arXiv subjects

Arne Scholtissek

Publications and source records attributed to Arne Scholtissek.

At least 19 recordsLinked to original sources

arXiv2025

Spray flamelet structures in a tubular counterflow configuration

In this work, spray flamelet structures subject to curvature are systematically studied, emphasizing the ways in which this quantity modifies the budgets of the corresponding flamelet equations and their stretch-induced extinction limit. More specifically, a theoretical extension of the tubular counterflow configuration is first proposed, which allows the injection of droplets from the inner cylinder. After appropriate mathematical descriptions for this new configuration in physical and composition space are introduced, several ethanol/air tubular counterflow flames are studied. It is found that increasing curvature leads to major modifications of the resulting flamelet structures, which is attributable to its influence on the evaporation profiles. Further, it is found that increasing curvature considerably reduces the stretch-induced extinction limit, which can be directly related to a corresponding reduction of the maximum mixture fraction within the flamelet. Finally, it is concluded that extinction in tubular counterflow spray flames occurs through a mechanism significantly different from what has been previously observed for gas flamelets.

physics.flu-dyn↗

arXiv2025

Solutions of the spray flamelet equations in a non-monotonic mixture fraction space

Solving the spray flamelet equations in composition space is very challenging, which is attributable to the fact that the maximum value of the mixture fraction, $Z_\mathrm{max}$, is a priori unknown in such flames. In this work, an analytical solution for this quantity is proposed, which allows its determination in spray flames subject to imposed quadratic evaporation profiles. It is then illustrated how the proposed approach allows to effectively cover the solution space of the spray flamelet equations. The employed strategy works very well for the considered cases and the generality of the evaporation profile definition provides flexibility for explorations of other parametric choices in the future.

physics.flu-dyn↗

arXiv2025

Unsteady solutions of the spray flamelet equations

Solutions of the spray flamelet equations reported in the literature during the last decade have been limited to very specific situations presenting steady evaporation profiles only. In contrast, intrinsically unsteady interactions between the liquid and gas phases have received little attention so far. In this work, the spray flamelet equations are closed by means of a Lagrangian description of the liquid phase in mixture fraction space, which allows solving them for unsteady situations. The resulting formulation is then employed to conduct parametric analyses of the effects of initial droplet radius and velocity variations on ethanol/air non-premixed gas flamelets perturbed by sprays generated with different droplet injection strategies. Special emphasis is given to the differences between continuous and discontinuous droplet injection. The results illustrate how the latter can considerably increase the temperature and stability of flamelet structures, provided the spray parameters are appropriately selected.

physics.flu-dyn↗

arXiv2025

Numerical Analysis of the Stability of Iron Dust Bunsen Flames

This article presents numerical simulations of the response of an iron dust Bunsen flame to particle seeding changes. A validated numerical model is used to study the impact of particle seeding fluctuations on flame stability. Simulations are conducted for the Bunsen setup in the right-side up and up-side down configuration. No significant differences in flame response are identified in flame stability between the right-side up and up-side down configurations. We find that the Bunsen flame is surprisingly robust to abrupt changes in particle loading. The sudden change in particle loading does not excite any intrinsic instabilities in the flame. Based on our results, the iron dust flames are robust to imposed fluctuations. We hypothesize that this is due to the lack of a feedback mechanism between the burned temperature and the heat release rate. This mechanism is present in conventional, chemistry-driven, gaseous flames. However, such a mechanism is absent in iron dust flames because the combustion of individual iron particles is limited by oxygen diffusion, which is insensitive to temperature.

physics.flu-dyn↗

arXiv2025

Analyzing Iron Dust Bunsen Flames using Numerical Simulations

This article presents numerical simulations of an iron dust Bunsen flame. The results are validated against experimental results. The burning velocity is extracted from the 3D simulation results, as in the experiments. The agreement of the burning velocity between the model and experiment is the best to date for iron dust flames. A comparison is performed between 3D and 1D simulations to improve our understanding of how the 3D Bunsen flame deviates from an ideal 1D flame. This comparison reveals that the co-flow mixes with the post-flame zone, increasing the oxygen concentration in the reaction layer, which increases the burning velocity. Moreover, the analysis also reveals that stretch and curvature affect the burning velocity. These results are valuable for the future development of experimental setups aimed at measuring the burning velocity.

physics.flu-dyn↗

arXiv2024

Critical nanoparticle formation in iron combustion: single particle experiments with in-situ multi-parameter diagnostics aided by multi-scale simulations

The formation of iron oxide nanoparticles (NPs) presents challenges such as efficiency losses and fine dust emissions in practical iron combustion systems, highlighting the need for deeper understanding of the formation mechanisms and thermochemical conditions. This study combines experiments and multi-scale simulations to analyze NP clouds generated by single iron particles burning in high-temperature oxidizing environments. The ambient gas conditions were provided by a laminar flat flame burner, with post-flame oxygen mole fractions varied between 20, 30, and 40 vol% at a constant temperature of ~1800K. High-speed in-situ diagnostics were used to measure particle size, NP initiation, NP cloud evolution, and microparticle surface temperature history. The experimental setup utilized three 10kHz imaging systems: one for two-color pyrometry and two for diffusive-backlight illumination (DBI), targeting particle size and NP measurements. The findings showcase the powerful capabilities of multi-physics diagnostics in quantifying NP initiation time and temperature, which depend on particle size and ambient oxygen concentration. CFD simulations revealed enhanced convection velocity driven by increased Stefan flow, which transported NPs toward parent iron particles under high-oxygen conditions. This delayed the detection of NP clouds, leading to higher microparticle temperatures at NP initiation. Molecular dynamics (MD) simulations uncovered FeO2(g) as a key NP precursor, forming when Fe atoms dissociate from the liquid phase. The initial temperature significantly influenced the resulting nanocluster composition, with Fe(II) dominating at higher temperatures and Fe(III) at lower temperatures. This integrated approach enhances understanding of NP formation in iron combustion, offering insights into the conditions affecting nanoparticle characteristics.

physics.flu-dyn↗

arXiv2024

Advantages of the adoption of a generalized flame displacement velocity as a central element of flamelet theory

In combustion theory, flames are usually described in terms of the dynamics of iso-surfaces of a specific scalar. The flame displacement speed is then introduced as a local variable quantifying the progression of these iso-surfaces relative to the flow field. While formally defined as a scalar, the physical meaning of this quantity allows relating it with a vector pointing along the normal direction of the scalar iso-surface. In this work, this one-dimensional concept is extended by the introduction of a generalized flame displacement velocity vector, which is associated with the dynamics of iso-surfaces of two generic scalars, $α$ and $β$. It is then shown how a new flamelet paradigm can be built around this velocity vector, which leads to a very compact and generic set of two-dimensional flamelet equations for thermochemical quantities and the conditioning scalar gradients, $g_α = \lvert \nabla α\rvert$ and $g_β = \lvert \nabla β\rvert$. The most important features of the developed framework are discussed in the context of partially-premixed flames, which provides significant insights into several aspects of the theory, including the nature of the different contributions to the flamelet equations for the conditioning scalar gradients and the fact that different flamelet coordinate systems (orthogonal and non-orthogonal) can be characterized by the same flame displacement velocity vector. This approach opens an entire spectrum of possibilities for the definition of new two-dimensional composition spaces, which represents a very promising basis for the development of new variants of flamelet theory.

physics.flu-dyn↗

arXiv2024

On the closure of curvature in 2D flamelet theory

So far, flamelet theory has treated curvature as an independent parameter requiring specific means for closure. In this work, it is shown how the adoption of a two-dimensional orthogonal composition space allows obtaining formal mathematical relations between the flame curvatures and the gradients of the conditioning scalars (also called flamelet coordinates). With these, both curvatures become a flame response to the underlying flow, which conveniently allows removing them from the corresponding set of flamelet equations. While the demonstration is performed in the context of partially premixed flames, the approach is general and applicable to any orthogonal coordinate system.

physics.flu-dyn↗

arXiv2024

Carrier-phase DNS of ignition and combustion of iron particles in a turbulent mixing layer

Three-dimensional CP-DNS of reacting iron particle dust clouds in a turbulent mixing layer are conducted. The simulation approach considers the Eulerian transport equations for the reacting gas phase and resolves all scales of turbulence, whereas the particle boundary layers are modelled employing the Lagrangian point-particle framework for the dispersed phase. The CP-DNS employs an existing sub-model for iron particle combustion that considers the oxidation of iron to FeO and that accounts for both diffusion- and kinetically-limited combustion. At first, the particle sub-model is validated against experimental results for single iron particle combustion considering various particle diameters and ambient oxygen concentrations. Subsequently, the CP-DNS approach is employed to predict iron particle cloud ignition and combustion in a turbulent mixing layer. The upper stream of the mixing layer is initialised with cold particles in air, while the lower stream consists of hot air flowing in the opposite direction. Simulation results show that turbulent mixing induces heating, ignition and combustion of the iron particles. Significant increases in gas temperature and oxygen consumption occur mainly in regions where clusters of iron particles are formed. Over the course of the oxidation, the particles are subjected to different rate-limiting processes. While initially particle oxidation is kinetically-limited it becomes diffusion-limited for higher particle temperatures and peak particle temperatures are observed near the fully-oxidised particle state. Comparing the present non-volatile iron dust flames to general trends in volatile-containing solid fuel flames, non-vanishing particles at late simulation times and a stronger limiting effect of the local oxygen concentration on particle conversion is found for the present iron dust flames in shear-driven turbulence.

physics.flu-dyn↗

arXiv2024

Can flamelet manifolds capture the interactions of thermo-diffusive instabilities and turbulence in lean hydrogen flames? -- An a-priori analysis

Flamelet-based methods are extensively used in modeling turbulent hydrocarbon flames. However, these models have yet to be established for (lean) premixed hydrogen flames. While flamelet models exist for laminar thermo-diffusively unstable hydrogen flames, for which consideration of curvature effects has resulted in improved model predictions, it is still unclear whether these models are directly applicable to turbulent hydrogen flames. Therefore, a detailed assessment of stretch effects on thermochemical states in a turbulent lean premixed hydrogen-air slot flame through finite-rate chemistry simulations is conducted. Strain and curvature are examined individually using a composition space model, revealing their distinct influences on thermochemical states. An a-priori analysis confirms that the previously developed tabulated manifolds fall short of capturing all turbulent flame phenomena, necessitating a novel manifold incorporating both strain and curvature variations. These results underscore the significance of these variations in developing manifold-based combustion models for turbulent lean hydrogen flames.

physics.flu-dyn↗

arXiv2023

Dynamic stabilization of a hydrogen premixed flame in a narrow channel

Combustion of hydrogen can help in reducing carbon-based emissions but it also poses unique challenges related to the high flame speed and Lewis number effects of the hydrogen flame. When operated with conventional burners, a hydrogen flame can flashback at higher volumetric flow rates than a methane flame due to the difference in stabilization mechanisms of the two fuels. Due to these differences, conventional burners cannot offer similar operational ranges for hydrogen than that for hydrocarbon flames. An exploration into the unique stabilization behaviour of hydrogen flames is required which could help in envisioning non-conventional burner concepts for keeping hydrogen flames stable. Stability conditions, which describe the kinematics of premixed flames with spatially and temporally changing flow parameters, are crucial for such an exploration. Stability conditions are usually hypothesized for stable flames, where a flame upon perturbation is assumed to return to its original position. Alternatively, in the case of flashback/blow-off, it refers to a flame moving upstream of the burner or being convected out of the domain. However, it is also of interest to understand how and why a flame could move to a new location when the velocity and strain fields are varying with time and space at the original and the new location. In this paper, we investigate the flame stabilization by 1) observing the hydrogen flame's upstream movement in a multi-slit configuration when a geometrical change is made, and 2) changing strain and velocity fields in a dynamic and periodic manner using numerical tools such that the unique behaviour of a hydrogen flame can be captured.

physics.flu-dyn↗

arXiv2023

Application of dense neural networks for manifold-based modeling of flame-wall interactions

Artifical neural networks (ANNs) are universal approximators capable of learning any correlation between arbitrary input data with corresponding outputs, which can also be exploited to represent a low-dimensional chemistry manifold in the field of combustion. In this work, a procedure is developed to simulate a premixed methane-air flame undergoing side-wall quenching utilizing an ANN chemistry manifold. In the investigated case, the flame characteristics are governed by two canonical problems: the adiabatic flame propagation in the core flow and the non-adiabatic flame-wall interaction governed by enthalpy losses to the wall. Similar to the tabulation of a Quenching Flamelet-Generated Manifold (QFM), the neural network is trained on a 1D head-on quenching flame database to learn the intrinsic chemistry manifold. The control parameters (i.e. the inputs) of the ANN are identified from thermo-chemical state variables by a sparse principal component analysis (PCA) without using prior knowledge about the flame physics. These input quantities are then transported in the coupled CFD solver and used for manifold access during simulation runtime. The chemical source terms are corrected at the manifold boundaries to ensure boundedness of the thermo-chemical state at all times. Finally, the ANN model is assessed by comparison to simulation results of the 2D side-wall quenching (SWQ) configuration with detailed chemistry and with a flamelet-based manifold (QFM).

physics.flu-dyn↗

arXiv2023

A comparison of mechanistic models for the combustion of iron microparticles and their application to polydisperse iron-air suspensions

Metals can serve as carbon-free energy carriers, e. g. in innovative metal-metal oxide cycles as proposed by Bergthorson (Prog. Energy Combust. Sci., 2018). Iron powder is a suitable candidate since it can be oxidized with air. Nevertheless, the combustion of iron powder in air is challenging especially with respect to flame stabilization which depends on the particle size distribution among other factors. Models for the prediction of reaction front speed in iron-air suspensions can contribute to overcoming this challenge. To this end, three different models for iron particle oxidation are integrated into a laminar flame solver for simulating reaction fronts. The scientific objective of this work is to elucidate the influence of polydispersity on the reaction front speed, which is still not satisfactorily understood. In a systematic approach, cases with successively increasing complexity are considered: From single particle combustion to iron-air suspensions prescribing binary particle size distributions (PSDs), generic PSDs, and a PSD measured for a real iron powder sample. The simulations show that, dependent on the PSD, particles undergo thermochemical conversion in a sequential manner according to their size and every particle fraction exhibits an individual combustion environment. The local environment can be leaner or richer than the overall iron-to-air ratio would suggest and can be very different from single particle experiments. The contribution of individual particle fractions to the overall reaction front speed depends on its ranking within the PSD. The study further demonstrates, that although the three particle models show good agreement for single particle combustion, they lead to very different reaction front speeds. This is due to the different ignition behavior predicted by the particle models, which is shown to strongly influence the reaction front characteristics.

physics.flu-dyn↗

arXiv2023

The potential of retrofitting existing coal power plants: a case study for operation with green iron

Storing electrical energy for long periods and transporting it over long distances is an essential task of the necessary transition to a CO$_2$-free energy economy. An oxidation-reduction cycle based on iron and its oxides represents a very promising technology in this regard. The present work assesses the potential of converting an existing modern coal-fired power plant to operation with iron. For this purpose, a systematic retrofit study is carried out, employing a model that balances all material and energy fluxes in a state-of-the-art coal-fired power plant. Particular attention is given to components of the burner system and the system$'$s heat exchanger. The analysis provides evidence that main components such as the steam generator and steam cycle can be reused with moderate modifications. Major modifications are related to the larger amounts of solids produced during iron combustion, for instance in the particle feeding and removal systems. Since the high particle densities and lower demand for auxiliary systems improve the heat transfer, the net efficiencies of iron operation can be one to two percentage points better than coal operation, depending on operating conditions. This new insight can significantly accelerate the introduction of this innovative technology by guiding future research and the development of the retrofit option.

eess.SY↗

arXiv2023

Techno-economic assessment of long-distance supply chains of energy carriers: Comparing hydrogen and iron for carbon-free electricity generation

Effective usage of renewable energy requires ways of storage and delivery to balance energy demand and availability divergences. Carbon-free chemical energy carriers are proposed solutions, converting clean electricity into stable media for storage and long-distance energy trade. Hydrogen (H$_2$) is the subject of significant investment and research. Metal fuels, such as iron (Fe), are promising solutions for a clean energy supply, but establishing an interconnected ecosystem still requires considerable research and development. A model is proposed to assess the supply chain of hydrogen and iron as clean, carbon-free energy carriers and then examines case studies of possible trade routes between the potential energy exporters Morocco, Saudi Arabia, and Australia and importers Germany and Japan. The work comprehends the assessment of economic (levelized cost of electricity - LCOE), energetic (thermodynamic efficiency) and environmental (CO$_2$ emissions) aspects, quantified by the comprehensive model accounting for the most critical processes in the supply chain. Sensitivity and uncertainty analyses identify the main drivers for energy costs. Iron is shown to be lower-cost and more efficient to transport in longer routes and for long-term storage, but potentially more expensive and less efficient than H$_2$ to produce and convert. Uncertainties related to the supply chain specifications and the sensitivity to the used variables indicate that the path to viable energy carriers fundamentally depends on efficient synthesis, conversion, storage, and transport. A break-even analysis demonstrated that clean energy carriers could be competitive with conventional energy carriers at low renewable energy prices, while carbon taxes might be needed to level the playing field. Thereby, green iron is an important potential energy carrier for long-distance trade in a globalized clean energy market.

physics.soc-ph↗

arXiv2023

Flamelet modeling of thermo-diffusively unstable hydrogen-air flames

In order to reduce CO2 emissions, hydrogen combustion has become increasingly relevant for technical applications. In this context, lean H2-air flames show promising features but, among other characteristics, they tend to exhibit thermo-diffusive instabilities. The formation of cellular structures associated with these instabilities leads to an increased flame surface area which further promotes the flame propagation speed, an important reference quantity for design, control, and safe operation of technical combustors. While many studies have addressed the physical phenomena of intrinsic flame instabilities in the past, there is also a demand to predict such flame characteristics with reduced-order models to allow computationally efficient simulations. In this work, a H2-air spherical expanding flame, which exhibits thermo-diffusive instabilities, is studied with flamelet-based modeling approaches both in a-priori and a-posteriori manner. A recently proposed Flamelet/Progress Variable (FPV) model, with a manifold based on unstretched planar flames, and a novel FPV approach, which takes into account a large curvature variation in the tabulated manifold, are compared to detailed chemistry (DC) calculations. First, both FPV approaches are assessed in terms of an a-priori test with the DC reference dataset. Thereafter, the a-posteriori assessment contains two parts: a linear stability analysis of perturbed planar flames and the simulation of the spherical expanding flame. Both FPV models are systematically analyzed considering global and local flame properties in comparison to the DC reference data. It is shown that the new FPV model, incorporating large curvature variations in the manifold, leads to improved predictions for the microstructure of the corrugated flame front and the formation of cellular structures, while global flame properties are reasonably well reproduced by both models.

physics.flu-dyn↗

arXiv2022

Numerical modeling of pulverized iron flames in a multidimensional hot counterflow burner

Pulverized iron flames stabilized in a multidimensional hot counterflow burner are simulated using a numerical model, which is extended from the state-of-the-art model developed by Hazenberg and van Oijen (PCI, 2021) considering unsteady effects. The results are compared to available experimental data (McRae et al., PCI, 2019), including particle image velocimetry measurements, a direct flame photo, the flow field velocity and the flame speed for different iron and oxygen concentrations. The comparison shows that the particle dynamics and flame shape can be reasonably well predicted. The flow field velocity and flame speed also show quantitative agreement between the simulation and the experiment. Based on the validated simulation results, the iron combustion characteristics, including the thermal structures and the multidimensional effects, are analyzed for different oxidizer environments. The analysis shows that the iron particles undergo a transition from kinetic-controlled regime (up to ignition) to a diffusion-controlled regime (burning) at the central axis for both environments with the particle temperature being higher than the gas temperature at the flame front, which is indicated by the Damköhler number. For the hot counterflow burner, there exist multidimensional effects, i.e., the temperature and Damköhler number change along the radial direction.

physics.flu-dyn↗

arXiv2022

Effect of flame retardants on side-wall quenching of partially premixed laminar flames

A combined experimental and numerical investigation of partially premixed laminar methane-air flames undergoing side-wall quenching (SWQ) is performed. A well-established SWQ burner is adapted to allow the seeding of the main flow with additional gaseous products issued from a (secondary) wall inlet close to the flame's quenching point. First, the characteristics of the partially premixed flame that quenches at the wall are assessed using planar laser-induced fluorescence measurements of the OH radical, and a corresponding numerical simulation with fully-resolved transport and chemistry is conducted. A boundary layer of enriched mixture is formed at the wall, leading to a reaction zone parallel to the wall for high injection rates from the wall inlet. Subsequently, in a numerical study, the wall inflow is mixed with dimethylmethylphosphonat (DMMP), a phosphor-based flame retardant. The DMMP addition allows the assessment of the combined effects of heat loss and flame retardants on the flame structure during flame-wall interaction. With an increasing amount of DMMP in the injected mixture, the flame stabilizes further away from the wall and shows a decrease in the local heat-release rate. Thereby, the maximum wall heat flux is significantly reduced. That results in a lower thermal load on the quenching wall. The flame structure analysis shows an accumulation of the intermediate species HOPO at the wall similar to the CO accumulation during the quenching of premixed flames without flame retardant addition. The study shows how the structure of a partially premixed flame is influenced by a wall that releases either additional fuel or a mixture of fuel and flame retardant. The insights gained from the canonical configuration can lead to a better understanding of the combined effects of flame retardants and heat losses in near-wall flames.

physics.flu-dyn↗