Laminar Flame Speed Correlation Research Articles

New laminar flame speed correlation for lean mixtures of hydrogen combustion with water addition under high pressure conditions

Hydrogen may become a substitute for liquid fossil fuels, contributing to greenhouse gas emissions reductions in internal combustion engines. Numerical simulations play a critical role in the advancement of these engines, with laminar flame speed being the main input. Experimental data of hydrogen flame speed at elevated pressures are scarce, due to the instability of the flames. Nonetheless, stable hydrogen flames can be predicted using chemical kinetics models. Moreover, the injection of water into the hydrogen fuelled engine could offer several benefits to engine combustion and emission performance, as it modulates the laminar flame speed within the combustion chamber and this effect has not been completely understood. Currently, no correlation exists to predict the laminar flame speed of hydrogen-air combustion with water addition under lean mixture engine operating conditions. In this study, we have extended the newly developed laminar flame speed correlation of hydrogen-air combustion to account for the effects of water addition under engine relevant conditions by using chemical kinetic laminar flame speed values. The laminar flame speed correlation was derived for pressures from 10 to 70 bar, temperatures from 400 to 800 K, equivalence ratios from 0.35 to 1 and water addition by mole from 0 to 20%. The hydrogen laminar flame speed correlation was expressed using polynomial forms with reduced order and number of terms with optimized values of coefficients. Additionally, a new exponential term was proposed to the power term α of the laminar flame speed correlation to capture the coupled effects of pressure and temperature on laminar flame speeds under engine-relevant lean burn water-diluted operating conditions.

Measurements and a new correlation of methanol laminar flame speeds at temperatures up to 916 K and elevated pressures behind reflected shock waves

A side-wall-imaging shock tube was utilized to measure the laminar flame speed of methanol in 21% O2/79% Ar (airgon) behind reflected shock waves. The flames, initiated via laser-induced spark ignition, were tracked through schlieren imaging, which allowed the extraction of unstretched methanol flame speeds using an area-averaged linear curvature model. Methanol/airgon flame speeds were measured at atmospheric pressure at four equivalence ratios (0.8, 1, 1.2, and 1.4) and at temperatures between 640 K and 916 K; elevated-pressure measurements at pressures up to 2.6 atm were performed at 750 K for three equivalence ratios (0.8, 1, and 1.2) and at pressures up to 2 atm at 820 K for the stoichiometric mixture. Four Kinetic models, NUIG 1.3, Aramco 3.0, FFCM-2, and Zhang 2017, were used to compare with the present measurements. The model-simulated flame speeds are observed to diverge at temperatures above 650 K, with the Zhang 2017 and NUIG 1.3 models showing the best agreement with experimental data. The Aramco 3.0 model is shown to consistently underpredict methanol laminar flame speeds at high temperatures and elevated pressures, whereas the FFCM-2 model shows overprediction at temperatures under 750 K, particularly at rich conditions. The methanol/airgon flame speed data were then mixture-scaled to obtain the equivalent methanol/air results, which trend closely with lower-temperature flame speed measurements reported in the literature. The first methanol/air laminar flame speed correlation validated at temperatures up to 916 K was constructed by fitting mixture-scaled methanol/air data and available literature values using a non-Arrhenius form.

An integrated 0D/1D/3D numerical framework to predict performance, emissions, knock and heat transfer in ICEs fueled with NH3–H2 mixtures: The conversion of a marine Diesel engine as case study

In the maritime transportation, e-fuels represent a valid alternative to fossil energy sources, in order to accomplish the European Union goals in terms of climate neutrality. Among the e-fuels, the ammonia-hydrogen mixtures can play a leading role, as the combination of the two allows to exploit the advantages of each one, simultaneously compensating their gaps.The main goal of the present publication is the proposal of a robust numerical framework based on 0D, 1D and 3D tools for CFD analyses of internal combustion engines fueled with ammonia-hydrogen mixtures.The 1D engine model provides boundary conditions for the multi-dimensional investigations and estimates the overall engine performance. 3D in-cylinder detailed analyses are proficiently used to predict combustion efficiency (via the well-established G-equation model supported by laminar flame speed correlations for both ammonia and hydrogen) and emissions (with a detailed chemistry based approach). Heat transfer and knock tendency are evaluated as well, by in-house developed models. As for the 0D/1D chemical kinetics calculations, firstly they support 3D analyses (for example via the generation of ignition delay time tables). Moreover, they allow insights on aspects such as NOx formation, to individuate mixture qualities able to strongly reduce the emissions.The present paper offers also indications for designers on the effectiveness of NH3–H2 as valid substitutes of traditional fuels in marine applications. In fact, the proposed framework is preliminary tested to convert an existing marine Diesel engine to ammonia-hydrogen mixtures. Different proportions (among the two fuels) are investigated, included pure ammonia and pure hydrogen and the NH3–H2 80mol%-20mol% mixture is selected as the best compromise. Thanks to both the anti-knock quality of the ammonia and the (limited) addition of hydrogen as combustion enhancer, the engine supplied with NH3–H2 mixtures is able to operate free of knock and keep the same performance as the original one. Simultaneously (neglecting the presence of lubricant oil in the combustion chamber) unburned hydrocarbons, carbon monoxide and carbon dioxide are eliminated. Still in comparison with the Diesel version, the heat transfer is similar, the NOx formation is much greater and the engine range (here evaluated in terms of working hours) noticeably reduces.

Impact of fuel surrogate formulation on the prediction of knock statistics in a single cylinder GDI engine

The statistical tendency of an optically accessible single-cylinder direct-injection spark-ignition engine to undergo borderline/medium knocking combustion is investigated using 3D-CFD. Focus is made on the role of fuel surrogate formulation for the characterization of anti-knock quality and flame speed of the actual fuel. An in-house methodology is used to design surrogates able to emulate laminar flame speed and autoignition delay times of the injected fuel. Two different surrogates, characterized by increasing level of complexity, are compared. The most complex one (six components) improves the representation of the real fuel, highlighting the crucial role of accurate fuel kinetics to predict flame propagation and unburnt mixture reactivity. A devoted chemical mechanism including the oxidation pathways for all the species in the surrogate is also purposely developed for the current analysis. Knock is investigated using a proprietary statistical knock model (GruMo-UNIMORE Statistical Knock Model, GK-PDF), which can infer the probability of knocking events within a RANS formalism. Predicted statistical distributions are compared to measured counterparts. The proposed numerical/experimental comparison demonstrates the possibility to efficiently integrate complex-chemistry driven information in 3D-CFD combustion simulations without online solving chemical reactions: a combination of laminar flame speed correlations, ignition delay look-up tables, and a statistics-based knock model is adopted to estimate the percentage of knocking cycles in a GDI engine while limiting the computational cost of the simulations.

Laminar flame speed measurements of ethanol, iso-octane, and their binary blends at temperatures up to 1020 K behind reflected shock waves

A shock tube augmented with a laser-spark ignition system was used to measure the atmospheric-pressure (1 atm ±2%) laminar flame speeds of neat ethanol and iso-octane as well as ethanol/iso-octane blends in a 21% O2-79% Ar (so-called “airgon”) oxidizer. Low-temperature (453 K–524 K) validation experiments of atmospheric-pressure, stoichiometric ethanol/air were first performed for comparison with literature. Two fuel blends with 50% and 85% ethanol (by volume) in iso-octane (E50 and E85, respectively) were then studied in airgon at high unburned-gas temperatures (637 K–1020 K) and at various equivalence ratios (0.75, 1, and 1.2). The low-temperature ethanol/air measurements showed good agreement with literature data, confirming the accuracy of flame speeds measured using a shock tube. High-temperature fuel/airgon flame speeds were simulated with various kinetic models, and significant model disagreement was found, highlighting the value of the new flame speed data in the high-temperature regime. To make the present fuel/airgon measurements compatible with practical engineering applications using air as the oxidizer, the ratio of fuel/airgon and fuel/air flame speeds was calculated using 1D simulations for each experiment, then subsequently used to perform mixture-scaling for the fuel/airgon data to obtain equivalent fuel/air flame speeds. Fuel/air laminar flame speed correlations as a function of the unburned-gas temperature were determined for the studied fuel blends and equivalence ratios. Finally, flame speed sensitivity analyses conducted for high-temperature ethanol/airgon and iso-octane/airgon flames revealed three key governing reactions. Assigning common rate expressions to these reactions across different kinetic models improved the agreement between model-predicted and experimentally measured ethanol flame speeds, but did not have a similar effect for iso-octane, suggesting the need for further exploration of iso-octane-specific reaction rates to facilitate better model agreement.

An improved TRF mechanism for a new turbulent premixed combustion model with application to engine combustion CFD

In the present work, an existing TRF (toluene reference fuel) chemical kinetic mechanism has been improved. In the improvement, the existing TRF mechanism was found to have unphysically higher laminar flame speeds at temperatures greater than 750 K which are typical in engine compression-combustion operating conditions. Eight dominant reactions which mainly control the laminar flame speeds under higher temperatures ( T > 750 K) have been identified. Based on a recently published power-law laminar flame speed correlation which is able to provide good predictions under a wide range of engine conditions, correct reaction rate constants for the eight high-temperature dominant reactions have been determined for physical laminar flame speeds. The identification and improvement of the eight high-temperature dominant reactions are the first novelty of this work. In order to simulate ethanol and gasoline blends, a latest ethanol sub-mechanism has been implemented into the TRF mechanism. The improved TRF mechanism was validated using available experimental data in the literature. Based on the improved TRF mechanism, a new mechanism library has been generated, which can be used for an improved mechanism-dynamic-selection turbulent premixed combustion model in which turbulent diffusivity is constructed as a function of local turbulence/thermodynamics conditions. The dynamic turbulent diffusivity sub-model is the second novelty of this work. The improved TRF mechanism and its combination with the improved mechanism-dynamic-selection turbulent premixed combustion model were successfully applied to the prediction of combustion and emissions of GTDI (gasoline turbocharged direct injection) engines under both warm-up and transient cold start operating conditions, indicating that the improved models in this work are beneficial for predictive engine combustion CFD (computational fluid dynamics).

Laminar flame speed correlations for dual fuel flames in high-pressure conditions

The usage of methane as a fuel stands out as a transition strategy to zero carbon energy conversion processes. However, methane is difficult to be made to burn due to its strong molecular structure. To mitigate this issue, a mixture of methane and a more ignitable fuel may be a promising solution for large scale combustion applications. In many of those, the corresponding CFD setup can be very demanding and detailed chemistry may be replaced by simplified approaches, such as laminar flame speed methods. To contribute with robust and reliable modeling of combustion processes fueled by dual fuel mixtures, novel formulations are proposed to improve accuracy of laminar flame speed correlations. To accomplish this task, numerical simulations of 1D freely propagating laminar flames based on detailed chemistry are conducted for various pressures and equivalence ratios. Kinetic mechanisms are obtained from the literature and evaluated with available experimental data. For the best performing mechanism, the resulting laminar flame speeds are mapped based on fresh mixture composition and correlated through power-law wholly empirical equations. The proposed formulations show clear improvements when compared to other existing methods.

Laminar flame speed correlations of ammonia/hydrogen mixtures at high pressure and temperature for combustion modeling applications

Ammonia/hydrogen mixtures are among the most promising solutions to decarbonize the transportation and energy sector. The implementation of these alternative energy carriers in practical systems requires developing suitable numerical tools, able to estimate their burning velocities as a function of both thermodynamic conditions and mixture quality. In this study, laminar flame speed correlations for ammonia/hydrogen/air mixtures are provided for high pressures (40 bar–130 bar) and elevated temperatures (720 K–1200 K), and equivalence ratios ranging from 0.4 to 1.5. Based on an extensive dataset of chemical kinetics simulations for ammonia/hydrogen blends (0-20-40-60-80-90-100 mol% of hydrogen), dedicated correlations are derived using a regression fitting. Besides these blend-specific correlations, a generalized (i.e., hydrogen-content adaptive) formulation, with hydrogen content used as additional parameter, is proposed and compared to the dedicated correlations.

FlameFoam: An open source CFD solver for turbulent premixed combustion

Severe accidents in nuclear power plants may cause significant and long-lasting harm to the environment in the case of containment integrity loss. Numerical simulation of phenomena threatening containment integrity, e.g., hydrogen combustion and explosion, is essential for the effective severe accident management.International research projects and benchmarks are organized in order to promote improvement of simulation codes adequacy and accuracy in the severe accident relevant conditions. Results of the most recent benchmarks show that current state-of-the-art simulations were able to mostly reproduce the observable flame speed at the considered conditions, but there are still inadequacies in estimating velocity and its maximum value.Addressing a need for an accessible open source turbulent premixed combustion simulation tool, a new solver is being developed. An OpenFOAM version 7 based combustion solver called flameFoam has been built by implementing a progress variable approach and turbulent flame closure model for premixed combustion simulation. Turbulent flame speed is defined as a function of laminar flame speed and turbulence parameters, and is evaluated using one of three different correlations. Laminar flame speed can be set as a constant or calculated using a laminar flame speed correlation. The basis for the solver are standard compressible OpenFOAM solvers rhoPimpleFoam, buoyantPimpleFoam and chtMultiRegionFoam.The article presents initial public flameFoam version 0.8 and provides examples of its validation. Initial validation and verification of general solver features are represented by the simulations of shock tube and backward facing step cases. Solver capabilities in relation to the current turbulent premixed combustion modeling state-of-the-art are demonstrated with the simulations of ETSON-MITHYGENE benchmark experiments.It is shown that flameFoam version 0.8 is capable of adequate simulation of general flow features, turbulent compressible flow phenomena and turbulent premixed combustion of lean hydrogen-air mixtures at the considered conditions. Solver code is publicly hosted and being developed further.

Effect of CH4, Pressure, and Initial Temperature on the Laminar Flame Speed of an NH3-Air Mixture.

Ammonia (NH3) is not only expected to be used as a hydrogen energy carrier but also expected to become a carbon-free fuel. Methane (CH4) can be used as a combustion enhancer for improving the combustion intensity of NH3. In addition, it is important to understand the flame characteristics of NH3–air at elevated pressures and temperatures. The laminar flame speed of NH3–CH4–air is numerically investigated, where the mole fraction of CH4 ranges from 0 to 50% in binary fuels and the pressure and initial temperature are up to 10 atm and 1000 K, respectively. The calculated value from the Okafor mechanism is in excellent agreement with experimental data. The CH4 in the fuel affects the flame speed by changing the main species of free radicals in the flame; the high pressure not only increases the rate-limiting reaction rate in the flame but also reduces the amount of H, O, and OH radicals in the flame, so as to restrain the propagation of the flame. At a higher initial temperature, the faster flame speed is mainly due to the higher adiabatic flame temperature. The laminar flame speed correlation equation has a consistent trend with the simulation results, though with a slight underestimation at higher pressures and temperatures. It is a more effective way to calculate the laminar flame speeds of NH3–air for a given pressure and temperature.

Prediction of Ultra-Lean Spark Ignition Engine Performances by Quasi-Dimensional Combustion Model With a Refined Laminar Flame Speed Correlation

AbstractThe turbulent combustion in gasoline engines is highly dependent on laminar flame speed SL. A major issue of the quasi-dimensional (QD) combustion model is an accurate prediction of the SL, which is unstable under low engine speeds and ultra-lean mixture. This work investigates the applicability of the combustion model with a refined SL correlation for evaluating the combustion characteristics of a high-tumble port gasoline engine operated under ultra-lean mixtures. The SL correlation is modified and validated for a five-component gasoline surrogate. Predicted SL values from the conventional and refined functions are compared with measurements taken from a constant-volume chamber under micro-gravity conditions. The SL data are measured at reference and elevated conditions. The results show that the conventional SL overpredicts the flame speeds under all conditions. Moreover, the conventional model predicts negative SL at equivalence ratio ϕ ≤ 0.3 and ϕ ≥ 1.9, while the revised SL is well validated against the measurements. The improved SL correlation is incorporated into the QD combustion model by a user-defined function. The engine data are measured at 1000–2000 rpm under engine load net indicated mean effective pressure (IMEPn) = 0.4–0.8 MPa and ϕ = 0.5. The predicted engine performances and combustions are well validated with the measured data, and the model sensitivity analysis also shows a good agreement with the engine experiments under cycle-by-cycle variations.

Monte-Carlo based laminar flame speed correlation for gasoline

Laminar flame speed and autoignition properties of gasoline play key role in the overall performance of spark-ignition and modern engines. Since gasoline is a complex fuel containing hundreds of species, it is not feasible to model all components present in gasoline. Researchers tend to employ surrogates, comprising of few components, that mimic targeted physical and chemical properties of gasoline. Detailed kinetic models of the surrogates can still be prohibitively large for CFD simulations and/or fuel-screening studies. For fuel-engine optimization efforts, it is highly desirable to have simple methods which can be used to accurately predict autoignition and laminar flame speed of real fuels. In this work, a laminar flame speed correlation is proposed for typical gasolines. This correlation is based on Monte-Carlo simulations of randomly generated mixtures comprising of 21 gasoline-relevant molecules. Laminar flame speed of each molecule is numerically computed over a wide range of thermodynamic conditions using detailed chemical kinetic models, and flame speed of each mixture is estimated with a suitable mixing rule. The proposed correlation is validated against experimentally-measured laminar flame speeds of various gasoline fuels.

Gasoline-ethanol blend formulation to mimic laminar flame speed and auto-ignition quality in automotive engines

Several environment agencies worldwide have identified biofuels as a viable solution to meet the stringent targets imposed by future regulations in terms of on-road transport emissions. In the last decades, petroleum-based gasoline has been increasingly blended with oxygenated fuels, mostly ethanol. Blending ethanol with gasoline has two major effects: an increase of the octane number, thus promoting new scenarios for engine efficiency optimization, and a potential reduction of soot emissions.3D-CFD simulations represent a powerful tool to optimize the use of ethanol-gasoline blends in internal combustion engines. Since most of the combustion models implemented in 3D-CFD codes are based on the “flamelet assumption”, they require laminar flame speed as an input. Therefore, a thorough understanding of the gasoline-ethanol blend chemical behavior at engine-relevant conditions is crucial. While several laminar flame speed correlations are available in literature for both gasoline and pure ethanol at ambient conditions, none is available, to the extent of authors’ knowledge, to describe laminar flame speed of gasoline-ethanol blends (for different ethanol volume contents) at engine relevant conditions. For this reason, in the present work, laminar flame speed correlations based on 1D detailed chemical kinetics calculations are derived targeting typical full-load engine-like conditions, for different ethanol-gasoline blends. A methodology providing a surrogate able to match crucial properties of a fuel is presented at first and validated against available experimental data. Then, laminar flame speed correlations obtained from 1D chemical kinetics simulations are proposed for each fuel blend surrogate.

A Novel Laminar Flame Speed Correlation for the Refinement of the Flame Front Description in a Phenomenological Combustion Model for Spark-Ignition Engines

A Novel Laminar Flame Speed Correlation for the Refinement of the Flame Front Description in a Phenomenological Combustion Model for Spark-Ignition Engines

Gasoline flame behavior at elevated temperature and pressure

Freely propagating laminar premixed flames of stoichiometric mixtures of gasoline surrogate and iso-octane with air are computed using three chemical kinetics mechanisms of varied complexity and detail. A good agreement of the computed burning velocities with past experimental data is observed. The burning velocities of these mixtures at temperature of 850⩽T⩽950 decrease with pressure up to about 3 MPa and starts to increase beyond this pressure. This contrasting behavior is related to the role of pressure dependent reaction involving OH and the influence of this radical on the fuel consumption rate. The results suggest that the overall order of the combustion reaction is larger than two at pressures higher than 3 MPa. Hence, one must be cautious in extending the commonly used laminar flame speed correlation with pressure to thermo-chemical conditions of interest for future engines.

Flow and thermal field effects on cycle-to-cycle variation of combustion: scale-resolving simulation in a spark ignited simplified engine configuration

Premixed, spark ignited combustion of lean methane at fuel to air equivalence ratio of 0.58 is numerically investigated in a piston-cylinder assembly. The simplified numerical configuration is tailored to emulate the intake, compression and spark ignition processes in engines. Large-eddy simulation is employed in the core flow along with a zonal hybrid wall treatment in near-wall regions. The G-equation level-set method is used to simulate flame propagation with a detailed chemistry based laminar flame speed correlation developed herein. The main numerical findings of this paper are as follows: (1) Despite the geometrical simplicity, the present set-up is shown to exhibit relatively large cycle-to-cycle variation for the three investigated cycles. (2) The local thermodynamics and fluid dynamic conditions around the spark close to the ignition location initiate the first discrepancies between the cycles. (3) These early variations are then amplified due to the subsequent differences in the early growth of flame area. (4) The cycle-to-cycle variation in the present set-up is shown to be largely a consequence of the local flow fluctuations close to the spark position and timing, while the results indicated a less dominating role of thermal stratification on cycle-to-cycle variation. (5) The asymmetric combustion behavior was explained to be a combined effect of burning rate and convection velocity, while convection velocity proved to be the major contributor. (6) Finally, a numerical test in the present model setup indicated large spark kernels being less prone to cycle-to-cycle variations than small kernels.

Impact of the laminar flame speed correlation on the results of a quasi-dimensional combustion model for Spark-Ignition engine

In the present study, the impact of the laminar flame speed correlation on the prediction of the combustion process and performance of a gasoline engine is investigated using a 1D numerical approach. The model predictions are compared with experimental data available for full- and part-load operations of a small-size naturally aspirated Spark-Ignition (SI) engine, equipped with an external EGR circuit.A 1D model of the whole engine is developed in the GT-Power™ environment and is integrated with refined sub-models of the in-cylinder processes. In particular, the combustion is modelled using the fractal approach, where the burning rate is directly related to the laminar flame speed.In this work, three laminar flame speed correlations are assessed, including both experimentally- and numerically-derived formulations, the latter resulting from the fitting of laminar flame speeds computed by a chemical kinetic solver.Each correlation is implemented within the combustion sub-model, which is properly tuned to reproduce the experimental performance of the engine at full load. Then, the reliability of the considered flame speed formulations is proved at part-loads, even under external EGR operations.

Development of Chemistry-Based Laminar Flame Speed Correlation for Part-Load SI Conditions and Validation in a GDI Research Engine

Development of Chemistry-Based Laminar Flame Speed Correlation for Part-Load SI Conditions and Validation in a GDI Research Engine

Laminar flame properties of C1-C3 alkanes/hydrogen blends at gas engine conditions

The use of fuel blending is encouraged in order to achieve more flexibility in gas engines. In order to design such engines effectively, relevant information about the laminar flame speeds and laminar flame thickness are necessary. Hydrodynamic and thermo-diffusive instabilities at gas engine conditions prevent the acquisition of reliable data experimentally. One-dimensional numerical simulations with detailed chemistry can be a solution. A huge database of laminar flame speeds is generated covering a broad range of gas engine applications, pressure (p) 0.1–20 MPa, fresh gas temperature (Tu) 300–1100 K, air-fuel equivalence ratio (λ) 0.9–2.5, methane 100–60 vol%, ethane 0–40 vol%, propane 0–40 vol%, hydrogen 0–30 vol% and exhaust gas recirculation (EGR) 0–30 m%. The detailed reaction mechanisms GRI 3.0 and AramcoMech 1.3 are used for the generation of flame speed data for the mentioned conditions. A laminar flame speed correlation for 100% hydrogen extending up to elevated pressure and temperature conditions is developed. A blending law based on Le Chatelier’s rule is investigated. It is observed that the HC-ratio has a very determining effect for the laminar flame properties of different C1-C3 alkane blends. Based on this observation, it was possible to derive a very efficient correlation for both laminar flame speed and laminar flame thickness for the group of natural gas blends with methane, ethane, propane and hydrogen, and as well as including relevant EGR, which corresponds within 7% accuracy to the calculated database of about 73,000 points. The developed laminar flame speed correlation is incorporated in an engine process simulation code. It is validated with the measured in-cylinder pressure traces from the single cylinder research engine experiments for different gas blends and EGR ratios.

Benchmarking of Improved DPAC Transient Deflagration Analysis Code

The deflagration pressure analysis code (DPAC) has been upgraded for use in modeling hydrogen deflagration transients. The upgraded code is benchmarked using data from vented hydrogen deflagration tests conducted at the HYDRO-SC Test Facility at the University of Pisa. DPAC originally was written to calculate peak pressures for deflagrations in radioactive waste storage tanks and process facilities at the Savannah River Site. Upgrades include the addition of a laminar flame speed correlation for hydrogen deflagrations and a mechanistic model for turbulent flame propagation, incorporation of inertial effects during venting, and inclusion of the effect of water vapor condensation on vessel walls. In addition, DPAC has been coupled with chemical equilibrium with applications (CEA), a NASA combustion chemistry code. The deflagration tests are modeled as end-to-end deflagrations. The improved DPAC code successfully predicts both the peak pressures during the deflagration tests and the times at which the pressure peaks.