to the droplet, but also different degrees of oscillation depending on excitation frequency and droplet position within the waveguide; there are also oscillations in localized flame front chemiluminescent intensity. Results for differences in flame dynamics are explored in the context of the well-known Rayleigh criterion, with implications for other non-premixed reactive systems. [Preview Abstract] |
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