Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session D35: Focus Sessions: Fluid Dynamics of Fire: From Small to Large ScalesReacting
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Chair: Michael Gollner, University of Maryland Room: 301 |
Sunday, November 19, 2017 2:15PM - 2:28PM |
D35.00001: The critical conditions for the onset of pool-fire puffing Wilfried Coenen, Daniel Moreno-Boza, Jaime Carpio, Antonio L. Sánchez, Forman A. Williams Pool fires are known to exhibit a self-sustained oscillatory behavior, shedding large toroidal coherent structures at a well established frequency that scales with the macroscopic properties of the flow, a phenomenon referred to in the literature as \emph{puffing}. This behavior influences the rate of air entrainment, the radiated heat output, the flame height, and also the spreading of the flame. Pool-fire puffing has been studied extensively in the literature, and it is well known that under normal conditions of temperature and pressure typical hydrocarbon fuel pools of a few centimeters in diameter puff with a frequency on the order of 10Hz. Nevertheless, no detailed account has been given of the critical conditions for the \emph{onset} of puffing. The present work focuses on the latter aspect, encompassing experiments, linear global stability analyses, and direct numerical simulations. In particular, we obtain the critical Rayleigh number, which is the only governing parameter of the problem, for different fuels, obtaining results that show good agreement between experiments and theory/numerics. [Preview Abstract] |
Sunday, November 19, 2017 2:28PM - 2:41PM |
D35.00002: A temperature wall function for large-eddy simulation of natural convection based on modified Rayleigh number scaling Randall McDermott, Marcos Vanella, Bradley Henderson Natural convection heat transfer may be responsible for roughly half of the surface preheating in wildfire flame spread. Modeling flame spread, and convective heat transfer in general, is a challenge for large-eddy simulation (LES) since resolution requirements for wall-resolved LES scale as distance from the wall. In this work, we illustrate a deficiency in loglaw-based wall functions for LES with near-wall modeling of natural convection and propose a new wall function based on modified Rayleigh number (Ra) scaling. A thermal length scale based on heat flux similar to that of Yuan (1995) is employed. A local modified Nusselt number (Nu) is formulated in terms of nondimensional thermal wall units. The new model is implemented in a low-Mach LES code called the Fire Dynamics Simulator (FDS). Both the new wall model and a loglaw-based model are used in LES of natural convection in vertical and horizontal enclosed cavities for a broad range of Ra. The bulk Nu at steady state are compared with correlations in the literature. It is shown that the loglaw model, designed principally for forced convection, requires substantially higher grid resolution to achieve accurate Ra scaling. [Preview Abstract] |
Sunday, November 19, 2017 2:41PM - 2:54PM |
D35.00003: The Structure of the Blue Whirl Sriram Bharath Hariharan, Yu Hu, Huahua Xiao, Michael Gollner, Elaine Oran Recent experiments have led to the discovery of the blue whirl, a small, stable regime of the fire whirl that burns typically sooty liquid hydrocarbons without producing soot. The physical structure consists of three regions -- the blue cone, the vortex rim and the purple haze. The physical nature of the flame was further investigated through digital imaging techniques, which suggest that the transition (from the fire whirl to the blue whirl) and shape of the flame may be influenced by vortex breakdown. The flame was found to develop over a variety of surfaces, which indicates that the formation of the blue whirl is strongly influenced by the flow structure over the incoming boundary layer. The thermal structure was investigated using micro-thermocouples, thin-filament pyrometry and OH* spectroscopy. These revealed a peak temperature around 2000 K, and that most of the combustion occurs in the relatively small, visibly bright vortex rim. The results of these investigations provide a platform to develop a theory on the structure of the blue whirl, a deeper understanding of which may affirm potential for applications in the energy industry. [Preview Abstract] |
Sunday, November 19, 2017 2:54PM - 3:07PM |
D35.00004: Convection-driven melting in an n-octane pool fire bounded by an ice wall Hamed Farmahini Farahani, Ulises Alva, Ali Rangwala, Grunde Jomaas Burning of the liquid fuels adjacent to ice bodies creates a lateral cavity due to melting of the ice. The formation of lateral cavities are noticed recently and only a few experimental studies have addressed them. One study has shown lateral cavity formation with length of 12 cm for 5 minutes burning of oil. Based on the hypothesis that melting is facilitated by the convection in the liquid fuel, a series of PIV tests were conducted on burning of n-octane in a square glass tray with a 3 cm thick ice wall placed on one side of the tray. Marangoni generates a flow below the surface of the fuel and near the ice from hot to cold regions. The flow measurements by a 2D PIV system indicated the existence of different flow regimes. Before ignition, combined surface tension and buoyancy effects led to a one roll structure. After ignition the flow field began transitioning toward an unstable regime with an increase in velocity magnitude. Unfortunately, the PIV quality declined in the unstable regime, but indications of a multi-roll structure separating from a primary horizontal flow on the top driven by Marangoni convection were observed. The knowledge gained from these experiments will help determine the influential parameters in ice melting during burning of oil in ice-infested waters. [Preview Abstract] |
Sunday, November 19, 2017 3:07PM - 3:20PM |
D35.00005: Flame Structure and Dynamics for an Array of Premixed Methane-Air Jets Siddharth P. Nigam, Caelan Lapointe, Jason D. Christopher, Nicholas T. Wimer, Torrey R. S. Hayden, Gregory B. Rieker, Peter E. Hamlington Premixed flames have been studied extensively, both experimentally and computationally, and their properties are reasonably well characterized for a range of conditions and configurations. However, the premixed combustion process is potentially much more difficult to predict when many such flames are arranged in a closely spaced array. These arrays must be better understood, in particular, for the design of industrial burners used in chemical and heat treatment processes. Here, the effects of geometric array parameters (e.g., angle and diameter of jet inlets, number of inlets and their respective orientation) and operating conditions (e.g., jet velocities, fuel-air ratio) on flame structure and dynamics are studied using large eddy simulations (LES). The simulations are performed in OpenFOAM using multi-step chemistry for a methane-air mixture, and temperature and chemical composition fields are characterized for a variety of configurations as functions of height above the array. Implications of these results for the design and operation of industrial burners are outlined. [Preview Abstract] |
Sunday, November 19, 2017 3:20PM - 3:33PM |
D35.00006: Direct Numerical Simulation of Wildland Fires at Small Scales Nicholas Wimer, Amanda Mackoweicki, Chad Hoffman, Alexei Poludnenko, John Daily, Gregory Rieker, Peter Hamlington Preliminary results are presented from a new research effort focused on understanding and characterizing wildland fire spread at small scales (roughly 1m-1mm) using direct numerical simulations (DNS). The simulations are intended to directly resolve, with high physical accuracy, all small-scale fluid dynamic and chemical processes relevant to wildland fire spread. Simulation of wildland fires is an incredibly complex and challenging problem due to the vast difference in scales associated with the problem. An understanding is needed not just of the burning of fuel, but also of the atmospheric conditions, weather patterns, topography, and turbulence-flame dynamics. This work is focused on the sub-meter scales associated with wildland fire; in particular, the dynamics of small-scale diffusion flames. Here, preliminary results are presented for DNS of centimeter-scale gaseous pool fires coupled with multi-step chemical reaction mechanisms. The results are connected to the fundamental structure and spread of wildland fires, and an outlook is provided for the future expansion of these DNS studies. [Preview Abstract] |
Sunday, November 19, 2017 3:33PM - 3:46PM |
D35.00007: The origin and structure of streak-like instabilities in laminar boundary layer flames Michael Gollner, Colin Miller, Wei Tang, Mark Finney Streamwise streaks are consistently observed in wildland fires, at the base of pool fires, and in other heated flows within a boundary layer. This study examines both the origin of these structures and their role in influencing some of the macroscopic properties of the flow. Streaks were reproduced and characterized via experiments on stationary heated strips and liquid and gas-fueled burners in laminar boundary layer flows, providing a framework to develop theory based on both observed and measured physical phenomena. The incoming boundary layer was established as the controlling mechanism in forming streaks, which are generated by pre-existing coherent structures, while the amplification of streaks was determined to be compatible with quadratic growth of Rayleigh-Taylor Instabilities, providing credence to the idea that the downstream growth of streaks is strongly tied to buoyancy. These local instabilities were also found to affect macroscopic properties of the flow, including heat transfer to the surface, indicating that a two-dimensional assumption may fail to adequately describe heat and mass transfer during flame spread and other reacting boundary layer flows. [Preview Abstract] |
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