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The combustion of metal fuels as energy carriers in a closed-cycle carbon-free process is a promising approach for reducing CO2 emissions in the energy sector. For a possible large-scale implementation, the influence of process conditions on particle properties and vice versa has to be well understood. In this study, the influence of different fuel–air equivalence ratios on particle morphology, size and degree of oxidation in an iron–air model burner is investigated by means of small- and wide-angle X-ray scattering, laser diffraction analysis and electron microscopy. The results show a decrease in median particle size and an increase in the degree of oxidation for leaner combustion conditions. The difference of 1.94 μm in median particle size between lean and rich conditions is twentyfold greater than the expected amount and can be connected to an increased intensity of microexplosions and nanoparticle formation for oxygen-rich atmospheres. Furthermore, the influence of the process conditions on the fuel usage efficiency is investigated, yielding efficiencies of up to 0.93. Furthermore, by choosing a suitable particle size range of 1 to 10 μm, the amount of residual iron content can be minimized. The results emphasize that particle size plays a key role in optimizing this process for the future.

In this work, the engineering performance of an innovative furnace concept developed for continuous drying and curing of paint-coated metal sheets (coil coating process) is investigated through advanced modeling and numerical simulation techniques. Unlike the traditional and wide-spread coil coating furnaces – which operate according to the so-called convective air-drying technology –, the present furnace concept relies on infrared radiative heating to drive solvent evaporation and curing reactions. Radiative heat is provided by the operation of radiant porous burners which are fed with evaporated solvents. The current furnace concept consists of two main chambers (the radiant burner section and the curing oven section) with different gas compositions (atmospheres) that are separated by a semi-transparent window. The window allows energy transfer and prevents gas mixing between the two sections. To utilize the solvent-loaded atmosphere available in the curing oven section as fuel – and to prevent the development of explosive conditions therein –, a novel inertization concept shielding the curing oven section from the external environment is considered. The current furnace concept aims at improving process intensification and promoting energy efficiency. For the current furnace concept, numerical simulation results support a suitable and competitive performance for drying the applied coatings in comparison with the traditional approach. Simultaneously, a safe operation is predicted, without (i) solvent leakage from the furnace and (ii) oxygen entrainment from the surrounding ambient into the furnace. These conditions are satisfied demonstrating a safe operation and a complete evaporation of solvents from applied liquid film coatings.

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 Kas and a considerable decrease of the total stretch rate Katot for the SWQ flame. The flame consumption speed SL decreases with decreasing normal stretch due to curvature Kac while approaching the cold wall, but it increases with decreasing Kac for the FF case, leading to an inversion of the Markstein number Matot based on Katot 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 SL has been found to be almost equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature.

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.

In this paper the evolution of morphological and nanostructural properties of soot primary particles during low-temperature oxidation with oxygen is presented. Soot primary particles and aggregates from three soot samples with varying morphological and nanostructural initial properties were investigated by high-resolution transmission electron microscopy (HRTEM) at consecutive steps of oxidation. Soot samples were deposited on a TEM grid and then oxidized repetitively. After each oxidation step labelled identical aggregates were scanned and reanalyzed by HRTEM. Size distributions of primary particles and their nanostructure was tracked using an image analysis algorithm. The oxidation conditions (temperature 850 K, O2 concentration 5% by volume) correspond to those of the real application, e.g. for regeneration of particle filters. The tracking of different aggregates revealed that the morphological changes of the primary particles are determined by the predominant oxidation mode, which is governed by the nanostructure of the primary particles. Extended graphene layers favor internal oxidation, while bent, short and reactive layers favor both, external and internal oxidation. In surface oxidation mode, primary particles do not shrink homogeneously, but rather form a blackberry-like cluster of smallest nucleated particles. While a partly expansion of graphene layers is detected in the early stages of oxidation, the number density of graphene layers gradually decreases with increasing oxidation time. Additionally, graphene layers are oxidized by fragmentation until final degradation. Tracking of selected primary particles at different positions within the aggregate brought about that each primary particle is subjected to its own oxidation history. Therefore, each observed and analyzed primary particle possibly exhibits a different oxidation stage at the same oxidation time.

The numerical investigation of quenching distances in laminar flows is mainly concerned with two setups: head-on quenching (HOQ) and side-wall quenching (SWQ). While most of the numerical work has been conducted for HOQ with good agreement between simulation and experiment, far less analysis has been done for SWQ. Most of the SWQ simulations used simplified diffusion models or reduced chemistry and achieved reasonable agreement with experiments. However, it has been found that quenching distances for the SWQ setup differ from experimental results if detailed diffusion models and chemical reaction mechanisms are employed. Side-wall quenching is investigated numerically in this work with steady-state 2D and 3D simulations of an experimental flame setup. The simulations fully resolve the flame and employ detailed reaction mechanisms as well as molecular diffusion models. The goal is to provide data for the sensitivity of numerical quenching distances to different parameters. Quenching distances are determined based on different markers: chemiluminescent species, temperature and OH iso-surface. The quenching distances and heat fluxes at the cold wall from simulations and measurements agree well qualitatively. However, quenching distances from the simulations are lower than those from the experiments by a constant factor, which is the same for both methane and propane flames and also for a wide range of equivalence ratios and different markers. A systematic study of different influencing factors is performed: Changing the reaction mechanism in the simulation has little impact on the quenching distance, which has been tested with over 20 different reaction mechanisms. Detailed diffusion models like the mixture-averaged diffusion model and multi-component diffusion model with and without Soret effect yield the same quenching distances. By assuming a unity Lewis number, however, quenching distances increase significantly and have better agreement with measurements. This was validated by two different numerical codes (OpenFOAM and FASTEST) and also by 1D head-on quenching simulations (HOQ). Superimposing a fluctuation on the inlet velocity in the simulation also increases the quenching distance on average compared to the reference steady-state case. The inlet velocity profile, temperature boundary condition of the rod and radiation have a negligible effect. Finally, three dimensional simulations are necessary in order to obtain the correct velocity field in the SWQ computations. This however has only a negligible effect on quenching distances.

Large eddy simulations (LES) have been carried out to calculate turbulent flame propagation in a fan-stirred combustion bomb and a Bunsen-type burner. Objective of the work is to reveal the main mechanism of increased flame wrinkling due to elevated pressure and to assess the ability of the turbulent flame-speed closure (TFC-class) combustion model to reproduce the enhancement of flame wrinkling or burning rate at elevated pressures. The simulations have been performed for a premixed methane/air mixture at equivalence ratio 0.9 and the pressure has been varied from 1 bar to 5 bar. The turbulent kinetic energy is found to increase with pressure in the high frequency range, indicating reinforced small-scale turbulent fluctuations at elevated pressure. The reason is attributed to the increased turbulent Reynolds number with pressure, which shifts the turbulent energy spectra to the higher wave number range. A reinforced flame wrinkling and an increased total burning rate are obtained at elevated pressure, which is in accordance with results from previous high-pressure combustion experiments. In addition, applying the same method to a quiescent flow in the bomb vessel reveals a decrease of the overall burning rate with pressure, which agrees with the behaviour of laminar flame speed at elevated pressures. Therefore, the beneficial effect of increased burning rate or flame wrinkling at elevated pressure can be explained by the enhanced small-scale turbulent fluctuations along with formation of small-scale vortices and flame structures, which over-compensate the reduced local laminar burning velocity at high pressures. The calculated amplification rates of flame wrinkling factor at increased pressures show a reasonable agreement with measured data for both fan-stirred bomb and Bunsen flame configurations, without using any additional adjusting parameters for considering the pressure effect. The results justify the applicability of the current TFC-LES method for high-pressure combustion processes.

Motivated by the recent success in finding coherent structures in turbulent flows and describing their noise emission using the Resolvent Analysis (RA), the method is applied to a turbulent jet flame at Re = 15 , 000 and a corresponding non-reacting flow to investigate their axisymmetric dynamics. The RA, which in this study assumes a passive flame and neglects compressibility effects, is based on the governing equations linearized around the temporal mean state. It is validated against Spectral Proper Orthogonal Decomposition (SPOD) obtained from time-resolved snapshots. Both the temporal mean state and the snapshots are obtained by Large Eddy Simulation (LES). The SPOD reveals that an axisymmetric, convective Kelvin–Helmholtz (KH) instability is the dominant hydrodynamic mechanism in the jet flame within a narrow frequency band and incorporates up to 40 % of the turbulent kinetic energy. Results show that the RA is capable of reproducing the mode shapes seen in the SPOD. The RA furthermore allows to address the origin of these hydrodynamic structures: While the corresponding KH mode in a non-reacting turbulent jet flow is most sensitive to perturbations in the nozzle boundary layer, the same dominant mode in the turbulent Bunsen flame is most receptive to perturbations in the region between the nozzle edge and the annular pilot burner. The results suggest that the strong density gradients in this region initiate perturbations in the baroclinic torque, which are feeding the KH mode. Finally, a linear stability analysis proves that the high sensitivity of the KH structure is due to resonance with stable linear eigenmodes, which explains its high energy content. By applying the RA to a turbulent reacting flow, this study opens up a new pathway in analyzing the role of hydrodynamic structures in reacting flows.

The performance of horizontal dilution tubes is investigated by Reynolds-averaged Navier–Stokes and large-eddy simulations. The flame gas enters the dilution tube through a pinhole. The orifice flow and the dilution process inside the tube are studied. The volume flow through the orifice is shown to be proportional to the square root of the pressure drop. The discharge coefficient is 0.9 ± 0.3 in the cold air (calibration) case and drops to 0.35 under hot (flame) conditions. The resulting dilution ratio is roughly a factor of five below typical literature data. The gas sample remains in the wall boundary layer and the mixing process is not complete at the end of the dilution tube. Turbulence decays rapidly behind the tube inlet, which shifts the flow into the laminar to turbulent transition regime. Turbulence increases significantly in the outlet section which has much smaller pipe cross-sections. Despite its relatively low Reynolds number, the outlet flow to the particle sizer (or to the gas analyzer) is clearly turbulent, and interactions with the wall are probable. The results are in agreement with previous findings from laminar jets in cross-flow. Guidelines for optimization of the sampling conditions are suggested.

Numerical simulations have been conducted for a novel double-concentric swirl burner, which is specifically designed for combustion of sulfur with a high power density. The burner serves as a major component of an enclosed conversion cycle, which uses elemental sulfur as a carbon-free chemical energy carrier for storing solar energy. The focus of the work is to assess operability of the burner and NO

This study focuses on the effects of continuous volumetric discharge of sinusoidal plasma actuator at 20 kHz coupled directly with methane-air premixed flame in the near field of the injector exit. A plasma actuator composed of a needle-type electrode placed at the center of the nozzle, connected with high-voltage, while the nozzle was acted as a grounded electrode with different input electrical power values was designed to enhance lean blowout performance in a swirl model combustor. The ionic wind induced by the electrical body force given by the flow ionization leads to velocity disturbance and subsequently affects the flame. To investigate the possible mechanism of the combustion control by the plasma through the aerodynamic effect high speed flow visualization was analyzed under quiescent conditions. Flow visualizations showed that the plasma discharge affects the flow dynamics near the burner exit. It was observed that by increasing the electrical power used for the actuation a recirculation zone is formed in the non-reacting flow field. Furthermore, comparative experiments between conventional and plasma-assisted combustion were carried out to analyze the combustion enhancement in terms of lean blowout performance. The effect of the input electric power of the plasma actuator was studied, and it was seen that at coupled plasma powers corresponding to less than 1% of the thermal output power, there is a significant improvement in the blow-out limit.

This work presents a numerical study of the initial turbulent flow characteristics prior to ignition in a fan-stirred combustion vessel. The moving mesh methodology has been applied to account for ...

As an alternative to usual combustion processes with air as oxidant, oxy-fuel or oxygen-enhanced processes are attractive in high temperature thermal or thermochemical processes, novel power plant concepts or in gasification processes. Especially for production of basic chemicals or synthetic fuels, high-purity synthetic gas without nitrogen dilution is required for further industrial processing. In case of oxy-fuel combustion, the absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. Because of the increased reactivity, most combustors are designed in non-premixed configuration. In the present work, the extinction limits of non-premixed CH4/N2/O2 flames were numerically calculated using the CHEMKIN Pro opposed flow code together with the GRI 3.0 mechanism. The inlet compositions were varied from high nitrogen dilution to pure oxy-fuel conditions. Additionally, the stoichiometric mixture fraction was varied by shifting the (inert) N2-fraction stepwise from the oxidizer to the fuel side. Therewith, the equilibrium temperature of the mixture is constant for a fixed N2-dilution. The results show, that as expected, the extinction occurs at higher strain rates with increasing the flame temperatures by reducing the N2-diluation. Additionally, the extinction limits strongly increase for a fixed N2-dilution with increasing the stoichiometric mixture fraction. The shift of the N2 from the oxidizer to the fuel side leads to a movement of the stagnation plane towards the fuel side and hence, a significant change of the temperature field and the flame structure. Thus, the radical chain branching occurs at higher temperature level, which strengthen the flame resistance against strain. The increase of the stoichiometric mixture leads to a shift of the flame towards the fuel side and crosses at high values the stagnation plane.

To comply with future emission regulations for internal combustion engines, system-related cold-start conditions in short-distance traffic constitute a particular challenge. Under these conditions, pollutant emissions are seriously increased due to internal engine effects and unfavorable operating conditions of the exhaust aftertreatment systems. As a secondary effect, the composition of the exhaust gases has a considerable influence on the deposition of aerosols via different deposition mechanisms and on fouling processes of exhaust gas-carrying components. Also, the performance of exhaust gas aftertreatment systems may be affected disadvantageously. In this study, the exhaust gas and deposit composition of a turbocharged three-cylinder gasoline engine is examined in-situ upstream of the catalytic converter at ambient and engine starting temperatures of −22 °C to 23 °C using a Fourier-transform infrared spectrometer and a particle spectrometer. For the cold start investigation, a modern gasoline engine with series engine periphery is used. In particular, the investigation of the behavior of deposits in the exhaust system of gasoline engines during cold start under dynamic driving conditions represents an extraordinary challenge due to an average lower soot concentration in the exhaust gas compared to diesel engines and so far, has not been examined in this form. A novel sampling method allows ex-situ analysis of formed deposits during a single driving cycle. Both, particle number concentration and the deposition rate are higher in the testing procedure of Real Driving Emissions (RDE) than in the inner-city part of the Worldwide harmonized Light vehicles Test Cycle (WLTC). In addition, reduced ambient temperatures increase the amount of deposits, which consist predominantly of soot and to a minor fraction of volatile compounds. Although the primary particle size distributions of the deposited soot particles do not change when boundary conditions change, the degree of graphitization within the particles increases with increasing exhaust gas temperature.

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston.

The main objective of this research is to assess an innovative, low nitrogen oxides emission combustor concept, which has the potential to achieve the long term European emissions goals for aircraft engines. Lean lifted spray flames and their very low nitrogen oxides emissions are combined with an inclination of burners in annular combustor leading to a more compact combustor with superior stability range. The presented combustor concept was developed in the frame of the European research project CHAIRLIFT (Compact Helical Arranged combustoRs with lean LIFTed flames). CHAIRLIFT combustor concept is based on “low swirl” lean lifted spray flames, which features a high degree of premixing and consequently significantly reduced nitrogen oxides emissions and flashback risk compared to conventional swirl stabilized flames. In the CHAIRLIFT combustor concept, the lifted flames are combined with Short Helical Combustors arrangement to attain stable combustion by tilting the axis of the flames relative to the axis of the turbine to enhance the interaction of adjacent flames in a circumferential direction. A series of experimental tests were conducted at a multi-burner array test rig consisting of up to five modular burners at different burner inclination angles (0 and 45), equivalence ratios, and relative air pressure drop at ambient conditions. For all investigated configurations, a remarkable high lean blow out for nonpiloted burners (fLBO = 0.29 - 0.37), was measured. The multiburner configurations were observed having a superior stability range in contrast to the typical decrease in stability from single to high swirl multi-burner. The unwanted flow deflection of highly swirled flames in Short Helical Combustors arrangement, could be avoided with the investigated low swirl lifted flames. Moreover, the flame chemiluminescence (OH) measurements were used to provide a qualitative characterization of the flame topology. Complementary numerical investigations were carried out using different numbers of burners to evaluate the effect of boundary conditions

This work presents a numerical study on the effect of flame-wall interaction (FWI) from the viewpoint of flame dynamics. For that purpose, direct numerical simulations (DNS) employing detailed calculations of reaction rates and transport coefficients have been applied to a 2D premixed methane/air flame under atmospheric condition. Free flame (FF) and side-wall quenching (SWQ) configurations are realized by defining one lateral boundary as either a symmetry plane for the FF or a cold wall with fixed temperature at 20 oC for the SWQ case. Different components of flame stretch and Markstein number regarding tangential, normal (due to curvature) and total stretch, Kas, Kacand Katot= Kas+ Kac, as well as their correlations with respect to the local flame consumption speed SL have been evaluated. It has been shown that the FWI zone is dominated by negative flame stretch. In addition, SL decreases with decreasing normal stretch due to curvature Kacwhile approaching the cold wall. However, SLincreases with decreasing Kac while approaching the symmetry boundary for the free flame case, leading to an inversion of the Markstein number Matot based on Katot from positive in the free flame case to negative in the SWQ case. The quenching distance evaluated based on wall-normal profiles of SLhas been found to be approximately equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature. The flame speed has been confirmed to scale quasi-linearly with the stretch in the FWI zone. The results reveal a distinct correlation during transition between FWI and FF regarding flame dynamics, which brings a new perspective for modeling FWI phenomena by means of flame stretch and Markstein number. To do this, the quenching effect of the wall may be reproduced by a reversed sign of the Markstein number from positive to negative in the FWI zone and by applying the general linear Markstein correlation (SL/SL,0= 1− Ma ·Ka), leading to a decrease of the flame speed or the reaction rate in the near-wall region.

To meet future climate goals, the efficiency of combustion devices has to be increased. This requires a better understanding of the underlying physics. The simulation of turbulent flames is a challenge because of the multi-scale nature of combustion processes: relevant length scales span over five orders of magnitude and time scales over more than ten. Because of this, the direct numerical simulation (DNS) of turbulent flames is only possible on large supercomputers. A DNS solver for chemically reacting flows implemented in the open-source framework OpenFOAM is presented. The thermo-chemical library Cantera is used to compute detailed transport coefficients based on kinetic gas theory. The multi-scale nature of time scales, which are much lower for the combustion chemistry than for the flow, are bridged by an operator splitting approach, which employs the open-source solver Sundials to integrate chemical reaction rates. Because the direct simulation of turbulent flames has to be performed on supercomputers, special care has been taken to improve the computational performance. A tool was developed which generates highly optimized C++ source code for the computation of chemical reaction rates. Additionally, a load balancing approach specifically made for the computation of chemical reaction rates is employed. In total, these optimizations can reduce total simulation times by up to 70 %. The accuracy of the new solver is assessed from different canonical testcases: 2D and 3D Taylor-Green vortex simulations show that the solver can reach up to fourth order convergence rates and that results differ by less than 1 % when compared to spectral DNS codes. Molecular diffusion and chemical reaction rates are compared to solutions of 1D flames from Cantera, showing perfect agreement. The solver is used to simulate the Sydney/Sandia burner. The simulation is performed on one of Germany's largest supercomputer on 28 800 CPU cores, employing 150 million cells and a chemical reaction mechanism with 19 species and about 200 reactions. Comparison with experimental data shows excellent agreement for time averaged and RMS values.

Due to worldwide increasingly sharpened emission regulations, the development of Gasoline Direct Injection and Diesel Direct Injection engines not only aims at the reduction of the emission of nitrogen oxides but also at the reduction of particulate emissions. Regarding present regulations, both tasks can be achieved solely with the help of exhaust after treatment systems. For the reduction of the emission of particulates, Gasoline (GPF) and diesel Particulate Filters (DPF) offer a solution and their implementation is intensely promoted. Under optimal conditions particulates retained on particulate filters are continuously oxidized with the exhaust residual oxygen so that the particulate filter (PF) is regenerated possibly without any additional intervention into the engine operating parameters. The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at exhaust gas temperatures. The reaction rates of soot particles from internal combustion engines (ICE) often are discussed in terms of order/disorder on the particle nanoscale, the concentration and kind of functional groups on the particle surfaces, and the content of (mostly polycyclic aromatic) hydrocarbons in the soot. In this work the reactivity of different kinds of soot (soot from flames, soot from ICE, carbon black) under oxidation conditions representative for PF regeneration is investigated. Soot reactivity is determined in dynamic Temperature Programmed Oxidation (TPO) experiments and the soot primary particle morphology and nanostructure is investigated by High-Resolution Transmission Electron Microscopy (HRTEM). An image analysis method based on known methods from the literature and improving some infirmities is used to evaluate morphology and nanostructural characteristics. From this, primary particle size distributions, length and separation distance distributions as well as tortuosities of fringes within the primary particle structures are obtained. Further, UV–visible spectroscopy and Raman scattering and other diagnostic techniques are used to study the properties connected to the reactivity of soot and to corroborate the experimental findings. It is found that nanostructural characteristics predominantly affect reactivity. Oxidation rates are derived from TPO and interpreted on a molecular basis from quantum chemistry calculations revealing a replication/activation oxidation mechanism.

The article Quasi-DNS Dataset of a Piloted Flame with Inhomogeneous Inlet Conditions written by Thorsten Zirwes, Feichi Zhang, Peter Habisreuther, Maximilian Hansinger, Henning Bockhorn, Michael Pfitzner, Dimosthenis Trimis, was originally published Online First without Open Access.

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O2 from easily measurable optical properties. First, ex-situ experiments utilizing thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) serve to evaluate the kinetics of soot oxidation with O2 and relate reactivity to particle morphology and nanostructure. Second, ultraviolet–visible (UV-VIS) absorption spectra provide wavelength-dependent absorption cross sections and refractive-index functions E ( m ~ , λ ) . From these, optical band gap energies, EOG, and coefficients ξ∗ for single parameter functions describing the wavelength-dependency of E ( m ~ , λ ) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E ( m ~ , λ i ) / E ( m ~ , λ j ) at three excitation wavelengths and primary particle size distributions are acquired. The ex-situ experiments show that the size of the graphene layers predominantly determines soot reactivity against oxidation. Graphene layer size and, therefore, soot reactivity are reflected in the UV-VIS absorption spectra and E ( m ~ , λ ) , EOG, and ξ∗, respectively. Similarly, scattering-corrected ratios E ( m ~ , λ i ) / E ( m ~ , λ j ) from TR-LII also reflect graphene layer size and, hence, soot reactivity. The established strong correlations between the optical properties, nanostructural characteristics and reactivity against oxidation make UV-VIS spectroscopy as well as TR-LII useful fast in-situ diagnostic methods for soot reactivity.

Identifying combustion regimes in terms of premixed and non-premixed characteristics is an important task for understanding combustion phenomena and the structure of flames. A quasi-DNS database of the compositionally inhomogeneous partially premixed Sydney/Sandia flame in configuration FJ-5GP-Lr75-57 is used to directly compare different types of flame regime markers from literature. In the simulation of the flame, detailed chemistry and diffusion models are utilized and no turbulence and combustion models are used as the flame front and flow are fully resolved near the nozzle. This allows evaluating the regime markers as a post-processing step without modeling assumptions and directly comparing regime markers based on gradient alignment, drift term analysis and gradient free regime identification. The goal is not to find the correct regime marker, which might be impossible due to the different set of assumptions of every marker and the generally vague definition of the partially premixed regime itself, but to compare their behavior when applied to a resolved turbulent flame with partially premixed characteristics.

OpenFOAM is one of the most popular open source tools for CFD simulations of engineering applications. It is therefore also often used on supercomputers to perform large eddy simulations or even direct numerical simulations of complex cases. In this work, general guidelines for improving OpenFOAM’s performance on HPC clusters are given. A comparison of the serial performance for different compilers shows that the Intel compiler generally generates the fastest executables for different standard applications. More aggressive compiler optimization options beyond O3 yield performance increases of about 5 % for simple cases and can lead to improvements of up to 25 % for reactive flow cases. Link-time optimization does not lead to a performance gain. The parallel scaling behavior of reactive flow solvers shows an optimum at 5000 cells per MPI rank in the tested cases, where caching effects counterbalance communication overhead, leading to super linear scaling. In addition, two self-developed means of improving performance are presented: the first one targets OpenFOAM’s most accurate discretization scheme “cubic”. In this scheme, some polynomials are unnecessarily reevaluated during the simulation. A simple change in the code can reuse the results and achieve performance gains of about 5 %. Secondly, the performance of reactive flow solvers is investigated with Score-P/Vampir and load imbalances due to the computation of the chemical reaction rates are identified. A dynamic-adaptive load balancing approach has been implemented for OpenFOAM’s reacting flow solvers which can decrease computation times by 40 % and increases the utilization of the HPC hardware. This load balancing approach utilizes the special feature of the reaction rate computation, that no information of neighboring cells are required, allowing to implement the load balancing efficiently.

International audience; The pyrolysis of chlorobenzene under dilute atmosphere and quasi-atmospheric pressure was studied at temperatures from 800 to 1150 K using a fused silica jet stirred reactor (JSR) and from 800 to 1250 K in an alumina tubular reactor. Chlorobenzene was chosen as a surrogate to model the thermal decomposition of polychlorinated biphenyls (PCBs). In the jet stirred reactor, a maximum chlorobenzene conversion of 48.5 % was observed at a residence time of 2 s, a temperature of 1150 K and an inlet mole fraction of chlorobenzene of 0.005. The following species were quantified: benzene (the major product), HCl, methane, ethylene, acetylene, biphenyl, 1-, 2-and 3-chlorobiphenyl, biphenylene and six isomers of dichlorobiphenyls. In the tubular reactor, a maximum chlorobenzene conversion of 95% was observed at a temperature of 1250 K under the same conditions as in the jet stirred reactor. The same reaction products were detected but with a larger formation of acetylene and methane and a smaller production of chlorinated and bicyclic compounds. The effect of two H-atom donors, methane and hydrogen, has been investigated in the JSR. Hydrogen addition has a strong inhibiting effect on the formation of chlorinated and bicyclic products. A new detailed kinetic model was developed and gave a good prediction of the global reactivity, the formation of most major products, and the effect of the addition of hydrogen and methane. Flow rate and sensitivity analyses have been made to explain the effect of H-atom donors.

This work assesses the effect of using different closure concepts for the spatially filtered mean reaction rate on the resolved spectral response of turbulent heat release in large eddy sim...

This article presents the experimental results of a novel Power-to-Gas (PtG) concept combining a pressurized high-temperature steam electrolysis (SOEC) and a CO2-methanation module in stand-alone and thermally integrated operation. For the electrolyser, steam conversion and energy demands at pressures up to 15 bar were examined. In terms of the methanation module, cooling performance, steam production and product gas quality were of main interest. Additionally, temperature profiles inside the fixed beds were gathered by a multipoint thermocouple at pressures up to 30 bar and load modulations from 20 to 100%. With less than 2 vol% H2 and over 97 vol% CH4 in the finally produced synthetic natural gas (SNG), it can be directly injected into the existing German natural gas grid without further gas cleaning and without capacity limitations. The achieved overall PtG efficiency of 76% is significantly higher than state of the art plants and has the potential to reach 80% in industrial scale.

The complete evaporation process inside an asymmetrically heated porous channel, under steady-state condition, has been numerically investigated in this article, based on the modified enthalpy formulation along with the assumption of Local Thermal Non-Equilibrium (LTNE). The governing equations have been discretised using finite volume method on both staggered and non-staggered grid arrangements and solved iteratively in a SIMPLE-like manner. All simulations have been carried out by applying the proposed smoothing algorithm for the effective diffusion coefficient in order to avoid the non-physical jump in the predicted temperature distributions. The performance of staggered and non-staggered grid layouts have been compared only on orthogonal coordinates for various parameters and the results show that the accuracy and effectiveness of the non-staggered and staggered grid layouts are identical. Different models for the partitioning of the wall heat flux have no influence on the total evaporated volume fraction. Effects of various parameters on the temperature and liquid saturation distributions have been carefully investigated, which clearly indicate that imposed heat flux, Reynolds number, Darcy number and thermal conductivity of the solid phase strongly influence the initiation and termination of phase change process, whereas the porosity has only a minor impact. Therefore, operating conditions and properties of porous media are required to be properly designed in order to achieve the desired objective. In addition, the results obtained using the modified h - and H -formulations have been compared and excellent agreement have been observed. It has been found that the modified h -formulation requires considerably less computation time as compared to that for the H -formulation and hence the method is strongly recommended for the future use. Nevertheless, it should now be extended in order to accommodate the multi-dimensional complex geometries that require the employment of curvilinear coordinates using non-staggered grid layout.

This work deals with the coupling between high temperature steam electrolysis using solid oxide cells (SOEC) and carbon dioxide methanation to produce a synthetic natural gas (SNG) directly injecta...

SSCI-VIDE+ING+FRM:NOG:YSC; International audience; In this study, a robust and efficient decentralized fuel processor based on the direct autothermal reforming (ATR) of biogas with a nominal production rate of 50 Nm3/h of hydrogen and a plant efficiency of about 65% was developed and tested. The ATR unit is composed of a structured catalyst support for the biogas reforming close coupled to a catalytic wall-flow filter to retain eventual soot particles.The performance of the conventional random foam and homogeneous lattice supports structures for the production of hydrogen from the ATR reaction was investigated. 15–0.05 wt%-Ni-Rh/MgAl2O4-SiSiC structured catalyst and LiFeO2-SiC monolith were selected for the conversion of biogas to hydrogen and for the syngas post-treatment process, respectively. For all the experiments, a model synthetic biogas was used and the catalytic activities were evaluated in three different experimental facilities: lab bench, pilot test rig and demonstration plant. High methane conversions (>95%) and hydrogen yields (>1.8) reached in the lab bench were also achieved in the pilot and demonstration plant operating at different GHSV.Results of duration test using a foam coupled to the filter has demonstrated that the pre-commercial processor is reliable while offering a satisfactory reproducibility and negligible pressure drop. A thermodynamic equilibrium and a cold gas efficiency of 90% were reached for an inlet temperature of 500 °C, O/C: 1.1 and S/C: 2.0, as predicted with the Aspen simulation.

This study investigates the particle size distribution's effect on the microstructure and effective thermal conductivity (ETC) of alumina castables. The ETC was measured by the transient plane source method and predicted numerically based on a two-scale model describing the structure on a fine and coarse scale. The prediction considered particle and pore size distributions, porosity (around 20%) and grain morphology. The microstructure was investigated by scanning electron microscopy. For a constant fines content, increasing the coarse grain fraction while decreasing the medium fraction enhanced sintering of the matrix. Small pores (≤250 nm) increased the sintering activity. The densest castable contained the most small pores. The particles’ and pores’ contributions to the sintering activity led to intensified microcracking and a decreased ETC. The numerical model did not consider constituents ≤500 nm like the small pores and microcracks and the calculated ETC values consequently deviated from the measured values.

Industrial furnaces are analyzed continuously to identify optimization potentials. Process data from the glass industry suggest that the size of a plant has an important influence on the specific energy consumption. This leads to incorrect results when using key performance indicators (KPIs) for comparison purposes. Therefore, the evaluation of innovative industrial technologies, as the use of microwaves, is often affected by incomplete assumptions, since economies of scale are often disregarded. A thermodynamic model for energy consumption was developed for analysing the scale dependencies on the specific energy consumption. It contains a correction factor for KPIs. This factor will be compared and validated with industrial process data from literature and databases as well as experimental data for the microwave process. The paper shows the impact of existing scale dependencies and their importance for a comprehensive technology comparison.

As an alternative to usual combustion processes with air as oxidant, oxy-fuel processes are attractive in high temperature thermal or thermochemical processes, novel power plant concepts or in gasification processes. Especially for production of basic chemicals or synthetic fuels, high-purity synthetic gas without nitrogen dilution is required for further industrial processing. In case of oxy-fuel combustion, the absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. In the present work, freely propagating methane-oxygen-nitrogen flames were numerically calculated using a 1D model from lean to rich conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations were performed for equivalence ratios of 0.5 The SAFT phenomenon was identified based on temperature and species profiles. Additionally, a detailed analysis of convection, diffusion and chemical source of both, temperature and species, were performed in order to investigate the role of physical transport processes on SAFT. The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios Φ > 0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in slightly rich conditions (1.0 2.0). A first maximum of temperature difference is observed at an equivalence ratio of Φ = 1.5. Here, an exaggeration of approximately 120–180 K, depending on the applied reaction mechanism, was found for standard conditions. The first maximum at Φ = 1.5 correlates with the maximum concentration of the H-radical, which plays a key role in the first SAFT regime. A minimum temperature difference of 50 K was identified at an equivalence ratio of Φ = 2.1. While increasing the equivalence ratio further, the maximum flame temperature exceeded the equilibrium up to almost 400 K at Φ = 3.0 in the second SAFT regime. An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT in the second regime.