Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R17: Reacting Flows: Experiments II |
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Chair: Chenning Tong, Clemson University Room: D131 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R17.00001: Fire Whirls, Vortex Breakdown(?), and Blue Whirls Elaine Oran, Huahua Xiao, Michael Gollner As we were investigating the efficiency of fire-whirl burning on water, we observed the usual transformation of a pool fire to a fire whirl, and then suddenly, we saw the fire undergo a third transition. A blue cup appeared around the base of the fire whirl, surrounding the yellow flame, the yellow flame receded into the cup and finally disappeared. What remained was a small, rapidly spinning blue flame that burned until the fuel on the water was consumed. The blue whirl was shaped like a spinning cup, closed at the bottom near the water surface, and spreading in radius moving upwards towards the rim. Above the blue cup lip, there was a purple cone-shaped mist. The fuel was usually n-heptane, but at one point it was crude oil, and still the blue whirl formed naturally. The height of the fire whirl on the laboratory pan was larger than a half meter, and this evolved into a blue whirl about 4–8 cm high. Occasionally the blue whirl would become ``unstable’’ and revert to a transitional state of blue cup holding a yellow flame. When the blue whirl formed, turbulence seemed to disappear, and the flame became quiet. We will show videos of how this happened and discuss the evolution of the fire whirl to the blue whirl in vortex-breakdown concepts. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R17.00002: Time-resolved PIV investigation of flashback in stratified swirl flames of hydrogen-rich fuel Rakesh Ranjan, Noel Clemens Hydrogen is one of the promising alternative fuels to achieve greener power generation. However, susceptibility of flashback in swirl flames of hydrogen-rich fuels acts as a major barrier to its adoption in gas turbine combustors. The current study seeks to understand the flow-flame interaction during the flashback of the hydrogen-rich flame in stratified conditions. Flashback experiments are conducted with a model combustor equipped with an axial swirler and a center-body. Fuel is injected in the main swirl flow via the fuel ports on the swirler vanes. To achieve mean radial stratification, these fuel ports are located at a radial location closer to the outer wall of the mixing tube. Stratification in the flow is assessed by employing Anisole PLIF imaging. Flashback is triggered by a rapid increase in the global equivalence ratio. The upstream propagation of the flame is investigated by employing time-resolved stereoscopic PIV and chemiluminescence imaging. Stratification leads to substantially different flame propagation behavior as well as increased flame surface wrinkling. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R17.00003: The effect of stratification on premixed swirl-flame flashback by using porous center-body injection Andrew McCaslin, Rakesh Ranjan, Noel Clemens Boundary layer flashback must be prevented in order to stably operate stationary gas turbines. One strategy to avoid flashback is to create equivalence-ratio stratification, such as by reducing the fuel/air ratio in the boundary layer below the flammability limit. Typically, stratification is achieved by using radially non-uniform fuel injection. The goal of the current study is to reduce the propensity of flashback in a premixed annular swirl combustor that uses a premix section with center-body. A porous metal center-body (10 micron pore size) is used to bleed air directly into the boundary layer and thus locally reduce the equivalence ratio. Planar laser-induced fluorescence imaging of anisole-seeded flow is carried out to assess the stratification in the flow. Time-resolved PIV and chemiluminescence imaging are used to investigate flashback at atmospheric pressure conditions. A comparative study between fully premixed and stratified flame flashback is conducted to determine how stratification influences flashback physics. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R17.00004: Investigation of lean combustion stability and pressure drop in porous media burners Sadaf Sobhani, Bret Haley, David Bartz, Jared Dunnmon, John Sullivan, Matthias Ihme The stability and thermal durability of combustion in porous media burners (PMBs) is examined experimentally and computationally. For this, two burner concepts are considered, which consist of different pore topologies, porous materials, and matrix arrangements. Long-term material durability tests at constant and cycled on-off conditions are performed, along with a characterization of combustion stability, pressure drop and pollutant emissions for a range of equivalence ratios and mass flow rates. Experimental thermocouple temperature measurements and pressure drop data are presented and compared to results obtained from one-dimensional volume-averaged simulations. Experimental and model results show reasonable agreement for temperature profiles and pressure drop evaluated using Ergun's equations. Enhanced flame stability is illustrated for burners with Yttria-stabilized Zirconia Alumina upstream and Silicon Carbide in the downstream combustion zone. Results reinforce concepts in PMB design and optimization, and demonstrate the potential of PMBs to overcome technological barriers associated with conventional free-flame combustion technologies. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R17.00005: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R17.00006: Influence of Aerodynamic Strain Rate on Local Extinction in Turbulent Non-premixed Jet Flames Aravind Ramachandran, Venkateswaran Narayanaswamy, Kevin Lyons 2-D velocity field measurements obtained from Particle Image Velocimetry (PIV) are used to obtain aerodynamic strain rate information in regions of local extinction in lifted turbulent non-premixed methane jet flames in coflow. Diluting the coflow to reduce the oxygen molefraction results in increased occurrences of local extinction. Statistical analysis is performed to correlate regions of high local strain rate with local extinctions in both air coflow and diluted coflow cases to study the influence of strain rate against vortical structures in extinguishing the flame front. A comparison is also made with heated and vitiated coflow cases, where autoignition is a flame stabilization mechanism and influenced by local strain rate. At high jet exit velocities (Ux $>>$ Ur), the out-of-plane strain rate component can be neglected but the convection of extinguished pockets into the measurement plane needs to be resolved by stereoscopic (3-D) measurements which will be done in a future work. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R17.00007: Shear Layer Interactions in the Helical Hydrodynamic Structures of Swirling, Reacting Jets Travis Smith, Kiran Manoharan, Benjamin Emerson, Santosh Hemchandra, Tim Lieuwen Swirling jets~with density stratification~are~a canonical~combustor~flow field.~ This~work consisted of coupled experimental and theoretical analysis of~the spatial structure of the most amplified modes in~an annular jet, with a specific focus on the~radial~mode shapes~of the shear layer disturbances, which we characterize as inner shear layer (ISL) motion relative to outer shear layer (OSL) motion.~ The stability analysis identifies spatial structures dominated by~ISL motion, modes dominated by OSL~motion, and modes with mixed ISL and OSL motion.~ These mixed modes are further classified as sinuous or varicose radial structures, depending on the relative motions of the two shear layers.~~The presence and spatial dependencies of~these~spatial modes are demonstrated~experimentally with a 5 kHz stereo PIV measurement of a reacting swirling jet.~ In the experiment, we demonstrate that external excitations of various spatial configurations can be used to elicit hydrodynamic responses~of axisymmetric and helical motions in~either the ISL, the OSL, or the sinuous or varicose radial modes. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R17.00008: Effects of mean shear and scalar initial length scale on three-scalar mixing in turbulent coaxial jets Chenning Tong, Wei Li, Mengyuan Yuan, Campbell Carter We investigate three-scalar mixing in a turbulent coaxial jet, in which a center jet and an annular flow, consisting of acetone-doped air and ethylene respectively, are mixed with the co-flow air. We investigate the effects of the velocity and length scale ratios of the annular flow to the center jet. Planar laser-induced fluorescence and Rayleigh scattering are employed to image the scalars. The results show that the velocity ratio alters the relative mean shear rates in the mixing layers between the center jet and the annular flow and between the annular flow and the co-flow, modifying the scalar fields through mean-flow advection, turbulent transport, and small-scale mixing. The length scale ratio determines the degree of separation between the center jet and the co-flow. The results show that while varying the velocity ratio can alter the mixing characteristics qualitatively, varying the annulus width only has quantitative effects. The evolution of the mean scalar profiles are dominated by the mean-flow advection, while the shape of the joint probability density function is largely determined by the turbulent transport and molecular diffusion. The results in the present study have implications for understanding and modeling multiscalar mixing in turbulent reactive flows. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R17.00009: Characterizing Laminar Flame Interactions with Turbulent Fluidic Jets and Solid Obstacles for Turbulence Induction. Jessica Chambers, Stephen Gerdts, Kareem Ahmed A detonation engine's fundamental design concept focuses on enhancing the Deflagration to Detonation Transition (DDT), the process through which subsonic flames accelerate to form a spontaneous detonation wave. Flame acceleration is driven by turbulent interactions that expand the reaction zone and induce mixing of products and reactants. Turbulence in a duct can be generated using solid obstructions, fluidic obstacles, duct angle changes, and wall skin friction. Solid obstacles have been previously explored and offer repeatable turbulence induction at the cost of pressure losses and additional system weight. Fluidic jet obstacles are a novel technique that provide advantages such as the ability to be throttled, allowing for active control of combustion modes. The scope of the present work is to expand the experimental database of varying parameters such as main flow and jet equivalence ratios, fluidic momentum ratios, and solid obstacle blockage ratios. Schlieren flow visualization and particle image velocimetry (PIV) are employed to investigate turbulent flame dynamics throughout the interaction. Optimum conditions that lead to flame acceleration for both solid and fluidic obstacles will be determined. [Preview Abstract] |
Tuesday, November 22, 2016 3:27PM - 3:40PM |
R17.00010: Experimental testing of a liquid bipropellant rocket engine using nitrous oxide and ethanol diluted with water Jeff Phillip, Rudy Morales, Stewart Youngblood, W. Venner Saul, Mark Grubelich, Michael Hargather A research scale liquid bipropellant rocket engine testing facility was constructed at New Mexico Tech to perform research with various propellants. The facility uses a modular engine design that allows for variation of nozzle geometry and injector configurations. Initial testing focused on pure nitrous oxide and ethanol propellants, operating in the range of 5.5-6.9 MPa (800-1000 psi) chamber pressure with approximately 667 N (150 lbf) thrust. The system is instrumented with sensors for temperature, pressure, and thrust. Experimentally found values for specific impulse are in the range of 250-260 s which match computational predictions. Exhaust flow visualization is performed using high speed schlieren imaging. The engine startup and steady state exhaust flow features are studied through these videos. Computational and experimental data are presented for a study of dilution of the ethanol-nitrous oxide propellants with water. The study has shown a significant drop in chamber temperature compared to a small drop in specific impulse with increasing water dilution. [Preview Abstract] |
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