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 L26: Reactive Flows VI: Fire, Soot, and Spray Combustion |
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Chair: Kal Seshadri, University of California, San Diego Room: 31B |
Monday, November 19, 2012 3:35PM - 3:48PM |
L26.00001: Effects of buoyancy on heat transfer under an inclined flat plate Michael Gollner, Antonio Sanchez, Forman Williams A recent study of flame spread over and under a plastic fuel at different angles of inclination revealed new flame-spread behavior, where peak rates of flame spread were found on the underside of fuel surfaces, in contradiction with the traditional assumption that maximum spread rates occur in a vertical configuration (Gollner et al, Proc. Comb. Inst, 2012). Because flame spread is governed by heat transfer from flames to unignited fuel, a natural analogy can be drawn with heat transfer from an inclined, heated flat plate. Kierkus (IJHMT, 1968) performed a first-order perturbation analysis of this problem, however in taking the boussinesq approximation, the lack of density variation within the boundary layer resulted in no differences in the results between the under and over flat plate configurations. In this analysis, an attempt is made to perform a second-order perturbation analysis without invoking the boussinesq approximation, taking into account density differences within the boundary layer. These results are compared to heat-flux measurements made during flame spread both over-and-under inclined fuels to see if this observation is in fact caused by buoyancy effects within the boundary layer. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L26.00002: A Numerical Study on Effects of Pressure and Gravity on Opposed Flow Flame Spread Rate over Thin Fuels Ranjit Shukla, Amit Kumar In the recent years there has been renewed interest on effects of low pressure on combustion processes especially because of increased human endeavors in space. As access to space is expensive and so researchers have tried emulating effect of reduced gravity with reduced pressures at normal gravity. One such area of interest has been studies on spreading flames over condensed fuels. These studies are primarily driven by need of fire safety in low convection space environment. In quiescent space environment flame spread against the flow has been known to exist even where concurrent flame spread is not possible. Therefore, here in this work a 2D numerical model has been formulated to analyze the effects of pressure and gravity on flame spread behavior in an opposed flow configuration. An attempt is also made to arrive at pressure-gravity equivalence. The numerical model comprises of governing conservation equations for solid phase and gas phase. The 1D solid phase model (thin fuel) for ideally pyrolysing fuel is coupled to the gas phase by boundary conditions. Simulations carried over a range of gravity level from microgravity to the normal gravity and sub atmospheric pressures up to flame extinction show that the flame spread behavior and extinction is qualitatively the same from partial gravity of about 0.1g to 1g but quite different at near zero gravity. While the former is amenable to pressure-gravity equivalence modeling, the latter is not. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L26.00003: Physics-based Modeling of Shrub Fires: Study of Distribution of Bulk Density and Moisture Content Ambarish Dahale, Babak Shotorban, Shankar Mahalingam We utilized a physics-based model to investigate the influence of the spatial variation of solid-fuel bulk density and the solid fuel-moisture content on the behaviour of a shrub fire. The model accounts for the interaction of fluid dynamics, combustion of solid and gas phases, convective and radiative heat transfer, and thermal degradation of solid fuel. The turbulence was dealt with large eddy simulation and the gas-phase combustion was modeled through filtered flame surface density approach [Zhou \& Mahalignam, Phys. Fluids, 2002]. Predictions from the model were compared against the experimental results, and fairly good agreement was observed between them. Vertical fire spread rate within the shrub and the time to initiate the ignition within the shrub were significantly affected by the spatial variation of the bulk density. They were also significantly influenced by the variation of the fuel moisture content. The amount of fuel burnt was also impacted by the change of fuel moisture content. The specific mechanisms responsible for the reduction in propagation speed in presence of higher bulk densities and/or moisture content were identified. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L26.00004: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 4:27PM - 4:40PM |
L26.00005: Mathematical Modeling of Wildfire Dynamics Kevin Del Bene, Donald Drew Wildfires have been a long-standing problem in today's society. In this paper, we derive and solve a fluid dynamics model to study a specific type of wildfire, namely, a two dimensional flow around a rising plume above a concentrated heat source, modeling a fire line. This flow assumes a narrow plume of hot gas rising and entraining the surrounding air. The surrounding air is assumed to have constant density and is irrotational far from the fire line. The flow outside the plume is described by a Biot-Savart integral with jump conditions across the position of the plume. The plume model describes the unsteady evolution of the mass, momentum, energy, and vorticity inside the plume, with sources derived to model mixing in the style of Morton, et al.\. [1956]. The fire is then modeled using a conservation derivation, allowing the fire to propagate, coupling back to the plume model. The results show that this model is capable of capturing the complex interaction of the plume with the surrounding air and fuel layer. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L26.00006: Flamelet Radiation Modeling Jeffrey Doom, Krishnan Mahesh A flamelet model is proposed that couples soot and radiation. The soot model from Carbonell et al. ({\it Combust. Flame.} 2009) is used. The radiation model is the $P_1$ gray and non--grey model from Modest ({\it Academic Press.} 2003) which are cast into the flamelet equations. A sooty ethylene flame is studied and a series of canonical calculations are performed. Results associated with the soot and radiation will be shown and compared to experiment. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L26.00007: DNS of soot formation in three-dimensional turbulent non-premixed jet flames Antonio Attili, Fabrizio Bisetti, Michael E. Mueller, Heinz Pitsch A set of three-dimensional Direct Numerical Simulations (DNS) of soot formation in a three-dimensional n-heptane/air turbulent non-premixed jet flame has been performed to investigate the coupling between turbulence, chemistry, and soot dynamics with varying Damk\"{o}hler number. Finite rate chemistry of Polycyclic Aromatic Hydrocarbons (PAH) is included in the chemistry model. Soot is described with a bivariate distribution in volume-surface sample space, and a selected number of moments of the distribution are transported via a recently proposed transport Lagrangian scheme. Closure of the soot moment equations is achieved via the Hybrid Method of Moments (HMOM). It is observed that, for smaller Damk\"{o}hler number, the mass fraction of soot particles decreases while the number density stays approximately constant. In addition, Lagrangian statistics are used to study the evolution and transport of soot aggregates during their movement in physical and mixture fraction space. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L26.00008: Validation of an LES Model for Soot Evolution against DNS Data in Turbulent Jet Flames Michael Mueller An integrated modeling approach for soot evolution in turbulent reacting flows is validated against three-dimensional Direct Numerical Simulation (DNS) data in a set of $n$-heptane nonpremixed temporal jet flames. As in the DNS study, the evolution of the soot population is described statistically with the Hybrid Method of Moments (HMOM). The oxidation of the fuel and formation of soot precursors are described with the Radiation Flamelet/Progress Variable (RFPV) model that includes an additional transport equation for Polycyclic Aromatic Hydrocarbons (PAH) to account for the slow chemistry governing these species. In addition, the small-scale interactions between soot, chemistry, and turbulence are described with a presumed subfilter PDF approach that accounts for the very large spatial intermittency characterizing soot in turbulent reacting flows. The DNS dataset includes flames at three different Damk\"{o}hler numbers to study the influence of global mixing rates on the evolution of PAH and soot. In this work, the ability of the model to capture these trends quantitatively as Damk\"{o}hler number varies is investigated. In order to reliably assess the LES approach, the LES is initialized from the filtered DNS data after an initial transitional period in an effort to minimize the hydrodynamic differences between the DNS and the LES. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L26.00009: Direct numerical simulations of temporally developing turbulent reacting liquid-fueled jets Shashank Shashank, Heinz Pitsch Liquid fueled engines are ubiquitous in the transportation industry because liquid fuel minimizes the weight and volume of propulsion systems. The combustion that occurs in these engines is an inherently multi-physics process, involving fuel evaporation, reaction kinetics, and high levels of turbulence. A desire for high fidelity data that explains complex interaction between different physical mechanisms motivates the consideration of direct numerical simulation (DNS) as an investigation tool. In this study three-dimensional DNS of a reacting n-heptane liquid fueled temporal jet have been performed to study auto-ignition and subsequent burning in conditions that are representative of a diesel engine environment. In these simulations the continuous phase is described using an Eulerian representation whereas Lagrangian particle tracking is used to model the dispersed phase. The results of this study will demonstrate the importance of unsteady effects, and of accounting for the interaction between different modes of combustion, when simulating spray combustion. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L26.00010: Droplet evaporation and vapor mixing characteristics in a high-speed liquid jet spray Junji Shinjo, Akira Umemura Droplet evaporation and vapor mixing in the early stage of a dense autoigniting spray are studied by detailed numerical simulation. Due to the relative velocity between droplets and air, heat and mass transfer is enhanced around the droplets. In the region of low droplet number density, the behavior is similar to that of a single droplet. In the region of high droplet number density, the interaction between neighboring droplets affects the transfer characteristics. Non-spherical geometry effect of droplets and ligaments will be also studied. The fuel/air mixture is formed non-uniformly due to the non-uniform droplet distribution and flow structure, which are determined during the spray formation. Reaction initiation is strongly affected by this mixture formation. Extension of temporal and spatial scales is finally sought for future effort in applying the results for real-scale combustors. [Preview Abstract] |
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