Simulation Of Ignition Research Articles

Direct numerical simulation of ignition of a single particle freely moving in a uniform flow

In this study, a direct numerical simulation (DNS) of the ignition of a single particle freely moving in a uniform flow is performed to investigate the particle’s ignition behavior in detail. The Arbitrary Lagrangian-Eulerian (ALE) method is employed to compute the six degrees of freedom motion of a particle (Zhang et al., 2015). The computational setting follows the experiment designed by Lee and Choi (2015). The volatile gas that is composed of methane blows out at the particle surface. Its velocity is calculated by Ex-CPD model (Umemoto et al., 2017) and its direction is set perpendicular to the particle surface. The ignition behavior is compared with that observed in the experiment. The effect of the particle’s shape is also investigated. Results show that the ignition delay of the particle and the flame inclined angle are in good agreement with that of the experiment. While examining the combustion of the gas phase by considering the variation of Flame Index (FI), it is found that a premixed and diffusion regions are formed around the particle after the devolatilization starts. The gas phase ignites at the boundary of the premixed and diffusion regions and the flame propagates towards the particle. This causes a rapid increasing of the temperature and the volatile velocity on the particle surface. Finally, a diffusion flame is formed and reaches a stable state around the particle. It is also revealed that the flame keeps spherical despite the spheroidal shape of the particle.

Influence of mixture content on the minimum sparkplug ignition energy of a coal dust suspension in the air

This paper provides a numerical simulation of a coal-dust suspension sparkplug ignition in the air. The mathematical model is based on the dual-velocity two-phase model of the reacting gas-dispersion medium. The model consists of the continuity equation, the impulse–conversation equation, the gas and the particle energy-conversation equations, the particle burn-out equation and the component mass-conversation equations which take into account diffusion and burn-out. The energy equations consider the thermal conductivity and chemical reactions on the particle surface. The research investigates the influence of particle size on the minimum sparkplug ignition energy.

Turbulent jet ignition assisted combustion in a rapid compression machine

Numerical simulations of turbulent jet ignition (TJI) and combustion in a rapid compression machine (RCM) are conducted by a hybrid Eulerian–Lagrangian large eddy simulation/filtered mass density function (LES/FMDF) computational model. TJI is a novel method for initiating combustion in ultra-lean mixtures and often involves one or several hot combustion product turbulent jets, rapidly propagating from a pre-chamber (PCh) to a main chamber (MCh). An immersed boundary method is developed and used together with LES to handle complex geometries and to decrease the complexity and computational cost of the Monte Carlo (MC) particle operations, while maintaining the high accuracy of the hybrid LES/FMDF model. Analysis of numerical data suggests three main combustion phases in the RCM-TJI: (i) cold fuel jet, (ii) turbulent hot product jet, and (iii) reverse fuel-air/product jet. The effects of various parameters (e.g., the igniter location, mixture composition, and wall heat transfer) on these phases are studied numerically. It is found that the turbulent jet features and the MCh combustion are very much dependent on the PCh ignition details. Igniting the PCh at the lower locations close to the nozzle forces the PCh charge to fully participate in the PCh combustion and prevents the unburned fuel leaking to the MCh. It also leads to longer discharge of the PCh hot products into the MCh with more uniform jet velocity, enhancing the MCh combustion. The results predicted by LES/FMDF are found to be comparable with the available experimental data, both qualitatively and quantitatively.

Large eddy simulation of spark ignition of a bluff-body stabilized burner using a subgrid-ignition model coupled with FGM-based combustion models

PurposeSafety improvement and pollutant reduction in many practical combustion systems and especially in aero-gas turbine engines require an adequate understanding of flame ignition and stabilization mechanisms. Improved software and hardware have opened up greater possibilities for translating basic knowledge and the results of experiments into better designs. The present study deals with the large eddy simulation (LES) of an ignition sequence in a conical shaped bluff-body stabilized burner involving a turbulent non-premixed flame. The purpose of this paper is to investigate the impact of spark location on ignition success. Particular attention is paid to the ease of handling of the numerical tool, the computational cost and the accuracy of the results.Design/methodology/approachThe discrete particle ignition kernel (DPIK) model is used to capture the ignition kernel dynamics in its early stage of growth after the breakdown period. The ignition model is coupled with two combustion models based on the mixture fraction-progress variable formulation. An infinitely fast chemistry assumption is first done, and the turbulent fluctuations of the progress variable are captured with a bimodal probability density function (PDF) in the line of the Bray–Moss–Libby (BML) model. Thereafter, a finite rate chemistry assumption is considered through the flamelet-generated manifold (FGM) method. In these two assumptions, the classical beta-PDF is used to model the temporal fluctuations of the mixture fraction in the turbulent flow. To model subgrid scale stresses and residual scalars fluxes, the wall-adapting local eddy (WALE) and the eddy diffusivity models are, respectively, used under the low-Mach number assumption.FindingsNumerical results of velocity and mixing fields, as well as the ignition sequences, are validated through a comparison with their experimental counterparts. It is found that by coupling the DPIK model with each of the two combustion models implemented in a LES-based solver, the ignition event is reasonably predicted with further improvements provided by the finite rate chemistry assumption. Finally, the spark locations most likely to lead to a complete ignition of the burner are found to be around the shear layer delimiting the central recirculation zone, owing to the presence of a mixture within flammability limits.Research limitations/implicationsSome discrepancies are found in the radial profiles of the radial velocity and consequently in those of the mixture fraction, owing to a mismatch of the radial velocity at the inlet section of the computational domain. Also, unlike FGM methods, the BML model predicts the overall ignition earlier than suggested by the experiment; this may be related to the overestimation of the reaction rate, especially in the zones such as flame holder wakes which feature high strain rate due to fuel-air mixing.Practical implicationsThis work is adding a contribution for ignition modeling, which is a crucial issue in various combustion systems and especially in aircraft engines. The exclusive use of a commercial computational fluid dynamics (CFD) code widely used by combustion system manufacturers allows a direct application of this simulation approach to other configurations while keeping computing costs at an affordable level.Originality/valueThis study provides a robust and simple way to address some ignition issues in various spark ignition-based engines, namely, the optimization of engines ignition with affordable computational costs. Based on the promising results obtained in the current work, it would be relevant to extend this simulation approach to spray combustion that is required for aircraft engines because of storage volume constraints. From this standpoint, the simulation approach formulated in the present work is useful to engineers interested in optimizing the engines ignition at the design stage.

Analysis of operator splitting errors for near-limit flame simulations

High-fidelity simulations of ignition, extinction and oscillatory combustion processes are of practical interest in a broad range of combustion applications. Splitting schemes, widely employed in reactive flow simulations, could fail for stiff reaction–diffusion systems exhibiting near-limit flame phenomena. The present work first employs a model perfectly stirred reactor (PSR) problem with an Arrhenius reaction term and a linear mixing term to study the effects of splitting errors on the near-limit combustion phenomena. Analysis shows that the errors induced by decoupling of the fractional steps may result in unphysical extinction or ignition. The analysis is then extended to the prediction of ignition, extinction and oscillatory combustion in unsteady PSRs of various fuel/air mixtures with a 9-species detailed mechanism for hydrogen oxidation and an 88-species skeletal mechanism for n-heptane oxidation, together with a Jacobian-based analysis for the time scales. The tested schemes include the Strang splitting, the balanced splitting, and a newly developed semi-implicit midpoint method. Results show that the semi-implicit midpoint method can accurately reproduce the dynamics of the near-limit flame phenomena and it is second-order accurate over a wide range of time step size. For the extinction and ignition processes, both the balanced splitting and midpoint method can yield accurate predictions, whereas the Strang splitting can lead to significant shifts on the ignition/extinction processes or even unphysical results. With an enriched H radical source in the inflow stream, a delay of the ignition process and the deviation on the equilibrium temperature are observed for the Strang splitting. On the contrary, the midpoint method that solves reaction and diffusion together matches the fully implicit accurate solution. The balanced splitting predicts the temperature rise correctly but with an over-predicted peak. For the sustainable and decaying oscillatory combustion from cool flames, both the Strang splitting and the midpoint method can successfully capture the dynamic behavior, whereas the balanced splitting scheme results in significant errors.

Mathematical modelling of sparkplug ignition of a coal-dust monodisperse suspension in a methane-air mixture

The paper provides the numerical simulation of a coal-dust monodisperse suspension sparkplug ignition in a methane-air mixture. The methane-air mixture is not stoichiometric. The aim of the research is a design-theoretical determination of the minimum ignition energy of coal-dust-methane-air mixture (CDMAM) depending on the methane volume fraction. Simulation has shown that the increase of the methane volume fraction leads to the decrease of the CDMAM minimum ignition energy, and with the expansion of coal particles the minimum ignition energy rises.

Numerical simulation of ignition in pulverized coal combustion with detailed chemical reaction mechanism

A two-dimensional direct numerical simulation with a detailed chemical reaction mechanism that considers 158 species and 1804 reactions is applied to pulverized coal combustion occurring in a mixing layer and the ignition phenomena is investigated in detail. The pulverized coal particles which are initially heated up to 2000K are distributed in a fully developed mixing layer. The distributions of temperature and various chemical species concentrations in terms of the mixture fraction are analyzed during ignition. The results show that the ignition occurs in the rich mixture fraction condition (ξ∼0.2), as the gas temperature is also high in this condition because of the high initial temperature set for the central coal particles. However, once ignition occurs, the combustion reaction dramatically takes place in the condition indicating ξ=0.03–0.06, which is a slightly lean value compared with the stoichiometric value and shows the shortest ignition delay time in the calculation for autoignition of homogeneous mixtures of volatile matter and air (zero-dimensional calculation). In addition, in the combustion reaction zone, high OH radical mass fraction is observed around the conditions of stoichiometric mixture fraction, whereas high CH radical mass fraction is observed in the rich mixture fraction conditions.

A skeletal iso-octane reaction mechanism for low temperature plasma ignition with ozone surrogate

In this work, we present a skeletal reaction mechanism for modeling ignition of iso-octane with low temperature plasma. The low temperature plasma was modeled by its main product ozone as a surrogate. A key chain branching reaction of oxygen radical attack on iso-octane was identified from reaction pathway analysis. A new skeletal reaction mechanism was developed by incorporating the key reaction along with ozone chemistry to the skeletal iso-octane reaction mechanism from Ra and Reitz [19]. Reaction rate constants of the key reactions and five other related reactions were calibrated to match predictions from the comprehensive reaction mechanism from Curran et al. [Combust. Flame 129 (2002) 253–280]. The optimized reaction mechanism shows good agreement with the comprehensive mechanism. It provides sufficient accuracy and computational simplicity to enable practical CFD simulation of low temperature plasma ignition in internal combustion engines.

Three-dimensional simulations of ignition of composite solid propellants

This paper reports three-dimensional direct numerical simulations of ignition of a composite AP (ammonium perchlorate)–HTPB (hydroxyl-terminated polybutadiene) propellant subjected to a constant flux ϕ. The model includes solid heat transfer, gas-phase combustion with global kinetics and explicit description of propellant microstructure. Simulations show that ignition starts from AP particles because of primary AP/binder flame. Go/no-go computations reveal an unreported intermediate regime between go and no-go with apparent quenching followed by a delayed ignition. This delay is linked to a slow flame spreading from localized scattered hot spots on surface. In accordance with experiments, ignition delays deviate from classical ϕ−2 scaling for high flux and low pressure conditions. For intense flux levels, simulations attest deradiation extinction upon flux termination meaning that there is a critical flux above which ignition is no longer possible. The role of AP particle shape and size distribution on ignition delay is studied and predicted to be limited. The effect of propellant surface conditions is also investigated and can lead to substantial effects on ignition delay. Finally, semi-transparent propellants are also considered and low absorption materials result in longer ignition times, reduced ϕ-dependence, and absence of deradiation extinction. This work eventually highlights the importance of microstructure-based details in the physics of composite propellant ignition and opens the way towards better understanding of the role of AP particles.

ERENA: A fast and robust Jacobian-free integration method for ordinary differential equations of chemical kinetics

We herein propose a fast and robust Jacobian-free time integration method named as the extended robustness-enhanced numerical algorithm (ERENA) to treat the stiff ordinary differential equations (ODEs) of chemical kinetics. The formulation of ERENA is based on an exact solution of a quasi-steady-state approximation that is optimized to preserve the mass conservation law through use of a Lagrange multiplier method. ERENA exhibits higher accuracy and faster performance in homogeneous ignition simulations compared to existing popular explicit and implicit methods for stiff ODEs such as VODE, MTS, and CHEMEQ2. We investigate the effects of user-specified threshold values in ERENA, to provide trade-off information between the accuracy and the computational cost.

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability Pign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high Pign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the ST-versus-u′ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.

Computational investigations of low-emission burner facilities for char gas burning in a power boiler

Various variants for the structure of low-emission burner facilities, which are meant for char gas burning in an operating TP-101 boiler of the Estonia power plant, are considered. The planned increase in volumes of shale reprocessing and, correspondingly, a rise in char gas volumes cause the necessity in their cocombustion. In this connection, there was a need to develop a burner facility with a given capacity, which yields effective char gas burning with the fulfillment of reliability and environmental requirements. For this purpose, the burner structure base was based on the staging burning of fuel with the gas recirculation. As a result of the preliminary analysis of possible structure variants, three types of early well-operated burner facilities were chosen: vortex burner with the supply of recirculation gases into the secondary air, vortex burner with the baffle supply of recirculation gases between flows of the primary and secondary air, and burner facility with the vortex pilot burner. Optimum structural characteristics and operation parameters were determined using numerical experiments. These experiments using ANSYS CFX bundled software of computational hydrodynamics were carried out with simulation of mixing, ignition, and burning of char gas. Numerical experiments determined the structural and operation parameters, which gave effective char gas burning and corresponded to required environmental standard on nitrogen oxide emission, for every type of the burner facility. The burner facility for char gas burning with the pilot diffusion burner in the central part was developed and made subject to computation results. Preliminary verification nature tests on the ТP-101 boiler showed that the actual content of nitrogen oxides in burner flames of char gas did not exceed a claimed concentration of 150 ppm (200 mg/m3).

Numerical investigation of ethanol fuelled HCCI engine using stochastic reactor model. Part 1: Development of a new reduced ethanol oxidation mechanism

Ethanol is considered a potential biofuel for internal combustion engines. In this study, homogeneous charge compression ignition (HCCI) simulations of ethanol engine experiments were performed using stochastic reactor model (SRM). Detailed ethanol oxidation mechanism is developed by including NOx reaction in existing detailed oxidation mechanism with 57 species and 383 reactions. Detailed ethanol mechanism with NOx used in this study contains 76 species and 495 reactions. This mechanism was reduced by direct relation graph (DRG) method, which was validated with the experimental results. Existing Lu’s 40-species skeletal mechanism with NO formation were also compared with detailed and reduced mechanisms for predicting maximum cylinder pressure, maximum heat release rate and crank angle position of maximum cylinder pressure in HCCI engine. Reduced mechanism developed in this study exhibited the best resemblance with the experimental data. This reduced mechanism was also validated by measured engine cylinder pressure curves and measured ignition delays in constant volume reactors. The results showed that reduced mechanism is capable of predicting HCCI engine performance parameters with sufficient accuracy. Sensitivity analysis was conducted to determine the influential reactions in ethanol oxidation. Results also show that detailed and reduced mechanism was able to predict NOx emission in good agreement with the corresponding experimental data.

Development and Validation of a Reduced Chemical Kinetic Mechanism for Computational Fluid Dynamics Simulations of Natural Gas/Diesel Dual-Fuel Engines

A reduced chemical kinetic mechanism consisting of 141 species and 709 reactions has been constructed to simulate the combustion of both natural gas and diesel fuels in a dual-fuel engine. Natural gas is modeled as a mixture of methane, ethane, and propane, while the diesel fuel is modeled as n-heptane. The new reduced mechanism combines reduced versions of a detailed n-heptane mechanism and a detailed methane through n-pentane mechanism, each of which was reduced using a direct relation graph method. The reduced dual-fuel mechanism is validated against ignition delay computations with full detailed mechanisms, adiabatic homogeneous charge compression ignition simulations with full detailed mechanisms, experimental premixed laminar flame speeds of CH4/O2/He mixtures at 40 and 60 atm, ignition delay and lift-off length from a diesel spray experiment in a constant-volume chamber, and finally against dual-fuel engine experiments using multidimensional computational fluid dynamics simulations. The engine simu...

Third O2 addition reactions promote the low-temperature auto-ignition of n-alkanes

Comprehensive low-temperature oxidation mechanisms are needed to accurately predict fuel auto-ignition properties. This paper studies the effects of a previously unconsidered third O2 addition reaction scheme on the simulated auto-ignition of n-alkanes. We demonstrate that this extended low-temperature oxidation scheme has a minor effect on the simulation of n-pentane ignition; however, its addition significantly improves the prediction of n-hexane auto-ignition under low-temperature rapid compression machine conditions. Additional simulations of n-hexane in a homogeneous charge compression ignition engine show that engine-operating parameters (e.g., intake temperature and combustion phasing) are significantly altered when the third O2 addition kinetic mechanism is considered. The advanced combustion phasing is initiated by the formation and destruction of additional radical chain-branching intermediates produced in the third O2 addition process, e.g. keto-dihydroperoxides and/or keto-hydroperoxy cyclic ethers. Our results indicate that third O2 addition reactions accelerate low-temperature radical chain branching at conditions of relevance to advance engine technologies, and therefore these chemical pathways should also be considered for n-alkanes with 6 or more carbon atoms.

超希薄・高 EGR 率ヘプタン・空気予混合気の着火特性に関する研究

Direct numerical simulations (DNS) of ignition of an ultra-lean n-heptane/air mixture with high EGR at high pressure have been performed using a reduced kinetic mechanism to investigate the effect of the strain rate of turbulence on ignition criterion and its mechanism. An initial high-temperature region is set for ignition at the center of the computational domain. From one-dimensional DNS results, it is suggested that ignition is supported by the cool flame. Two-dimensional DNS results show that the ignition delay time becomes longer in proportion to the square of strain rate averaged in the high temperature region in the induction period.

Trigger Detection for Adaptive Scientific Workflows Using Percentile Sampling

The increasing complexity of both scientific simulations and high-performance computing system architectures are driving the need for adaptive workflows, in which the composition and execution of computational and data manipulation steps dynamically depend on the evolutionary state of the simulation itself. Consider, for example, the frequency of data storage. Critical phases of the simulation should be captured with high frequency and with high fidelity for postanalysis; however, we cannot afford to retain the same frequency for the full simulation due to the high cost of data movement. We can instead look for triggers, indicators that the simulation will be entering a critical phase, and adapt the workflow accordingly. In this paper, we present a methodology for detecting triggers and demonstrate its use in the context of direct numerical simulations of turbulent combustion using S3D. We show that chemical explosive mode analysis (CEMA) can be used to devise a noise-tolerant indicator for rapid increase in heat release. However, exhaustive computation of CEMA values dominates the total simulation, and thus is prohibitively expensive. To overcome this computational bottleneck, we propose a quantile sampling approach. Our sampling-based algorithm comes with provable error/confidence bounds, as a function of the number of samples. Most importantly, the number of samples is independent of the problem size, and thus our proposed sampling algorithm offers perfect scalability. Our experiments on homogeneous charge compression ignition and reactivity controlled compression ignition simulations show that the proposed method can detect rapid increases in heat release, and its computational overhead is negligible. Our results will be used to make dynamic workflow decisions regarding data storage and mesh resolution in future combustion simulations.

Large-Eddy Simulation of Ignition and Flame Propagation in a Trisector Combustor

Understanding ignition and flame propagation in multisector combustors is critical for aeronautical gas turbine manufacturers. To study this problem, large-eddy simulations using an Euler–Euler approach for sprays were performed in a trisector configuration. The sensitivity of the simulation to the injection diameter, the energy input to the mixture, and the time of fueling were assessed. The flame structure following ignition was shown to be complex and time dependent. The flame propagation through the chamber was analyzed using a formulation for the velocity of isosurfaces of temperature, showing that it was dominated by the gas velocity. In particular, both the swirling flow issuing from the injectors and the dilatation effect that modifies the gas flow aid the flame propagation through the chamber.

Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature and composition inhomogeneities relevant to HCCI and SCCI combustion

The effects of temperature and composition stratifications on the ignition of a lean n-heptane/air mixture at three initial mean temperatures under elevated pressure are investigated using direct numerical simulations (DNSs) with a 58-species reduced mechanism. Two-dimensional DNSs are performed by varying several key parameters: initial mean temperature, T0, and the variance of temperature and equivalence ratio (T′ and ϕ′) with different T−ϕcorrelations. It is found that for cases with ϕ′ only, the overall combustion occurs more quickly and the mean heat release rate (HRR) increases more slowly with increasing ϕ′ regardless of T0. For cases with T′ only, however, the overall combustion is retarded/advanced in time with increasing T′ for low/high T0 relative to the negative-temperature coefficient (NTC) regime resulting from a longer/shorter overall ignition delay of the mixture. For cases with uncorrelated T−ϕfields, the mean HRR is more distributed over time compared to the corresponding cases with T′ or ϕ′ only. For negatively-correlated cases, however, the temporal evolution of the overall combustion exhibits quite non-monotonic behavior with increasing T′ and ϕ′ depending on T0. All of these characteristics are found to be primarily related to the 0-D ignition delays of initial mixtures, the relative timescales between 0-D ignition delay and turbulence, and the dominance of the deflagration mode during the ignition. These results suggest that an appropriate combination of T′ and ϕ′ together with a well-prepared T−ϕdistribution can alleviate an excessive pressure-rise rate (PRR) and control ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. In addition, critical species and reactions for the ignition of n-heptane/air mixture through the whole ignition process are estimated by comparing the temporal evolution of the mean mass fractions of important species with the overall reaction pathways of n-heptane oxidation mechanism. The chemical explosive mode analysis (CEMA) verifies the important species and reactions for the ignition at different locations and times by evaluating the explosive index (EI) of species and the participation index (PI) of reactions.

Improved combustion kinetic model and HCCI engine simulations of di-isopropyl ketone ignition

Di-isopropyl ketone (DIPK) is considered a promising biofuel candidate produced using endophytic fungal conversion. In the current study, homogeneous charge compression ignition (HCCI) simulations of DIPK engine experiments were conducted with single-zone and multi-zone engine models. Both adiabatic and non-adiabatic single-zone HCCI models were explored. The non-adiabatic model employed the Woschni correlation to account for heat transfer observed in the experiments. The HCCI simulations utilized a literature DIPK kinetic model with slight modifications to better reproduce experimental data. The modifications were done by including additional intermediate species and radical reactions, which were not considered in the original reaction mechanism. In addition, zero dimensional simulations were conducted to validate the updated model against limited shock tube and pyrolysis experimental data available in the literature. The single-zone model of HCCI engine with the updated kinetic model provided good agreement for pressure during compression until ignition, however, it over predicted the peak pressure, as expected. The improved kinetic mechanism was able to predict the pressure, heat release rate, and temperature in a 5-zone model of HCCI engine with good agreement to the experiments. Brute force sensitivity analyses revealed that the most sensitive reaction in which DIPK participates is the H-abstraction reaction from the fuel by HO2. Discussions are provided on the validity of the DIPK model in comparison with the parametric engine data over a range of temperature, pressure, equivalence ratio, and engine speed.