Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C05: Sprays/Droplet Combustion and Ignition |
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Chair: Jacqueline H. Chen, Sandia National Laboratories Room: 204 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C05.00001: Flame structure analysis and flame stabilization in a turbulent swirling spray flame Danyal Mohaddes, Wenwen Xie, Matthias Ihme The quantitative prediction of the flow and combustion dynamics within highly turbulent environments encountered in modern aviation gas turbine engines remains an important challenge for the numerical combustion community. The use of large eddy simulation (LES) has become well-established for the analysis of such flows. Although a multitude of modelling approaches exist for combustion chemistry with known limitations in accuracy and computational cost, specific effects of a combustion model on a given simulation cannot be known a-priori. In this study, a turbulent swirling n-dodecane spray flame at ambient pressure is investigated using LES employing the Lagrangian point-particle approach for the liquid phase and the gas-phase reaction chemistry is described using finite-rate chemistry. Comparisons with experiments are performed to assess the accuracy of the simulation and physical submodels. The flame structure is analyzed and effects of the combustion model on the spray are examined. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C05.00002: Liquid Oxygen Droplet Combustion in Hydrogen under Microgravity Conditions Florian Meyer, Christian Eigenbrod, Volker Wagner, Wolfgang Paa, James Hall, Michael Zody, Jon Frydman, James Hermanson In liquid rocket propulsion the liquid oxygen (LOX) and liquid hydrogen system is widely used. Single oxygen droplets burning in gaseous hydrogen surrounds are investigated, representing the most basic element of this spray combustion process. The basic processes of droplet vaporization, mixture formation, ignition and combustion under cryogenic conditions in microgravity are studied. Experiments in the ZARM 4.7 s drop tower are conducted using a cryogenically-cooled test chamber that allows for pressures up to 60 bar. Initial experiments indicate that the hydrogen-oxygen diffusion flame is formed relatively close to the droplet surface. During the combustion process the surface of the LOX droplet appears to become covered by a water-ice layer, which ruptures to produce discrete, gaseous oxygen jets. External to the flame zone, the water vapor combustion product is observed to condense or freeze, forming a spherical shell around the burning droplet. The flame standoff distance and the droplet regression rate are investigated with shadowgraphy and OH chemiluminescence imaging. The experimental results are compared with the findings of numerical simulations conducted by the University of Washington. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C05.00003: Numerical Simulation of Liquid Oxygen Droplet Combustion in Hydrogen James Hall, Michael Zody, Jon Frydman, James Hermanson, Florian Meyer, Christian Eigenbrod, Volker Wagner, Wolfgang Paa In liquid rocket propulsion oxygen normally enters the combustion chamber as a dispersed phase, while the hydrogen fuel rapidly evaporates into a continuous, vapor phase. The ignition and combustion of a single, liquid oxygen droplet in gaseous hydrogen surroundings is thus the essential, first step in the subsequent spray combustion process. The OpenFOAM platform is used to calculate species concentrations, temperatures, heat release, and reactant consumption for this system. The simulations suggest that ignition, subsequent to the initial diffusion of gaseous oxygen into hydrogen, initially results in the appearance of two flame zones. As quasi-steady combustion is approached, these two flames merge. The resulting stable, quasi-steady flame, combined with the conductive heat transfer from the flame to the droplet surface is used to predict the oxygen droplet-combustion lifetime. These numerical simulations are conducted in parallel with drop-tower tests at ZARM. The calculated lifetime of a 1-mm liquid oxygen droplet undergoing combustion in gaseous hydrogen at 1 bar pressure and an initial temperature of 100 K is comparable to that observed in the drop-tower tests. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C05.00004: ABSTRACT WITHDRAWN |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C05.00005: Ignition Kernel Dynamics in a $M = 3$ Flame holder Esteban Cisneros-Garibay, David Buchta, Jonathan Freund The coupled mixing and reaction time scales of ignition in a supersonic flame-holding cavity flow are studied with detailed numerical simulations. A round jet ejects ethylene into the cavity under a $M = 3$, $T = 440$ $\mathrm{K}$ free-stream. The ignition (and subsequent sustained flame) are studied in detail, including direct comparisons with corresponding measurement. Two injection configurations are simulated: (i) vertical, from the cavity floor; and (ii) horizontal, from the cavity’s $45^{\circ}$ back wall. Ignition is seeded by a laser-induced breakdown (LIB), which creates radical species and locally heats the gas. Comparisons are made against measured excited hydroxyl radical (OH*) to assess prediction accuracy. The injection geometry significantly affects the direction in which ignition kernels (as quantified by OH mass fraction) advect and grow. Over the first $75$ $\mu\mathrm{s}$, the turbulence mechanics that produce this and the observed large variance of ignition kernel statistics are studied and compared. These variances are greater than dependencies on the chemical kinetic parameters. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C05.00006: Direct Numerical Simulation of Multi-Injection Ignition in Low-Temperature Compression Ignition Environments Martin Rieth, Marc Day, Shubhangi Bansude, Tianfeng Lu, Chol-Bum Kweon, Jacob Temme, Jacqueline Chen Multi-injection strategies in combustion engines are known to be able to reduce pollutant emissions and improve ignition reliability. The latter is especially important in advanced compression ignition engine concepts relying on low-temperature high-efficiency operation or for engines operating at extreme high-altitude conditions. Under low-temperature conditions, fluid from the first injection does not ignite prior to mixing with fluid from the second injection, but provides pre-ignition chemical species and potentially a moderately elevated temperature. Fluid from the second injection mixes with this partially reacted mixture and its ignition is accelerated. However, the exact details of how pre-ignition species accelerate the ignition of the fluid from the second injection are not known. We will explore this with Direct Numerical Simulations (DNS) and, in particular, we will focus on understanding how particular chemical species and reaction pathways are responsible for accelerated ignition, identify the combustion modes and flame topologies present during ignition, and how these are affected by turbulent mixing and entrainment. To this end, we utilize a chemical explosive mode analysis, reaction-diffusion balances and related techniques. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C05.00007: The impact of ignition on the occurrence and dynamics of multi-stage flames under shock tube conditions Tianhan Zhang, Yiguang Ju The laminar flame speeds and structures of ignition-assisted cool and warm n-heptane/air flames are studied computationally and analytically. The primary objective is to understand the effects of the ignition Damkohler number, mixture temperature, equivalence ratio, and pressure on the dynamics and structures of cool and warm flame propagation under shock tube conditions. Different transitions among cool, warm, and hot flames are examined. The results show that both cool and warm flame structures and propagation speeds change dramatically with the increase of the ignition Damkohler number and are affected by the initial temperature, equivalence ratio, pressure, and flame regimes. Furthermore, within the hot flame flammability limits, it shows that the cool flame speed has a non-monotonic dependence on the initial mixture temperature due to the negative temperature coefficient (NTC) effect, while the hot flame speed is divergently different and only increases monotonically with the initial temperature. Finally, compared with recent flame speed measurements using the shock tube, the simulation agrees well with the experimental data and demonstrates clearly how ignition and multi-stage flames affect the experimental observation and result in the NTC behavior. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C05.00008: Three-dimensional effects in vorticity production, cellular instabilities, and transition to turbulence in focused-laser-induced ignition kernels Jonathan F. MacArt, Jonathan M. Wang, Jonathan B. Freund Ignition of combustible mixtures via laser-induced breakdown (LIB) involves interactions between thermal, chemical, and hydrodynamic processes. An initially axisymmetric plasma core has large density, pressure, and velocity gradients that lead to vorticity production, collapse of the plasma core, and a transition to three-dimensional evolution. These stages and their co-evolution with the thermochemical state are investigated in lean hydrogen-oxygen premixtures using three-dimensional detailed numerical simulations, in which the compressible Navier-Stokes and reactive species equations are solved assuming local thermodynamic equilibrium and charge neutrality. The initial LIB and gasdynamics are simulated in auxiliary calculations employing a two-temperature non-local thermodynamic equilibrium kinetic model and a radiative transfer equation. Three-dimensional vorticity is observed in the plasma core, but this initial vorticity diffuses during the collapse of the core and transition to laminar burning. After an initial period of laminar propagation, the flame develops a cellular instability, which accelerates the flame-front and produces flame-front vorticity leading to sustained turbulence. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C05.00009: Hydrodynamic Ejection from a Laser-induced Breakdown and its Implications for Ignition Jonathan Wang, David Buchta, Jonathan MacArt, Jonathan Freund Optical breakdown of a gas by a focused laser produces a high-temperature, high-pressure plasma kernel that expands rapidly and, in some cases, ejects hot gas along the laser axis. Traveling as a hot vortex ring, this ejected gas can reach distances several times the size of the plasma kernel and, in a combustible mixture, initiate flame growth away from the laser focal region. Under certain conditions (e.g. sub-atmospheric pressure), however, the ejection can fail to form or even reverse direction, altering the distribution of heat and radical species necessary for ignition. Detailed simulations of a model kernel, confirmed to reproduced key experimental observations, show how the ejection and its reversal are caused by two primary mechanisms of vorticity production: tangential variations in the strength of the shock produced by the breakdown, and baroclinic generation in the trailing rarefaction. Relatively mild changes in the early-time kernel geometry alone---including, for example, a 20{\%} increase in overall aspect ratio---can alter the subsequent interaction of auto-advecting vorticity and ultimately precipitate ejection failure or reversal. The ejection influences ignition via the competing effects of strain and heating. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C05.00010: Thermal-Chemical Instability in Plasma-Assisted Combustion Hongtao Zhong, Mikhail Shneider, Mikhail Mokrov, Yiguang Ju In the flow of weakly ionized plasma, the transition from a diffusive volumetric discharge to a contracted localized filament is called plasma thermal instability. Traditionally, plasma thermal instability is controlled by the well-known thermal-ionization mechanism. However, endothermic/exothermic chemical reactions may bring new couplings between the reactive flow of weakly ionized plasma and chemical kinetics. In this work we developed a one-dimensional numerical model and studied the dynamics of thermal-chemical instability in a reactive mixture. Several key parameters including flow speed, gas pressure, initial temperature and mixture compositions are identified and discussed for their influence on triggering the thermal-chemical instability. This work will fill the knowledge gap in understanding the chemical kinetic effect on plasma instability in combustible mixtures and provide support for the future development of volumetric plasma ignition for ultra-lean combustion in advanced engines. [Preview Abstract] |
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