Hydrocarbon Flames Research Articles

Chemical effects of hydrogen addition on low-temperature oxidation of premixed laminar methane/air flames

Hydrogen addition exhibits its chemical incentive on hydrocarbon flames via its reactivity in elementary reactions and the heat enthalpy release in self-oxidation. In the study, a one-step reaction kinetics was provided to fictitious hydrogen addition for separating the hydrogen reactivity effect on radical formation from the heat release effect on flame temperature change. The numerical investigation was conducted by adding the fictitious hydrogen into premixed flame modeling. The structures of the flame preheat zone were compared in terms of the profiles of flame temperature, the build-up of active radical pool and the consumption rates of methane and hydrogen addition. Comparative analyses showed that the reactivity and the heat release of hydrogen addition could both lead to the early boosting of flame oxidation, but showed different dependences on hydrogen enrichment rate. Reaction path analyses were conducted to explore the kinetic reasons for the above observations. The results suggested that OH + H2 = H + H2O was an apparent reaction route to show the reactivity of hydrogen addition, whereas the enhanced triangular reaction loops between OH, H and HO2 were the key factors for strengthening the low-temperature oxidation of hydrogen-enriched methane/air flames. Besides, the effects of the heat release and the reactivity of hydrogen addition on the early build-up of flame kernel were quantified.

Flame shapes and thermal flux of vertical hydrocarbon flames

Hydrocarbon fuels are involved in most fire accidents occurring in industrial facilities, which may lead to catastrophic consequences in terms of life and property losses. The prediction of the related-fire effects can contribute to provide effective protection measures, hence mitigating the consequences of hydrocarbon flames in processing environments. In this regard, there is an essential need for developing state-of-the-art correlations able to accurately estimate the flame shapes and heat fluxes for a wide range of possible hydrocarbon fires. The present paper collects a vast data bank of vertical hydrocarbon fire experiments involving multiple fuels, burner diameters, mass flow rates and wind speeds. Correlations are developed, based on the data collected, to describe the flame-geometry descriptors and the heat fluxes of vertical hydrocarbon flames under different flow regimes.

The Reaction NCN + H2: Quantum Chemical Calculations, Role of 1NCN Chemistry, and 3NCN Absorption Cross Section.

The NCN radical plays a key role for modeling prompt-NO formation in hydrocarbon flames. Recently, in a combined shock tube and flame modeling study, the so far neglected reaction NCN + H2 and the related chemistry of the main product HNCN turned out to be significant for NO modeling under fuel-rich conditions. In this study, the reaction has been thoroughly revisited by detailed quantum chemical rate constant calculations both for the singlet 1NCN and triplet 3NCN pathways. Optimized geometries and vibrational frequencies of reactants, products, and transition states were calculated on B3LYP/aug-cc-pVQZ level with single-point energy calculations carried out against the optimized structures using CASPT2/aug-cc-pVQZ. The determined rate constants for the 1NCN + H2 reaction as well as the newly measured high temperature absorption cross section of 3NCN made a reevaluation of the shock tube data of the previous work necessary, finally revealing quantitative agreement between experiment and theory. Moreover, the new directly measured Doppler-limited absorption cross section data, σ(3NCN, λ = 329.1302 nm) = 2.63 × 109 × exp(-1.96 × 10-3 × T/K) cm2/mol (±23%, p = 0 bar, T = 870-1700 K), are in agreement with previously reported values based on detailed spectroscopic simulations. Hence, a long-standing debate about a reliable high temperature 3NCN absorption cross section has been resolved. Whereas 3NCN + H2 resembles a simple abstraction type reaction with the exclusive products HNCN + H, the singlet radical reaction is initiated by the insertion into the H-H bond. Up to pressures of 100 bar, the main products of the subsequent decomposition of the H2NCN intermediate are HNCN + H as well, with minor contributions of CN + NH2 toward higher temperatures. Although much faster than the triplet reaction, the singlet radical insertion is actually rather slow, due to the necessary reorganization of the HOMO electron density in 1NCN that is equally distributed over the two N atom sites. In general, the distinct reactivity differences call for a separate treatment of 1NCN and 3NCN chemistry. However, as the main reaction products in case of the H2 reaction are the same and as the population of the 1NCN in thermal equilibrium remains low, a properly weighted effective rate constant k(NCN + H2 → HNCN + H) = 2.62 × 104 × (T/K)2.78 × exp(-97.6 kJ/mol/RT) cm3 mol-1s-1(±30%, 800 K < T < 3000 K, p < 100 bar) is recommended for inclusion into flame models that, as yet, do not explicitly account for 1NCN chemistry.

The temperature dependence of the laminar burning velocity and superadiabatic flame temperature phenomenon for NH3/air flames

Combustion of ammonia (NH3) as a carbon-free alternative fuel has been recently widely studied, with vast majority of the burning velocity data obtained at room temperature. In the present study, the laminar burning velocity SL of NH3/air mixtures has been measured at unburnt gas temperature Tu from 298 K to 448 K, covering equivalence ratios from 0.85 to 1.25 and at 1 atm using the heat flux method. Kinetic simulations were made with five literature mechanisms developed for NH3 combustion, i.e., Nakamura et al., Otomo et al., San Diego, Okafor et al., and Mei et al. mechanisms, and the influence of radiation heat losses was considered. Using the obtained burning velocity data at different temperatures, the temperature dependence coefficients α in SLSL0=(TuTu0)α were derived, and compared with different models’ predictions. Further analyses of the temperature dependence of SL were carried out through examination of the overall activation energy, temperature and species profiles as well as the reaction paths, and a unique flame structure at the rich side of adiabatic NH3/air flames was found, which resembles ‘over-rich’ phenomena in hydrocarbon flames. At equivalence ratio larger than 1.1 ± 0.05, the NH3/air flames become so rich that (1) the NH2 radical overwhelms the H and OH radicals in maximum mole fraction; (2) after the flame front, H2O converts back to H2 with NO formed at the same time, causing the superadiabatic flame temperature phenomena, i.e. adiabatic flame temperature being lower than the maximum achieved in the flame. Moreover, local minimum NO concentration is found right after the over-rich NH3/air flame front, which may be helpful in reducing NO emissions from NH3 flames in practical applications.

The effects of hydrogen addition on the chemical kinetics of hydrogen-hydrocarbon flames: A computational study

In this paper, the effects of hydrogen addition on the chemical kinetics of hydrogen–hydrocarbon flames were investigated numerically. Profiles of maximum OH, O and H radical mole fractions, flame temperature and emission levels of hydrogen-methane, hydrogen-ethane and hydrogen-propane flames were computational obtained using Kintecus solver code. The simulations were performed by incorporating Konnov's hydrocarbon combustion mechanism at the stoichiometric condition and the constant pressure of 1 atm. It was found that a small hydrogen increase in the flame mixture can modestly affects the temperature and mole fraction profiles, however, the significant increase can be observed upon 40%, 60% and 80% of hydrogen addition for hydrogen-methane, hydrogen-ethane and hydrogen-propane flames respectively can lead to decrement of CO and CO2 emissions but increment in the combustion kinetics and the adiabatic flame temperature plus an extension of the flame stability limits. A comparison of the flames free radical profiles and the laminar burning velocity of the flames showed a strong correlation.

Flame propagation in the mixtures of O2/N2 oxidizer with fluorinated propene refrigerants (CH2CFCF3, CHFCHCF3, CH2CHCF3)

ABSTRACT A kinetic model is presented for high-temperature oxidation and combustion of the refrigerants: 2,3,3,3-tetrafluoropropene (R-1234yf), 1,3,3,3-tetrafluoropropene (R-1234ze(E)), and 3,3,3-trifluoropropene (R-1243zf) at atmospheric pressure. The kinetic model is based on: GRI-Mech-3.0 and previously developed models for the inhibition of hydrocarbon flames by 2-bromo-3,3,3-trifluoropropene (2-BTP) and C1-C2 hydrofluorocarbons. The model includes 1001 reactions and 105 species. Thermodynamic equilibrium calculations indicate a maximum combustion temperature of 2047 K, 2037 K, and 2312 K for R-1234yf, R-1234ze(E), and R-1243zf, respectively, in air for standard conditions. Calculations of the 1D, steady, adiabatic, laminar burning velocity for these refrigerants with air or oxygen-enriched air indicate reasonable agreement with experimental data from the literature when the burning velocity is above 10 cm/s. The simulations are used to understand the relevant reactions. Despite the relatively high F/H ratio in the reactants (2), the combustion is dominated by reactions with radical pool radicals typical of hydrocarbons (O, OH, and H). The combustion of R-1234yf or R-1234ze(E) is characterized by a two-zone flame, the second of which is a slow reaction zone accounting for CO and CF2O consumption and additional temperature rise of a few hundred K. Simulations of the effects of water vapor on the burning velocity of R-1234yf and R-1234ze(E) capture qualitatively the trends in the experimental results. For certain values of the equivalence ratio and oxygen content of air, the premixed flame structure shows temperature peak in the main reaction zone higher than the equilibrium value.

Effects of radiation reabsorption of C1-C6 hydrocarbon flames at normal and elevated pressures

The radiative preheating of the unburned hydrocarbon fuels by the burned combustion products in counterflow diffusion flame at pressures from 1 to 100 atm was investigated. The radiative characteristics of hot H2O, CO2 and cold hydrocarbon gases were calculated from HITEMP 2010, CDSD-4000 and HITRAN 2016 databases respectively. A simplified radiation reabsorption model was given based on the properties of the radiative reabsorption energy at T = 1500–2000 K, P = 1–100 atm. By using this model, the critical strain rate was calculated for eight C1-C6 hydrocarbon species at T = 2000 K, P = 1–100 atm. The results show that radiation reabsorption should be taken into account when the strain rate is less than 2.9 s−1 at P = 100 atm. The comparison results of the radiation time and the residence time in the CH4 counterflow diffusion flame shows that the radiation time scale would dominate at elevated pressures.

Proposal of an efficient method for handling turbulent fluctuation on numerical simulations of radiative heat transfer in the large-scale industrial furnaces including hydrocarbon flame

Proposal of an efficient method for handling turbulent fluctuation on numerical simulations of radiative heat transfer in the large-scale industrial furnaces including hydrocarbon flame

A physics-based approach to modeling real-fuel combustion chemistry – V. NOx formation from a typical Jet A

Real transportation fuels are complex mixtures of a variety of hydrocarbon components. Predicting NOx formation in practical combustors burning real fuels is usually made with the assumption that the NOx submodels developed and tested for small hydrocarbon combustion are applicable to mixtures of large hydrocarbons as found in real fuels. Additionally, NOx data are scarce for flames of real fuels. The aims of the current study are (i) to provide reliable NOx data in flames of a typical jet fuel, and (ii) to test our capability to predict these data by combining a recently proposed HyChem reaction model of jet A combustion (Xu et al., 2018) with the NOx submodel of Glarborg (2018). Specifically, NOx concentrations were measured in stretch-stabilized premixed flames of methane and Jet A (POSF10325) from fuel lean to rich conditions and of ethylene at a fuel-rich equivalence ratio. This range of stoichiometries allows both thermal NO and prompt NO pathways to be tested. The results show reasonably good agreement between the experimental data and model predictions for all flames tested, although the model appears to underpredict NOx concentrations in the Jet A flames under fuel rich conditions. Sensitivity analyses were conducted to illustrate the influence of the reaction pathways and flame boundary conditions on NOx predictions. The analyses also suggest that additional prompt NO reaction pathways may play a role in flames of large hydrocarbons.

Confined spherically expanding flame method for measuring laminar flame speeds: Revisiting the assumptions and application to C1[sbnd]C4 hydrocarbon flames

The spherically expanding flame under constant volume method was introduced in 1934 by Lewis and von Elbe as a means to study laminar flame propagation at engine-relevant conditions. Despite its potential, this method has not been utilized extensively due to concerns regarding the underlying assumptions and data uncertainty. In the present study, the intricacies of the experimental approach as well as the models and assumptions involved during data interpretation were reassessed with the aid of direct numerical simulations. Results confirmed that stretch effects are negligible during the compression stage of the experiment for a wide range of Lewis numbers. Additionally, it was shown that the laminar flame speed is sensitive to the flame radius stressing thus the requirement that the modeling of flame radius needs to be done with the highest possible accuracy by accounting properly for product dissociation and thermal radiation from the burned gases. It was also shown, that the equilibrium assumption is valid for modeling the flame radius as a function of pressure and, as expected, the kinetic and transport effects are negligible. Subsequently, laminar flame speeds were measured for methane, ethane, ethylene, propane, propylene, n-butane, 1-butene, and isobutene flames for 8–30 atm pressures and 400–520 K unburned mixture temperatures. A hybrid thermodynamic/radiation model was utilized to interpret the experimental observables and derive the laminar flame speed by accounting for spectrally dependent emission from and absorption by the burned gases as well as product dissociation during compression. The data were found to be consistent with measurements obtained in spherically expanding flame experiments under constant pressure conditions and predictions using a number of current kinetic models.

Experimental and kinetic modeling investigation on the laminar flame propagation of ammonia under oxygen enrichment and elevated pressure conditions

Ammonia is attracting more and more attentions due to its role as both a carbon-free fuel for gas turbines and an effective H2 carrier. Only a limit number of investigations on the laminar flame propagation and laminar burning velocity of ammonia have been performed on elevated pressures, which were focused on ammonia/air mixtures and suffered strong buoyancy effect. In this work, laminar flame propagation of ammonia/O2/N2 mixtures covering wide ranges of equivalence ratios, oxygen contents and initial pressures was investigated in a high-pressure constant-volume cylindrical combustion vessel. The oxygen enrichment speeds up the spherically expanding flames and consequently reduces buoyancy effect on the laminar flame propagation of ammonia. The laminar burning velocity was observed to increase with the increasing oxygen content, but decrease with the increasing initial pressure. A kinetic model of ammonia combustion consisting 38 species and 265 reactions was constructed from previous models with updated rate constants of important reactions. The present model can reasonably reproduce the laminar burning velocity data in this work and literature, as well as the ignition delay time and speciation data in literature. Based on the model analysis, effects of oxygen enrichment, equivalence ratio and initial pressure on laminar burning velocities of ammonia were analyzed in detail. It is revealed that the enhanced flame propagation with oxygen enrichment is mainly due to the increase of adiabatic flame temperature which in turn leads to higher concentrations of key radicals like H, OH and NH2. For NH3 and its major decomposition products like NH2 and NH, reactions with oxygenated species such as OH, O, O2 and NO are generally more important in the lean flames, while the role of reactions with H, NH and NH2 becomes crucial in the rich flames. The calculated pressure dependent coefficient indicates that NH3/O2/N2 flames exhibit clear pressure dependence, while this pressure dependence is weaker than those of the hydrocarbon and biofuel flames.

The effects of differential diffusion in counter-flow premixed flames with dilution and hydrogen enrichment

The effects of differential diffusion on local flamelet velocities, turbulent burning rates, and structure of lean turbulent premixed flames in the thin reaction zone regime are investigated using aerodynamically stabilized flames in a counter-flow apparatus. Various fuel–oxidizer–inert mixtures with different transport properties, representative of distinct effective Lewis numbers, are studied. In order to minimize the effects of mixture reactivity in these experiments, unstretched laminar flame speed is kept constant during mixture dilution, and hydrogen enrichment of hydrocarbon flames, through changing the mixture equivalence ratio. Furthermore, bulk-flow properties and stagnation surface temperature are kept constant; hence, the study focuses on the effects of differential diffusion, which is the change in transport properties of the mixture, i.e., fuel and heat diffusivities, in the context of fuel flexibility. Highly strained laminar flame measurements are also reported as a reference of comparison. Local instantaneous statistics of various flame parameters within the imaged plane, such as flame location, flame velocity, and flame-front topology, are quantified using high-speed particle image velocimetry (PIV) and Mie scattering flame tomography at two levels of turbulence intensity. These parameters are presented as probability density functions using sufficiently large data sets to ensure statistical accuracy. The results for various flame-parameter statistics, which are measured over a wide range of Lewis numbers, show that the effects of differential diffusion are important in turbulent flames in the thin reaction zone regime. At constant turbulence intensity, differential diffusion increases the burning rates of turbulent flames in thermo-diffusively unstable mixtures through two main mechanisms: (1) increasing local flamelet displacement velocity, and (2) increasing flame-surface area. The relative contribution of these two parameters in increasing turbulent burning rates is approximately 76% and 24%, respectively, which is not dependent on the fuel, oxidizing-gas mixture, or turbulence intensity, and the results overlap over a wide range of Lewis numbers.

Simulation of flame speed enhancement of a hydrocarbon flame with a microwave field

This study models the effect of microwave electric field on a premixed flame to achieve an accurate prediction of flame speed enhancement by considering non-thermal electron for both kinetics and mass transport. The results compare well against experimental data, and show that using electron energy distribution function (EEDF) to calculate recombination rates of electron is the key to improve the prediction of the electron number density and the flame speed. The resulting technique also agrees well with the flame speed theory to explain the mechanism of flame speed increase by ohmic heating, which is by increasing the flame temperature. The model can also predict the efficiency of flame speed enhancement for a microwave electric field. Finally, the model can be improved by incorporating recombination cross sections of major ions, if they are available.

Emissivity Characteristics of Hydrocarbon Flame and Temperature Measurement by Color Image Processing

Non-gray radiation should be considered in the temperature and emissivity measurements of hydrocarbon flames. In this paper an improved ratio pyrometry by spectral analysis and color image processing is proposed. A Newton-type iterative method is utilized to analyze the spectrometer signals for the detection of monochromatic emissivity, and then the ratio pyrometry based on color image processing is corrected by the detected monochromatic emissivity without making approximations of the filter profiles of CCD camera. The experiments were conducted on a tubular heating furnace with coal gas and a propane flame. The spectral and spatial distributions of emissivity of hydrocarbon flame were detected, and the temperature measurement results at four conditions coincided with the thermocouple with relative errors less than 8.34%. The soot volume fractions in the turbulent diffusion hydrocarbon flame were approximately estimated from the detected emissivity, and are influenced by the O/C in the combustion. This study will provide a simple and effective method for the detection of non-gray radiation of hydrocarbon flames in the combustion industry.

Fire Safety of 1, 2 and 2l Refrigerants: Myths and Reality

For many years fire safety measures for refrigerants were based on experimental data on their burning velocity and flammability limits in air. The analysis conducted in the paper and obtained experimental results show that knowledge of the flammability limits and the burning velocities is not enough to ensure the fire safety of the refrigerant. “Low flammable” substances may form explosive mixtures with air in wide range of conditions (initial concentration, pressure, temperature, ignition source (open hydrocarbon flame, hot surface)), even being a part of non-combustible mixture. One of the most underestimated things is an influence of open hydrocarbon flame as ignition source for halogenated substances not just from Class 2L (low flammable), but also from Class 1 (non-flammable). Mixtures of Class 1, 2L and 2 (lower flammable) refrigerants with lubricant oil should be also tested more carefully for their fire and explosion safety. In some cases the danger of “low flammable” substances may be underestimated due to the mismatch between test conditions and real conditions of use. It is also important that due to peculiarities of the process of mixing “low flammable” partially halogenated substances may form explosive mixtures with air at any mass of partially halogenated substance – the influence of mass factor must be also estimated more carefully.

Algorithmic Aspects of the LES-PBE-PDF Method for Modeling Soot Particle Size Distributions in Turbulent Flames

ABSTRACTIn recent times, the LES-PBE-PDF framework has been developed to couple large eddy simulation (LES) and population balance models (PBE) for the description of soot formation in turbulent hydrocarbon flames. This approach is based on a modeled evolution equation for the LES-filtered probability density function (pdf) associated with the instantaneous gas composition and soot particle size distribution. Here, the interaction of turbulence with chemical reactions and soot formation can be represented without approximations on part of the chemical and soot formation kinetics, while effects due to turbulent transport and molecular diffusion require closure. In view of an efficient numerical solution scheme, we previously proposed to combine a statistically equivalent reformulation of the joint scalar-number density pdf based on Eulerian stochastic fields with a time-explicit adaptive grid discretization in particle size space and a fractional time stepping scheme. In this article, we present algorithmic aspects and relay implementational details for a consistent semi-discrete formulation of the PBE fractional step as well as an effective dynamic load balancing scheme for both the chemical reaction and PBE fractional steps. Considering soot formation in the Delft III turbulent diffusion flame as a test case, we show that the persisting load imbalance is almost negligible on average and give evidence of linear strong scaling on a modern high performance computer for moderate numbers of compute nodes.

Ion current and carbon monoxide release from an impinging methane/air coflow flame in an electric field

This research examines changes in the ion current and the release of carbon monoxide (CO) from surface-impinging coflow diffusion flames when subjecting those flames to high DC electric fields. Carbon monoxide results from the incomplete oxidation of hydrocarbon fuels and, while CO can be desirable in some syngas processes, it is usually a dangerous emission from forest fires, gas heaters, stoves, or furnaces where the core reaction insufficiently oxidizes the fuel to carbon dioxide and water. Electrical aspects of flames, specifically, the production of chemi-ions in hydrocarbon flames and the convective flows driven by these ions, have been investigated in ranges of application, and this research examines the use of electric fields as one mechanism for governing combustion as flames are partially extinguished when impinging on nearby surfaces. We evaluate the ion current and study the flame behavior at saturation current in response to the impinging plate location. We also measure the changes in CO emission, as correlated with variations in flame structure observed using OH chemiluminescence and OH planar laser induced fluorescence (PLIF), as a function of burner-to-plate distance and electrical potential applied to the flame. Three major saturation current impact regions in plate distance is observed, and it uses chemiluminescence with PLIF to illuminate the relative locations of the peak heat release zone and the extended oxidative zone changing by external electric fields. The results show that CO release correlates strongly with changes in location and extent of high concentration regions of OH in the surface-impinging diffusion flames. The results also show that electric fields affect the CO emission but that the burner-to-plate distance has the dominant influence. The detailed findings indicate that a continuous monitor of ion current from the flame to the impinging surface can be an effective sensing index of flame behavior and CO emission.

New water-based fire extinguishant: Elaboration, bench-scale tests, and flame extinguishment efficiency determination by cupric chloride aqueous solutions

A new aqueous fire-extinguishing agent (AFEA) – the 40% aqueous solution of CuCl2 – was elaborated. The AFEA was prepared by dissolving copper(II) chloride dihydrate in water. The resulting solution was characterized by a series of physicochemical methods. The ability of the CuCl2-containing AFEA to suppress the hydrocarbon flame has been investigated under conditions of an enclosed space according to the all-Union State Standard 3789 and open space by means of elaborated technique. The testing results have revealed a short-term exposure of the 40% aqueous solution of CuCl2 directly onto flame causes the efficient inhibition of fire. It has been ascertained that under conditions of the experiment, the extinguishment duration of the seat of fire (the special fire of B1 class) by the CuCl2-containing AFEA aerosol was 0.6 s; this is 26 times more efficient than the same extinguishment by water aerosol. The experimental ascertained values of the AFEA aerosol feed intensity (IAFEA) and the minimal expenses of the AFEA to extinguish the seat of fire of B1 class are 0.057 L⋅m−2⋅s−1 and 0.034 L⋅m−2, respectively. These data have become a reliable basis to explain the hydrocarbon flame suppression mechanism by aqueous solutions of copper(II) salts.

Fire extinguishing efficiency of hydrocarbon and fluorinated film-forming foaming agent

This article analyzes the fire extinguishing efficiency of hydrocarbon and fluorinated film-forming foaming agent. It was revealed that fluorinated blowing agents have a number of significant advantages over hydrocarbon. The main ones are: ensuring inertness when feeding from foam of various heights and distances, long-term prevention of reignition, extinguished petroleum product, the possibility of extinguishing hydrocarbon flames by supplying foam to the base of the tank, directly into the fuel layer

Fire-extinguishing efficiency and environmental performance of foam for extinguishing a warehouse space with oil

This article analyzes the fire extinguishing efficiency of hydrocarbon and fluorinated film-forming foaming agent. It was revealed that fluorinated blowing agents have a number of significant advantages over hydrocarbon. The main ones are: ensuring inertness when feeding from foam of various heights and distances, long-term prevention of reignition, extinguished petroleum product, the possibility of extinguishing hydrocarbon flames by supplying foam to the base of the tank, directly into the fuel layer.