Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session A26: Reacting Flows: Emissions and Soot |
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Chair: Yuan Xuan, Pennsylvania State University Room: E146 |
Sunday, November 20, 2016 8:00AM - 8:13AM |
A26.00001: Effects of nucleating species on soot formation in turbulent non-premixed sooting jet flames Abhishek Jain, Yuan Xuan Soot nucleation is one of the most unknown processes in the soot life cycle, and it is believed to occur from Polycyclic Aromatic Hydrocarbons (PAH) generated from the combustion of various fuel sources under locally fuel-rich conditions. Current soot nucleation models may include as few as one (typically naphthalene) or as many as a dozen of nucleating species. In this study, the effects of PAH inclusion in the soot nucleation model on soot yield and distribution are studied by means of Large-Eddy Simulations (LES) of two piloted turbulent non-premixed sooting jet flames, using ethylene and a jet fuel surrogate, respectively. Two sets of simulations are performed for each flame, one considering only a single nucleating PAH (naphthalene) and the other one considering a range of nucleating PAH from naphthalene to cyclopenta[cd]pyrene. Flamelet-based chemistry tabulation is used for the major thermochemical quantities, and a recently developed relaxation model is used for PAH species to account for the interactions between turbulence and their chemistry. The effects of nucleating PAH species on soot are highlighted by comparing the mean soot volume fraction distributions and statistical characteristics of soot obtained from both sets of simulations against experimental measurements. [Preview Abstract] |
Sunday, November 20, 2016 8:13AM - 8:26AM |
A26.00002: Radiation and Turbulence-Chemistry-Soot-Radiation Interactions in a High-Pressure Turbulent Spray Flame S. Ferreyro, C. Paul, A. Sircar, A. Imren, D. C. Haworth, S. Roy, M. F. Modest Simulations are performed of a transient high-pressure turbulent n-dodecane spray flame under engine-relevant conditions. An unsteady RANS formulation is used, with detailed chemistry, a two-equation soot model, various radiation heat transfer models, and a particle-based transported composition probability density function (PDF) method to account for composition and temperature. The PDF model results are compared with those from a locally well-stirred reactor (WSR) model to quantify the effects of turbulence-chemistry-soot-radiation interactions. Computed liquid and vapor penetration versus time, ignition delay, and flame lift-off are in good agreement with experiment, and relatively small differences are seen between the WSR and PDF models for these global quantities. Computed soot levels and spatial distributions from the WSR and PDF models show large differences, with PDF results being in better agreement with experimental measurements. A photon Monte Carlo method with line-by-line spectral resolution is used to compute the spectral intensity distribution of the radiation reachingthe wall. This provides new insight into the relative importance of molecular gas radiation versus soot radiation, and the importance of unresolved turbulent fluctuations on radiative heat transfer. [Preview Abstract] |
Sunday, November 20, 2016 8:26AM - 8:39AM |
A26.00003: A priori analysis of a LES subfilter model for soot-turbulence-chemistry interactions Jeffry K. Lew, Michael E. Mueller In a turbulent flame, soot interacts with turbulence and combustion chemistry at the smallest scales. An existing LES subfilter model [Mueller et al. Phys. Fluids 23 (2011)] proposes that soot-turbulence interactions are independent of chemistry due to the time scale separation between slow soot formation and rapid heat-releasing reactions. However, interactions between soot, turbulence, and chemistry occur even after the nucleation of soot from polycyclic aromatic hydrocarbon (PAH) dimers. In fact, the interplay of soot and gas-phase chemistry may be intensified during oxidation and surface growth. To capture these effects, a dependence on the local mixture fraction has been introduced into the subfilter model. This modified model is evaluated a priori using a direct numerical simulation (DNS) database of soot evolution in a turbulent non-premixed $n$-heptane/air jet flame. [Preview Abstract] |
Sunday, November 20, 2016 8:39AM - 8:52AM |
A26.00004: Direct numerical simulations of temporally developing hydrocarbon shear flames at elevated pressure: effects of the equation of state and the unity Lewis number assumption Ayse Korucu, Richard Miller Direct numerical simulations (DNS) of temporally developing shear flames are used to investigate both equation of state (EOS) and unity-Lewis (Le) number assumption effects in hydrocarbon flames at elevated pressure. A reduced $Kerosene/Air$ mechanism including a semi-global soot formation/oxidation model is used to study soot formation/oxidation processes in a temporarlly developing hydrocarbon shear flame operating at both atmospheric and elevated pressures for the cubic Peng-Robinson real fluid EOS. Results are compared to simulations using the ideal gas law (IGL). The results show that while the unity-Le number assumption with the IGL EOS under-predicts the flame temperature for all pressures, with the real fluid EOS it under-predicts the flame temperature for 1 and 35 atm and over-predicts the rest. The soot mass fraction, $Y_s$, is only under-predicted for the 1 atm flame for both IGL and real gas fluid EOS models. While $Y_s$ is over-predicted for elevated pressures with IGL EOS, for the real gas EOS $Y_s$' s predictions are similar to results using a non-unity Le model derived from non-equilibrium thermodynamics and real diffusivities. Adopting the unity Le assumption is shown to cause misprediction of $Y_s$, the flame temperature, and the mass fractions of CO, H and OH. [Preview Abstract] |
Sunday, November 20, 2016 8:52AM - 9:05AM |
A26.00005: Effects of Flamelet Generated Manifolds on Turbulent Flame Structure and Pollutant Emissions A. Cody Nunno, Temistocle Grenga, Michael E. Mueller Heat losses substantially modify turbulent combustion processes, especially the formation of pollutant emissions such as nitrogen oxides, which are highly sensitive to temperature. To account for heat loss effects in Large Eddy Simulation (LES) with flamelet models, \textit{a priori} flamelet solutions are computed at reduced enthalpy. In this work, two methods for generating flamelets of lower enthalpy are compared to determine under what conditions the different methods produce different flame structure and different pollutant emissions in order to determine their validity limits. In the first method, a variable heat loss is introduced into the flamelet solutions that mimics a real heat loss, reducing the enthalpy primarily in the post-flame region of the flamelet. In the second method, fuel and oxidizer are converted to products in the unburned gases while retaining a constant unburned temperature, reducing the enthalpy over the entire flamelet. The two methods are compared in methane-air piloted turbulent premixed planar jet flames with increasing levels of dilution with both water and carbon dioxide that maintain a constant adiabatic flame temperature. The ``product conversion'' method is expected to mirror some of the same effects as physical dilution. [Preview Abstract] |
Sunday, November 20, 2016 9:05AM - 9:18AM |
A26.00006: A novel scaling approach for sooting laminar coflow flames at elevated pressures Ahmed Abdelgadir, Scott A. Steinmetz, Antonio Attili, Fabrizio Bisetti, William L. Roberts Laminar coflow diffusion flames are often used to study soot formation at elevated pressures due to their well-characterized configuration. In these expriments, these flames are operated at constant mass flow rate (constant Reynolds number) at increasing pressures. Due to the effect of gravity, the flame shape changes and as a results, the mixing field changes, which in return has a great effect on soot formation. In this study, a novel scaling approach of the flame at different pressures is proposed. In this approach, both the Reynolds and Grashof's numbers are kept constant so that the effect of gravity is the same at all pressures. In order to keep the Grashof number constant, the diameter of the nozzle is modified as pressure varies. We report both numerical and experimental data proving that this approach guarantees the same nondimensional flow fields over a broad range of pressures. In the range of conditions studied, the Damkoehler number, which varies when both Reynolds and Grashof numbers are kept constant, is shown to play a minor role. Hence, a set of suitable flames for investigating soot formation at pressure is identified. [Preview Abstract] |
Sunday, November 20, 2016 9:18AM - 9:31AM |
A26.00007: DNS of turbulent premixed slot flames with mixture inhomogeneity: a study of NO$_x$ formation Stefano Luca, Antonio Attili, Fabrizio Bisetti A set of Direct Numerical Simulations of three-dimensional methane/air lean flames in a spatially developing turbulent slot burner are performed. The flames are in the thin-reaction zone regimes and the jet Reynolds number is 5600. This configuration is of interest since it displays turbulent production by mean shear as in real devices. The gas phase hydrodynamics are modeled with the reactive, unsteady Navier-Stokes equations in the low Mach number limit. Combustion is treated with finite-rate chemistry. The jet is characterized by a non-uniform equivalence ratio at the inlet and varying levels of incomplete premixing for the methane/air mixture are considered. The global equivalence ratio is 0.7 and temperature is 800 K. All simulations are performed at 4 atm. The instantaneous profiles of the mass fractions of methane and air at the inlet are sampled from a set of turbulent channel simulations that provide realistic, fully turbulent fields. The data are analyzed to study the influence of partial premixing on the flame structure. Particular focus is devoted to the assessment of heat release rate fluctuations and NO$_x$ formation. In particular, the effects of partial premixing on the production rates for the various pathways to NO$_x$ formation are investigated. [Preview Abstract] |
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