Volumetric Heat Release Rate Research Articles

Ammonia as a Fuel for Gas Turbines: Perspectives and Challenges

The paper addresses the prospects and challenges associated with the use of ammonia in gas turbine plants. The complexity of the chemical kinetic mechanisms of ammonia combustion and the insufficient amount of experimental data that can confirm the results still require a lot of scientific research. In this sense, this application is even less technologically mature than other sectors such as the marine engine one. From the point of view of combustion in gas turbines, the techniques in use and being perfected to deal with flame instability, low laminar burning velocity (about 20% of that of methane-air flames), the long ignition delay time and low volumetric heat release rate. In particular, the use of ammonia blended with other combustion enhancers such as hydrogen, methane and alcohols has been analyzed. Mild combustion is also being analyzed as a valid technique for dealing with the difficulties of ammonia combustion. The combustion of ammonia generates significant quantities of nitrogen oxides, both for the thermal mechanism and for the “fuel” one, which must therefore be mitigated, together with the emission of unburnt ammonia. Even in this case, blending with other fuels allows for improved emissions performance. The paper compares the different techniques and the relative advantages both in terms of combustion efficiency and emissions. Finally, the perspectives for the use of ammonia in gas turbines are outlined, highlighting the challenges still to be overcome, in a panorama of energy transition towards an increasingly decarbonized future, with the aim of mitigating climate change as much as possible.

A Multiple-Burner Approach to Passive Control of Multiple Longitudinal Acoustic Instabilities in Combustors

Abstract Presently, passive methods of controlling combustion instability fall short when one considers stabilizing multiple acoustic modes. In this paper, we present a passive control approach based on the locations of the burners to stabilize multiple acoustic modes. The approach is demonstrated using linear stability analysis performed on a canonical open–open rectangular tube enclosing a flame. A linear flame model based on the kinematic description of the flame surface is used. A simultaneous solution method, as opposed to a segregated method, is developed to calculate the mean flow and to evaluate mode shapes and eigenvalues. The stability analysis is performed both on single and on multiple-burner combustors. In the latter case, the axial and transversal arrangement of burners considered preserves the net volumetric heat release rate and exit temperature. The problem of stabilizing the first three acoustic modes is cast as a multi-objective optimization problem for both types of combustors using the location(s) of the burner(s) as decision variable(s). We show that the use of multiple burners markedly increases the stability of the first three modes while not disturbing the combustor design parameters.

Research on neutron diffusion equation and nuclear thermal coupling method based on gradient updating finite volume method

To solve the problem that the neutron diffusion equation has infinite solutions when the deterministic method is used to calculate the neutron diffusion equation, this paper uses the power calculation equation as a supplementary equation to form a differential and integral equation groups with the neutron diffusion equation, and designs a numerical analysis and machine learning coupling algorithm based on the gradient updating finite volume method that can be used to solve the above differential and integral equation groups. The algorithm decouples the multigroup neutron diffusion equations by source iteration. Then the finite volume method is used to solve the one group neutron diffusion equation and the reactor core power. Based on the power difference calculated by two adjacent iterations, the neutron flux density is updated by the gradient descent method until it converges. Finally, the algorithm can be used to calculate the neutron flux density and volumetric heat release rate of each energy group under the specified power. Taking 5 × 5 rod bundle channel as an example, the feasibility of the algorithm is verified in Fluent 18.0 by writing the UDF script. In addition, the thermal–hydraulic parameters such as temperature and velocity in the reactor can be further calculated according to the volumetric heat release rate, and a same set of grids can also be used to achieve the nuclear thermal coupling calculation. This algorithm can provide a new calculation method for the neutron diffusion equation, and also provide a theoretical basis for the development of application algorithms in the field of reactor physics using deep learning.

Inlet pulsation-induced extinction and plasma-assisted stabilization of premixed swirl flames

• The flame and flow dynamics under air flow pulsations are resolved by performing validated LES and simultaneous OH-PLIF/PIV. • Temporal variations of stretch rate and heat release rate can be used as a criterion for flame extinction/reignition. • A novel decoupled simulation which integrates the plasma source terms and the CFD model is developed. • Thermal and kinetic effects of microsecond repetitively pulsed discharge on combustion are quantitatively interpreted. This paper intensively investigates the inlet pulsation-induced extinction of a premixed swirl flame (PSF) and its plasma-assisted stabilization using the microsecond repetitively pulsed (MRP) discharge. A well-designed low-frequency flow pulsation is applied to the air feedline for the mimicking of transient operations/disturbances in practical engines, which exhibits significant deterioration on the lean blowout (LBO) limit. The MRP discharge is utilized to improve the stability of perturbed flames. It extends the LBO limit at a proper time delay between the air flow pulsation and the discharge, with discharge power less than 1% of the combustion power. Further, the validated large-eddy simulations (LES) and the simultaneous OH planar laser-induced fluorescence/particle imaging velocimetry (OH-PLIF/PIV) measurements are performed to capture the unsteady evolution of flames approaching lean blowout. The nonlinear flame response to the flow pulsation quantitatively reveals the phase difference between the maximum local stretch rates ( κ max ) and volumetric heat release rates ( q ̇ c ). A combined effect of the excessive stretch and the reduction of heat release due to flow pulsation can be used to interpret the flame extinction. Finally, a novel LES-ZDPlasKin combined approach, which decouples the discharge and combustion processes, is dexterously developed to simulate plasma-assisted combustion behaviors. The reignition and stabilization due to the plasma effects are well reproduced in the predictive model, pronouncing the indispensable and synergistic thermal and kinetic effects of the MRP discharge on flame stabilization.

Effect of sub-atmospheric pressure on the characteristics of concurrent/upward flame spread over a thin solid

The variation of ambient pressure is a potential tool for studying the driving parameters of fire dynamics and heat release in low-pressure environments such as high-altitude locations, aircraft, and spacecraft. The study of upward flame spread over a solid fuel has direct implications on material flammability and fire development, and low pressure environments have recently gained more attention for the possible comparison with the reduced gravity conditions encountered during space missions. In this work, we consider upward spreading flames over thin acrylic sheets in ambient pressures between 30 and 100 kPa. A forced flow velocity of 20 cm/s is added to the naturally-driven buoyant flow, creating a mixed flow field (natural and forced) that varies with pressure. Flame characteristics such as spread rate and standoff distance are measured from the video analysis of the experiments. It is observed that the former decreases with pressure while the latter increases. The larger flame stand-off distance at low pressures partially explains the decrease of the flame spread rate since the convective heat flux from the flame to the solid decreases. Additionally, volumetric concentrations of the combustion products are measured during the experiments. The results show lower O2 consumption and CO2 production rates at lower pressures. Based on these rates, we could calculate the heat release rate from upward spreading flames at low pressure, providing fundamental information for better understanding pressure-gravity correlations. According to the results, the volumetric heat release rate is proportional to pressure, which is consistent with previous studies on pressure modeling of fires. This suggests that chemical kinetics is not a constraint for the conditions tested in this study, which could help make future flammability tests comparable to low gravity ones.

Generation of skeletal and reduced reaction mechanisms for bio-derived syngas generated from biomass gasification – Experimental and numerical approach

Skeletal and reduced reaction mechanisms replicate the behavior of full reaction mechanism within the band/regime of optimization criteria and provide specific computational advantages for resource-intense multi-physics domain analysis. The current work reports on the development of skeletal and reduced mechanisms for bio-derived Producer gas and Hydrogen-rich Syngas by using GRI Mech 3.0 mechanism. The mechanisms are generated adopting graph-based approach, and timescale analysis are validated for laminar flame speed based on experiments in the equivalence ratios regime of 0.6–1.6 adopting a flame tube apparatus built in-house to mimic a freely propagating double infinity domain premixed reactor. Extending the analysis, the reduced/skeletal mechanisms are numerically validated for ignition delay time, major species profile, and volumetric heat release rate with validity established within the 5% tolerance limit. The current work is a first of its kind to propose optimized mechanism for compositions typical of bio-derived Producer gas and Syngas.

Вариационная формулировка математической модели процесса стационарной теплопроводности с учетом пространственной нелокальности

In creating and using new structurally sensitive materials, it is essential to construct mathematical models which make it possible to describe the materials’ behavior in widely changing external influences. Modeling of non-local media is a class of methods of generalized continuum mechanics. The capability of analyzing mathematical models of a continuous medium can be extended through variational methods. The paper describes the construction of a functional for the problem of stationary heat conduction in a homogeneous body, taking into account the effects of nonlocality and with a temperature-independent heat conduction coefficient. A variational formulation used in the case of a linear problem in combination with acceptable admissible functions allowed us to quantify this effect. The study gives an example of using the variational formulation, which takes into account the influence of the spatial nonlocality of the stationary heat conduction process on the parameters that determine the well-known phenomenon of thermal explosion with an exponential temperature dependence of the volumetric heat release rate in the plate. The quantitative analysis was carried out for two versions of the function that is admissible for the constructed integral functional and describes the possible temperature distribution over the thickness of the plate under consideration. Having compared the obtained results, we opted for the admissible function, as its use leads to a smaller difference between the maximum value of the parameter characterizing the intensity of heat release in the plate and the known result found with no account for the influence of spatial nonlocality

Theoretical Study of Combustion Processes in Aero Gas Turbines for Air Craft Engines

Combustion process involves various physical and chemical processes which govern and control flames initiation in aero gas turbine engines. During certain flying conditions, at full load, unexpected critical situation may take place in such engines called blow off conditions, which leads to flames diminishing in the combustion chamber of such engines. Gas motion, flow velocity and turbulence kinetic energy are the most important parameters in ensuring flame stabilities. These parameters play a tremendous role and effects on this phenomenon. In gas turbines, the flame exists within a high velocity, non-uniform and intensely turbulent flow field, therefore careful temperature control is vital. Another important factor which must be considered to avoid blow off conditions, is mixture strength. Nearly, all modern gas turbines, due to emissions restrictions, operate on lean mixture conditions which are hard to ignite and lower flame temperatures and thus more risk to reach blow off conditions which leads to a complete flame extinction. These conditions may exist in an air craft engines due to sharp changes in loading parameters, (θL): pressure (Pu), temperature (Tu), mass flow rate (), and cross sectional area (Au). At present there is no detailed theory of gas turbine combustion. Therefore, we must resort to simple models and experimental correlations. This paper investigates the blow-off phenomena in aero gas turbine engines, its causes and estimation of required energy to ensure recovery (re-ignition) again inside the combustion chamber. Identifying the conditions at which blow-off takes place and associated loading parameters (θL) which are a function of (A, T, P, and ). The paper also, quantify the recovery conditions (required energy to re-ignition, change in loading parameter (Δq) Power, Required VHRR: (Volumetric Heat Release Rate) and changes in other loading variables (ρ: density, T: Temperature, P: Pressure, and : mass flow rate) tarts with discussing causes of blow off along with effecting operating conditions.

Lewis number effects on lean premixed combustion characteristics of multi-component fuel blends

Variation in natural gas composition, alongside the potential for H2 enrichment, creates the potential for significant changes to premixed flame behaviour. To strengthen fundamental understanding of lean multi-component alternative fuel blends, an outwardly propagating spherical flame was employed to measure the flame speeds and Markstein lengths of C1C4 hydrocarbons, alongside precisely mixed blends of CH4/C2H6, CH4/C3H8 and CH4/H2. Theoretical relationships between Markstein length and Lewis Number are explored alongside effective Lewis number formulations. Under lean conditions, equal volumetric additions of H2 and C3H8 (30% vol.) to CH4 resulted in similar augmentation of burning velocity, however, opposite susceptibility to preferential diffusional instability was noted. At a fixed equivalence ratio of 0.65, limited changes in composition provide a marked change in the premixed flame response with the addition of C2H6 and C3H8 to CH4. For lean CH4/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat-release model resulted in better agreement for lean CH4/C3H8 blends. Modelling work suggests that measured enhancement of lean CH4 flames upon H2 or C3H8 is strongly correlated to changes in volumetric heat release rates and production of H radicals. Furthermore, a systematic analysis of the flame speed enhancement effects (thermal, kinetic, diffusive) of H2 and C3H8 addition to methane was undertaken. Augmented flame propagation of CH4/H2 and CH4/C3H8 was demonstrated to be principally an Arrhenius effect, predominantly through reduction of associated activation energy. Finally, plausible short-term variations in composition with hydrogen-enriched multi-component natural gas flames were investigated experimentally and numerically. At the leanest conditions, small variations in CH4:C3H8 content at a fixed H2 fraction resulted in discernible changes in stretch related behaviour, a reflection of the thermo-diffusive behaviour of each fuel's response.

Effect of environmental conditions on fire combustion regimes in mechanically-ventilated compartments

This publication deals with under-ventilated combustion regimes in case of pool fire in a confined- and mechanically-ventilated enclosure. The objective is to improve the understanding of combustion regimes in a ventilated compartment focusing on the effect of fire size and the ventilation rate. It is based on dodecane pool fire tests conducted on a reduced scale (1.875 m3) enclosure and considering two varying parameters, the pool diameter and the ventilation air flow rate prior to ignition. Three combustion regimes were identified: two steady burning regimes with extinction by lack of fuel or oxygen (fuel burnout) and a transient burning regime with extinction by lack of oxygen. A representation of these regimes is proposed with two scaling parameters: the reference global equivalence ratio, φo, and the volumetric fire heat release rate. Results demonstrate that increasing φo reduces the burning rate in comparison to its value in an open atmosphere. While for a low volumetric fire heat release rate (HRR), the reduction in burning rate is mainly due to oxygen depletion, gas temperature has an additional effect at higher volumetric HRR (about 16 kW/m3). The effect of gas temperature compensates for the effect of oxygen depletion and increases the burning rate. The conditions leading to the highest gas temperature inside the enclosure were obtained in fire scenarios with high volumetric fire HRR and moderate φo.

Laminar flame propagation in nitrogen-diluted stoichiometric H2-N2O mixtures – a numerical study

The paper reports results of the detailed chemical modelling of isobaric deflagrations in stoichiometric H2-N2O-N2 mixtures (N2 = 20 - 60 vol%) at various initial temperatures (300 - 500 K) and various initial pressures (1 - 10 bar): laminar burning velocities, and profiles of temperature, volumetric heat release rate and species concentrations across the flame front. The dependence of the laminar burning velocity on temperature and pressure is examined by empirical power laws, for restricted ranges of pressure (or temperature) variation. For each examined composition, the laminar burning velocity correlates well with the volumetric rate of heat release and with the sum of mass fractions of reactive species in the flame front.

Pengaruh Variasi Arah Aliran Udara pada Stove terhadap Karak-teristik Pembakaran Wood Pellet

This research was conducted to investigate the effect of inlet airflow direction on the combustion characteristics of a wood pellet stove. The direction of the airflow into the wood pellet stove is varied for four methods, namely inlet I, inlet II, inlet III, and inlet IV. At inlets, I, II, and III air is injected into the plenum in the radial direction with the injection points at r = -8, 0, and 8 cm respectively, whereas at inlet IV the direction of airflow into the plenum is in the axial direction with the injection point at r = 0. The combustion characteristics were observed in the wood pellet stove with a continuous fuel feeding system. The combustion characteristics investigated in this research consist of flame visualization, flame temperature, combustion rate, and the efficiency of the wood pellet stove. The results showed that wood pellet stoves with inlet IV had a lower combustion rate and flame height, however, this stove indicates a higher flame temperature and stove efficiency. Air entrance through the inlet IV induces most of the airflow to enter the combustion chamber through the primary channel, compared to that through the secondary and tertiary channels. The primary airflow through the wood pellet encourages a better devolatilization and combustion process. These conditions conduce the flame dimension which is a zone where the combustion reaction occurs is smaller with a higher flame temperature, due to higher volumetric heat release rate. This matter results in better heat transfer from the flame to the test fluid and higher stove efficiency.

Development of extended formulations of the relative concentration of chain carrier method for knock prediction in spark-ignited internal combustion engines fueled with gaseous fuels

This work contributes to improve the understanding of the link between the well-known integral method and the kinetics involved for predicting ignition delay time and knock occurrence crank angle. Extensions of the Relative Concentration of Chain Carriers method, originally developed for predicting ignition delay time under variable thermodynamic conditions of surrogate liquids fuels, were proposed. These extensions are composed of four formulations, which were deduced from the Glassman's kinetic model for predicting knock occurrence crank angle in spark-ignition engines fueled with gaseous fuels. Cases of study of engines fueled with CH4/H2, H2/CO/CO2 and iC8H18/nC7H16 mixtures under Spark Ignition and Homogeneous Charge Compression Ignition mode were considered to assess the performance of the formulations working with hydrogen peroxide (H2O2), formaldehyde (CH2O), hydroperoxyl (HO2) and hydroxyl (OH) as chain carriers. According to the experimental measurements of knock occurrence crank angle for mixtures of CH4/H2, the formulation 1 defined as CC/CCcrit=1, had the best performance working with H2O2 as a chain carrier. Moreover, to understand the role of the so-called chain carrier, a thermal sensitivity analysis, based on the relative contribution of key elementary reactions of the GriMech3.0 chemical kinetic mechanism on the total volumetric heat release rate was carried out. The analysis revealed that the elementary reactions that play an important role in the autoignition of a stoichiometric mixture of CH4/H2/50/50 were the chain propagating reactions R36, R116, and R168, the third body reactions R85 and R33, and finally, the chain branching reaction R119.

Study on the Layering Length in a Railway Tunnel with Semi-transverse Extraction Ventilation

In this study, the long-distance subway tunnel in Qingdao of China adopted semi-transverse extraction ventilation is taken as the research object. STAR-CCM+ software is used to simulate the smoke movement status and study the influence of different exhaust volumetric flow and heat release rates on the smoke layering length in tunnel with the semi-transverse extraction ventilation. Results show that the layering length has a linear decreasing relation with the natural logarithm of the exhaust volumetric flow under different heat release rates. There is opposite relation between the layering length and the natural logarithm of the heat release rate. Based on dimensional analysis and numerical simulations, the prediction model of the layering length and the critical exhaust volumetric flow in tunnel with semi-transverse extraction ventilation is proposed.

Reduced Gas-Phase Kinetic Models for Burning of Douglas Fir

New skeletal chemical kinetic models have been obtained by reducing a detailed model for the gas-phase combustion of Douglas Fir pyrolysis products. The skeletal models are intended to reduce the cost of high-resolution wildland fire simulations, without substantially affecting accuracy. The reduction begins from a 137 species, 4533 reaction detailed model for combustion of gas-phase biomass pyrolysis products, and is performed using the directed relation graph with error propagation and sensitivity analysis method, followed by further reaction elimination. The reduction process tracks errors in the ignition delay time and peak temperature for combustion of gas-phase products resulting from the pyrolysis of Douglas Fir. Three skeletal models are produced as a result of this process, corresponding to a larger 71 species, 1179 reaction model with less than 1\% error in ignition delay time compared to the detailed model, an intermediate 54 species, 637 reaction model with 24\% error, and a smaller 54 species, 204 reaction model with 80\% error. Using the skeletal models, peak temperature, volumetric heat release rate, premixed laminar flame speed, and diffusion flame extinction temperatures are compared with the detailed model, revealing an average maximum error in these metrics across all conditions considered of less than 1\% for the larger skeletal model, 10\% for the intermediate model, and 24\% for the smaller model. All three skeletal models are thus sufficiently accurate and computationally efficient for implementation in high-resolution wildland fire simulations, where other model errors and parametric uncertainties are likely to be greater than the errors introduced by the reduced kinetic models presented here.

Modelling of tree fires and fires transitioning from the forest floor to the canopy with a physics-based model

Wildland fires can take different forms such as surface fire or an elevated crown fire or combination of both. Crown fires are normally originated from surface fires spreading either along the bark of the tree trunks or direct flame contact to low branches with leaves and needles. In the past, surface fire (grassfire) spread simulations were conducted using physics-based models with fidelity. Here, we firstly seek numerically converged results for the burning of a single tree. Previously, numerical convergence for such physics-based fire simulations has been elusive. Subsequently, the linear and Arrhenius thermal degradation sub-models are appraised. For both thermal degradation sub-models grid convergence of the mass-loss rate is achieved with a 50 mm grid. The grid converged simulations also agree with experimental results of a single burning Douglas fir tree. A fire in a modelled tree plantation is then simulated using the linear thermal degradation sub-model. The aim of this part is twofold: one to demonstrate a good modelling practice; and secondly to assess the model capability to simulate transitioning from a forest floor fire to a crown fire leading to a quasi-steady rate of spread. The Kolmogorov–Smirnov test is used to rigorously demonstrate that the simulations on different grid and domain sizes have converged — that is, the results have become independent of the numerical parameters imposed upon the simulation.A physics-based model reproduces many observed features of surface fire to forest fire transition. The crown fire propagates with a quasi-steady rate-of-spread after an initial development period. Analysis of the volumetric heat release rate shows that a surface fire propagates under the crown fire and supplies energy to support the burning of the crown. Overall many features are qualitatively in agreement with other tree and crown fire studies.

Reduction of Nitrous Oxide Emissions of the Boiler E-135-3,2-420 DG During Combustion of the Gaseous Oil Shale Processing Products

The technological and constructive measures implemented on the boiler E-135-3,2-420 DG are provided, which allow reducing the formation of nitrous oxides during combustion of the gaseous oil shale processing products. By reducing the volumetric heat release rate of the furnace, employing special burners, arranging for a staged combustion and recirculating flue gases, it became possible to ensure stable fuel burning throughout the entire operating range of the boiler with a nitrous oxide concentration in flue gases of less than 100 mg/m3 (at O2 = 3%). Incomplete combustion was practically eliminated.

Auto-ignition of near-ambient temperature H2/air mixtures during flame-vortex interaction

This paper demonstrates auto-ignition in reactants at approximately 350 K, upstream of curved H2/air flame surfaces during flame/vortex interaction. Temperature fields were measured using laser Rayleigh scattering during head-on interactions of toroidal-vortices with stagnation flames. Repeatable ignition occurred along the ring of the vortex – slightly towards the center – when it was approximately 1 mm upstream of the wrinkled flame surface. The resultant outwardly propagating toroidal flame led to approximately twice the volumetric heat release rate over the duration of the interaction. The ignition occurred in a region of low kinetic energy dissipation rate that was farther from the flame than the region of maximum vorticity. This region was upstream of positively curved flame segments. The advective time over which the vortex transports flame-generated products to the ignition site corresponded well to the time between the beginning of the flame/vortex interaction and ignition. It therefore is hypothesized that the vortex transports HO2 and H2O2 to the low-temperature, low-dissipation region wherein ignition is promoted by preferential diffusion of H due to the positively curved flame. Evidence of additional ignition pockets was found upstream of other flame wrinkles, preferentially near the highest magnitude flame curvatures. These results provide a novel test case for validating diffusion and low-temperature kinetic models, and also have potential implications for reaction rate closure models.

Thermal Characteristics of a CH4 Jet Flame in Hot Oxidant Stream: Dilution Effects of CO2 and H2O

This study investigates the diluent-dependent diffusion flames of an axisymmetric methane jet in annular hot coflow (JHC) of oxidizer diluted by “inert” N2, CO2, and H2O, respectively. The fuel jet issues at an exit Reynolds number of ≈10 000, while the hot (1300 K) coflow oxygen level (molar fraction) varies between 6 and 23%. To identify the chemical and physical factors, simulations of the diffusion flames diluted by fictitious gases XH2O and XCO2 are also conducted. Inspections and analyses to combustion radicals and heat release rates are made on the stoichiometric sheet. Results show that the stoichiometric length, temperature, and volumetric heat release rate vary drastically with coflow oxygen level or dilution extent. The flame volume increases greatly when replacing the diluent N2 with CO2 but reduces substantially under the H2O dilution. Such discrepancies are found to stem mainly from specific physical properties of N2, CO2, and H2O. The CO2 and H2O dilutions show more chemical impact on the formations of key intermediate species. Besides, the dilution by CO2 weakens all the main reactions, thus producing the mildest flame, whereas that by H2O promotes the formations of radicals OH and H2, enhancing the heat release rate. Last, the global heat contributions of C-included and C-excluded (H2–O2) reactions are examined so as to understand the roles of the H2–O2 kinetics in oxy-combustion.

The impacts of three flamelet burning regimes in nonlinear combustion dynamics

Axisymmetric simulations of a liquid rocket engine are performed using a delayed detached-eddy-simulation (DDES) turbulence model with the Compressible Flamelet Progress Variable (CFPV) combustion model. Three different pressure instability domains are simulated: completely unstable, semi-stable, and fully stable. The different instability domains are found by varying the combustion chamber and oxidizer post length. Laminar flamelet solutions with a detailed chemical mechanism are examined. The β probability density function (PDF) for the mixture fraction and Dirac δ PDF for both the pressure and the progress variable are used. A coupling mechanism between the volumetric Heat Release Rate (HRR) and the pressure in an unstable cycle is demonstrated. Local extinction and reignition are investigated for all the instability domains using the full S-curve approach. A monotonic decrease in the amount of local extinctions and reignitions occurs when pressure oscillation amplitude becomes smaller. The flame index is used to distinguish between the premixed and non-premixed burning mode in different stability domains. An additional simulation of the unstable pressure oscillation case using only the stable flamelet burning branch of the S-curve is performed. Better agreement with experiments in terms of pressure oscillation amplitude is found when the full S-curve is used.