Prediction Of Laminar Burning Velocity Research Articles

Numerical Modeling of Chemical Kinetics, Spray Dynamics, and Turbulent Combustion towards Sustainable Aviation

With growing interest in sustainable civil supersonic and hypersonic aviation, there is a need to model the combustion of alternative, sustainable jet fuels. This work presents numerical simulations of several related phenomena, including laminar flames, ignition, and spray flames. Two conventional jet fuels, Jet A and JP-5, and two alternative jet fuels, C1 and C5, are targeted. The laminar burning velocities of these fuels are predicted using skeletal and detailed reaction mechanisms. The ignition delay times are predicted in the context of dual-mode ramjet engines. Large Eddy Simulations (LES) of spray combustion in an aeroengine are carried out to investigate how the different thermodynamic and chemical properties of alternative fuels lead to different emergent behavior. A novel set of thermodynamic correlations are developed for the spray model. The laminar burning velocity predictions are normalized by heat of combustion to reveal a more distinct fuel trend, with C1 burning slowest and C5 fastest. The ignition results highlight the contributions of the Negative Temperature Coefficient (NTC) effect, equivalence ratio, and hydrogen enrichment in determining ignition time scales in dual-mode ramjet engines. The spray results reveal that the volatile alternative jet fuels have short penetration depths and that the flame of the most chemically divergent fuel (C1) stabilizes relatively close to the spray.

Quantitative evaluation of several chemical mechanisms for gasoline surrogate-ethanol blends and study of dominant chemistry under super-knock relevant conditions

A comprehensive database that covers wide ranges of temperature, pressure and equivalence ratio conditions was established for gasoline surrogate fuels composed of n-heptane, iso-octane, toluene with and without ethanol addition. The database includes ignition delay time measurements in shock tubes and laminar flame velocity measurements which are relevant combustion properties under super-knock relevant conditions. Due to the lack of reaction pathways for low-temperature iso-octane combustion and ethanol oxidation in Caltech mechanism, an updated mechanism has been constructed. The update has been carried out as follows: (i) changing the iso-octane sub-mechanism, (ii) adding an ethanol sub-mechanism, and (iii) modifying thermodynamic and kinetic parameters for few species and reactions for removing collision limit violation. The predictive capabilities of the modified Caltech mechanism and of three reaction models developed for n-heptane/iso-octane/toluene/ethanol based mixtures have been quantified using the model evaluation method of Olm. Significant improvement of the combustion properties predictions by the modified Caltech model has been demonstrated by the lower error value. For the prediction of ignition delay time, the updated Caltech mechanism shows better performance than the other models, especially in the high-temperature regime. Error maps were drawn over wide ranges of thermodynamic conditions to show the overall performance of all the evaluated mechanisms. For the prediction of laminar burning velocity, the modified Caltech mechanism performs better for most cases. Laminar burning velocity is always over-estimated by Livermore’s mechanism. The new mechanism was employed to determine the most important chemical pathways under super-knock relevant conditions corresponding to the following thermodynamic states: (630 K, 2.3 MPa), (750 K, 6.0 MPa), (720 K, 2.8 MPa), and (1000 K, 12.0 MPa). The modified Caltech model, or reduced versions of it, could be employed in future work to investigate super-knock for realistic gasoline surrogate.

Machine learning model to predict the laminar burning velocities of H2/CO/CH4/CO2/N2/air mixtures at high pressure and temperature conditions

An empirical model based on machine learning is developed for predicting the variation of the laminar burning velocities of H2/CO/CH4/CO2/N2/air mixtures with volumetric fractions as the independent variables at different elevated mixture temperatures and pressures. The proposed model is derived partly based on the measured burning velocities of syngas-air mixtures at elevated temperatures and pressures using diverging channel method, and partly established from the predictions using the FFCM-1 detailed kinetic model. The experiments at elevated pressures and temperature strongly agree with the predictions of the FFCM-1 kinetic model for PG1 (H2/CO/CO2/N2 = 15/15/15/55) syngas composition. Based on the detailed analysis of the experimental results, a power-law correlation considering the α, β variations is proposed: Su = Su,o * (Tu/Tu,o)α0+α1 (1−Pu/Pu,o) * (Pu/Pu,o)β0+β1 (1−Tu/Tu,o).Machine learning model (multiple linear-regression) was trained for the variables (Su,o, αo, α1, β0, β1) in the power-law correlation to enable the prediction of laminar burning velocity at various pressure and temperature conditions. The empirical model was developed with mole fractions of various components (H2/CO/CH4/CO2/N2) in the syngas composition and equivalence ratio as independent variables. The developed model was intended for low-calorific value syngas mixtures, and it performs exceedingly well without solving detailed governing equations, detailed chemistry, and transport equations. The proposed model is accurate for a wide range of syngas-air mixtures reported in the literature. A detailed comparison showed that the empirical model accurately predicts the laminar burning velocity with error <10%, for a wide range of H2/CO/CH4/CO2/N2/Air mixtures with 0.25 <XH2<0.70, 0.25 < XCO < 0.70, 0 <XCH4< 0.15, 0 <XCO2< 0.50, 0 <XN2< 0.70, for equivalence ratios of ϕ = 0.5–2.5, mixture temperatures from 300 to 650 K, and pressures from 1 to 5 atm.

Parametrization of the temperature dependence of laminar burning velocity for methane and ethane flames

The power exponent α in the temperature dependence of laminar burning velocity SLSL0=TuTu0α is usually considered an empirical parameter extracted from measurements performed at different temperatures. In this paper an analytical derivation of α is proposed, calculating the power exponent from the overall activation energy as: αTu0→Tu=Ea2R·X+x. This relation is verified against experimental burning velocity data measured with the heat flux method and chemical kinetic models for flames with equivalence ratios, Φ, from 0.6 to 1.6 at up to 368 K unburned gas temperature and 1atm. Both methane and ethane were used as fuel. Laminar burning velocity predictions at elevated temperatures are made using proposed relation and the resulting values are in good agreement with existing data for methane flames up to 500 K. This indicates that the proposed mathematical derivation of α is accurate. In addition to providing a reliable extrapolation of the burning velocity at varying temperatures, isolating the temperature dependence of the power exponent α enables more accurate quantification of other factors, e.g., Φ, the unburned gas temperature and pressure, that influence laminar burning velocity. Additionally, it provides a simple means to evaluate the overall activation energy, Ea.

Experimental and chemical kinetic studies of the effect of H2 enrichment on the laminar burning velocity and flame stability of various multicomponent natural gas blends

The effect of the addition of hydrogen to various multicomponent natural gas (NG) blends is experimentally and numerically investigated. All the experiments are performed at a pressure of 0.1 MPa, a temperature of 300 ± 3 K, and a range of equivalence ratios (Φ = 0.6 to 1.4), using a constant pressure freely propagating spherical flame method. Numerical simulations are performed using the CHEMKIN-PRO® simulation software, with three different chemical kinetic mechanisms. Laminar burning velocity (LBV) and burned gas Markstein length (Lb) of the various NG-H2 blends at three different levels of hydrogen in the fuel, viz., 25%, 50%, and 75%, are experimentally evaluated to assess the effect of the simultaneous presence of H2 and higher hydrocarbons (HC) in various NG blends. The addition of H2 enhances the combustion chemistry of all the NG blends, and hence, increases the LBV. However, the effect is more prominent for the NG6-H2 blend, which has a higher mole fraction of CH4. The NG5-H2 blend, which has a higher mole fraction of C3H8 maintains a positive Lb for a wider range of equivalence ratios (0.7–1.4). The LBV prediction using the GRI-MECH 3.0 mechanism is within the range of experimental uncertainty, for the blends with up to 50% H2 in the fuel. The prediction of LBV using GRI-MECH 3.0 is the closest to the experimental results for the blends with 75% H2 in the fuel when compared with those using San Diego and USC-MECH 2.0 mechanisms.

Laminar Burning Velocity Predictions of Single-Fuel Mixtures of C1-C7 Normal Hydrocarbon and Air

Laminar Burning Velocity Predictions of Single-Fuel Mixtures of C1-C7 Normal Hydrocarbon and Air

Direct prediction of laminar burning velocity and quenching distance of hydrogen-air flames using an annular stepwise diverging tube (ASDT)

Flame propagation characteristics of hydrogen-air mixtures were investigated using an improved annular stepwise diverging tube (ASDT). An infrared detectable camera was used to visualize flame behaviors. Flame propagation velocity (FPV) was examined using the variation of equivalence ratio (or premixing ratio) and flow rate. Conclusively, it was found that hydrogen-air flames have critical FPVs that are not significantly affected by the channel gap scales. The critical FPVs were measured over a range of equivalence ratio of 0.27 to 6.50, and the maximum critical FPV was approximately 285 cm/s at the equivalence ratio of 1.7. These values were comparable to the laminar burning velocities. Quenching distances were also measured using the identical ASDT system, and the minimum value was about 0.60 mm at approximately stoichiometric conditions.

Investigations on the Formation of Planar Flames in Mesoscale Divergent Channels and Prediction of Burning Velocity at High Temperatures

Flame stabilization studies of preheated mesoscale channels of various divergent angles and aspect ratios are reported in this article. Flame propagation modes such as planar flame and negatively and positively stretched flames were observed for a range of mixture flow rates and equivalence ratios. The present investigation is focused on the formation of the planar flames in these channels. The effect, of various parameters such as channel aspect ratio, divergence angle, and heating rate on the formation of planar flames and thereby on the prediction of laminar burning velocity of various fuel–air mixtures at high temperatures are discussed. Detailed investigations show that stretch-free planar flames can be stabilized in high-aspect-ratio channels with linear velocity and temperature gradient in the axial direction. Detailed numerical simulations confirm a negligible effect of heat loss on the burning velocity due to external preheating of the channel walls. Apparatus independence of the burning velocity is confirmed through experiments in channels of various aspect ratios and divergence angles for various fuel–air mixtures. The proposed method of burning velocity measurement at high temperature is quite promising.

Direct prediction of laminar burning velocity using an adapted annular stepwise diverging tube

In a combustion study, laminar burning velocities of premixed flames are essential data. A number of experimental results for various fuels have been reported. Although there are many reliable experimental methods that provide precise burning velocities, these methods are not suitable for in situ monitoring in the field. In response to this limitation, a method using an annular diverging tube (ADT) was introduced in the previous study, and its feasibility was verified. In this study, an advanced technique using an annular stepwise diverging tube (ASDT) was introduced to enhance flow uniformity and to formulate flatter flames in the majority of experimental conditions. The configuration of the burner was gradually improved based on their flame propagation characteristics. An adapted burner configuration was developed and used to predict the laminar burning velocities of methane, propane, and DME. Two types of measuring methods were compared: a transient method, which was similar to the previous study, and a static method, which was used for a direct prediction of burning velocity. The static method predicts similar burning velocities when compared to the transient method. The results of both methods were sufficiently similar to the other’s previous results. Quenching distances could also be directly predicted using this adapted ASDT system. Therefore, the static method may be the easiest and fastest method to predict burning velocity, and quenching distance at the instant. This method can be applied to the energy field and aid in the understanding of flame characteristics in narrow combustion spaces.

Laminar burning velocity predictions by meso-scale flames in an annular diverging tube

Flame behavior in an annular diverging tube (ADT) consisting of an outer quartz tube and a tapered inner core column was investigated as a basic model for small combustion devices of various combustion space scales. Flames can be stabilized at suitable locations where the mean flow velocity is matched to the spatial average propagation velocity (SAPV). Transient variations of wall temperature and the SAPV were compared for various experimental parameters: inner core materials, burner configurations, and flow rates. It was found that a critical propagation velocity (CPV) exists that is least affected by the flow rates. The CPVs of methane, propane, and dimethyl ether (DME) were measured and a good agreement was shown between the measured CPVs and the laminar burning velocities presented in the literatures. Therefore, the ADT method can be a model for small combustion devices of various combustion space scales; furthermore, this study can be beneficial in designing and operating small combustion devices. The ADT method can also be applied in the field for in situ monitoring of the burning velocities.