This work focuses on flame structure analysis and composition space modeling of a multidimensional premixed hydrogen flame transitioning from a laminar stable condition to a thermodiffusively ...unstable state. Specifically, budget and a priori analyses are conducted based on a detailed chemistry simulation of a 2D expanding, thermodiffusively unstable hydrogen flame, and the recurring issues for modeling differential diffusion, the strain rate and curvature in the thermodiffusively unstable flame are addressed in a single newly proposed flamelet tabulation method. The model is based on recently developed self-contained strained-curved premixed flamelet equations in composition space (Scholtissek et al., 2019, CNF), which inherently incorporate the interactions among differential diffusion, the strain rate and curvature. The validity of the newly proposed flamelet tabulation method is evaluated based on the representative strongly strained-curved flamelets extracted from the reference simulation, featuring wide ranges of strain rates and curvatures. The advance realized in the proposed flamelet model is confirmed by comparing it with a conventional flamelet tabulation method. Through the budget analysis, the effects of curvature on the diffusion along the flame front (i.e., tangential diffusion) are quantified. Through the a priori analysis, the suitability of the proposed flamelet tabulation method in predicting differential diffusion is confirmed. For the prediction of the strain rate and curvature, it is found that introducing the strain rate and curvature themselves as the trajectory variables does not necessarily improve the prediction accuracy in the reaction zone, compared to the flamelet model based on the 1D freely-propagating premixed flame with differential diffusion. To remedy this, the trajectory variables that can characterize the flame structure’s internal response to the strain rate and curvature are identified. The results show that the thermo-chemical variables in the thermodiffusively unstable flame at atmospheric pressure can be accurately predicted by the newly introduced trajectory variables based on the 1D strained-curved flamelet equations.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
This paper is focused on the characterization of local heat release rate and its correlation with flame structures under atmospheric conditions in a side-wall quenching geometry. The influence of ...different wall temperatures ranging from 330 K to 670 K is compared for stoichiometric and lean (
ϕ
= 1.0 and 0.83) methane/air as well as dimethyl ether (DME)/air flames. Simultaneous formaldehyde (CH
2
O) and hydroxyl radical (OH) planar laser-induced fluorescence (PLIF) is used to determine relative local heat release rates (HRRs). Based on the resulting relative HRR distributions, flame front positions are determined and flame curvatures are evaluated. In the laminar flame configuration, for both fuel types at wall-distances
y
< 0.08 mm, a steep gradient of CH
2
O-signal is observed. From ensemble-averaged HRR images, quenching distances are deduced and compared. With increasing wall temperature, quenching distances decrease due to increased laminar flame speeds. Compared to methane/air flames, DME/air flames are quenched at smaller quenching distances which is as well attributed to increased laminar flame speeds. In the turbulent flame configuration, fluctuations prevail in the flame-wall interaction (FWI) zone which is associated with local flame curvature. Comparing the near-wall curvature for different wall temperatures, flame curvature decreases with higher wall temperatures due to the increased viscosity of higher temperatures of near-wall gases. The correlation of HRR, flame curvature and wall-normal distance is further investigated and indicates an influence of Lewis-number effects upon turbulent premixed flames near walls.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OBVAL, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Current methods for combustion regime characterization, such as the flame index, rely on 3D gradient information that is not accessible with available experimental techniques. Here, a method is ...proposed for reaction zone detection and characterization, which can be applied to instantaneous 1D Raman/Rayleigh line measurements of major species and temperature as well as to results of laminar and turbulent flame simulations, without the need for 3D gradient information. Several derived flame markers, namely the mixture fraction, the heat release rate, and the chemical explosive mode, are combined to detect and characterize premixed versus non-premixed reaction zones. The methodology is developed and evaluated using fully resolved simulation data from laminar flames. The fully resolved 1D simulation data are spatially filtered to account for the difference in spatial resolution between experiment and simulation. Then, starting from just temperature and major species, a constrained homogeneous constant pressure, constant temperature reactor calculation gives an approximation of the full thermochemical state at each sample location along the line. Finally, the chemical explosive mode and the heat release rate are calculated from this approximated state and compared to those calculated directly from the simulation data. As a further test, experimental uncertainty is superimposed onto the filtered numerical data to produce a Raman/Rayleigh equivalent state before running the constrained homogeneous reactor, and results are again compared. After successful tests using the numerical data, the approach is applied to Raman/Rayleigh line measurements from laminar counterflow flames and a mildly turbulent lifted flame. The results confirm that the reaction zones can be reliably detected and characterized using experimental data. Furthermore, the relative importance of premixed and non-premixed reaction zones within the same flame can be qualitatively assessed as demonstrated in the results.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Cycle-to-Cycle Variations (CCV) in IC engines is a well-known phenomenon and the definition and quantification is well-established for global quantities such as the mean pressure. On the other hand, ...the definition of CCV for local quantities, e.g. the velocity or the mixture distribution, is less straightforward. This paper proposes a new method to identify and calculate cyclic variations of the flow field in IC engines emphasizing the different contributions from large-scale energetic (coherent) structures, identified by a combination of Proper Orthogonal Decomposition (POD) and conditional averaging, and small-scale fluctuations. Suitable subsets required for the conditional averaging are derived from combinations of the the POD coefficients of the second and third mode. Within each subset, the velocity is averaged and these averages are compared to the ensemble-averaged velocity field, which is based on all cycles. The resulting difference of the subset-average and the global-average is identified as a cyclic fluctuation of the coherent structures. Then, within each subset, remaining fluctuations are obtained from the difference between the instantaneous fields and the corresponding subset average. The proposed methodology is tested for two data sets obtained from scale resolving engine simulations. For the first test case, the numerical database consists of 208 independent samples of a simplified engine geometry. For the second case, 120 cycles for the well-established Transparent Combustion Chamber (TCC) benchmark engine are considered. For both applications, the suitability of the method to identify the two contributions to CCV is discussed and the results are directly linked to the observed flow field structures.
Les variations cycle à cycle (VCC) dans les moteurs à combustion interne sont un phénomène bien connu, et leur définition et quantification sont bien établies pour des quantités globales telles que la pression moyenne. Toutefois, la définition des VCC pour des quantités locales, par exemple la vitesse ou le mélange, est moins simple. Le présent article propose une nouvelle méthode visant à identifier et à calculer des variations cycliques de l’écoulement dans les moteurs à combustion interne, en mettant l’accent sur les différentes contributions provenant de structures énergétiques aux grandes échelles (cohérentes), identifiées en combinant la détermination d’une moyenne conditionnelle et par décomposition orthogonale aux valeurs propres (Proper Orthogonal Decomposition, POD) et de fluctuations aux petites échelles. Des sous-ensembles adaptés requis pour l’établissement d’une moyenne conditionnelle sont tirés de la combinaison de coefficients POD des second et troisième modes. Dans chaque sous-ensemble, la vitesse est moyennée et ces moyennes sont comparées à la moyenne d’ensemble du champ de vitesse basée sur l’ensemble des cycles. La différence résultante entre la moyenne du sous-ensemble et la moyenne globale est identifiée comme une fluctuation cyclique des structures cohérentes. Ensuite, dans chaque sous-ensemble, les fluctuations résiduelles sont obtenues à partir de la différence entre les champs instantanés et la moyenne correspondante de sous-ensemble. La méthodologie proposée est testée pour deux ensembles de données obtenues à partir de simulations de moteur à résolution d’échelle. Pour le premier cas, la base de données numérique est constituée de 208 réalisations indépendantes pour une géométrie moteur simplifiée. Pour le second cas, 120 cycles pour le moteur de référence à chambre de combustion transparente (Transparent Combustion Chamber, TCC) sont considérés. Pour les deux applications, l’adéquation de la méthodologie permettant d’identifier les deux contributions aux VCC est discutée, et les résultats sont directement reliés aux structures de l’écoulement observé.
For the development of reliable numerical models of entrained flow gasifiers it is crucial to understand and to model correctly the interactions between the hot gas-flow and the coal particles ...undergoing pyrolysis and char conversion. In this work a laboratory-scale entrained flow coal gasifier was investigated numerically. Based on the detailed analysis of the particle trajectories, the interaction between the flow field and coal particles of different sizes was carefully discussed and analyzed. For small, medium, and large particles, the primary regions for devolatilization, oxidation, and gasification were detected, and the conversion behavior for these particle fractions was quantified in terms of pyrolysis time, residence time, and overall conversion rate. Additional particle-resolved CFD calculations were carried out for representative particles of different sizes in order to study the heat and mass transfer of the isolated particles in detail. Pyrolyzing coal particles and combusting or gasifying char particles were considered at several positions in the reactor, where each location is characterized by different gas compositions and temperatures. For these particles, the flow field, and the species and temperature distributions in the boundary layer are discussed. The particle-resolved calculations demonstrate the impact of the particle Reynolds number and the influence of the mass flow from the particle to the surrounding gas on the overall heat and mass transfer.
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•Advanced CFD simulations of a laboratory-scale entrained flow gasifier•Representative particles sampled from the CFD reactor simulation•Analysis of the gas-particle interaction during pyrolysis and char conversion•Particle-resolved CFD simulations of the particle boundary layers for pyrolysis and char gasification
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Non-catalytic partial oxidation (POX) of hydrocarbon fuels is an important process for producing syngas. Quantitative experimental data under the demanding conditions relevant for POX reactions, e.g. ...long residence times, rich stoichiometries and high temperatures, respectively, are rare in literature. Here, the DLR high-temperature flow reactor setup was used to obtain a unique experimental data set for validation of reaction models and general understanding of fuel-rich hydrocarbon chemistry. A systematic experimental speciation data set for rich methane conditions with relevance to partial oxidation/gasification processes is presented. Both fast oxidation and slow reforming reactions are considered here. Quantitative data is obtained in the DLR high temperature flow reactor setup with coupled molecular beam mass spectrometry (MBMS) detection. Five test case scenarios are investigated, featuring rich methane conditions (ϕ=2.5) for the temperature range from 1100–1800K under atmospheric conditions. CO, CO2 and acetylene in two different amounts is added to the system for systematic analysis for addressing phenomena related to partial oxidation. The new experimental database includes quantitative species profiles of major and intermediate species and is available as Supplemental material. The experimental data is compared with results from a 0D modeling approach using the GRI 3.0, USC-II, Chernov and a reduced model based on the full Chernov mechanism. The comparisons reveal significant differences in the model predictions among themselves and with respect to the experimental data, underlining the relevance of this unique data set for further mechanism development and/or optimization.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Synthetic fuels, especially oxygenated fuels, which can be used as blending components, make it possible to modify the emission properties of conventional fossil fuels. Among oxygenated fuels, one ...promising candidate is oxymethylene ether-3 (OME
3
). In this work, the sooting propensity of ethylene (C
2
H
4
) blended with OME
3
is numerically investigated on a series of laminar burner-stabilized premixed flames with increasing amounts of OME
3
, from pure ethylene to pure OME
3
. The numerical analysis is performed using the Conditional Quadrature Method of Moments combined with a detailed physico-chemical soot model. Two different equivalence ratios corresponding to a lightly and a highly sooting flame condition have been investigated. The study examines how different blending ratios of the two fuels affect soot particle formation and a correlation between OME
3
blending ratio and corresponding soot reduction is established. The soot precursor species in the gas-phase are analyzed along with the soot volume fraction of small nanoparticles and large aggregates. Furthermore, the influence of the OME
3
blending on the particle size distribution is studied applying the entropy maximization concept. The effect of increasing amounts of OME
3
is found to be different for soot nanoparticles and larger aggregates. While OME
3
blending significantly reduces the amount of larger aggregates, only large amounts of OME
3
, close to pure OME
3
, lead to a considerable suppression of nanoparticles formed throughout the flame. A linear correlation is identified between the OME
3
content in the fuel and the reduction in the soot volume fraction of larger aggregates, while smaller blending ratios may lead to an increased number of nanoparticles for some positions in the flame for the richer flame condition.
The effective usage of renewable energy sources 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, long-distance energy trade and on-demand electricity generation. Among them, hydrogen (H2) is noteworthy, being the subject of significant investment and research. Metal fuels, such as iron (Fe), represent another promising solution for a clean energy supply, but establishing an interconnected ecosystem still requires considerable research and development. This work proposes a model to assess the supply chain characteristics 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 the energy importers Germany and Japan. The work comprises the assessment of economic (levelized cost of electricity - LCOE), energetic (thermodynamic efficiency) and environmental (CO2 emissions) aspects, which are quantified by the comprehensive model accounting for the most critical processes in the supply chain. The assessment is complemented by sensitivity and uncertainty analyses to identify the main cost drivers. 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 H2 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 shows potential to become an important energy carrier for long-distance trade in a globalized clean energy market.
•H2 and iron are promising energy carriers, adoption depends on viable ecosystems.•Techno-economic assessment of long-distance supply chains (efficiency, cost, CO2).•Good production location with access to cheap renewable energy of vital importance.•Additional production steps for iron overcompensated by more favorable transport.•Cost-competitiveness of H2/iron to fossil fuels likely depends on CO2 taxation.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Detailed numerical analyses of pulverised solid fuel flames are computationally expensive due to the intricate interplay between chemical reactions, turbulent multiphase flow, and heat transfer. The ...near-burner region, characterised by a high particle number density, is particularly influenced by these interactions. The accurate modelling of these phenomena is crucial for describing flame characteristics. This study examined the reciprocal impact between the discrete phase and the continuous phase using Reynolds-averaged Navier–Stokes (RANS) simulations. The numerical model was developed in Ansys Fluent and equipped with user-defined functions that adapt the modelling of combustion sub-processes, in particular, devolatilisation, char conversion, and radiative heat transfer under oxyfuel conditions. The aim was to identify the appropriate degree of detail necessary for modelling the interaction between discrete and continuous phases, specifically concerning mass, momentum, energy, and turbulence, to effectively apply it in high-fidelity numerical simulations. The results of the numerical model show good agreement in comparison with experimental data and large-eddy simulations. In terms of the coupling schemes, the results indicate significant reciprocal effects between the discrete and the continuous phases for mass and energy coupling; however, the effect of particles on the gas phase for momentum and turbulence coupling was observed to be negligible. For the investigated chamber, these results are shown to be slightly affected by the local gas phase velocity and temperature fields as long as the global oxygen ratio between the provided and needed amount of oxygen as well as the thermal output of the flame are kept constant.
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•The semi-conservative and fully conservative implementation in the two-fluid model are validated.•Different test cases for turbulent gas-liquid dispersions in a bubble column are ...simulated.•The drag force is calculated by the linear blending method and the hyperbolic blending method.•The blending method is more versatile than the traditional method and offers significant advantages applications.
Bubbly flows are found in a large number of chemical engineering applications. For the computational fluid dynamics (CFD) simulations of such multi-phase flows, both physical models and numerical treatment are crucial to obtain robust and accurate results. In this numerical study, we investigate the two-fluid model (TFM) under challenging conditions such as phase segregation and inversion. For the phase segregation, a singular problem arises in the phase momentum and the two-phase k-ε equations when one phase fraction approaches zero. Another numerical issue is the accurate calculation of the drag coefficient, e.g., during the phase inversion. To address the singular problem, previous studies used a non-conservative formulation after dividing by the phase fraction; in our approach, we present a robust methodology for semi-conservative and fully conservative formulations. A special numerical treatment is introduced to the phase momentum equations and the turbulence equations, which avoids the singular problem in case of phase segregation. Concerning the drag force, two novel methods, the linear and the hyperbolic blending method, respectively, are presented to obtain accurate results. For testing the new numerical treatment, the analytical solution of a two-dimensional test case is first compared with the results predicted using a semi-conservative and a fully conservative formulation. The second test case investigated is a bubble column with different superficial velocities. The results from three-dimensional simulations using the novel formulations show good agreement with the literature data. Especially when phase segregation occurs, the semi-conservative and the fully conservative formulations using the two-phase k-ε model formulation converge.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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