Correction to: Quasi-DNS Dataset of a Piloted Flame with Inhomogeneous Inlet Conditions

Flow, Turbulence and Combustion - The article Quasi-DNS Dataset of a Piloted Flame with Inhomogeneous Inlet Conditions written by Thorsten Zirwes, Feichi Zhang, Peter Habisreuther, Maximilian...     

Numerical investigation of the partial oxidation process in porous media based reformer

Numerical investigation of the thermal partial oxidation process of Methane in porous media based reformer is performed. A finite volume based CFD code, including radiation modeling, in combination with a detailed chemical kinetics scheme is used to perform the numerical simulation. A heterogeneous approach for the heat transport modeling in porous media (separate coupled energy equations for the gas and solid phases) was used. Validation of the model with experimental data is also performed. The model was able to predict the temperature behavior in the reformer reasonably well. However, the concentrations of H₂ and CO were under predicted while the H₂O concentration was over predicted.     

Flamelet-Modeling of Inverse Rich Diffusion Flames

An experimental and numerical investigation of a confined laminar inverse diffusion flame (IDF) with pure oxygen as oxidizer and carbon dioxide diluted methane as fuel with a global stoichiometry of partial oxidation processes (equivalence ratio of 2.5) is presented. The present burner setup allows studying both the flame and the post-flame zone in a simplified geometry considering typical operating conditions as found in large-scale gasifiers. This partial oxidation flame setup is characterized by very high temperatures close to the stoichiometric oxidation zone due to oxy-fuel combustion, whereas lower temperatures and slow endothermic post-flame conversion reactions with long residence times are found in the fuel rich post-flame region. The scope of this paper is to investigate different modeling approaches suitable for both regimes by comparing the simulation results to detailed experimental data. Planar OH laser-induced fluorescence (OH-LIF) was performed for measuring the hydroxyl radical in the reaction zone and the results are compared to CFD calculations. Based on this comparison, the necessary level of detail of diffusion flux modeling, which includes Soret and Dufour effects, is analyzed and established. Finally, steady and unsteady non-premixed flamelet approaches based on a single mixture fraction are used in order to study their applicability for both the oxidation and post-flame zone. Significantly different time scales are obtained using different flamelet paths. Their influence on the results is investigated in the steady flamelet and the Lagrangian flamelet approach.     

Conduction–radiation interaction in 3D irregular enclosures using the finite volume method

Interaction of conduction and radiation in 3D enclosures is carried out with a gray participating media. Application of block structured grid is shown with the finite volume method (FVM). Radiation modeling is performed with the FVM and is coupled with an ‘in-house’ code to solve the set of transport equations. The detailed numerical results are presented for a cubical and a cylindrical enclosure as these results are not available in the literature. The numerical simulation for the cylindrical enclosure is performed using a block-structured ‘O’ grid. Two additional geometries are considered in order to show the applicability of the present work. Results of temperature, radiative heat flux and total heat flux distributions are presented for different optical thicknesses, scattering albedoes, emissivities and conduction–radiation parameters. The 3D results are validated with the available 2D results or results with pure radiation problems as limiting cases.     

Measurement of instantaneous flow rates in periodically operating injection systems

Research and development work carried out to provide a method to measure accurately instantaneous flow rates in periodically operating injection systems is summarized. The instantaneous flow rate is reconstructed from axial velocity time series measured by a laser Doppler anemometer on the center-line of a capilary pipe flow. The theoretical background, on which the evaluation of the instantaneous flow rate is based is provided. It is shown that the axial mean velocity is sufficient to reconstruct the periodically varying flow rate and the pressure gradient. The application of the proposed method is described and an instrument is suggested that can be employed in many fields where fast, periodically varying flow rates occur and instantaneous information is needed.List of Symbols C _j complex Fourier coefficients - C.C. complex conjugate of different variables - D inner diameter of pipe - f ₀ repetition frequency of valve motion - i complex unit - J ₀ Bessel function of zeroth order - J _l Bessel function of first order - L distance between valve and measuring volume - m integrated fluid mass within one period - P/z mass flow rate - P/z pressure gradient along the pipe axis - p ₀,p _n normalized amplitude of the complex pressure gradient - r radial position - R pipe radius - Re Reynolds number - t time - T time of the period of the repetition valve opening - Ta Taylor number - U axial velocity - V integrated fluid volume within one period - instantaneous volumetric flow rate - viscosity - kinematic viscosity - fluid density - normalized open valve time - angular velocity of Fourier modes     

In-situ flame particle tracking based on barycentric coordinates for studying local flame dynamics in pulsating Bunsen flames

Flame particles (FP) are massless, virtual particles which follow material points on the flame surface. This work presents a tracking algorithm for FPs which utilizes barycentric coordinates. The methodology can be used with any cell shape in the computational mesh and allows computationally fast spatial interpolation as well as efficient determination of the intersection of FP trajectories with iso-surfaces. In contrast to previous flame particle tracking (FPT) approaches, the code is fully parallelized and can therefore be used in-situ during the simulation. It also includes fully parallelized computation of flame consumption speed by integrating reaction rates along a line normal to the flame surface at each FP position. Direct numerical simulations of laminar pulsating premixed hydrogen–air Bunsen flames serve as validation cases and showcase the added value of tracking material points for studying local flame dynamics. Exciting the inlet flow harmonically with frequencies equal to the inverse flame time scale leads to a pulsating mode where the flame front is corrugated. Ten times higher frequencies nearly resemble the steady state solution. The FPs are seeded along the flame surface and are used to track the unsteady diffusive, convective and chemical contributions at arbitrary points on the flame front over time. Their trajectories reveal a phase shift between the unsteady flame stretch rate and local flame speed of the order of 0.1 flame time scales for rich hydrogen flames. This is caused by a time delay between straining and stretch due to curvature. The reason is that diffusive processes follow the time signal of curvature while chemical processes are most strongly affected by the straining rate, which dominates the high Lewis number hydrogen flames investigated. This time history effect may help to explain the large scattering in the correlation of local flame speed with flame stretch found in turbulent flames.     

Near Wall Dynamics of Premixed Flames

Highly-resolved numerical simulations employing detailed reaction kinetics and molecular transport have been applied to flame-wall interaction (FWI) of laminar premixed flames. A multiple plane-jet flame (2D) has been considered, which is operated with premixed methane/air mixtures at atmospheric conditions and with different equivalence ratios. Free flame (FF) and side-wall quenching (SWQ) conditions have been accomplished by defining one lateral boundary as either a symmetry plane for FF or a cold wall with fixed temperature for SWQ. An equidistant grid with a resolution of 20 µm is used to resolve the FWI zone. The GRI-3.0 mechanism is used for computing chemical reaction rates. The flame is tangentially compressed when approaching the cold wall, and elongated in the FF case, causing an inversion of the sign of the tangential strain rate Ka_s and a considerable decrease of the total stretch rate Ka_tot for the SWQ flame. The flame consumption speed S_L decreases with decreasing normal stretch due to curvature Ka_c while approaching the cold wall, but it increases with decreasing Ka_c for the FF case, leading to an inversion of the Markstein number Ma_tot based on Ka_tot from positive in FF to negative in the SWQ case. The results reveal a strong correlation of flame dynamics during transitions from FWI to freely propagating flames, which may bring a new perspective for modeling FWI phenomena by means of flame dynamics. To do this, the quenching effect of the wall may be reproduced by an inversion of the Markstein number from positive to negative in the FWI zone and applying the general linear Markstein correlation, leading to a decrease of the flame consumption speed. In addition, the quenching distance evaluated from S_L has been found to be almost equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature.     

多孔介质中甲烷预混气火焰传播特性实验研究

本文对甲烷预混气在多孔介质中的火焰传播特性进行了实验研究,在开口竖直管中充填多孔介质,通过改变预混气氧含量使火焰在不同多孔介质中传播并测量火焰传播速度。预混气中氧含量最高达到29％。实验结果表明:多孔介质中甲烷可燃预混气火焰传播速度大于其层流火焰传播速度,可达到5倍以上(当量比的甲烷-空气预混气);多孔介质当量孔直径越大,或预混气层流火焰速度越高,则预混气火焰传播速度越高;多孔介质中可燃混气的火焰传播界限变小,当量孔直径大的多孔介质其界限值较大。实验结果与Babkin提出的多孔介质中的火焰传播机理相符。     

Thermochemistry and kinetics of the 2-butanone-4-yl CH3C(=O)CH2CH2• + O2 reaction system

Thermochemistry and kinetic pathways on the 2-butanone-4-yl (CH₃C(=O)CH₂CH₂•) + O₂ reaction system are determined. Standard enthalpies, entropies, and heat capacities are evaluated using the G3MP2B3, G3, G3MP3, CBS-QB3 ab initio methods, and the B3LYP/6-311g(d,p) density functional calculation method. The CH₃C(=O)CH₂CH₂• radical + O₂ association reaction forms a chemically activated peroxy radical with 35 kcal mol⁻¹ excess of energy. The chemically activated adduct can undergo RO−O bond dissociation, rearrangement via intramolecular hydrogen transfer reactions to form hydroperoxide-alkyl radicals, or eliminate HO₂ and OH. The hydroperoxide-alkyl radical intermediates can undergo further reactions forming ketones, cyclic ethers, OH radicals, ketene, formaldehyde, or oxiranes. A relatively new path showing a low barrier and resulting in reactive product sets involves peroxy radical attack on a carbonyl carbon atom in a cyclic transition state structure. It is shown to be important in ketones when the cyclic transition state has five or more central atoms.     

Oxidation of the 1-naphthyl radical C10H7• with oxygen: Thermochemistry,kinetics, and possible reaction pathways

The reaction of the 1-naphthyl radical C₁₀H₇• (A2•) with molecular (³O₂) and atomic oxygen, as part of the oxidation reactions of naphthalene, is examined using ab-initio and DFT quantum chemistry calculations. The study focuses on pathways that produce the intermediate final products CO, phenyl and C₂H₂, which may constitute a repetitive reaction sequence for the successive diminution of six-membered rings also in larger polycyclic aromatic hydrocarbons. The primary attack of ³O₂ on the 1-naphthyl radical leads to a peroxy radical C₁₀H₇OO• (A2OO•), which undergoes further propagation and/or chain branching reactions. The thermochemistry of intermediates and transition state structures is investigated as well as the identification of all plausible reaction pathways for the A2• + O₂ / A2• + O systems. Structures and enthalpies of formation for the involved species are reported along with transition state barriers and reaction pathways. Standard enthalpies of formation are calculated using ab initio (CBS-QB3) and DFT calculations (B3LYP, M06, APFD). The reaction of A2• with ³O₂ opens six main consecutive reaction channels with new ones not currently considered in oxidation mechanisms. The reaction paths comprise important exothermic chain branching reactions and the formation of unsaturated oxygenated hydrocarbon intermediates. The primary attack of ³O₂ at the A2• radical has a well depth of some 50 kcal mol⁻¹ while the six consecutive channels exhibit energy barriers below the energy of the A2• radical. The kinetic parameters of each path are determined using chemical activation analysis based on the canonical transition state theory calculations. The investigated reactions could serve as part of a comprehensive mechanism for the oxidation of naphthalene. The principal result from this study is that the consecutive reactions of the A2• radical, viz. the channels conducting to a phenyl radical C₆H₅•, CO₂, CO (which oxidized to CO₂) and C₂H₂ are by orders of magnitude faster than the activation of naphthalene by oxygen (A2 + O₂ → A2• + HO₂).