Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G40: Ignition |
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Chair: Stefan Hickel, Delft University of Technology Room: Sheraton Back Bay D |
Monday, November 23, 2015 8:00AM - 8:13AM |
G40.00001: Mach Number Effects on Ignition and Mixing Processes in a Reacting Shock-Bubble Interaction Stefan Hickel, Felix Diegelmann, Volker Tritschler We investigate reacting shock-bubble interactions (RSBI) by direct numerical simulations (DNS) with detailed chemical reaction kinetics. The bubble contains a stoichiometric $H_2$-$O_2$ gas mixture and is surrounded by pure $N_2$. The interaction with a planar shock wave induces Richtmyer-Meshkov instability. Secondary instabilities develop into a turbulent mixing zone at the bubble interface. The transmitted shock focuses at the downstream pole of the bubble and may ignite the bubble gas. To trigger different reaction wave types, we performed DNS of RSBI for shock Mach numbers in the range of $Ma = 2.13$ - $2.50$ at a constant initial pressure of $p_0 = 0.50$ atm. Deflagration, dominated by $H$, $O$ and $OH$ production, is observed for a shock Mach number of $Ma = 2.13$. Increasing the shock Mach number reduces the induction time and eventually leads to deflagration-detonation transition. Ignition by a $Ma = 2.50$ shock wave directly leads to a detonation wave, driven by $HO_2$ and $H_2O_2$ high-pressure chemistry. Richtmyer-Meshkov instability, subsequent Kelvin Helmholtz instabilities, and bubble expansion are highly affected by the reaction wave. Mixing is significantly decreased by both reaction waves types. In particular detonation waves reduce the mixing distinctly. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G40.00002: Shock induced ignition and DDT in the presence of mechanically driven fluctuations Wentian Wang, James G. McDonald, Matei I. Radulescu The present study addresses the problem of shock induced ignition and transition to detonation in the presence of mechanical and thermal fluctuations. These departures from a homogeneous medium are of significant importance in practical situations, where such fluctuations may promote hot-spot ignition and favor the flame transition to detonation. The problem is studied in 1D, where a piston-induced shock ignites the gas. The fluctuations in the shock-compressed medium are controlled by allowing the piston’s speed to oscillate around a mean, with controllable frequency and amplitude. A Lagrangian numerical formulation is used, which allows to treat exactly the transient boundary condition at the piston head. The hydrodynamic solver is coupled with the reactive dynamics of the gas using Cantera. The code was verified by comparison with steady state ZND solutions and previous shock induced ignition results in homogeneous media. Results obtained for different fuels illustrate the strong relation of the DDT amplification length to mechanical fluctuations in systems with a high effective activation energy and fast rate of energy deposition, consistent with experiments performed on fast flame acceleration in the presence of strong mechanical perturbations. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G40.00003: Diffusion-flame ignition by shock-wave impingement on a supersonic mixing layer Antonio L. Sanchez, Cesar Huete, Forman A. Williams, Javier Urzay Ignition in a supersonic mixing layer interacting with an oblique shock wave is investigated analytically and numerically under conditions such that the post-shock flow remains supersonic. The study requires consideration of the structure of the post-shock ignition kernel that is found to exist around the point of maximum temperature, which may be located either near the edge of the mixing layer or in its interior. The ignition kernel displays a balance between the rates of chemical reaction and of post-shock flow expansion, including the acoustic interactions of the chemical heat release with the shock wave, leading to increased front curvature. The analysis, which adopts a one-step chemistry model with large activation energy, indicates that ignition develops as a fold bifurcation, the turning point in the diagram of the peak perturbation induced by the chemical reaction as a function of the Damköhler number providing the critical conditions for ignition. Subsequent to ignition the lead shock will rapidly be transformed into a thin detonation on the fuel side of the ignition kernel, and, under suitable conditions, a deflagration may extend far downstream, along with the diffusion flame that must separate the rich and lean reaction products. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G40.00004: Acoustic timescale characterization of hot spot ignition in thermally stratified mixtures Fynn Reinbacher, Jonathan Regele Thermal stratification and the formation of hot spots in reactive mixtures are of key interest to characterize the autoignition behavior of charges in internal combustion engines. Critical gradient conditions and local maximum sizes of a finite hot spot centers can be used to describe such a hot spot. In previous work, one- and two-dimensional hot spots consisting of a linear temperature gradient and constant plateau have been characterized on an acoustic timescale. In the present work, random one-dimensional temperature fields, derived from Fourier superposition for temperature fluctuations with a temperature spectrum similar to Passot-Pouquet kinetic energy spectrum, are analyzed. The linear gradient constant plateau model is compared to a more realistic hot spot temperature profile. Hot spots in the one-dimensional temperature fields are modeled with linear gradients and constant plateaus in order to be characterized with acoustic time scale analysis. Probability distributions for different excitation-to-acoustic timescale ratios are calculated for a range of engine conditions. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G40.00005: Prediction of strong and weak ignition regimes in turbulent reacting flows with temperature fluctuations: A direct numerical simulation study Pinaki Pal, Mauro Valorani, Hong Im, Margaret Wooldridge The present work investigates the auto-ignition characteristics of compositionally homogeneous reactant mixtures in the presence of thermal non-uniformities and turbulent velocity fluctuations. An auto-ignition regime diagram is briefly discussed, that provides the framework for predicting the expected ignition behavior based on the thermo-chemical properties of the reactant mixture and flow/scalar field conditions. The regime diagram classifies the ignition regimes mainly into three categories: \textit{weak} (deflagration dominant), \textit{reaction-controlled strong} and \textit{mixing-controlled strong} (volumetric ignition/spontaneous propagation dominant) regimes. Two-dimensional direct numerical simulations (DNS) of auto-ignition in a lean thermally-stratified syngas/air turbulent mixture at high-pressure, low-temperature conditions are performed to assess the validity of the regime diagram. Various parametric cases are considered corresponding to different locations on the regime diagram, by varying the characteristic turbulent Damk\"{o}hler and Reynolds numbers. Detailed analysis of the reaction front propagation and heat release indicates that the observed ignition behaviors agree very well with the corresponding predictions by the regime diagram. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G40.00006: Numerical investigation of kinetic energy dynamics during autoignition of n-heptane/air mixture Paulo Lucena Kreppel Paes, James Brasseur, Yuan Xuan Many engineering applications involve complex turbulent reacting flows, where nonlinear, multi-scale turbulence-combustion couplings are important. Direct representation of turbulent reacting flow dynamics is associated with prohibitive computational costs, which makes it necessary to employ turbulent combustion models to account for the effects of unresolved scales on resolved scales. Classical turbulence models are extensively employed in reacting flow simulations. However, they rely on assumptions about the energy cascade, which are valid for incompressible, isothermal homogeneous isotropic turbulence. A better understanding of the turbulence-combustion interactions is required for the development of more accurate, physics-based sub-grid-scale models for turbulent reacting flows. In order to investigate the effects of reaction-induced density, viscosity, and pressure variations on the turbulent kinetic energy, Direct Numerical Simulation (DNS) of autoignition of partially-premixed, lean n-heptane/air mixture in three-dimensional homogeneous isotropic turbulence has been performed. This configuration represents standard operating conditions of Homogeneous-Charge Compression-Ignition (HCCI) engines. The differences in the turbulent kinetic energy balance between the present turbulent reacting flow and incompressible, isothermal homogeneous isotropic turbulence are highlighted at different stages during the autoignition process. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G40.00007: ABSTRACT WITHDRAWN |
Monday, November 23, 2015 9:31AM - 9:44AM |
G40.00008: Characterization of Diesel and Gasoline Compression Ignition Combustion in a Rapid Compression-Expansion Machine using OH* Chemiluminescence Imaging Sundar Rajan Krishnan, Kalyan Kumar Srinivasan, Matthew Stegmeir Direct-injection compression ignition combustion of diesel and gasoline were studied in a rapid compression-expansion machine (RCEM) using high-speed OH* chemiluminescence imaging. The RCEM (bore $=$ 84 mm, stroke $=$ 110-250 mm) was used to simulate engine-like operating conditions at the start of fuel injection. The fuels were supplied by a high-pressure fuel cart with an air-over-fuel pressure amplification system capable of providing fuel injection pressures up to 2000 bar. A production diesel fuel injector was modified to provide a single fuel spray for both diesel and gasoline operation. Time-resolved combustion pressure in the RCEM was measured using a Kistler piezoelectric pressure transducer mounted on the cylinder head and the instantaneous piston displacement was measured using an inductive linear displacement sensor (0.05 mm resolution). Time-resolved, line-of-sight OH* chemiluminescence images were obtained using a Phantom V611 CMOS camera (20.9 kHz @ 512 x 512 pixel resolution, $\sim$ 48 $\mu $s time resolution) coupled with a short wave pass filter (cut-off $\sim$ 348 nm). The instantaneous OH* distributions, which indicate high temperature flame regions within the combustion chamber, were used to discern the characteristic differences between diesel and gasoline compression ignition combustion. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G40.00009: Ignition and propagation of premixed methane flame by successive laser-induced breakdowns Lydia Wermer, Moon Soo Bak, Seong-kyun Im The ignition and the propagation of premixed methane flame by two successive laser-induced breakdowns were investigated. The ignition and flame propagation were visualized using a high-speed schlieren imaging technique. Experiments were performed for various time intervals between the two pulses ranging from nanoseconds to milliseconds and were compared to the ignition by a single laser breakdown. For time intervals in the nanosecond range, the second pulse energy coupled with the first breakdown increasing energy absorption in the breakdown. For time intervals in the microseconds and milliseconds, the blast wave from the second breakdown interacted with the propagating flame induced by the first breakdown. The interaction triggered Richtmyer-Meshkov instability enhancing flame propagation. It is observed that there are time intervals inhibiting the second breakdown due to the heating either by the first breakdown or by combustion. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G40.00010: Investigation of Laser Ignition Behavior of Iso-octane and Ethanol Blends Nathan Peters, Patrina Bailey, Deshawn Coombs, Benjamin Akih-Kumgeh Laser-induced ignition is a promising technology for combustion initiation in gas turbines and internal combustion engines. There is renewed interest in this technology in recent years due to its ability to ignite lean mixtures which are desirable for cleaner combustion. Research in this area has mainly focused on methane combustion. Effects of pressure, temperature, and ignition energy have been studied. Another fuel of practical interest which has not been studied as extensively is iso-octane. Due to the complexities of the laser ignition process, there is still a lot that to be understood, especially during the early stages of ignition. In this work we study the ignition of iso-octane and blends including ethanol, induced by focused light pulse from an Nd:YAG laser emitting at 532 nm. Experiments are carried out in a cylindrical stainless steel vessel, equipped with 6 optical accesses. Schlieren imaging and laser interferometry are used to image the ignition process. We seek to understand the multiphysics of the early stages of ignition including shock wave velocity, plasma to flame kernel transition, and flame kernel quenching under lean conditions. [Preview Abstract] |
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