Detailed Chemistry Simulations Research Articles

Quantum chemistry study on inhibition of coal spontaneous combustion based on anti-gastric acid bionics: Na+, Mg2+ inhibition and CO2 inerting

In view of the limitations of single means of inhibiting coal spontaneous combustion and the lack of research on the joint mechanism of different inhibition means, this paper constructed a joint oxygen shielding structure based on the bionic principle that gastric mucosa and gastric mucus cooperate with each other to resist gastric acid and prevent gastric acid from eroding the gastric wall, and studied the microscopic mechanism of joint inhibition of coal spontaneous combustion by using Na+, Mg2+ blocking and CO2 inerting at the same time. In this paper, the change of CO2 adsorption strength before and after ionic inhibition is used to demonstrate that the two have a synergistic effect of inhibiting coal spontaneous combustion, and the micro-mechanism of the combination of these two means is not seen in related studies. In addition, this paper also made detailed quantum chemistry simulation calculations on the water absorption of ions, the existence of coordination bonds, and the chemical stability of coal molecules after inhibition, which is a refinement and supplement to the theory of coordination inhibition. The results showed that the coal molecules themselves were attractive to water, with an adsorption energy of -12.43 kJ/mol; the adsorption strength of coal molecules to water after Na+ inhibition was more than four times of that before the inhibition, at -53. 65 kJ/mol, and the effect of Mg2+ was even better, with the adsorption energy to water increasing to -180.98 kJ/mol, and from the point of view of adsorption of water, the ions exhibited a positive coal spontaneous combustion inhibition effect. The inhibitory effect of Na+ on aldehyde group is stronger than that of Mg2+; the initial adsorption energy between CO2 and coal molecules was -12.34 kJ/mol, and the adsorption energy between the coal molecular structure and CO2 after ionic inhibition was -18.12 kJ/mol ∼ -28.62 kJ/mol, which was significantly enhanced compared with that before the inhibition, suggesting that the combination of the two means can synergistically inhibit spontaneous combustion of coal, and the synergistic effect of Mg2+ with inert gases is stronger than that of Na+.

Hydrogen, methane and one of their fuel blends combustion: CFD analysis and numerical-experimental comparisons of fixed and mobile applications

The capabilities of Computational Fluid Dynamics (CFD) coupled with detailed chemistry simulations are examined in both steady jet diffusion flames and in an internal combustion engine case fuelled with hydrogen. Different approaches to turbulence-chemistry interaction such as the “Laminar Flame Concept” the “Eddy Dissipation Concept” and the “Turbulent Flame Speed Closure” are considered and tested. The results are compared with the experimental data available. Concerning the jet diffusion flames, the combustion processes of hydrogen, methane and one of their fuel blends are investigated on two burner geometries. Different sensitivities (i.e. mesh, turbulence model, turbulent Schmidt number, chemical mechanism) are performed. The study demonstrates that despite the burner geometry considered and the chemical composition of the fuel, the Complex Chemistry with “Eddy Dissipation Concept” is the model that better describes the behaviour of the turbulent flames. On the other hand, the “Laminar Flame Concept” sub-model is characterized by an higher fuel consumption rate, which causes an overestimation of the temperature peak. As for the in-cylinder unsteady simulations, the hydrogen combustion process is better described by the “Turbulent Flame Speed Closure” sub-model, which, unlike the other two, requires the specification of both laminar and turbulent flame speed. Despite different variations being considered, the “Laminar Flame Concept” adoption leads to an unphysically high burning rate, while the Eddy Dissipation Concept sub-model is characterized by an underestimation of the apparent heat release rate, and thus of the pressure peak inside the combustion chamber.

Reduced chemical kinetic model for CH[formula omitted]-air non-premixed flames including excited and charged species

Electric fields can impact small laminar flames by changing their shape and overall behavior by acting on charged species produced in combustion. However, no reduced chemical kinetic model has been developed considering both major species and minor species related to flame characterization and flame behavior in the presence of an electric field. This study presents a reduced chemical kinetic model for methane-air combustion which includes minor excited species (CH* and OH*) and charged species (H3O+, HCO+, C2H3O+, CH5O+, O2−, OH−, e−, CO3−, CHO2−, O−, CHO3−). The results employing the reduced chemistry model have been validated for a two-dimensional flame geometry by comparison with (i) detailed chemistry simulation results for species location and peak values, and (ii) experimental CH* chemiluminescence location, considering the self-repulsion of charges yet without externally applied electric field to the flame.This reduced chemical kinetic model, with 45 species and 216 reactions, shows a computational demand one-third that of employing its equivalent detailed chemistry (83 species and 394 reactions). The reduction is modest but significant considering that high fidelity is needed to capture the behavior of the chemi-ion and chemiluminescent species. Future works will involve the use of this model for simulations predicting flame behavior with applied electric field (i.e., field strength ≠0 kV/cm).

Reaction-Diffusion Manifolds including differential diffusion applied to methane/air combustion in strong extinction regimes

Detailed chemistry simulations of turbulent reacting flows involving combustion of hydrocarbons can easily exceed the available computational resources, depending on the dimensions of the chemical system. Previous work of the authors showed how the combination of the Eulerian Stochastic Fields (ESF) model with tabulated chemistry based on 2-dimensional Reaction-Diffusion Manifolds (REDIM) provided a significant computational speed-up, compared to the finite rate ESF solver. In this work, the behaviour for flame F, featuring a strong degree of extinction, is further investigated. A comparison is performed for 2D and 3D databases, both using simplified and detailed transport, where the scalar dissipation rate is included as the third table parameter. The results show that the upstream sections are well captured by the REDIM built for detailed transport, while the downstream sections are better captured by the simplified transport database. While a 3D-REDIM based on simplified transport seems to be necessary to capture the extinction events, a 2D-REDIM with differential diffusion already provides satisfactory results. Overall, the use of a 3D-REDIM with differential diffusion better describes the global behaviour of flame F.

Flame structure analysis and composition space modeling of thermodiffusively unstable premixed hydrogen flames — Part I: Atmospheric pressure

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.

Flame structure analysis and composition space modeling of thermodiffusively unstable premixed hydrogen flames — Part II: Elevated pressure

This paper is a continuation of the work we carried out in Part I (Wen et al., 2021), studying the flame structure analysis and composition space modeling of an outwardly expanding thermodiffusively unstable premixed hydrogen flame at atmospheric pressure. In this work, we focus on the case of elevated pressure (5 atm), which is particularly challenging for the model proposed in Part I due to wider ranges of curvatures and strain rates induced by promoted intrinsic instabilities in the flame studied. A detailed chemistry simulation of the thermodiffusively unstable premixed hydrogen flame at 5 atm is first conducted, and the flame structure is comprehensively analyzed by comparing the simulation results with the asymptotic theory proposed by Matalon and co-workers (PCI, 2002; JFM, 2003). Then, budget terms of the generalized premixed flamelet equations are calculated to identify the dominating processes of the cells with different length scales. Finally, the performance of the proposed tabulation method based on premixed flamelet equations in composition space presented in Part I is evaluated through an a priori analysis by comparing tabulated values with the reference results. The critical flame radius and average cell size calculated with the asymptotic theory are found to agree well with the reference results. The calculated critical Péclet number at the elevated pressure is smaller than that under the condition of atmospheric pressure, suggesting a smaller critical flame radius as the pressure increases. The budget analysis shows that for either small- or large-scale cells, tangential diffusion is only important for flamelets with negative curvature. The a priori analysis shows that the proposed tabulation method performs well in predicting the intermediate species in regions with negative curvature, although the peak values of some species mass fractions are under-predicted. The reasons are explained and guidelines for future research to improve the model are outlined.

Comparison of Flame Propagation Statistics Based on Direct Numerical Simulation of Simple and Detailed Chemistry. Part 2: Influence of Choice of Reaction Progress Variable

Flame propagation statistics for turbulent, statistically planar premixed flames obtained from 3D Direct Numerical Simulations using both simple and detailed chemistry have been evaluated and compared to each other. To achieve this, a new database has been established encompassing five different conditions on the turbulent combustion regime diagram, using nearly identical numerical methods and the same initial and boundary conditions. The discussion includes interdependencies of displacement speed and its individual components as well as surface density function (i.e., magnitude of the reaction progress variable) with tangential strain rate and curvature. For the analysis of detailed chemistry Direct Numerical Simulation data, three different definitions of reaction progress variable, based on CH4,H2O and O2 mass fractions will be used. While the displacement speed statistics remain qualitatively and to a large extent quantitatively similar for simple chemistry and detailed chemistry, there are pronounced differences for its individual contributions which to a large extent depend on the definition of reaction progress variable as well as on the chosen isosurface level. It is concluded that, while detailed chemistry simulations provide more detailed information about the flame structure, the choice of the reaction progress variable definition and the choice of the resulting isosurface give rise to considerable uncertainty in the interpretation of displacement speed statistics, sometimes even showing opposing trends. Simple chemistry simulations are shown to provide (a) the global flame propagation statistics which are qualitatively similar to the corresponding results from detailed chemistry simulations, (b) remove the uncertainties with respect to the choice of reaction progress variable, and (c) are more straightforward to compare with theoretical analysis or model assumptions that are mostly based on simple chemistry assumptions.

A new preferential diffusion model applied to FGM simulations of hydrogen flames

Preferential diffusion is very important in simulations of hydrogen flames. Flame stretch and curvature induce strong preferential diffusion effects in laminar premixed hydrogen flames, causing strong local deviations from the unburnt mixture fraction in the reaction zone. In tabulated chemistry methods, this necessitates the use of a partially premixed model even if the inlet mixture is purely premixed. Furthermore, in realistic combustion problems heat losses often play a dominant role. In this paper we derive a preferential diffusion model for constant but non-unity species Lewis numbers using three controlling variables, namely mixture fraction, progress variable and enthalpy. The model has been implemented in the Flamelet Generated Manifold (FGM) approach and validated by comparing with detailed chemistry simulations. As a test case we investigate a 2D laminar premixed hydrogen flame stabilised on an isothermal slit burner. Additionally, the model was compared with the standard treatment of preferential diffusion in FGM to show the increase in accuracy of the new model presented in this paper. The new model shows a significant improvement compared to the previous model, which can be attributed to the inclusion of cross-diffusion. The importance of the additional diffusion terms and its variation in mixture fraction for initially purely premixed hydrogen flames is highlighted.

A tabulated chemistry method for heterogeneous solid propellant combustion

Detailed chemistry simulations of heterogeneous solid propellant combustion at the microstructure scale are very costly as they involve large and stiff chemical schemes in the gas phase. A usual workaround is to use global chemistry to approximate gas phase reactions. A drawback of this approach is the need for an a priori calibration which necessitates reference data and often limits the operating conditions range of validity. An alternative method based on 1-D burner-stabilized flames tabulation is proposed in this paper. The theoretical and numerical frameworks are detailed. An appropriate tabulation space is introduced to store useful thermochemical variables as well as Jacobian terms used for time-integration. Jump conditions are also rewritten to cope with tabulated chemistry formalism. Several 1-D validations are performed on classical propellant compositions. Detailed comparisons with available experimental data demonstrate the ability of the proposed methodology to tackle inner flame structure (temperature and species) as well as the evolution of the burning rate with pressure and composition. Finally, 2-D laminate propellant simulations are conducted showing the ability of the methodology to capture complex flame regimes and associated burning rate modifications.

Capturing multi-regime combustion in turbulent flames with a virtual chemistry approach

Multiple flame regimes are encountered in industrial combustion chambers, where premixed, stratified and non-premixed flame regions may coexist. To obtain a predictive tool for pollutant formation predictions, chemical flame modeling must take into account the influence of such complex flame structure. The objective of this article is to apply and compare two reduced chemistry models on both laminar and turbulent multi-regime flame configurations in order to analyze their capabilities in predicting flame structure and CO formation. The challenged approaches are (i) a premixed flamelet-based tabulated chemistry method, whose thermochemical variables are parameterized by a mixture fraction and a progress variable, and (ii) a virtual chemical scheme which has been optimized to retrieve the properties of canonical premixed and non-premixed 1-D laminar flames. The methods are first applied to compute a series of laminar partially-premixed methane-air counterflow flames. Results are compared to detailed chemistry simulations. Both approaches reproduced the thermal flame structure but only the virtual chemistry captures the CO formation in all ranges of equivalence ratio from stoichiometry premixed flame to pure non-premixed flame. Finally, the two chemical models combined with the Thickened Flame model for LES are challenged on a piloted turbulent jet flame with inhomogeneous inlet, the Sydney inhomogeneous burner. Mean and RMS of temperature and CO mass fraction radial profiles are compared to available experimental data. Scatter data in mixture fraction space and Wasserstein metric of numerical and experimental data are also studied. The analyses confirm again that the virtual chemistry approach is able to account for the impact of multi-regime turbulent combustion on the CO formation.

Numerical Investigation of Local Heat-Release Rates and Thermo-Chemical States in Side-Wall Quenching of Laminar Methane and Dimethyl Ether Flames

The local heat-release rate and the thermo-chemical state of laminar methane and dimethyl ether flames in a side-wall quenching configuration are analyzed. Both, detailed chemistry simulations and reduced chemistry manifolds, namely Flamelet-Generated Manifolds (FGM), Quenching Flamelet-generated Manifolds (QFM) and Reaction-Diffusion Manifolds (REDIM), are compared to experimental data of local heat-release rate imaging of the lab-scale side-wall quenching burner at Technical University of Darmstadt. To enable a direct comparison between the measurements and the numerical simulations, the measurement signals are computed in all numerical approaches. Considering experimental uncertainties, the detailed chemistry simulations show a reasonable agreement with the experimental heat-release rate. The comparison of the FGM, QFM and REDIM with the detailed simulations shows the high prediction quality of the chemistry manifolds. For the first time, the thermo-chemical state during quenching of a dimethyl ether-air flame is examined numerically. Therefore, the carbon monoxide and temperature predictions are analyzed in the vicinity of the wall. The obtained results are consistent with previous studies for methane-air flames and extend these findings to more complex oxygenated fuels. Furthermore, this work presents the first comparison of the QFM and the REDIM in a side-wall quenching burner.

VLF Measurements and Modeling of the D-Region Response to the 2017 Total Solar Eclipse

In this paper, we report measurements in Colorado and Utah of the disturbed very-low-frequency (VLF) signals from the NML Navy transmitter in North Dakota during the 2017 solar eclipse. Using an occultation mask of solar fluxes together with detailed chemistry and VLF propagation simulations, we quantify the D-region response to the eclipse, in terms of electron density variation, as well as the expected signatures of VLF transmitter signals. The VLF measurements, including an anomalous amplitude enhancement recorded in UT, can be quantitatively explained using the Wait and Spies ionospheric profile with a sharpness parameter of $\beta = 0.3$ km−1 above ~55 km and an increase in the D-region ionosphere height of $\Delta h' \simeq 8$ km. This sharpness parameter is consistent with previously reported rocket measurements and first-principles calculations. The best-fit results suggest a reduction of D-region electron density by ~90% during the eclipse in the D-region, implying an occultation of Lyman- $\alpha $ by nearly 99%. This finding agrees with detailed calculations of time-dependent obscuration factors utilizing the He 30.4-nm images from Solar Dynamics Observatory as a proxy for the distribution of Lyman- $\alpha $ across the solar disk and limb. Moreover, the present results show that subionospheric VLF propagation is sensitive to the sharpness parameter of the electron density profile in the D-region. Previously reported first-principles simulations have shown that the sharpness parameter is mostly controlled by the background concentration of minor neutral species. Thus, the VLF technique can be likely used to remotely sense these neutral species at and below the effective reflection altitudes of VLF waves.

QFM: quenching flamelet-generated manifold for modelling of flame–wall interactions

This work introduces a new method to improve the accuracy of the flamelet-generated manifold (FGM) method under conditions of flame–wall interactions (FWI). Special attention is given to the prediction of the pollutant CO.In existing FGM methods, in order to account for heat loss, usually flamelets with constant enthalpy are utilised. These constant enthalpy flamelets used to generate the manifold, do not include the effects of wall heat loss on the manifold composition, resulting in simulation inaccuracies in the near-wall region, where large enthalpy gradients are present. To address this issue, the idea to utilise 1D head on quenching (HOQ) flamelets for tabulated chemistry is adopted and applied here in the context of the FGM method. The HOQ qualitatively resembles the general phenomena of FWI. However, the rates of wall heat loss and the accompanied effects on the chemical species composition may quantitatively differ between various FWI configurations. In addition, the magnitude of heat transfer rate may vary in space and time in general configurations. Therefore, in this work, a method is introduced to generate a 3D manifold, based on multiple HOQ-like flamelets, that includes the variation of the rate of heat loss as an extra table dimension. This dimension is parametrised by a second reaction progress variable for which a transport equation is solved next to the equations for enthalpy and the first progress variable.The new developed method, referred to as Quenching Flamelet-generated Manifold (QFM), is described in this work. Further, the method is validated against detailed chemistry simulations of a two-dimensional premixed laminar side-wall quenching of a methane-air flame. A comparison is presented, analysing the performance obtained using the existing 2D FGM method, a 2D QFM that is based on a single HOQ flamelet which does not account for a varying rate of wall heat loss and a 3D QFM, which does. Finally, it is shown that the 3D QFM tabulated chemistry simulation yields a very good level of accuracy and that the accuracy for prediction of CO concentrations near the wall is improved tremendously.

Reproducing curvature effects due to differential diffusion in tabulated chemistry for premixed flames

Tabulated chemistry reduces the cost of modeling reacting flows by transporting fewer scalars. It also has the benefit of introducing fewer terms that must be closed for Reynolds-averaged Navier-Stokes and large eddy simulation modeling. In this study of premixed lean hydrogen–air flames, an extension to an existing tabulated chemistry model was developed by including differential diffusion and thermal diffusion of multiple species, using a mixture-averaged diffusion model. The transport equations for the tabulation variables (a progress variable and a mixture fraction-like variable) have been formulated, and curvature effects are intrinsic to the transport equations derived from first principles. The mathematical derivation is general, such that other fuel/air mixtures could be examined. The new tabulated chemistry model was then validated using a range of configurations, including flames with curvature, extinction, and hot spots. The model validation indicated excellent agreement with detailed chemistry simulations for lean premixed hydrogen–air mixtures. Conditional means of the progress variable source term were shown to agree well with detailed chemistry simulations of two-dimensional freely propagating flames and three-dimensional turbulent flames. Finally, progress variable source terms three times higher than one-dimensional flat flame results were identified and validated with detailed chemistry, and super-adiabatic hot spots were also predicted in the turbulent flame configuration.

Multi-zone thermodynamic modeling of combustion and emission formation in CNG engines using detailed chemical kinetics

This study proposes an efficient and accurate methodology that combines detailed chemical kinetics with a validated multi-zone thermodynamic model to calculate the emissions of a spark-ignition, natural gas fueled engine. The relative air–fuel ratio has been changed from 0.8 to 1.53, and the nominal brake mean effective pressure has been varied from 0.2 MPa to 1.29 MPa.The multi-zone model in-cylinder pressure and temperature traces, as well as other experimental engine quantities, have been considered as input data for the chemical submodel. The objective has been to reproduce and interpret the measured engine-out data in order to obtain insight into the in-chamber combustion and pollutant formation processes. The concentrations of nitric oxide, carbon monoxide, hydrogen, carbon dioxide and hydrocarbon, as well as the oxygen concentration after ignition, have been compared with experimental data and some of them have been compared with the results of conventional models.The model results, based on detailed chemistry simulation, have been found to be in good agreement with experimental data for all the species at engine exhaust, and a higher prediction capability than that obtained through simplified reaction and chemical equilibrium methods has been shown. The influences of the uncertainties in RAFRs and unburned mass fractions on the calculated results are discussed. The unburned gas fraction, derived from the calculated and measured oxygen concentration at the engine exhaust, has been shown to be a way of correcting the hydrocarbon emissions.

Direct Numerical Simulation of Head-On Quenching of Statistically Planar Turbulent Premixed Methane-Air Flames Using a Detailed Chemical Mechanism

A three-dimensional compressible Direct Numerical Simulation (DNS) analysis has been carried out for head-on quenching of a statistically planar stoichiometric methane-air flame by an isothermal inert wall. A multi-step chemical mechanism for methane-air combustion is used for the purpose of detailed chemistry DNS. For head-on quenching of stoichiometric methane-air flames, the mass fractions of major reactant species such as methane and oxygen tend to vanish at the wall during flame quenching. The absence of text {OH} at the wall gives rise to accumulation of carbon monoxide during flame quenching because text {CO} cannot be oxidised anymore. Furthermore, it has been found that low-temperature reactions give rise to accumulation of text {HO}_{2} and mathrm {H}_{2}mathrm {O}_{2} at the wall during flame quenching. Moreover, these low temperature reactions are responsible for non-zero heat release rate at the wall during flame-wall interaction. In order to perform an in-depth comparison between simple and detailed chemistry DNS results, a corresponding simulation has been carried out for the same turbulence parameters for a representative single-step Arrhenius type irreversible chemical mechanism. In the corresponding simple chemistry simulation, heat release rate vanishes once the flame reaches a threshold distance from the wall. The distributions of reaction progress variable c and non-dimensional temperature T are found to be identical to each other away from the wall for the simple chemistry simulation but this equality does not hold during head-on quenching. The inequality between c (defined based on text {CH}_{4} mass fraction) and T holds both away from and close to the wall for the detailed chemistry simulation but it becomes particularly prominent in the near-wall region. The temporal evolutions of wall heat flux and wall Peclet number (i.e. normalised wall-normal distance of T = 0.9 isosurface) for both simple and detailed chemistry laminar and turbulent cases have been found to be qualitatively similar. However, small differences have been observed in the numerical values of the maximum normalised wall heat flux magnitude left ({Phi }_{max } right )_{mathrm {L}} and the minimum Peclet number (Pe_{min })_{mathrm {L}} obtained from simple and detailed chemistry based laminar head-on quenching calculations. Detailed explanations have been provided for the observed differences in behaviours of left ({Phi }_{max }right )_{mathrm {L}} and (Pe_{min })_{mathrm {L}}. The usual Flame Surface Density (FSD) and scalar dissipation rate (SDR) based reaction rate closures do not adequately predict the mean reaction rate of reaction progress variable in the near-wall region for both simple and detailed chemistry simulations. It has been found that recently proposed FSD and SDR based reaction rate closures based on a-priori DNS analysis of simple chemistry data perform satisfactorily also for the detailed chemistry case both away from and close to the wall without any adjustment to the model parameters.

Laminar near-wall combustion: Analysis of tabulated chemistry simulations by means of detailed kinetics

Chemistry pre-tabulation is well suited to include information of detailed reaction kinetics at reasonable computational costs to allow for simulations of realistic devices. In order to evaluate its accuracy in the near wall region, a study with numerical simulations of flame–wall-interaction is performed in this work. A laminar side-wall-quenching scenario is considered to judge on the prediction of the global flame behavior as well as on local species formation of practical relevance. The configuration considered represents a subsection of a side-wall-quenching burner introduced recently with the purpose of numerical validation in mind. The measured temperature and carbon-monoxide concentration are used to identify deficiencies of the tabulated chemistry approach. Furthermore, detailed chemistry simulations are carried out to identify the root cause for those deficiencies. The corresponding analysis is based on the transformation of the species transport equation into the composition space where the physical significance of the scalar dissipation rates provides clear indications regarding the pre-tabulation assumptions. The evaluation of individual terms allows to quantify the interaction of flamelets in the near-wall region where diffusive fluxes cause a departure from the presumed manifold. Based on this analysis, improvements are then suggested. First, as a proof of concept, the direct tabulation of the thermo-chemical states obtained by the detailed chemistry simulation is applied to evaluate whether the reduction to controlling variables is in general possible in such a physical scenario. Second, as an alternative way of pre-tabulating, the reaction-diffusion-manifold (REDIM) approach is then adopted. By building the REDIM based on a gradient estimate from a computationally inexpensive transient one-dimensional flame–wall-interaction simulation it is possible to obtain realistic dissipation rates without a-priory knowledge. By this approximation a significant gain in prediction is achieved when compared to the original tabulation.

TOPAS-nBio: An Extension to the TOPAS Simulation Toolkit for Cellular and Sub-cellular Radiobiology.

The TOPAS Monte Carlo (MC) system is used in radiation therapy and medical imaging research, having played a significant role in making Monte Carlo simulations widely available for proton therapy related research. While TOPAS provides detailed simulations of patient scale properties, the fundamental unit of the biological response to radiation is a cell. Thus, our goal was to develop TOPAS-nBio, an extension of TOPAS dedicated to advance understanding of radiobiological effects at the (sub-)cellular, (i.e., the cellular and sub-cellular) scale. TOPAS-nBio was designed as a set of open source classes that extends TOPAS to model radiobiological experiments. TOPAS-nBio is based on and extends Geant4-DNA, which extends the Geant4 toolkit, the basis of TOPAS, to include very low-energy interactions of particles down to vibrational energies, explicitly simulates every particle interaction (i.e., without using condensed histories) and propagates radiolysis products. To further facilitate the use of TOPAS-nBio, a graphical user interface was developed. TOPAS-nBio offers full track-structure Monte Carlo simulations, integration of chemical reactions within the first millisecond, an extensive catalogue of specialized cell geometries as well as sub-cellular structures such as DNA and mitochondria, and interfaces to mechanistic models of DNA repair kinetics. We compared TOPAS-nBio simulations to measured and published data of energy deposition patterns and chemical reaction rates (G values). Our simulations agreed well within the experimental uncertainties. Additionally, we expanded the chemical reactions and species provided in Geant4-DNA and developed a new method based on independent reaction times (IRT), including a total of 72 reactions classified into 6 types between neutral and charged species. Chemical stage simulations using IRT were a factor of 145 faster than with step-by-step tracking. Finally, we applied the geometric/chemical modeling to obtain initial yields of double-strand breaks (DSBs) in DNA fibers for proton irradiations of 3 and 50 MeV and compared the effect of including chemical reactions on the number and complexity of DSB induction. Over half of the DSBs were found to include chemical reactions with approximately 5% of DSBs caused only by chemical reactions. In conclusion, the TOPAS-nBio extension to the TOPAS MC application offers access to accurate and detailed multiscale simulations, from a macroscopic description of the radiation field to microscopic description of biological outcome for selected cells. TOPAS-nBio offers detailed physics and chemistry simulations of radiobiological experiments on cells simulating the initially induced damage and links to models of DNA repair kinetics.

3D Numerical Simulation of a Laminar Experimental SWQ Burner with Tabulated Chemistry

Flame-wall interaction (FWI) plays an important role in enclosed combustion systems. For avoiding the complexity of close to reality combustors, in this study an atmospheric premixed V-shaped flame interacting with an isothermal cold wall in a side wall quenching (SWQ) configuration is investigated. A stoichiometric methane/air mixture is used as fuel. A three-dimensional (3D) numerical simulation, which resolves all flow structures is combined with a tabulated chemistry approach (flamelet generated manifold, FGM). Results are compared with experimental data and two-dimensional simulations. The FGM approach is a suitable trade-off between computationally expensive detailed chemistry simulations and over simplified single step mechanisms. 2D simulations are used to investigate the influence of the uncertainty of the wall temperature, to show that the resolution in 3D is sufficient and that the influence of the flame thickening on the wall heat fluxes can be determined. Our results show that the 3D FGM approach is in close agreement to experimentally obtained flow and temperature fields. The dimensionless wall heat flux and Peclet number matches the expected values of 0.16 and 7, respectively. However, during FWI the measured CO mole fractions are not reproduced accurately showing that the transported variables in the present approach of tabulated chemistry do not recover premixed flame structures near walls.

Numerical analysis of laminar methane–air side-wall-quenching

Flame-wall-interaction (FWI) is investigated numerically using a premixed stoichiometric Side-Wall-Quenching configuration. Within the 2D fully resolving laminar simulation, detailed chemistry is used to study the stationary quenching of a methane–air (CH4) flame at an isothermal inert wall of 300 K. The investigation is related to a recent experimental study that revealed that the carbon-monoxide distribution substantially differs in the near-wall region when compared to an undisturbed flame. Simulations are carried out using different reaction mechanisms (GRI and Smooke) as well as diffusion treatments (unity Lewis and mixture averaged transport) and the results are compared to the measured temperature and CO concentrations. Specifically regarding the latter, being an important pollutant, recent attempts based on tabulated chemistry failed in predicting its near-wall accumulation. Accordingly, within this work the detailed chemistry simulations are used to investigate the origin of CO near the wall. Therefore, a Lagrangian analysis is applied to quantify the contribution of chemical production and consumption as well as diffusion to understand the root mechanism of the high CO concentrations measured. The analysis revealed that the high CO concentrations near the wall results from a transport originating from CO produced at larger wall distances. In that region being not submitted to large heat losses, a high chemical activity and corresponding CO production is found. Accordingly, a diffusion process is initiated towards the wall where the chemical sources itself were actually found to be negative.