Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session FQ: Reacting Flows II |
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Chair: David Forliti, SUNY Buffalo Room: Salt Palace Convention Center 251 E |
Monday, November 19, 2007 8:00AM - 8:13AM |
FQ.00001: Observations on Lifted Flame Oscillations and Flame Stability Near Blowout Nancy Moore, Kevin Lyons Studies are presented that examine the fluctuations in liftoff height of lifted flames in the presence of air co-flow. At a certain jet exit velocity, a flame will lift from the fuel nozzle and stabilize at some downstream position. The partially-premixed flame front of the lifted flame oscillates in the axial direction, with the oscillations becoming greater in flames stabilized further downstream. These oscillations are also observed in flames where blowout is imminent. This work attempts to determine the role of fuel velocity and air co-flow on flame oscillations in both stable and unstable regimes. The results of video imaging of a lifted methane-air diffusion flame are presented. Images are used to ascertain the changes in the reaction zone that influence these oscillations and relate the movement to blowout. [Preview Abstract] |
Monday, November 19, 2007 8:13AM - 8:26AM |
FQ.00002: Operating Characteristics of a Fluidic Premixed Dump Combustor Kareem Ahmed, Zakery Carr, David Forliti A transverse slot jet issuing into a channel flow has been shown to develop a large-scale recirculation zone. The current work involves both reacting and nonreacting flow studies of a fluidic dump combustor that utilizes a transverse slot jet in a planar channel flow. The motivation is to develop low thrust penalty flame holding methodologies that increase thrust and improve fuel economy. The reacting flow studies addressed the stabilization limits and combustion phenomena observed for the fluidic dump combustor. The fluidic stream consists of a mixture of methane fuel and air at an equivalence ratio matching that of the main combustor flow. A wall-mounted V-gutter was also studied to provide a comparison to a more traditional flame holder. The fluidic dump combustor has slightly degraded stabilization performance in terms of lean and rich blowout limits compared to the V-gutter. It also observed both stable and oscillatory combustion at different operating conditions. The combustion efficiency is higher for the fluidic dump combustor. The effect of the size of the slot jet was also explored. [Preview Abstract] |
Monday, November 19, 2007 8:26AM - 8:39AM |
FQ.00003: Flow Field Measurements of a Fluidic Dump Combustor Zakery Carr, Kareem Ahmed, David Forliti A fluidic-based dump combustor offers potential thrust and efficiency benefits for propulsion. The capability of fluidics for flame stabilization in a high-speed premixed reactant flow has been established. The current study documents detailed flow field measurements to help understand the fluidic dump combustor. Digital particle image velocimetry was used to study the flow field of both a fluidic and V-gutter based dump combustor. The effects of combustion on the mean and turbulent flow fields for the two configurations will be described. Measurements under steady and oscillatory combustion will be presented. Comparisons of the turbulence length and velocity scales as well as flame topology for the two configurations will be made to help understand the performance of the fluidic dump combustor. [Preview Abstract] |
Monday, November 19, 2007 8:39AM - 8:52AM |
FQ.00004: Combustion of Condensed Phase Alternative Fuels in an Acoustic Field Juan Rodriguez, Hann-Shin Mao, Sophonias Teshome, Alec Pezeshkian, Owen Smith, Ann Karagozian This experimental study focused on fuel droplet combustion characteristics for various liquids during exposure to external acoustical perturbations. Emphasis in the present study was placed on the combustion of a number of alternative liquid fuels, including ethanol, methanol, aviation fuels, and blends of aviation fuel and liquid synthetic fuel derived from coal gasification via the Fischer-Tropsch process. The study examined combustion during excitation conditions in which the droplet was situated at or near a velocity antinode (pressure node) and at or near a velocity node (pressure antinode). During acoustic excitation of burning droplets, flame orientation was consistent with the sign of the acoustic radiation force acting on the burning system, creating conditions where the flame deflection switched, depending on the relative location of the droplet. Differences in burning rates, the degree and orientation of flame deflection, and flame extinction mechanisms were identified for the range of fuels and acoustic excitation conditions. [Preview Abstract] |
Monday, November 19, 2007 8:52AM - 9:05AM |
FQ.00005: Dynamics of Isolated and Interacting Flame Structures in Strongly-Pulsed, Turbulent Jet Flames Mathieu Fregeau, Ying-Hao Liao, James Hermanson, Dennis Stocker, Uday Hegde The dynamics of the large-scale structures in strongly-pulsed, turbulent diffusion flames were studied in normal- and microgravity. Cross-correlation of temperature measurements and high-speed flame imaging were used to estimate the celerity of the flame structures. Both diagnostics indicate a marked increase in celerity with the increasing flame puff interaction as the jet off-time decreases. The celerity is also generally higher for shorter injection times, which yield more compact flame puffs. These trends are seen both for the case of fixed injection velocity as well as for the case of fixed fueling rate. The celerity correlates well with the inverse downstream distance scaled with an appropriate injection parameter, suggesting that the impact of buoyancy can be partially accounted for by the corresponding changes in the mean flame length. Differences in the values of celerity determined by the temperature and visual techniques can be attributed to nature of the evolution of the flame puffs with downstream distance. [Preview Abstract] |
Monday, November 19, 2007 9:05AM - 9:18AM |
FQ.00006: Autocatalytic Reaction Fronts in Confined Flows Ibrahin Bou Malham, Nolwenn Jarrige, Jerome Martin, Nicole Rakotomalala, Laurent Talon, Dominique Salin An autocatalytic reaction front between two reacting species is able to propagate as a solitary wave, that is at a constant velocity and with a stationary concentration profile, resulting from a balance between molecular diffusion and chemical reaction. We analyze both experimentally and with lattice BGK simulations the effect of hydrodynamic flows on the propagation and on the shape of the fronts in the Iodate-Arsenous Acid autocatalytic reaction. We address the issue of the flow induced by the buoyancy difference between reactants and products in micro-sized Hele-Shaw cells and in packed beads porous media. [Preview Abstract] |
Monday, November 19, 2007 9:18AM - 9:31AM |
FQ.00007: Loss of stability and re-stabilization of flat porous-plug burner flames Moshe Matalon, Vadim Kurdyumov In our recent studies of edge-flames we found that when the characteristic gas velocity exceeds a critical value the flame may undergo spontaneous oscillations. The oscillations are amplified as the flow rate increases reaching maximum amplitude and then decrease by further increasing the flow rate until the flame re-stabilizes. In this work we examine the concept of flame restabilization in a simpler but related problem -- the planar premixed flame on a porous-plug burner - which is amenable to a full stability analysis. We show the dependence of all possible steady states on the relevant parameters including the mass flow rate, the effective Lewis number of the mixture, the overall activation energy of the chemical reaction and the extent of heat release. A linear stability analysis is carried out to examine whether these steady states are stable to small disturbances. The analysis determines the critical conditions for the onset of instability as well as the nature of the instability. In particular we show that by decreasing the mass flow rate the flame, which is at first stable, starts to oscillate back and forth for a limited range of gas velocities but is then restabilized by further decreasing the mass flow rate. We also show that the properties of the plug, such as the thickness of the plate and its porosity, play a significant role on flame stabilization. [Preview Abstract] |
Monday, November 19, 2007 9:31AM - 9:44AM |
FQ.00008: Inner-Scale Effects of Heat Release in Reacting Turbulent Shear Flows Zachary Nagel, Werner J.A. Dahm Comparisons are presented from the first inner-scaled measurements of velocity gradient quantities in reacting and nonreacting versions of otherwise identical turbulent shear flows. Distributions of gradient quantities are obtained for outer-scale Reynolds numbers $Re_{\delta} \equiv u_{c} \delta / \nu$ from 7,200 to 200,000. The local outer length scale $\delta$ and velocity scale $u_c$ and associated inner scaling $\overline{ \left( \partial u_{i} / \partial x_{j} \right) ^{n} } \sim \left( \nu / \lambda^{2}_{\nu} \right)^n$ are used to identify the dominant physical mechanisms that produce heat release effects on the inner scales. In the nonreacting cases, classical inner scaling with the viscosity $\nu$ and inner (viscous) length scale $ \lambda_{\nu} \sim \delta \cdot Re_{\delta}$ removes most differences in distributions measured at different $Re_{\delta}$, with remaining differences being due to incomplete resolution of $\lambda_{\nu}$ with increasing $Re_{\delta}$. Inertial and dissipation range spectra allow the measurement resolution scale $\Delta^{*}$ and the proper resolution-corrected inner scaling to be determined, with the resulting scaling verifying near-perfect similarity for all $Re_{\delta}$. In the reacting cases, departures from this similarity reveal the true inner-scale changes due to heat release. Results clearly show that when inertial and body force effects on $\delta$ and $u_c$ are accounted for via the equivalent density, and viscous effects are accounted for via the mixture-fraction-averaged viscosity, the resolution-corrected inner scaling reveals remaining effects of heat release on turbulent shear flows to be remarkably small. [Preview Abstract] |
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