Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A34: Sprays and Emissions |
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Chair: Fabrizio Bisetti, King Abdullah University of Science and Technology Room: 2024 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A34.00001: Numerical investigation of spray ignition of a multi-component fuel surrogate Lara Backer, Krithika Narayanaswamy, Perrine Pepiot Simulating turbulent spray ignition, an important process in engine combustion, is challenging, since it combines the complexity of multi-scale, multiphase turbulent flow modeling with the need for an accurate description of chemical kinetics. In this work, we use direct numerical simulation to investigate the role of the evaporation model on the ignition characteristics of a multi-component fuel surrogate, injected as droplets in a turbulent environment. The fuel is represented as a mixture of several components, each one being representative of a different chemical class. A reduced kinetic scheme for the mixture is extracted from a well-validated detailed chemical mechanism, and integrated into the multiphase turbulent reactive flow solver NGA. Comparisons are made between a single-component evaporation model, in which the evaporating gas has the same composition as the liquid droplet, and a multi-component model, where component segregation does occur. In particular, the corresponding production of radical species, which are characteristic of the ignition of individual fuel components, is thoroughly analyzed. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A34.00002: Integral Length Scale Effects on JP-8 Spray Penetration and Ignition at Elevated Pressure and Temperature Conditions Matthew Kurman, Michael Tess, Luis Bravo, Chol-Bum Kweon The effect of the integral length scale on global spray diagnostics was examined for non-reacting and reacting JP-8 sprays. The scales were set by varying the nominal nozzle diameter from 90 $\mu$m, 100 $\mu$m, and 147 $\mu$m, resulting in the ranges of Re (6.7 x 10$^{4}$ - 9.9 x 10$^{4})$ and We (1.3 x 10$^{6}$ - 1.7 x 10$^{6})$ setting the spray in the fully atomization mode. A high temperature (900-1000 K) high pressure (60-100 bar) flow through chamber was used to conduct experiments at relevant compression ignition engine operating conditions. Each fuel injector was characterized with an injection analyzer to determine the rate of injection and injected fuel mass. High speed near simultaneous Mie and schlieren images were acquired to determine the liquid and vapor penetration lengths of the non-reacting spray. Ignition delay experiments were conducted by measuring the start of formation of OH radicals. A numerical investigation was also carried out to provide additional insights into the behavior of each spray with the specified conditions. The quantitative results presented will aid in the overall advancement of fuel injector designs and ultimately lead to optimized engines. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A34.00003: A spray-flamelet formulation using an effective composition-space variable Benedetta Franzelli, Aymeric Vi\'e, Matthias Ihme The modeling and simulation of spray flames is of primary importance as new combustion systems rely on the use of liquid fuels to feed the combustion process. The description of such flames is commonly performed using a mixture fraction variable that monotonically decreases from the fuel to the oxidizer side. Unfortunately, in the case of spray flames, this mixture-fraction variable is not monotonic as a result of the presence of an evaporation source term in the governing equations. To address this issue, a new composition space variable is defined, which is defined from the arc length~along the gas-liquid mixture-fraction space. This monotonic definition~enables~the complete description of the spray-flame~structure~in composition space and the formulation of a well-posed spray-flamelet equation. A closure model for the scalar dissipation rate is proposed, and the potential of this effective composition-space variable is demonstrated by~comparing simulation results in physical and composition space. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A34.00004: Investigation of Mixing and Chemical Reaction Interactions Using Rate-Controlled Constrained-Equilibrium Fatemeh Hadi, Mohammad Janbozorgi, Reza H. Sheikhi, Hameed Metghalchi The Rate-Controlled Constrained-Equilibrium (RCCE) method is applied to study the interaction between mixing and chemical reaction in a constant pressure Partially-Stirred Reactor (PaSR). The objective is to understand the influence of mixing on RCCE predictions. The RCCE is a computationally efficient method based on thermodynamics to implement the combustion chemistry. In the RCCE the dynamics of reacting systems is described by a small number of rate-controlling reactions and slowly-varying constraints. The method is applied to study methane combustion via 12 constraints and 133 reaction steps. Simulations are carried out over a wide range of initial temperatures and equivalence ratios. The RCCE predictions are assessed by comparing with those of detailed kinetics model, in which the same kinetics, involving 29 species and 133 reaction steps, is integrated directly. Chemical kinetics and mixing interactions are studied for different residence and mixing time scales. Results show that the RCCE accurately represents the effect of mixing with different mixing strengths. An assessment of numerical performance of the RCCE is also performed. It is shown that the method is effective to reduce the stiffness of the kinetics and thus allows simulations with much lower computation costs. [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A34.00005: Radiative cooling in a flameholder for NOx reduction Robert Breidenthal, Igor Krichtafovitch, Doug Karkow, Joseph Colannino Recent experiments have revealed dramatic reductions in NOx emissions using a ceramic honeycomb as a flameholder. A jet of fuel entrains and mixes air before entering the honeycomb. The honeycomb is positioned at a distance away from the jet nozzle such that the mixed fluid arriving at the upstream edge of the honeycomb is combustible. Combustion occurs within the honeycomb, transferring heat to the ceramic walls, which glow red hot. According to a simple physical model, radiation and thermal conduction transport energy toward the upstream end of the honeycomb, thereby heating the incident cold reactants to maintain combustion. The radiation also transports energy downstream and away from the honeycomb, toward a thermal load. This is an attractive characteristic in boiler applications, for example. Furthermore, the hot combustion products in intimate thermal contact with the walls of the radiating honeycomb are rapidly cooled, consistent with the low NOx emissions. Preliminary experiments with different honeycomb configurations are in accord with this model. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A34.00006: Eulerian methods for the description of soot: mathematical modeling and numerical scheme T.T. Nguyen, A. Wick, F. Laurent, R. Fox, H. Pitsch A development and comparison between numerical methods for soot modeling derived from the population balance equations (PBE) is presented. The soot mechanism includes nucleation, surface growth, oxidation, aggregation and breakage (Mueller et al., Proceed. Combust. Inst., 2009, 2011). For comparison, data from the ethylene premixed flame of Xu et al. (Combust. Flame 108, 1997) over a range of equivalence ratios are used. Two types of methods are introduced. The first is a moment method in which the closure is obtained through a reconstruction of the number density function (NDF). In particular, the NDF can be approximated by a sum of Gamma distribution functions (Yuan et al., J. Aero. Sci. 51, 2012). The second is Eulerian multi-fluid (MF), which is a size discretization method (Laurent et al., Combust. Theory Modelling 5, 2001) considering one or two moments per section. The case of one moment per section is also known as a sectional method. The accuracy of MF methods depends on the number of sections. Eventually, an extension of these two methods considering the surface area as a function of volume is taken into account to describe more precisely the geometry of soot particles. The solutions from these methods are compared with solutions from Monte Carlo method. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A34.00007: On the effects of gas-phase species Lewis number in turbulent nonpremixed sooting flames Fabrizio Bisetti, Antonio Attili, Michael Mueller, Heinz Pitsch Two large DNS of n-heptane/air turbulent nonpremixed combustion are compared to asses the effects of gas-phase species Lewis number on the dynamics of soot formation and growth. A detailed chemical mechanism, which includes PAHs, and a high-order method of moments for soot modeling are employed for the first time in the three-dimensional simulation of turbulent sooting flames. The results obtained employing a complex model (mixture average) for the transport of heat and mass $[$Attili {\it et al.} Comb. Flame, 161, 2014$]$ are compared with those calculated with Le=1 for all gas-phase species, including large soot precursor molecules. It is found that the statistics of temperature and other species governing the heat releasing chemistry are very similar in the two cases as the flow field achieves a fully turbulent state. The dynamics of the soot precursors and soot display quantitative differences between the two cases. Employing the Le=1 approximation, the total mass of soot precursors in the flame decreases by 10 to 20$\%$ only, but its field is less inhomogeneous in space and time. Due to the non-linearity of soot growth with respect to the concentration of gas-phase precursors, the domain-averaged rate of soot mass production decreases by a factor of 2. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A34.00008: LES of turbulent lifted CH4 /H2 flames using a novel FGM-PDF model S. Ebrahim Abtahizadeh, Jeroen van Oijen, Rob Bastiaans, Philip de Goey This study reports on numerical investigations of preferential diffusion effects on flame stabilization of turbulent lifted flames using LES with a FGM-PDF approach. The experimental test case is the Delft JHC burner to study Mild combustion; a clean combustion concept. In this burner, CH4 based fuel has been enriched from 0 to 25\% of H2. Since the main stabilization mechanism of these turbulent flames is autoignition, the developed numerical model should be able to predict this complex event. Furthermore, addition of hydrogen makes modeling even more challenging due to its preferential diffusion effects. These effects are increasingly important since autoignition is typically initiated at very small mixture fractions where molecular diffusion is comparable to turbulence transport (eddy viscosity). In this study, first, a novel numerical model is developed based on the Flamelet Generated Manifolds (FGM) to account for preferential diffusion effects in autoignition. Afterwards, the developed FGM approach is implemented in LES of the H2 enriched turbulent lifted jet flames. Main features of these turbulent lifted flames such as the formation of ignition kernels and stabilization mechanisms are thoroughly analyzed and compared with the measurements of OH chemiluminescence. [Preview Abstract] |
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