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One way to decarbonize the heat production sector is to gradually replace natural gas with hydrogen. The design of laminar premixed burners capable of operating with both methane and hydrogen is however challenging as these fuels have drastically different burning properties, which narrows the operating range. While methane flames face limitations in stabilization due to blow-off at high power, hydrogen flames tend to be susceptible to flashback at low power. This study investigates the effects of slits symmetry breaking on the blow-off of methane flames and flashback of hydrogen flames through two-dimensional direct numerical simulations of canonical asymmetrical slit configurations, revealing uneven interactions between the main openings and smaller auxiliary slits. The equations governing the reactive flow dynamics are coupled to a heat transfer solver in the solid phase to elucidate the thermal and hydrodynamic mechanisms determining the operability limits. Viscous dissipation in the small auxiliary slits together with a substantial preheating of the fresh gases by heat redistribution through the solid phase is found to govern flame stabilization in asymmetrical geometries, improving blow-off resistance of methane-air flames. Then, flashback of hydrogen-air flame is found to be driven by the competition between (i) the mass flow rate distribution between the main and auxiliary slits, (ii) preheating of the gas through the auxiliary slits and (iii) the ability of the main and auxiliary slits to quench the flame. The interplay of these phenomena gives rise to complex behaviors, wherein asymmetrical configurations could exhibit significantly enhanced resistance to flashback compared to symmetrical geometries. This conclusion, verified for different burner thicknesses and slit spacing, may be used to guide the design of fuel-flexible laminar burners. Novelty and significance statement 1. Analyzing the effect of slits symmetry breaking on blow-off of methane flames and flashback of hydrogen flames for multi-perforated laminar burners. 2. Defining a strategy for wall heat flux redistribution to assess the asymmetrical interaction between the main and auxiliary flames through the flame-holder. 3. Demonstrating and explaining the positive impact of symmetry breaking on the blow-off resistance of methane flames. 4. Highlighting the substantial and non-monotonous impact of symmetry breaking on hydrogen flame flashback. 5. Identifying the interplay of thermal and hydrodynamic mechanisms in asymmetrical slits leading to improvement of flashback resistance for hydrogen-air flames compared to symmetrical configurations. [ABSTRACT FROM AUTHOR]

The stability of detonation propagation at elevated pressure was investigated via linear stability analysis and numerical simulation in one dimension. The effect of finite molecular volume neglected in perfect gas was accounted for with a real gas model based on the Noble-Abel equation of state and was characterized with a non-dimensional covolume parameter (β). The neutral stability boundary was derived from linear stability analysis in various planes, including overdrive factor - heat content plane, activation energy - heat content plane and heat capacity ratio - overdrive factor plane. It was found that the finite molecular volume generally stabilizes the pulsating detonation compared to the perfect gas based solutions. On the contrary, in the limit of weak heat release and in the Newtonian limit, including the finite molecular volume destabilizes the pulsating detonation. These two effects were both enhanced as β increases. They were relatively minor when β was less than 1 and became significant as β was larger than 1. The above findings obtained from linear stability analysis were validated by performing numerical simulations using a solver developed to handle compressible reactive flow in non-ideal fluid. Satisfactory consistencies were obtained between the analytical solutions and simulations for both the stability of detonation as well as the period of oscillation of unstable cases. In order to explain the stabilization effect of the finite molecular volume on cellular detonation found in previous study, a new neutral stability boundary was proposed in the isentropic coefficient - covolume plane. The new boundary contribute to better understanding the variation of cellular structure regularity seen in previous simulations even though the present analysis was carried out for a lower spatial dimension. Detailed discussions are provided on the potential implications for detonation engine design and the implementation of numerical solvers at elevated pressure. Novelty and significance statement 1. For the first time, the effect of finite molecular volume on detonation stability was revealed via linear and non-linear stability analyses 2. The stabilizing effect of the finite molecular volume has potential strong practical implications since the stability has a dominant effect on the near-limit detonation behavior [ABSTRACT FROM AUTHOR]

Entropy waves are generated in many technically relevant flow processes such as combustion, mixing, or convective heat transfer. When accelerated, entropy waves generate acoustic waves that contribute to the overall sound emission and can lead to self-excited thermoacoustic instabilities, especially at low frequencies. In order to reduce or prevent these undesirable byproducts of the flow, an understanding of the generation mechanisms of entropy waves is key. This study derives the analytical source terms of entropy disturbances for moving sources in general three-dimensional reactive flows. In this general setup, the consistent derivation of the generation mechanisms requires the tracking of the moving source for which an Arbitrary Lagrangian–Eulerian (ALE) framework is utilized. The derived differential equations provide a fundamental understanding of the underlying source mechanisms. In addition, the general three-dimensional differential equations are reduced to a quasi-one-dimensional jump condition to unify the analysis of the entropy wave generation. This unified framework is used for an in-depth analysis of a premixed flame, where all source terms that generate entropy disturbances are analyzed and their relative importance are quantified. The dominant contribution of unsteady heat addition per unit mass to the generation of entropy waves is reaffirmed for lean premixed flames. Finally, by comparison with the entropy generation mechanisms of a heated gauze at rest, it is emphasized once more that a heat source at rest is an invalid model for a premixed flame. Novelty and significance statement The analytical terms of entropy wave generation are derived consistently for moving sources in a general three-dimensional reactive flow. It is shown and emphasized that a consistent derivation requires the tracking of the local entropy sources. Therefore, an Arbitrary Lagrangian–Eulerian framework is used. The derived equations provide a fundamental insight into the generation of entropy disturbances in reactive flows, e.g. premixed flames. Furthermore, the derived equations can be seen as a starting point to consistently extract sources of entropy waves from numerical simulations. • Source terms of entropy disturbances for moving sources in 3D reactive flows • Tracking of moving sources with an Arbitrary Lagrangian–Eulerian framework • Unified description from 3D differential equations to quasi 1D jump conditions • Exemplary in-depth analysis of entropy wave generation in partially premixed flames [ABSTRACT FROM AUTHOR]

Reactor-based approaches for handling the Turbulence–Chemistry-Interactions closure have the advantage of embedding finite-rate chemistry in the combustion model of RANS and LES simulations, which might be crucial for the solution accuracy when complex combustion regimes are investigated. However, the numerical solution of the chemical ODEs is burdened with stiffness and increased dimensionality, especially when large detailed mechanisms are required. To this end, the Sample-Partitioning Adaptive Reduced Chemistry (SPARC) methodology couples adaptive chemistry and machine learning to speed-up the chemistry integration in reactive flows simulations. It consists in building a library of skeletal mechanisms, associated to clusters of similar thermo-chemical states identified in a training dataset, and then, at run-time, each computational cell is assigned to a specific cluster, whose skeletal mechanism is retrieved and employed for the time integration. Such workflow builds on four interacting blocks , i.e., training dataset generation, clustering, mechanism simplification, and classification, and its success tightly depends on the quality of each block, which generally results from a combination of theoretical, methodological, and computational choices. In this paper, we explore the effects of the mechanism simplification strategy on the SPARC performance and we develop an ad-hoc procedure that automatically identifies the cluster-wise optimal reduction parameters, delivering a higher global reduction and therefore a larger computational speed-up compared to a standard approach, along with an explicit a-priori control on accuracy. We implement and test this novel procedure on a RANS simulation of the Adelaide Jet-in-hot-coflow (AJHC) burner, and we attain a ∼ 2 x CPU time improvement with respect to the simulation obtained with a 36-species detailed mechanism. Novelty and significance This work makes a contribution towards the acceleration of chemistry integration in reactive flows simulations. More specifically, the Sample-Partitioning Adaptive Reduced Chemistry method, which couples adaptive chemistry and machine learning, is enhanced with automatic target species definition and a-priori error estimation. The novelties lie in the utilization of the computational singular perturbation (CSP) reduction algorithm, which provides means for automatically identifying an adaptive set of target species, and in the definition of a novel strategy for assessing the performance of the reduced mechanisms in the pre-processing phase, with the goal of estimating a measure of accuracy of the upcoming CFD simulation. [ABSTRACT FROM AUTHOR]

Flame synthesis is a powerful and scalable method for generating nanoparticles for a wide range of applications. The chemical interaction of the flame and the precursor combined with the spatial and temporal temperature distribution determine the product properties. For controlled nanoparticle synthesis that can also be scaled to industrial production rates, detailed knowledge of the underlying chemical kinetics and their interaction with the reactive flow is essential. Laser diagnostics has the capability to analyze the process by probing the concentration of important intermediates in shock tubes and reactive flows. The gas-phase synthesis of silica nanoparticles from SiCl 4 in a premixed H 2 /O 2 low-pressure flame reactor is studied by laser-induced fluorescence imaging of SiO mole fractions and temperature. The literature value-based spectroscopy model of SiO used for fitting the LIF spectra are validated based on absorption cross-sections measurements in a shock tube, where SiO is formed under precisely defined conditions (temperature, pressure, mole fraction) using a well-known kinetics mechanism for SiH 4 /CO 2 /Ar decomposition. Based on literature sources, a reaction mechanism is assembled to describe the oxidation of SiCl 4 in the flame, which is then compared to the measured SiO mole fractions distribution to shed light on the current state of the understanding of SiCl 4 combustion chemistry and to direct further refinements. [ABSTRACT FROM AUTHOR]

Large Eddy Simulations (LES) of turbulent reacting flows carried out with detailed kinetic mechanisms have a key role for the discovery of the physical and chemical processes occurring in combustion systems, and are essential for the development of efficient, stable, and non-pollutant technologies. Nevertheless, these simulations require a large amount of computational resources, making their utilization for large-scale systems, such as industrial burners and gas turbines, impractical. In this work, we combine state-of-the-art machine learning algorithms and model reduction methods to deliver a fully automated strategy for performing LES with adaptive chemistry. This strategy is based on the Sample-Partitioning Adaptive Chemistry (SPARC) algorithmic procedure, which consists of four steps: the generation of a training dataset, its partitioning in clusters, the generation of a set of reduced chemical mechanisms specifically tailored to each cluster and, lastly, the numerical simulation of the case of interest with adaptive chemistry enabled by an on-the-fly classification of every grid point. The SPARC approach has already been demonstrated to substantially reduce the computational effort of reactive flows simulations. However a non-negligible level of user interventions is needed, upon which the method's success critically depend. Therefore, with the goal of boosting the performance of this workflow and minimise the user-specified degrees of freedom, we plug in and exploit the Local Principal Component Analysis augmented with an automated Bayesian-optimised search for optimal clustering solutions, and the Computational Singular Perturbation method with an additional layer of automation based on the Tangential Stretching Rate for minimally-sized reduced mechanisms. We employ a cheap and easy-to-generate 1-dimensional-flames training database and we demonstrate the efficiency, accuracy and robustness of this strategy with an application to LES of the Adelaide Jet in Hot Coflow (AJHC) burner, a turbulent reacting flow exhibiting intense turbulence-chemistry interactions. [ABSTRACT FROM AUTHOR]

Nanosecond repetitive pulse discharges (NRP) are a promising technology to promote the ignition of a lean reactive mixture. The success or failure of ignition results from complex interaction phenomena between the plasma generated by the discharge and the properties of the turbulent flow. This work presents two approaches for modeling plasma-assisted ignition scenarios resulting from NRP discharges. The first consists of a low-order model based on dimensionless analysis, while the second involves a Large Eddy Simulation (LES) of the reactive turbulent flow that requires high-performance computing. Both methods are presented and successfully applied to an experimental setup, which was specifically designed to study the influence of flow operating conditions and electrical discharge properties on plasma-assisted flame ignition. The simple dimensionless analysis can identify coupled and decoupled regimes, which always lead to ignition success or failure, respectively. However, the model is too crude to predict the fine interactions between plasma kinetics, combustion chemistry, and flow dynamics that control the formation of the reactive core in partially coupled regimes. The LES provides a means to capture these complex interaction mechanisms. A quantitative analysis of the reactive kernel growth has been performed by numerically reconstructing the OH-PLIF signal measured by the camera. A very good agreement is observed, validating the numerical strategy for plasma assisted combustion. [ABSTRACT FROM AUTHOR]

Transported probability density function (TPDF) methods are suitable for modelling turbulent reactive flows. One of the main challenges is to accurately model the molecular mixing terms. In TPDF mixing models, it is desired that the principle of localness is satisfied so that the molecular mixing is performed locally in both physical and composition spaces. The multiple mapping conditioning (MMC) mixing model can ensure mixing localness without violating other desired principles. The present study examines three MMC-like mixing models, including the original MMC (OMMC) mixing model, shadow-position mixing model (SPMM) and a conceptually simplified multiple mapping conditioning (SMMC) mixing model. Three direct numerical simulation (DNS) datasets modelling turbulent nonpremixed ethylene flames with increasing levels of extinction are used for model evaluation. The DNS datasets are also used to provide both initial conditions and inputs needed over the course of TPDF runs to remove the uncertainties caused by turbulence closure, allowing the study to focus on the molecular mixing model. The mixing model coefficients are specified analytically by reference to a canonical mean scalar gradient (MSG) flow in order to achieve specifiable dissipation rate and user-controllable localness. The results show that the MMC-like mixing models yield similar prediction of flame extinction and reignition if the coefficients are properly specified. The MMC-like mixing models can also be tuned to achieve a desired level of localness. For conditional statistics, the MMC-like mixing models can yield correct level of conditional variances. By changing localness, the MMC-like models can yield solutions resembling the interaction by exchange with the mean (IEM) or Euclidean minimum spanning tree (EMST) mixing model for extreme parameter choices. The models differ in their abilities to specify unconditional dissipation rates, at least in the flow considered, and in their ease of implementation. [ABSTRACT FROM AUTHOR]

A two-dimensional laboratory-scale DNS of a knocking experiment using a stoichiometric n -C 7 H 16 /O 2 /Ar mixture at engine-like condition with the latest reduced SIP isooctane kinetics was conducted. A compressible reactive flow solver PeleC combined with an in-house efficient chemical kinetics solver MACKS was used. The results of DNS were compared with a "knocking experiment by Kyushu University" conducted in a 14 × 14 × 80 mm rectangular constant volume chamber for the same mixture at an initial temperature of 480 K and pressure of 0.33 MPa. The comparisons showed the present DNS with the latest, validated reduced chemical kinetics successfully reproduced the knock onset timing within an error of 2% as well as the overall features of the experimental observations seen in the Schlieren images such as flame shape transitions, pressure history, and the characteristic faint increase in pressure due to the flame front attachment with the chamber wall. In addition, the present DNS captured that the cool flame ignition occurred homogeneously but accompanied by the rapid formation of density gradients vertical to the side walls in the unburned gas region. [ABSTRACT FROM AUTHOR]

This work demonstrates that a hybrid approach for linear thermoacoustic stability analysis that combines the Linearized Navier–Stokes Equations (LNSE) with a global Flame Transfer Function (FTF), generates spurious entropy waves when used to model acoustically forced premixed flames. The inability of the global FTF to account for the effects of flame movement is identified as the root cause of this unphysical behavior. Utilization of a local FTF, which resolves unsteady heat release on scales comparable to the reaction zone of the flame, suppresses the spurious entropy perturbations. This affirms that fine-grained resolution of the spatio-temporal distribution of heat release rate fluctuations in the combustion zone is required to model the movement of the flame front, even for acoustically and convectively compact flames. As an alternative to hybrid models, a Linearized Reactive Flow (LRF) approach is employed, which extends the LNSE by the linearized species transport equations as well as the reaction mechanism. Such a monolithic approach inherently accounts for the locally resolved flame dynamics, including the movement of the flame front, and does not require an external model for the flame-flow interaction. Thus the LRF eliminates the need for the cumbersome identification of a local FTF. Two configurations of lean premixed methane-air flames, i.e. a freely propagating 1D flame and a 2D flame anchored in a duct, are considered for validation. All results obtained with linearized modeling approaches and conclusions deduced thereof are validated against high resolution CFD results with excellent quantitative accuracy. [ABSTRACT FROM AUTHOR]

Recently, reconstruction integrating view registration (RIVR) has been demonstrated as an improved method to significantly enhance the accuracy of three-dimensional (3D) measurements in nonreactive flows. This work extended the RIVR method to 3D measurements of highly turbulent reactive flows with two specific goals. The first goal was to examine if the RIVR method can be effectively applied to highly turbulent flame structures, which display distinctively different spatial features from nonreactive flows. This examination of RIVR was performed specifically using two performance metrics, accuracy and spatial resolution. The second goal was to quantify the end benefits the RIVR can bring about on key flame properties involved in turbulence-chemistry interaction, such as flame surface density. The results demonstrated that the RIVR method can effectively enhance reconstruction accuracy of the thin flame front marked by CH radicals in 3D distribution. Compared to past methods, the RIVR method reduced the reconstruction errors by ~48% on average and improved the spatial resolution by ~26% on average. Such accuracy enhancement ultimately led to a ~15% improved accuracy on the determination of the 3D flame surface density as an indicator of the total burning rate. [ABSTRACT FROM AUTHOR]

We pursued a quantitative comparison of the continuum and mesoscale simulations of metal particle reactions in a condensed explosive. The continuum or homogeneous approximation assumed a single combustible mixture; contrastingly, in the present heterogeneous treatment of a mixture, aluminum and 1,3,5,7-tetranitro-1,3,5,7-tetrazocane were considered independent reactive granules of micron size with binder being inert material with strength. Both simulations similarly estimated the steady detonation velocity and peak pressure, while the early flame acceleration transients, marked by a complex flow structure behind the flame front, were observed only in the mesoscale simulation. The precise multi-interface processing techniques, combined with granule-level reactive flow models developed in this study, are deemed applicable to general heterogeneous solid fuel combustion studies related to granules of various size, shape, and reactivity. [ABSTRACT FROM AUTHOR]

A methodology is developed where the acoustic response of a swirl stabilized flame is obtained from a reduced set of simulations. Building upon previous analytical flame transfer functions, a parametrization of the flame response is first proposed, based on six independent physical parameters: a Strouhal number, the mean flame angle with respect to the main flow direction, the vortical structures convection speed, a swirl intensity parameter, a time delay between acoustic and vortical perturbations, as well as a phase shift between bulk and local velocity signals. It is then shown how these parameters can be deduced from steady and unsteady simulations. The methodology is applied to a laboratory scale premixed swirl stabilized flame exhibiting features representative of real aero-engines. In this matter, cold and reactive flow Large Eddy Simulations are first validated by comparing results with reference data from experiments. The high fidelity simulations are seen to be able to capture the flame structure and velocity profiles at different locations while forced flame dynamics for the frequency range of interest also match the experimental data. From the same analytical transfer function model, three methodologies of increasing complexity are presented for the determination of the model parameters, depending on the available data or computational resources. A first estimation of the flame acoustic response is obtained by evaluating parameters from a single stationary flame simulation in conjunction with analytical estimations for the acoustic-convective time delay. Flame dynamics and swirl related parameters can then be determined from a series of robust treatments on pulsed simulations data to improve the model accuracy. It is shown that good qualitative agreement for the flame transfer function can be obtained from a single non-forced simulation while quantitative agreement over the frequency range of interest can be obtained using additional reactive or non-reactive pulsed simulations at one single forcing frequency corresponding to a local gain minimum. The method also naturally handles different perturbation levels. [ABSTRACT FROM AUTHOR]

Large eddy simulations (LES) of non reactive and reactive flows in a cavity-based scramjet combustor configuration from the U.S Air Force Research Laboratory (AFRL) are performed. These simulations feature a 22 species and 206 reactions chemical scheme for ethylene/air. The ability of LES to reproduce the main features found in the experiment is first emphasised such as the average velocity field and the stability of the combustion for the case studied. The influence of the mesh resolution and of the thermal wall condition on the simulation results is also investigated along with the soundness of the use of a laminar model for the filtered source terms. The results of the simulations with the finest grid (resolution of 100 micrometers in the flame region) are then employed to gain understanding in the flame dynamics. This reactive simulation shows the persistence of the two recirculation zones already present in the non reactive flow. The globally high temperature into the cavity helps to sustain a reactive zone located in the mixing layer above the cavity. Combustion first occurs in a diffusion dominated regime followed by the efficient burning of a well stirred mixture (rich then lean). A significant diffusion dominated burning is also found inside the cavity. The links between the residence time inside the cavity and the efficiency of the combustion are explored along with the velocity/heat release correlation. [ABSTRACT FROM AUTHOR]

A Lattice-Boltzmann model for low-Mach reactive flows is presented, built upon our recently published model (Comb & Flame, 196, 2018). The approach is hybrid and couples a Lattice-Boltzmann solver for the resolution of mass and momentum conservation and a finite difference solver for the energy and species conservation. Having lifted the constant thermodynamic and transport properties assumptions, the model presented now fully accounts for the classical reactive flow thermodynamic closure: each component is assigned NASA coefficients for calculating its thermodynamic properties. A temperature-dependent viscosity is considered, from which are deduced thermo-diffusive properties via specification of Prandtl and component-specific Schmidt numbers. Another major improvement from our previous contribution is the derivation of an advanced collision kernel compatible of multi-component reactive flows stable in high shear flows. Validation is carried out first on premixed configurations, through simulation of the planar freely propagating flame, the growth of the associated Darrieus-Landau instability and three regimes of flame-vortex interaction. A double shear layer test case including a flow-stabilized diffusion flame is then presented and results are compared with DNS simulations, showing excellent agreement. [ABSTRACT FROM AUTHOR]

The 3D structure of a fire front propagating through a homogeneous porous solid-fuel layer was studied numerically at laboratory and field scales. At laboratory scale, wind-tunnel fires propagating through laser-cut cardboard fuel were numerically reproduced, while at field scale, simulations of grassland fires with quasi-infinite fire front were carried out for different wind speeds. These simulations were performed using FIRESTAR3D, based on a multiphase formulation that includes the main physical phenomena governing fire behavior. An unsteady RANS approach and a Large Eddy Simulation (LES) approach were used to simulate the reactive turbulent flow, whereas turbulent combustion was modeled using Eddy Dissipation Concept (EDC). Unlike other 3D wildfire tools available in the community, such as FIRETEC and WFDS, the model is based on an implicit, low-Mach number resolution of the governing equations, and makes no empirical assumptions in the resolution of the radiative transfer equation. The comparison with the experimental data concerned mainly the Rate of Spread (ROS) of fire, the fireline intensity, the flame-zone depth, and the wavelength characterizing the crest-and-trough structure of the fire front along the transverse direction. Particular attention was drawn to the similitude in the fire front dynamics between small and large scales. In order to highlight the physical mechanisms responsible for this dynamics, a dimensional analysis was carried out by introducing Byram's convection number N C based on the fireline intensity and Froude's numbers Fr based on the characteristic wavelength of the fire-front structure. The analysis shows that all the results (wind-tunnel fires and grassland fires, experimental and numerical) collapsed on a single scaling law in the form F r = N C − 2 / 3. [ABSTRACT FROM AUTHOR]

The increased modeling complexity needed to simulate high-pressure combustion in rocket engines results in significant computational costs–costs which are not always justified for engineering applications. Multicomponent diffusion computations are at least 40% more expensive than the constant Lewis number diffusion assumption for the simplest hydrogen/oxygen combustion; the computational penalty increases rapidly with the number of species under consideration. The higher-fidelity diffusion modeling is justified in cryogenic fuel combustion if it affects the phase-stability of the propellants. We investigate the impact of the mixture averaged, multicomponent, and constant Lewis number diffusion models of a laminar counterflow flame at trans- and supercritical conditions for typical rocket propellants, namely: hydrogen, methane, and kerosene. Using vapor liquid equilibrium (VLE) theory, we show that, even in the limit cases, the mass diffusion model has a limited effect on the phase stability and pseudo-phase change of the propellants; the majority of the differential diffusion errors are located in the high temperature/ideal gas regions of the flame. The differential diffusion error due to real fluid thermodynamics is at most about 20% of the differential diffusion error which occurs in the ideal gas region of the flame. As a result, the impact of the differential diffusion remains similar to low-pressure combustion conditions, thus supporting the use of engineering-level simplifications for the simulation of these complex high-pressure reactive flows. [ABSTRACT FROM AUTHOR]

An investigation of shock–particle interactions in reactive flows is performed using an Eulerian hydrodynamic method with a hybrid particle level-set algorithm to handle the material interface dynamics. The analysis is focused on the meso- to macro-scale numerical modeling of a granular metalized explosive containing randomly distributed metal particles intended to enhance its blast effect. The reactive flow model is used for the cyclotrimethylene-trinitramine (RDX) component, while thermally induced deflagration kinetics describes the aerobic reaction of the metal particles. The complex interfacial algorithm, which uses aligned level sets to track deforming surface between multi materials and to generate the random shape of granule elements, is described for aluminized and copperized RDX. Then, the shock-induced collapse of metal particles embedded in the condensed phase domain of a high explosive is simulated. Both aluminized and copperized RDX are shown to detonate with a shock wave followed by the burning of the metal particles. The energy release and the afterburning behavior behind the detonating shock wave successfully identified the precursor that gave rise to the development of deflagration of the metal particles. [ABSTRACT FROM AUTHOR]

To deal with the major challenges of combustion simulation with detailed chemical kinetics, a new dynamic adaptive chemistry method with mechanisms tabulation (DAC-ST: DAC with single tabulation) and a method in which DAC-ST is combined with the ISAT (In Situ Adaptive Tabulation) approach (DAC-DT: DAC with double tabulation) to accelerate numerical calculation of large chemical mechanisms in turbulent combustion flows are proposed. Compared to the conventional DAC, DAC-ST expedites calculations by tabulating and reusing the reduced mechanisms, and DAC-DT further expedites the calculations by reducing the number of direct ordinary differential equations (ODEs) integrations and tabulating and retrieving the solutions. The correlation of the two mechanisms is evaluated by a pre-specified criterion comprising temperature, pressure, fuel, oxygen, OH, CH 2 O, and HO 2 , and can be updated by a user-specified threshold value. The proposed methods were validated in a piloted partially premixed methane/air diffusion flame (Sandia Flame D) with the 53-species GRI-Mech 3.0 and a hydrogen fueled model scramjet combustor (DLR) with 9 species and 27 reactions. The results indicate that the two proposed methods can accurately capture the flame structure and that the relative percentage errors of temperature and species concentration are well-controlled and proportional to the threshold values. Further, compared to the conventional DAC, detailed diagnostics show that DAC-ST and DAC-DT reduce the computational overhead of mechanism reduction by factors of 41 and 96 for Flame D and 165 and 365 for DLR, respectively. They also speed up ODE integration by factors of 3.9 and 7.8 for Flame D and 1.6 and 4.0 for DLR, respectively. The successful validation demonstrates that the two proposed methods can be efficiently used in the simulation of reactive flow for detailed kinetic mechanisms, especially for the case with a large number of cells, e.g., DNS. [ABSTRACT FROM AUTHOR]

Aromatic formation in methane combustion is still not well understood. The first step to build kinetic models for aromatic formation is to understand fuel-rich oxidation and formation of aromatic precursors like acetylene and ethylene in varied temperature and equivalence ratio conditions. This understanding is also useful to optimize partial oxidation of methane in fuel-rich conditions. With this view, the current work presents investigation of fuel-rich but non-sooting premixed methane/air flames for equivalence ratios between 1.7 and 6.0 using a micro flow reactor with a prescribed temperature gradient from 300 K up to maximum temperature of 1200 K. Weak flames which represent the ignition behavior of fuel were studied. Concentrations of major stable species, CH 4 , CO, CO 2 , C 2 H 6 , C 2 H 4 and C 2 H 2 were measured using a gas chromatograph with a thermal conductivity detector (GC-TCD). Computations with 1-D reactive flow model using existing kinetic models, GRI 3.0, SD 2016 (San Diego mech), KAUST (USC II) and ARAMCO 1.3 were performed. Change in weak flame structure with change in equivalence ratio was studied. KAUST (USC II) predicted the weak flame positions very well at both equivalence ratios of 2.0 and 6.0, but species mole fractions were not so well predicted. It was found that models with higher rates of CH 3 +O 2 <=> CH 2 O+OH (SD 2016) and CH 3 +HO 2 <=> CH 3 O+OH (GRI 3.0) predicted upstream weak flame positions and thus higher reactivity. However, SD 2016 and GRI 3.0 predicted species mole fractions better, particularly at higher equivalence ratios. All the mechanisms used in the current work underpredicted the mole fractions of C 2 H 2 by factors of four to five at all equivalence ratios addressed here. Reaction path analysis showed significant differences among kinetic models. Particularly, consumption pathways of CH 3 vary greatly among kinetic models leading to different reactivity. Flexibility and applicability of existing mechanisms to predict the current fuel-rich to ultra-fuel-rich CH 4 combustion was discussed. HO 2 radicals, which are generally important at high pressures are found to be significant in current fuel-rich conditions. [ABSTRACT FROM AUTHOR]

Reaction-diffusion(-advection) problems are well-known in chemical engineering and computational fluid dynamics. A common feature of these systems is a linear (or non-stiff) transport sub-system and a non-linear (or stiff), highly coupled chemistry sub-system. The expected numerical effort often prohibits a fully coupled solution of the system. Therefore, the system is split into a transport and a reaction sub-system, and each sub-system is solved using specialized solvers. Operator splitting schemes are required to reconstruct the solution of the initial system from the sub-system solutions. Steady-state preserving splitting schemes are particularly essential for steady-state calculations since local time stepping (LTS) or other methods based on fictional time rely on large time steps to be efficient. This work formally analyzes common splitting schemes for reaction-diffusion problems by stability analysis and checking the steady-state preservation of a representative linear scalar problem. Balanced and Simpler Balanced splitting are the only steady-state conservative schemes analyzed. Three new steady-state preserving splitting schemes are proposed based on the findings of the formal analysis. To achieve steady-state preservation, the new schemes use splitting constants based on either the mixing derivative or the chemistry derivative. The formal analysis is accompanied by a dimensionless perfectly stirred reactor (PSR) case and a hydrogen combustion case, both known to be challenging for operator splitting schemes. The test case results are in line with the theoretical results but indicate that the scalar linear analysis is nonviable to capture the full effects of the chemical sub-system. The Simpler and the newly proposed Consistent Staggered splitting schemes give significantly better results than the remaining ones while being second-order and first-order accurate, respectively. If temporal accuracy is irrelevant, e.g., for steady-state solvers, the proposed Consistent Adaptive splitting scheme is promising since it preserves the steady-state solution first-order time accurate with less function evaluations. [ABSTRACT FROM AUTHOR]

With the dimensionless parameters obtained for syngas/air mixture, Bradley detonation peninsula is often used to determine the detonation development for hotspot autoignition (AI) in reactive flows. In this work, similar numerical simulations were carried out in order to identify the characteristics of detonation peninsula when considering other fuels. Three alternative C 0-1 fuels with detailed chemistry and transport were employed in a 1-D reaction wave propagation induced by temperature gradients, and different critical temperature gradients and hotspot sizes were considered. Meanwhile, the role of detonation parameters in the detonation development outside hotspot was addressed. First, the results show that different AI propagation modes can be well depicted using the dimensionless parameters for individual fuel at various critical temperature gradients. However, the quantitative difference in detonation development regime is significantly observed between different fuels with distinct physical–chemical properties even though similar regime distribution is observed. Second, the evolutions of AI reaction wave propagation outside hotspot were further studied, and combustion mode transitions involving detonation termination and formation were observed. The evolutions of the thermodynamic state of different flow particles show that detonation development is found to switch from constant-pressure to constant-volume combustion. Meanwhile, scaling analysis on combustion mode transitions indicates that besides the early-stage propagation controlled by reactivity gradient in hotspot interior, the reactivity of the mixture outside hotspot also plays an important role in detonation development. This can provide great insights into proposing integrated dimensionless parameters for determining detonation development in the whole reactive flows. [ABSTRACT FROM AUTHOR]

The flamelet method has been used extensively as an affordable turbulence-combustion interaction model that obviates the need to solve the evolution equations for the species mass fractions during a large-eddy or Reynolds-averaged Navier–Stokes calculation of a reactive flow field, leading to substantial savings in the simulation time and enabling modeling with relatively complex kinetic mechanisms. The canonical problem analyzed and stored in a look-up array in the flamelet procedure usually assumes some baseline fields; in particular, the pressure is often specified at a fixed value that is characteristic of the examined configuration. However, pressure in supersonic combustion has significant dynamical roles, unlike in low-Mach number or incompressible flows, and a constant pressure field will not be adequate for the former. To remedy this problem, reaction rate in the combustor is often assumed to scale squarely with pressure. This approach, which is probably acceptable for low-speed, high pressure combustors, is not suitable for dealing with the variable pressure conditions in supersonic combustion. This paper focuses on the assessment of the aforementioned scaling, in absolute sense, and also relative to an approach where pressure is added as a control parameter in the flamelet library. To achieve this, three classes of reactive systems with different levels of modeling complexities are investigated to show that representative chemical variables do not scale squarely with pressure. For the case of supersonic combustion, the scaling treatment in general leads to over-prediction of pressure and combustion and also tends to stabilize the flame. To the knowledge of the authors, no previous studies have reported on the issues addressed in the present paper. [ABSTRACT FROM AUTHOR]

Abstract Accurately predicting key combustion phenomena in reactive-flow simulations, e.g., lean blow-out, extinction/ignition limits and pollutant formation, necessitates the use of detailed chemical kinetics. The large size and high levels of numerical stiffness typically present in chemical kinetic models relevant to transportation/power-generation applications make the efficient evaluation/factorization of the chemical kinetic Jacobian and thermochemical source-terms critical to the performance of reactive-flow codes. Here we investigate the performance of vectorized evaluation of constant-pressure/volume thermochemical source-term and sparse/dense chemical kinetic Jacobians using single-instruction, multiple-data (SIMD) and single-instruction, multiple thread (SIMT) paradigms. These are implemented in pyJac, an open-source, reproducible code generation platform. Selected chemical kinetic models covering the range of sizes typically used in reactive-flow simulations were used for demonstration. A new formulation of the chemical kinetic governing equations was derived and verified, resulting in Jacobian sparsities of 28.6–92.0% for the tested models. Speedups of 3.40–4.08 × were found for shallow-vectorized OpenCL source-rate evaluation compared with a parallel OpenMP code on an avx2 central processing unit (CPU), increasing to 6.63–9.44 × and 3.03–4.23 × for sparse and dense chemical kinetic Jacobian evaluation, respectively. Furthermore, the effect of data-ordering was investigated and a storage pattern specifically formulated for vectorized evaluation was proposed; as well, the effect of the constant pressure/volume assumptions and varying vector widths were studied on source-term evaluation performance. Speedups reached up to 17.60 × and 45.13 × for dense and sparse evaluation on the GPU, and up to 55.11 × and 245.63 × on the CPU over a first-order finite-difference Jacobian approach. Further, dense Jacobian evaluation was up to 19.56 × and 2.84 × times faster than a previous version of pyJac on a CPU and GPU, respectively. Finally, future directions for vectorized chemical kinetic evaluation and sparse linear-algebra techniques were discussed. [ABSTRACT FROM AUTHOR]

Abstract A new Lattice-Boltzmann model for low-Mach reactive flows is presented. Based on standard lattices, the model is easy to implement, and is the first, to the authors’ knowledge, to pass the classical freely propagating flame test case as well as the counterflow diffusion flame, with strains up to extinction. For this presentation, simplified transport properties are considered, each species being assigned a separate Lewis number. In addition, the gas mixture is assumed to be calorically perfect. Comparisons with reference solutions show excellent agreement for mass fraction profiles, flame speed in premixed mixtures, as well as maximum temperature dependence with strain rate in counterflow diffusion flames. [ABSTRACT FROM AUTHOR]

Abstract This study proposes a clustered dynamic adaptive chemistry (CDAC) method, which uses an iterative K-means algorithm to partition the computational cells into different groups according to their similarity in terms of the temperature and significant species compositions. Taking advantage of the clustered cells, the averaged thermo-chemical properties of the cells in the respective clusters are then used to identify the adaptive dynamic reduced chemistry through the method of direct relation graph with error propagation (DRGEP). Moreover, the integration of the chemical source term, which commonly dominates the computational effort in reactive flow simulations, is now performed using the dynamic adaptive reduced chemistry at the cluster level instead of the cell level. With this CDAC method, the on-the-fly DRGEP process as well as the chemistry integration only needs to be conducted at the cluster level, dramatically reducing the unnecessary repeated computation for similar computational cells. In addition, the adaptive dynamic reduced chemistry further accelerates the chemistry integration process due to less ordinary differential equations (ODEs) to be solved for each cluster. This newly proposed CDAC method was tested in multi-dimensional homogeneous charged compression ignition engine (HCCI), direct injection compression ignition (DICI) engine and constant volume chamber combustion (CVCC) fueled with diesel, biodiesel and kerosene through the use of their respective surrogate fuel mechanisms under different operating conditions. The performance of CDAC in terms of accuracy and efficiency were extensively analyzed and discussed using different user-defined parameters, mechanisms with different surrogate components and number of species as well as different meshes with different number of grid cells. Based on this analysis, the error tolerances in DRGEP and the error tolerances of the temperature and significant species’ mass fraction are recommended as: ε d ≤ 0.001, ε T ≤ 20 K and ε Y ≤ 0.01 to achieve less than 0.1% integral error. With these recommended user-defined tolerances, it can be observed that the current CDAC method is able to accurately predict the in-cylinder pressure as well as the species profile in HCCI, DICI and the flame lift-off length in CVCC when compared with the full chemistry calculations as well as the experimental results. Moreover, with the recommended user defined error tolerances and a chemical kinetic model of 112 species, this CDAC method is capable of achieving a computational time speed-up factor of more than 3 compared to the conventional DAC and of almost 5 compared to the full chemistry. Finally, the CDAC method coupled CFD was applied to simulate a diesel DICI engine under three different engine speeds with a detailed primary reference fuel (PRF) mechanism of more than 1000 species. The 3-D simulation could be finished in acceptable CPU time while being able to well capture the experimental in-cylinder pressure and heat release rate for all the three engine speeds. [ABSTRACT FROM AUTHOR]

Experimental data from the Cambridge-Sandia burner are used to calculate the conditional average of temperature and the mass fractions of H 2 O, CO, and CO 2 in the context of the Uniform Conditional State (UCS) model. The performance of various presumed joint PDF models are examined for this burner working with different swirl and stratification levels. All of the presented PDF models are based on a few statistical moments of the conditioning variables which are extracted from the data at each measuring point. Primarily, only mixture fraction and reaction progress variable are employed as the conditioning variables. The β function is employed for modeling the marginal PDF of mixture fraction in all cases. For the progress variable, the β function and Modified Laminar Flamelet (MLF)-PDF are tested for modeling the marginal PDF. It is shown that even though the MLF-PDF results in more accurate predictions overall for most of the cases, its superiority with respect to the β -PDF is relatively small in the stratified flames. The performance of the product, Plackett, and Gaussian copulas are also investigated for employing the cross-correlation of mixture fraction and progress variable in building their joint PDF. It is found that the Gaussian copula shows a superior performance over the other two. Next, normalized total enthalpy is employed as a third conditioning variable, and different PDF models are examined for the three-condition version of UCS. It is found that even though adding the third conditioning variable can improve the predictions by considering the effect of heat transfer on the chemistry manifold, such improvements depend on having a proper 3D PDF model for the conditioning variables. In the vicinity of the flame brush, the models employed for the 3D PDFs represent an inferior performance compared to the two-condition version with the use of a Gaussian copula due to complications in the marginal PDF of normalized total enthalpy in these regions. To benefit from considering the impact of heat transfer on chemistry while also avoiding complexities in modeling the 3D joint PDF near the reaction zone, a switch between the two- and three-conditional models which is triggered by the variance of progress variable is proposed. It is shown that the predictions obtained by the use of this switch are almost always superior to all other PDF models investigated in this study. [ABSTRACT FROM AUTHOR]

While energetic material reactive burn processes such as the buildup to detonation are well understood for energetic materials like PBX 9501 that contain small amounts of porosity, the same is not true for highly cracked energetic materials. This work addresses this shortcoming by performing theoretical reactive burn calculations where thin submillimeter thick cracks permeating a PBX 9501 system are explicitly modeled. We use the Scaled Uniform Reactive Flow (SURF) model for the reactive burn calculations. Before considering highly cracked structures, we first show that SURF closely reproduces reactive burn experiments done on pristine and single air gap PBX 9501. For all cases of this investigation, a high-pressure shock is introduced by way of a high velocity impact to initiate a detonation in our cracked PBX 9501 structures. Using fully three-dimensional simulations and the physics code PAGOSA, we investigate the reactive burn process in cracked structures that have a propensity to have cracks either aligned with or orthogonal to the direction of wave propagation. While each crack structure has the same net crack porosity, we demonstrate that the reactive burn produced differs for the two cases. We also provide crack structure metrics, beyond porosity, that characterize the average crack orientation relative to the direction of wave propagation. Simulation results are provided for several variants of cracked structures. At the highest crack porosity studied here we observed the breakdown of stable detonation propagation. This high porosity case was achieved by doubling the average crack width, leaving all other features of the crack network unchanged. We conclude with a synthesis of our simulation observations to make general comments regarding how crack networks can affect points on the PBX 9501 detonation pop plot. [ABSTRACT FROM AUTHOR]

The concept of heterogeneous multiscale method (HMM) is applied into combustion chemistry integration in reactive flows in this study. It adopts a macro solver to integrate the slow variables with global time steps and a micro solver to integrate the fast variables with smaller inner time steps. Explicit or implicit schemes with different orders of accuracy could be applied in the macro and micro solvers. In this study, we formulated HMM with a first-order Euler forward scheme in the micro solver and a second-order trapezoidal rule in the macro solver. An adaptive time step method was proposed to better evaluate a proper time step and control the error in this HMM. This formulation of HMM with the adaptive time step method has been implemented and tested in homogeneous constant volume auto-ignition in hydrogen, methane and butane mixture under various initial temperatures, pressures and equivalence ratios. The speed-up factor of 100–700 has been achieved compared with the implicit solver DASAC. In addition, the numerical accuracy and stability of this HMM formulation have been discussed. It was also applied into multi-dimensional Reynolds-averaged Navier–Stokes (RANS) engine simulations and good efficiency and accuracy was accomplished. In order to better optimize its efficiency performance in multi-dimensional reactive flow simulations, a hybrid scheme with VODE and HMM was proposed and tested in non-homogeneous charged engine simulations. In sum, the concept of HMM in stiff combustion chemistry integration is numerically stable, accurate and efficient. [ABSTRACT FROM AUTHOR]

A correlated dynamic adaptive chemistry and transport (CO-DACT) method is developed to accelerate numerical simulations with detailed chemistry and transport properties in a reactive flow. Different sets of phase parameters, which govern the transport properties and chemical reaction pathways, respectively, are proposed to identify the correlated groups for transport properties and reaction pathways in both temporal and spatial coordinates. The correlated transport properties and reduced chemical mechanisms in phase space are dynamically updated by different user-specified threshold values. For the calculation of detailed transport properties, the mixture-averaged diffusion model is employed. For the on-the-fly generation of reduced chemistry, the multi-generation path flux analysis (PFA) method is used. In the present method, the chemical reduction and transport properties calculation are only conducted once for all the computation cells in the same correlated group within the pre-specified thresholds. Therefore, without sacrificing accuracy within the range of uncertainty of mechanisms and transport properties, the CO-DACT method can eliminate all redundant chemistry reductions and transport properties calculations in temporal and spatial coordinates when the transport properties and chemical reaction pathways are correlated due to the similarities in phase space. The CO-DACT method is further integrated with the hybrid multi-timescale (HMTS) method to achieve efficient integration of chemistry. Simulations of outward propagating spherical premixed flames and one dimensional (1D) diffusion ignitions of a jet fuel surrogate mixture, as well as an unsteady spherical propagating diffusion flame of a DME/air mixture are conducted to validate the present algorithm. The impact of the selection of threshold values as well as the dependence of numerical errors on pressure and equivalent ratio are also examined. The results demonstrate that the CO-DACT method can increase the computation efficiency for transport properties by at least two-order of magnitudes. Moreover, it is robust, accurate, and improves the overall computation efficiency involving a large kinetic mechanism. [ABSTRACT FROM AUTHOR]

Polymeric fuels with a cylindrical shape are widely found as forest combustibles, building components, and electrical cables and wires. Their flammability is commonly assessed using cone calorimetry, despite the fact that the exposed heat flux is well defined only for flat samples. This has led to great difficulties when trying to use the experimental data for calibrating pyrolysis models, which often treat the problem as one-dimensional. This study aims at increased understanding of the combustion of cylindrical fuels in cone calorimeter by carrying out experiments and two-dimensional numerical simulations on black, 20 mm diameter Poly(methyl methacrylate) rods. The solid-phase heat transfer and pyrolysis are modeled using a rectilinear 1 mm mesh, and the reactive flow field is solved by LES and a single-step mixing-controlled combustion reaction. The model is validated with the results of the gasification and flaming experiments with one or five rods under 50 kW m − 2 irradiation. For the single rod measurements, a steep vertical shrinkage in gasification, and roughly equal vertical and horizontal degradation rates in flaming were observed. Degradation patterns of the five rods experiments consisted of a ∪ -shape trend in gasification, and a ∩ -shape trend in flaming condition. The numerical simulations reproduce these deformation trends with a favorable accuracy for all cases. The ignition delay time of the five rods case was detected shorter than the single rod case, which is in consistency with the available literature. The model allowed for the extraction of comprehensive thermodynamic information on the surface of the samples. With flaming, the incident heat flux at the top of the sample increased by 15 kW m − 2 . Flame-induced convective heating is most significant on the sides of the single rod case or on the outermost rods in the five rods case, with a distribution that peaks to about 30 kW m − 2 . The current framework can be a basis for extension to modelling more complex cylindrical material such as cables and thermal insulations. [ABSTRACT FROM AUTHOR]

This work carries out one-dimensional simulations on spontaneous initiation and development of detonations from hot spots with initial temperature gradients embedded with non-autoignitive H 2 O 2 mixtures. Role of temperature-gradient steepness on the development of detonations inside and outside hot spots and the relationship of it to ozone sensitization are examined by changing gradient slope and O 3 addition. It is found that steepening temperature gradient has negative effect on initiations and a critical gradient is identified for different O 3 additions, below which spontaneous initiation fails. However, the sustenance of a developing detonation is not only related to the steepness, but also to the reactivity of mixtures with O 3 additions. High O 3 additions permit a detonation to be more likely to sustain outside hot spots; while for low O 3 additions, the sustenance depends nonmonotonically on the slope, which cannot be predicted by the idealized reactive flow system with a constant specific heat ratio and the reduced reaction model. The results clarify the suggestions of He & Clavin [1] that temperature gradient may prevent sustenance of a developing detonation and the transmission of detonation to the uniform fresh mixture. [ABSTRACT FROM AUTHOR]

A detailed study on the spray local flow and flame structure has been performed by means of PIV and laser-sheet LIF techniques under Diesel spray conditions. Operating conditions were based on Engine Combustion Network recommendations. A consistent comparison of inert and reacting axial velocity fields has produced quantitative information on the effect of heat release on the local flow. Local axial velocity has been shown to increase 50–60% compared to the inert case, while the combustion-induced radial expansion of the spray has been quantified in terms of a 0.9–2.1 mm radius increase. As a result, the drop in entrainment rate has been quantified around 25% compared to the inert case. Streamline analysis also hints at a reduced entrainment under reacting conditions. A 1D spray model under reacting condition has been used, which confirms the modifications obtained in the main flow metrics when moving from inert to reacting conditions. When comparing the flow evolution with the flame structure, little effect of chemical activity on the spray flow upstream the lift-off length has been evidenced, in spite of the presence of formaldehyde in such regions. Only downstream of the lift-off length, as defined by OH LIF, has a strong change in flow pattern been observed as a result of combustion-induced heat release. [ABSTRACT FROM AUTHOR]

Several assumptions of atmospheric-pressure (atmospheric- p ) single-phase turbulent reaction rate models are examined for high- p reactive flows having turbulent characteristics. The study uses a Direct Numerical Simulation (DNS) database described elsewhere (Bellan, 2017). This database was obtained with a model combining multi-species mixing under high- p conditions, a real-gas equation of state (EOS) and a single-step chemical reaction. The database, created in the configuration of a temporal mixing layer, probes the effect of the initial Reynolds number, Re 0 , of the initial pressure, p 0 , and of the initial composition of the two mixing-layer streams. The reaction is initiated in a turbulent flow and in each simulation the computations are pursued past a time when a maximum average-volumetric p is attained, t p p * . The examination of the vorticity and enstrophy equations at a time before reaction initiation and also at t p p * highlights the pivotal role of the high density-gradient magnitude regions (Bellan, 2017) in producing turbulence and the crucial role at t p p * of the baroclinic effect representing the misalignments of gradients of thermodynamic variables. Compared to the classical mixture fraction equation, the results show that the mixture fraction obeys an equation in which there is an additional diffusion term having a larger r.m.s. magnitude than that of the mixture-fraction typical diffusion term. An assessment of the Conditional Source-term Estimation (CSE) model assumptions found that, using an accurate mixture fraction probability density function (PDF), one can obtain a very good representation of the turbulent reaction rate in the most intense reaction regions, however the quality of the predictions of CSE deteriorated significantly when a common model for the PDF, the β -PDF, was used despite the β -PDF being constructed with the accurate moments extracted from the DNS. In the regions of minimal reaction, the CSE model using the exact PDF extracted from the DNS does not provide an accurate representation of the turbulent reaction rate, a fact which is attributed to the combined effect of lack of correlation between fluctuations of the reaction rate and of the mixture fraction, and to the strong correlation of the thermodynamic variables through the real-gas EOS in regions of colder and denser fluid. [ABSTRACT FROM AUTHOR]

The simulation of complex reactive flows in combustion applications generally requires a large amount of computational power, and the reactive part of the solution is usually the most time consuming. Detailed chemical kinetics mechanisms can reach up to tens of thousands species, while numerical simulations of complex reactive flows can handle mechanisms, at most, with a few hundred species. Virtual kinetic mechanisms are a very effective strategy to drastically reduce the computational time spent in the reactive part of numerical simulations. They are formed by artificial species and reaction paths, that are optimized to reproduce the important characteristics of a canonical problem of interest. This work presents the development of a virtual mechanism able to reproduce the temperature evolution and ignition delay times of the thermal ignition of a homogeneous, constant mass, constant pressure, adiabatic, fuel-air mixture. Three different reaction structures are built for a methane-air mixture and their predictions are tested over a range of temperatures from 1000 K to 1500 K and pressures from 1 atm to 3 atm. The limitations observed are due to the small number of reactions and parameters of the kinetic model. However, mechanism reduction methodologies, such as DRG, may present much larger errors if used to achieve the same mechanism size of the virtual mechanisms presented in the current study. [ABSTRACT FROM AUTHOR]

Jacobian clustering is proposed to reduce the computational cost associated with matrix operations encountered in the Newton iteration in fully coupled, fully implicit schemes for unsteady reactive flow simulations with detailed chemistry. The iterative solver is based on the Lower-Upper Symmetric Gauss-Seidel (LUSGS) algorithm and sparse matrix technique. The evaluation and sparse Lower-Upper (LU) factorization of the diagonal block of the system Jacobian are performed within clusters rather than individual cells for Computational Fluid Dynamics (CFD) simulations. Cells that are close in the state space are clustered to provide the averaged states for calculating the Jacobians of chemical source terms and transport fluxes. The cells retrieve the factorized sparse matrices from the belonging clusters to perform the necessary iterations. For the purpose of clustering, the spatial dependency of transport Jacobian in the diagonal block is eliminated. To further reduce the computational cost, the sparsity of chemical Jacobian is augmented by removing the insignificant matrix elements. The method is tested in various one-dimensional hydrocarbon flames with both the second order Crank-Nicolson scheme and a third order implicit Runge-Kutta scheme. Various chemical mechanisms with 9 to 111 species are used to test the performance of the iterative solver. Fast convergence of Newton iteration is achieved, and the formal order of accuracy is demonstrated with Jacobian clustering. The overall costs of evaluation and factorization of the block diagonal Jacobian are negligible compared to the cost of calculating transport fluxes and chemical source terms. The averaged costs of Jacobian evaluation, LU factorization and Newton iteration, all increase only linearly with the number of chemical species. The fully coupled, fully implicit Crank-Nicolson scheme with Jacobian clustering shows 4 to 42 times speedup in computational time compared to the decoupled implicit scheme with Strang operator splitting. Jacobian clustering is promising to increase the computational efficiency of high order fully coupled, fully implicit schemes for unsteady reactive flow simulations with detailed chemistry. [ABSTRACT FROM AUTHOR]

The development of energy efficient combustion systems is a critical technical objective in the engine and power generation industries. This pursuit often prompts using conditions at the limits of capability, dictating the need to expand the envelope of robust and reliable operation. Nanosecond-pulsed high-frequency discharges (NPHFD) have proven to be effective in extending ignition limits in both quiescent and flowing environments. However, limited research has been conducted comparing the kernel growth behavior in a flowing environment of a train of discrete nanosecond discharges to that of a conventional DC arc discharge. Therefore, this work focused on comparing these two types of discharges by matching the total energy deposited, total deposition time, and average power. For each system, three average power conditions (100, 180, and 400 W) were tested, each including five total discharge times (0.05, 0.15, 0.25, 0.50, and 1.00 ms) at two inter-electrode gap distances of 1 and 2 mm. The 1 mm gap distance results showed that the NPHFD system produced larger ignition kernel areas than the conventional system at the highest average power (therefore, highest pulsation frequency) setting. At all other average power conditions, the NPHFD system produced equivalent or smaller ignition kernels than the conventional system due to a lack of pulse-to-pulse coupling. At the 2 mm inter-electrode gap distance, the NPHFD ignition system produced larger kernels at all conditions except at the lowest average power and discharge duration setting. At the highest average power and longest discharge duration setting, the NPHFD kernel was nearly twice as large in area as the kernel produced by the conventional system at 5 ms after discharge. The larger kernel area created by the NPHFD system at the larger gap distance, higher average power, and longer total discharge duration conditions could be explained by the high level of pulse-to-pulse coupling and discharge-induced jetting behavior. The jetting behavior was a function of electrode polarity and mainly extended the reactive region of the kernel in the spanwise direction. [ABSTRACT FROM AUTHOR]

The interactions of a sinusoidal premixed flame interface with successive shock waves are studied by numerically solving the two-dimensional, reactive Navier–Stokes equations with high resolution schemes. The focus is the development of the perturbations on the flame interface due to the Richtmyer–Meshkov (RM) instability. A single irreversible chemical reaction for a C 2 H 4 +3O 2 +4N 2 mixture is used to represent the combustion process during the interactions. Three cases of interactions, with different initial incident shock wave strengths (Ma = 1.2, 1.7 and 2.2), are studied. The multiple interactions between the flame interface and the shock and its reflection waves lead to interesting new observations that have not been presented before. To parameterize the interactions, we introduce a time scale decoupling assumption, and propose to use two decoupled time scales, i.e., the decoupled chemical reaction time scale τ C and the decoupled RM instability time scale τ RM. Based on this assumption, τ C is considered to be decoupled from τ RM , so that both time scales can be defined and determined independently. Specifically, τ RM is defined as the reciprocal of vorticity magnitudes that can be obtained by performing the computations of two-dimensional RM instability in a chemically frozen flow that removes the effect of chemical reactions. On the other hand, τ C is defined as the ratio of reaction zone width of an un-stretched, laminar, premixed flame to propagating speed of the flame, during the one-dimensional laminar flame propagation without considering two-dimensional RM instability. Using these definitions, our calculations show that both τ C and τ RM decrease with the successive interactions of the shock waves with the flame interface in all three cases, and that τ C is larger than τ RM at early stages of interactions and then rapidly decreases to a level comparable with τ RM at later stages of interactions. We compare the two-dimensional visualizations of the perturbation development on the interface in reactive flows with those in chemically frozen flows. The comparison shows that RM instability plays an important role in early evolution, but chemical reactions dominate later, and that the evolution is correlated with the variations of τ C and τ RM for all cases studied here. The quantitative correlations between the ratio of τ C to τ RM and chemical heat release rate in all three cases are calculated. The good correlations indicate the advantage of the approaches to determine τ C and τ RM proposed in present study. The observation that the ratio of τ C to τ RM is independent of the initial strength of incident shock wave suggests that the ratio reflects the nature of reactive RM instability, hence provides a new dimensionless parameter for understanding such flows. [ABSTRACT FROM AUTHOR]

The one-dimensional turbulence (ODT) methodology is extended to include an efficient compressible implementation and a model for capturing shock-induced turbulence is presented. Lignell et al. recently introduced a Lagrangian ODT implementation using an adaptive mesh. As the code operates in the incompressible regime (apart from constant-pressure dilatation) it cannot handle compressibility effects and their interactions with turbulence and chemistry. The necessary algorithmic changes to include compressibility effects are highlighted and our model for capturing shock–turbulence interaction is presented. To validate our compressible solver, we compare results for the Sod shock tube problem against a finite volume Riemann solver. To validate our model for shock–turbulence interaction, we present comparisons for a non-reactive and a reactive case. First, results of a shock traveling from light (air) to heavy (SF 6 ) with reshock have been simulated to match mixing width growth data of experiments and turbulent kinetic energy results from LES. Then, for one-step chemistry calibrated to represent an acetylene/air mixture we simulate the interaction of a shock wave with an expanding flame front, and compare results with 2D simulation (2D-sim) data for flame brush formation and ensuing deflagration-to-detonation transitions (DDT). Results for the Sod shock tube comparison show that the shock speed and profile are captured accurately. Results for the non-reactive shock–reshock problem show that interface growth at all simulated Mach numbers is captured accurately and that the turbulent kinetic energy agrees in order of magnitude with LES data. The reactive shock tube results show that the flame brush thickness compares well to 2D-sim data and that the approximate location and timing of the DDT can be captured. The known sensitivity of DDT characteristics to details of individual flow realizations, seen also in ODT, implies that model agreement can be quantified only by comparing flow ensembles, which are presently unavailable other than in an ODT run-to-run sensitivity study that is reported herein. [ABSTRACT FROM AUTHOR]

This work presents the results of two-dimensional modeling of the effects of non-equilibrium excitation and electrode geometry on H 2 /air ignition in a nanosecond plasma discharge. A multiscale adaptive reduced chemistry solver for plasma assisted combustion (MARCS-PAC) based on PASSKEy discharge modeling package and compressible multi-component reactive flow solver ASURF+ is developed and validated. This model is applied to simulate the impact of non-equilibrium plasma excitation and electrode geometry and heat loss on the dynamics of the discharge from streamer to spark and ignition kernel development in a H 2 /air mixture with a pair of cylindrical electrodes. The results show that the plasma-generated species (N 2 (A), N 2 (B), N 2 (a ′), N 2 (C), O(1D), O and H) in the spark and afterglow significantly accelerate the ignition kernel development. The increase of discharge voltage at the same total discharge energy promotes the non-equilibrium active species production. It is found that the production of electronically excited species at higher reduced electric field strength is more efficient in enhancing ignition in comparison to the vibrational excitation and heating. Moreover, the 2D simulation clearly reveals that the electric field and active species distribution are highly non-uniform. The streamers are initiated at the sharp outer edges of the negative and positive electrodes by a strong electric field while the electric field is much weaker at the centerline of the electrodes. Furthermore, the simulations reveal that the ignition enhancement is sensitive to the variation of electrode shape, diameter, and gap size due to the changes of electric field distribution and location of streamer formation. A cylindrical electrode produces a larger discharge volume and ignition kernel than the parabolic and spherical electrodes, when the discharge is localized near the axis of the gap. It is found that there is a non-monotonic dependence of ignition kernel size on the electrode diameter and inter-electrode distance. The increase of electrode diameter and gap size above the optimal conditions leads to the reduction of ignition kernel volume, due to the decrease of active species concentration and gas temperature. At a larger electrode surface area and electrode diameter as well as smaller electrode gap size, the heat loss to electrode plays a greater role in reducing the ignition kernel size and slowing ignition kernel development. This work provides insights and guidance to understand the kinetic enhancement of non-equilibrium plasma and the effects of electrode geometries on ignition for the optimization ignitors in advanced engines. [ABSTRACT FROM AUTHOR]

Dimethyl ether (DME), a widely studied alternative fuel, is known to exhibit complex low- and high-temperature oxidation chemistry. It is also the smallest molecule in the families of symmetric ethers and oxymethylene ethers that receive attention as renewable fuels. Thanks to several studies performed in facilities such as shock tubes, jet-stirred reactors and flames, it can be assumed that the DME oxidation is well understood. However, DME oxidation in flow reactors has been addressed comparatively rarely, although this configuration presents an interesting system with influences of both kinetics and fluid dynamics on the reaction behavior. To examine the interplay of both influences and potential uncertainties resulting from such effects, DME oxidation was experimentally investigated over an extended range of conditions in a flow reactor equipped with mass-spectrometric analysis. Quantitative species profiles were obtained at near-atmospheric pressure in a temperature range of 400–1100 K for three equivalence ratios φ (0.8, 1.0 and 1.2), and three flow rates at each stoichiometry. These nine different cases were first analyzed using a detailed chemical reaction mechanism with a Plug-Flow Reactor (PFR) model. In-depth examination of experimental and reaction model uncertainties led to updates in the reaction mechanism that were performed on the basis of most recent, reliable kinetic information. In spite of the good agreement of the PFR model with the experimental data at selected conditions, especially in the low-temperature regime, substantial deviations in the reactivity and associated species profiles were noted in several cases, particularly for lean conditions at low flow rates and intermediate temperatures around and above 700 K. A two-dimensional (2D) computational fluid dynamics (CFD) model was therefore employed to characterize the reactive flow conditions more accurately. Significant contributions of fluid dynamics effects were observed in the cases that presented the most severe deviations, and overall good agreement within experimental uncertainty was obtained for the nine cases with the 2D simulations. With the aid of a mathematical curve matching procedure using a variety of recent, established kinetic mechanisms, it could be convincingly demonstrated under the current conditions that improvements in predictive modeling capability in the sensitive test cases were not a question of improved kinetics but were mainly achieved by considering the two-dimensional reactive flow. As a consequence, the present investigation can serve to alert the community to the potential major influences that might be neglected if standard PFR models are used to predict fuel oxidation without detailed analysis whether the conditions are suited to that approach. [ABSTRACT FROM AUTHOR]

Eulerian stochastic fields (ESF) based transported PDF method has evolved as one of the general purpose methods for the numerical simulation of complex chemically reactive turbulent flows in which the effect of turbulence on the chemical reaction rates can be computed directly without the need of modeling. It is found that ESF method may suffer from violation of the element mass conservation law due to the stochastic Wiener process and the highly non-linear dependence of the chemical reaction rates on the stochastic fields. This paper presents a modified ESF method to remove the error in the element mass conservation. The method is evaluated in numerical simulation of two turbulent flames, (i) Reynolds averaged Navier-Stokes (RANS) simulation of a turbulent non-premixed methane/air counterflow flame under stationary burning and transient flame extinction conditions, (ii) large eddy simulation of swirling turbulent methane/air non-premixed flames under local extinction and re-ignition conditions. The original ESF method violates the element mass conservation in both flames, and the element mass error would not disappear even if a large number of stochastic fields were used. The new method yields a satisfactory prediction of the element mass conservation even with a small number of stochastic fields. The new and original methods predict similar stationary flame structures but the results under flame extinction and re-ignition conditions are rather different. [ABSTRACT FROM AUTHOR]

We derive adjoint equations for the reactive compressible Navier–Stokes equations. Therein the reaction rate is modelled by an Arrhenius approach. The adjoint equations are validated by means of a comparison between the adjoint solution and a finite difference expression. An adjoint based optimisation framework for reactive compressible flows is presented. Formulations for different target functions are shown. One- and two-dimensional laminar flame configurations are presented. We find, that the adjoint approach works well despite the strong non-linearity of the reaction terms. [ABSTRACT FROM AUTHOR]

Abstract: Detailed chemical kinetics is an integral component for predictive simulation of turbulent flames and is important for reliable prediction of flames and emissions. Major challenges of incorporation of detailed chemistry in flame simulations are induced by the large number of chemical species and the wide range of timescales involved in detailed kinetics. In this work, dynamic adaptive chemistry (DAC) and in situ adaptive tabulation (ISAT) for efficient chemistry calculations in calculating turbulent reactive flows with detailed chemistry are studied in iso-octane/air homogeneous charge compression ignition (HCCI) and methane/air combustion in a partially-stirred reactor (PaSR). Chemistry calculations are accelerated by DAC via expediting the integration of ordinary differential equations (ODEs) governing chemical kinetics with local skeletal mechanisms obtained on-the-fly using the directed relation graph (DRG) method, and by ISAT via reducing the number of ODE integrations through tabulating and re-using the ODE solutions. It is shown that, in contrast to ISAT, the performance of DAC is mostly independent of the nature of combustion simulations, e.g., steady or unsteady, premixed or non-premixed combustion, and its efficiency increases with the size of chemical kinetic mechanisms. DAC is particularly suitable for transient combustion simulations with large mechanisms containing hundreds of species or more, such as those for gasoline or diesel fuels. A speedup factor of about 30 is achieved for HCCI combustion of iso-octane/air with good agreements in the histories of temperature and species concentrations. In contrast, ISAT performs better for simulations where chemistry calculations can be predominantly resolved by retrieving from the ISAT table, i.e., re-using the ODE solutions. It is shown that ISAT achieves speedup factors of about 100 with only about 10%, 0.1% and 0.01% incurred errors in NO, CO, and temperature, respectively, for the premixed methane/air PaSR simulations. Moreover, a coupled DAC and ISAT approach, namely ISAT–DAC, has been developed and demonstrated in this study to accelerate chemistry evaluation. It is shown that the incurred errors in temperature and species concentrations in ISAT–DAC are well controlled, and it can significantly enhance the performance of ISAT, when the fraction of direct ODE integration is significant, via accelerating the ODE integrations by DAC. [Copyright &y& Elsevier]

We investigate the sooting propensity of an Alcohol-to-Jet-Synthetic Kerosene with Aromatics (ATJ-SKA) biojet fuel. The soot volume fraction and primary particle size in the pre-vaporised diffusion flames using ATJ-SKA biojet and blends with Jet A-1 at atmospheric conditions were measured experimentally and compared to numerical simulations. The measurements were conducted using extinction calibrated laser induced incandescence (LII). The soot volume fractions measured using the ATJ-SKA fuel do not show significant differences relative to measurements with Jet A-1. A comparison of the chemical composition of the fuels suggests that the Degree of Unsaturation (DoU) may not determine the sooting propensity of biojet fuels. The SEM analysis shows that diffusion flames using neat Jet A-1 produce finer soot particles and larger number density compared to the biojet and biojet surrogate. The soot model employs a semi-detailed chemical kinetic mechanism and a physical model which integrates the population balance equation governing the soot particle size distribution with an in-house reactive flow solver for multicomponent ideal gases. The model predicts the maximum soot volume fraction (SVF m) in the neat biojet case and the blended cases with Jet A-1 fuels within an error margin of 13% of the measured values. However, the predicted soot volume fraction distribution patterns differ from the measured one and the possible causes are discussed. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of fuel distribution on flame stabilization and local combustion modes in a cavity-based scramjet. Two injection schemes with a global equivalence ratio of 0.4 are comparatively studied. Ethylene is injected only from upstream of the cavity in the first case, whereas in the second case 75% of the fuel is injected from upstream of the cavity and 25% of the fuel is injected from the cavity floor. The reactive flow in the scramjet is numerically investigated using a hybrid RANS/LES method. The agreement between calculation results and experimentally measured pressure profiles and CH* chemiluminescence images is good. The simulation results are further employed to gain a fundamental understanding of local combustion modes from different aspects: supersonic vs. subsonic, and premixed (fuel-rich vs. fuel-lean) vs. diffusion by using filter functions. In the upstream-only fueling case, it is found that the flame is stabilized by the cavity shear layer and most of the heat is released by supersonic-premixed combustion. Moreover, subsonic-premixed combustion and supersonic-diffusion combustion are significant, and the leading mode changes along the combustor. In the dual fueling case, a cavity assisted jet-wake stabilized flame is observed with higher combustion efficiency. Diffusion combustion becomes the dominant mode, with similar heat release in subsonic and supersonic regions. Nevertheless, the flashback of a premixed flame along the walls plays an important role, leading to diffusion combustion around the fuel jet. The results elucidate the link between the fuel injection schemes, flame stabilization locations, and local combustion modes. [ABSTRACT FROM AUTHOR]

Abstract: A numerical investigation of the overexpanded reactive gas flow in the ATAC nozzle of rectangular section, equipped with an H2 injection, is carried out. This study is aiming at reproducing the reacting phenomena occurring when air engulfed in the separated region at the tip of the extension meets the H2 rich overexpanded flow. Both steady Reynolds Averaged Navier Stokes (RANS) and unsteady Delayed Detached Eddy Simulation (DDES) approaches are used to model turbulence and compared to each other. The effect of the chemical nature of the mixture (either frozen or reacting) on the flow dynamics is discussed. The computations are compared to wall pressure measurements and flowfield visualizations by instantaneous Planar Laser Induced Fluorescence of OH (PLIF-OH) and OH spontaneous emission. A self-sustained flow oscillation at a frequency of around 1kHz is found numerically, in good agreement with the unsteady wall pressure measurements. An analysis of the space–time characteristics of the propagating disturbances contributes to a better understanding of this phenomenon. [Copyright &y& Elsevier]

This paper builds upon our previous work where we defined distinct physically-based parameters of the Scaled Uniform Reactive Flow (SURF) model of Shaw and Menikoff for heterogeneous high explosives (HE) reactive flow calculations. We call this P hysically- I nformed version of the model PiSURF, or πSURF. Here, we re-derived the model in a manner that resulted in a dependency on the fraction of the overall specific surface area (associated with the void space in a consolidated sample) that is 'activated' by a shock wave. This perspective is appealing and consistent with the mechanism of combustion in this class of materials at lower pressures. We apply the model to three case studies having slight perturbations in the nominal microstructure of the explosive PBX 9502, characterized by a void volume distribution in the 0.1 nm–10 µm void size range, over a wide pressure range (0.01–0.6 Mbar/1–60 GPa). We concluded that the model provided meaningful predictions that agreed with our intuitive expectations, qualitatively supported by experimental evidence, for the entire relevant pressure range in a manner consistent with a more generalized picture of combustion in heterogeneous materials. This approach takes a significant step forward towards the broader goal of a priori calculation of the hydrodynamic behavior of HE from measurable microstructural details. [ABSTRACT FROM AUTHOR]