1,007 research outputs found
A conservative cell-based unsplit Volume of Fluid advection scheme for three-dimensional atomization simulations
No full textThe Volume of Fluid (VoF) method is widely used for capturing the interface motion in multiphase flow simulations. In particular, unsplit geometrical advection schemes have proved well-suited for flows with complex topologies. In the cell-based approach, the computational cell is followed along its Lagrangian trajectories and provides a conceptually simple framework for advancing the interface. Thus, this is a semi-Lagrangian method, and enforcing conservation is difficult. This paper presents a cell-based three-dimensional (3D) unsplit advection scheme that is conservative. The method relies on the Hybrid Lagrangian Eulerian Method (HyLEM) of Le Chenadec and Pitsch [3] but additionally ensures discrete conservation by introducing a correction of the projected cell, which is inspired by the 3D flux-based method of Owkes and Desjardins [14] and the two-dimensional cell-based method of Comminal et al. [4]. While the projected cell vertices are evaluated as in HyLEM, additional vertices are introduced to modify the projected cell faces. The positions of those are obtained from conservative flux volumes. The proposed method provides the same accuracy as the method of Owkes and Desjardins [14], but is more efficient. The proposed method is tested in various benchmark cases and applied in an atomization case.</p
First-Principles Based Analysis of the Electrocatalytic Activity of the Unreconstructed Pt(100) Surface for Oxygen Reduction Reaction
No full textWe apply a rigorous computational procedure combining ab initio DFT calculations and statistical mechanics based methods to examine the electrocatalytic activity of the unreconstructed Pt(100) surface for oxygen reduction reaction. Using the cluster expansion formalism, we obtain stable interfacial water structures using Monte Carlo simulations carried out using parametrized interactions of water-water and water-metal. We find that both long-range and multibody interactions are important to describe the adsorbate interactions as a consequence of the mismatch between the preferred "hexagonal" water overlayer and the underlying square symmetry of the (100) surface. Our results indicate that the stable interfacial water structure is substantially different from that found on the Pt(111) surface. We compute the potential-dependent equilibrium coverages of oxygen-containing adsorbates, which shows that the surface is poisoned by strongly adsorbed OH. We construct the free-energy diagram of intermediates for oxygen reduction reaction on the Pt(100) surface and find that the limiting step is the reduction of the strongly adsorbed OH. We also find that, at a given potential, a higher degree of poisoning by OH is the reason unreconstructed (100) surfaces are catalytically less active than (111) surfaces. This study shows the importance of accurately capturing atomistic interactions beyond the nearest neighbor pairs. © 2012 American Chemical Society.
Analysis of turbulent reacting jets via principal component analysis
No full textThe interpretation of high-dimensional data, like those obtained from Direct Numerical Simulations (DNS) of turbulent reacting flows, constitutes one of the biggest challenges in science and engineering. Although these simulations are a source of key information to advance the knowledge of turbulent combustion, as well as to develop and validate modeling approaches, the dimensionality of the data often limits the full opportunity to leverage the detailed and comprehensive information stored in datasets. The Principal Component Analysis (PCA) and its local formulation (LPCA) are widely used in many fields, including combustion. During the last 20 years, they have been used in combustion for the identification of lowdimensional manifolds, data analysis, and development of reduced-order models. Lower dimensional structures, either global or local, can provide better insights on the underlying physical phenomena, and lead to the formulation of high-fidelity models. This chapter aims to offer to the reader a comprehensive introduction of the PCA potential for data analysis, firstly introducing the main theoretical concepts, and then going through all the required computational steps by means of a MATLAB® code. Finally, the methodology is applied to data obtained from a DNS of a turbulent reacting non-premixed n-heptane jet in air. The latter can be regarded as an optimal case for data analysis because of the complex physics characterized by turbulence- chemistry interaction and soot formation
Theoretical study of important phenylacetylene reactions in polycyclic aromatic hydrocarbon growth
No full textPhenylacetylene is of significance in polycyclic aromatic hydrocarbon (PAH) growth in combustion systems, especially as the key intermediate species in the hydrogen abstraction carbon addition sequence. The consumption of phenylacetylene via H-atom abstraction reactions forming ortho-C6H4C2H (o-C6H4C2H) radical and its subsequent reaction with carbon addition are important steps in PAH growth. Nevertheless, rate constants for H-atom abstraction reactions from phenylacetylene have not been explored theoretically in the literature. As regards carbon addition, besides acetylene, ethylene is also presented in large concentrations in e.g., ethylene and propene flames. However, its role in PAH growth has not been well addressed yet. In this study, the H-atom abstraction reactions from phenylacetylene aromatic ring sites (ortho-, meta-, para-) by both H atoms and OH radicals, and the subsequent C2H4 addition to o-C6H4C2H radical are accurately determined with electronic structure calculations. The main bimolecular product from the potential energy surface of the C2H4 addition to o-C6H4C2H is 2-ethynylstyrene + H, while naphthalene + H and 1-methyleneindene + H are preferred to form at high temperatures and low pressures. Rate constants are lumped with an automated master equation-based lumping approach and integrated into a recently developed chemical kinetic model. Updates of the phenylacetylene-related reactions are tested by ethylene and benzene copyrolysis in shock tube, and ethylene oxidation in laminar premixed and counterflow diffusion flames. Reaction flux analysis are additionally performed regarding the phenylacetylene consumption and naphthalene formation
Effects of differential diffusion on hydrogen flame kernel development under engine conditions
No full textThe early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel
Mechanism Comparison for PAH Formation in Pyrolysis and Laminar Premixed Flames
No full textPolycyclic aromatic hydrocarbons (PAHs) are known precursors of harmful carbonaceous particles. Accurate predictions of soot formations strongly rely on accurate predictions of PAHs chemistry. This work addresses the detailed kinetic modeling of PAH formation using two models: CRECK [8] and ITV [12], aiming to compare the model predictions with experimental data in olefin pyrolysis and laminar premixed flames. The two kinetic mechanisms are validated and compared highlighting similarities and differences in PAHs formation pathways. The validation highlights the critical role of resonance-stabilized radicals leading to the PAH formation
Effects of turbulence on variations in early development of hydrogen and iso-octane flame kernels under engine conditions
No full textThe understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.</p
Three-dimensional numerical investigation of flashback in premixed hydrogen flames within perforated burners
No full textPredicting flashback represents a pivotal challenge in the development of innovative perforated burners for household appliances, especially for substituting natural gas with hydrogen as fuel. Most existing numerical studies have utilized two-dimensional (2D) simulations to investigate flashback in these burners, primarily to reduce computational costs. However, the inherent complexity of flashback phenomena suggests that 2D simulations may inadequately capture the flame dynamics, potentially leading to inaccurate estimations of flashback limits. In this study, three-dimensional (3D) simulations are employed to examine the impact of the actual slit shapes on the flashback velocities of hydrogen-premixed flames. Steady-state simulations are conducted to compute flashback velocities for three equivalence ratios (ϕ=0.6, 0.8, and 1.0), investigating slits with fixed width W and varying length L. Additionally, transient simulations are performed to investigate the flashback dynamics. The results are compared with those from 2D configurations to assess the reliability of the infinite slit approximation. For stable flames, 2D simulations underpredict the burner plate temperature compared to slits with lengths typical of practical devices but match the 3D results as L→∞. Conversely, flashback velocities are consistently underpredicted in 2D simulations compared to 3D simulations, even as L→∞. This is due to the critical role of the slit ends in flashback dynamics, where favorable aerodynamics, preferential diffusion, the Soret effect, and higher preheating due to a higher surface-to-volume ratio trigger the initiation of flashback in those regions. These findings underscore the necessity of employing 3D simulations to accurately estimate the flashback velocities in domestic perforated burners. Novelty and significance statement This study presents a novel investigation into how finite slit lengths affect the critical flashback velocities in hydrogen-fueled perforated burners, using three-dimensional simulations. Our findings indicate that two-dimensional configurations, which are widely used in the literature, significantly underpredict flashback velocities because they fail to capture the crucial influence of slit ends. For the first time, we show that in slits of finite length and circular holes, the combined effect of favorable aerodynamic conditions and enhanced preheating, due to the increased surface area available for heat transfer, leads to higher flashback velocities compared to infinite-length slits. Additionally, we provide the first analysis of the temporal evolution of flashback dynamics in a realistic three-dimensional configuration, demonstrating that flashback initiation occurs at the slit ends. These insights are essential for the development of advanced numerical models that can inform the design of innovative perforated burners to prevent flashback effectively.</p
Mechanism optimization with a novel objective function: Surface matching with joint dependence on physical condition parameters
No full textThe prediction accuracy of chemical kinetics models can be improved efficiently by using automatic model optimization techniques. In the optimization, an objective function, which quantifies the differences between model responses and experimental data for quantities of interest, is minimized by calibrating the reaction rate parameters of a model within their uncertainty limits. Consequently, the values of the model predictions become closer to those of the measurements. Typically, a point-wise objective function, which is based on function components separately for each measurement over the investigated domain, is used in the model optimization. Quantities of interest are often functions of various physical condition parameters, such as temperature, pressure, and equivalence ratio. However, the point-wise objective function does not consider the correlation between data and their corresponding physical conditions. Thus, in this work, a new objective function is proposed, which uses a surface-matching (SM) method. It evaluates the similarity between surface shapes of the predicted and measured values, which is quantified in form of two user-defined physical condition parameters. By minimizing this function, the joint dependence of model predictions on physical conditions is optimized in conjunction with the point-wise model prediction accuracy. A chemical mechanism of oxymethylene ethers is optimized in this work as an example. The model is calibrated with the point-wise, curve-matching (CM)-based, and SM-based objective functions. The optimized models are compared and the results are discussed. It is shown that the optimization with the SM-based objective function yields improved results for certain cases compared to using the point-wise objective function. This model also provides the best prediction accuracy in terms of joint physical condition dependence. In addition, a better overall performance is achieved by adjusting the ratios between the component functions in the objective function, which demonstrates that the definition of objective functions plays a crucial role for model optimization
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