Reduction In Burning Velocity Research Articles

Effects of Premixed and Diffusion Conditions on the Iso-Octane/ammonia and Iso-Octane/ethanol Flames

ABSTRACT The combustion engines fueled by liquid hydrocarbon fuel significantly release emissions such as CO, CO2, and soot. Renewable fuels such as zero-carbon fuel and biofuel mitigate these emissions of IC engines when used with liquid hydrocarbon fuel. Previously, ammonia and ethanol were used in IC engines and also studied fundamentally, but these fuels were tested separately. In the current study, a comparative effect of these renewable fuels on the emission characteristics of hydrocarbon fuel was made on premixed and diffusion flame conditions. A kinetic mechanism was prepared by adding the sub-mechanisms of ammonia combustion, soot precursors formation and nitrogen oxide emissions to the detailed mechanism of iso-octane flame. The combustion conditions of premixed flow flame, counterflow flame and coflow flame were adopted. The Arrhenius model with soot surface reaction pathways was used in the premixed flow flame and counterflow flame analysis. The Moss-Brooke model with soot surface reaction pathways was added in OpenFOAM Solver to study the soot formation in normal and inverse coflow flames. The results tell that a higher reduction in peak combustion temperature was observed in premixed flame instead of diffusion flame thanks to alternative fuels. Ethanol greatly reduced the peak temperature of the premixed flame rather than ammonia. The impact of carbon-free fuel on total reduction in CO and CO2 emissions was higher than that of biofuel at premixed and diffusion flame conditions. Ethanol impact on total reduction in benzene formation was stronger than ammonia at premixed and normal coflow flames. Pyrene was reduced highly by ammonia rather than ethanol at diffusion flame conditions. The study suggests that biofuel can be used from 0% to 90% in iso-octane fuel, and better results may be achieved when its fraction is ≥ 70%. In contrast, the use of ammonia with iso-octane may be better if its fraction is < 70% because a higher reduction in burning velocity will occur with higher NO2 emissions at NH3 ≥70%.

Comparative study of NH3 and H2 impacts on combustion and emission characteristics of n-Hexadecane under different flame conditions

The addition of renewable fuel to hydrocarbon fuel mitigated the carbon monoxide, carbon dioxide and soot particle emissions of internal combustion engines. A comparative study of the ammonia and hydrogen effect on combustion and emission characteristics of n-hexadecane flames was executed at various flame conditions. The results showed that alternative fuels had a stronger impact on peak temperature at diffusion flame conditions rather than at premixed flame conditions. The premixed flame released carbon monoxide emission lower than the diffusion flames with/without additional fuels. The carbon monoxide emissions decreased highly with ammonia enrichment instead of hydrogen. At premixed and coflow diffusion flame conditions, the ammonia highly reduced carbon dioxide emission compared to hydrogen. Hydrogen addition reduced the nitrogen oxide emission, whereas it was increased by ammonia. The soot particle number density was reduced highly by hydrogen addition instead of ammonia at the premixed flow flame and diffusion coflow flame conditions. The impact of hydrogen on the reduction of soot mass concentration emissions was greater than that of ammonia at all combustion conditions. It is suggested that H2 ≥ 70% should be mixed in n-hexadecane fuel because a higher burning velocity can be achieved with a higher emission reduction. A reduction in the emission of carbon-related species can be realized with a small reduction in burning velocity at NH3 < 70%.

Experimental investigation of aluminum-air burning velocity at elevated pressure

Aluminum is an energy-dense metal that reacts exothermically with a range of oxidizers, making it a potentially useful fuel for certain thermal energy and power applications (e.g., boilers, swirl-stabilized burners, rockets, etc.). In addition to its reactive properties, aluminum is naturally abundant and commercially available in fine powders that are relatively inexpensive and chemically stable. Fine aluminum dusts, with particle diameters on the order of micrometers, can be aerosolized and mixed with a gaseous oxidizer such as air to create stable dust flames and heat for a thermodynamic cycle. In order to realize the potential utility of aluminum-air dust flames, the fundamental combustion behavior, such as the burning velocity, must be understood. In this work, an experimental system was developed that allows metal dust flames to be observed within an optically-accessible pressurized chamber. Using a high-speed camera, fuel-rich premixed aluminum-air jet flames were imaged through narrowband interference filters, and the data were used to calculate burning velocity. A single polydisperse size distribution of particles was tested at pressures ranging from 1 to 7.2 bar. Burning velocity was found to decrease with increased pressure (P), with an approximate proportionality to P−0.6 over the range of pressures tested (∼P−0.3 for only the laminar flow conditions). This reduction in burning velocity with increased pressure is hypothesized to occur due to increased oxidizer density along with a decrease in interparticle spacing, which may result in asymmetric particle heating and increased competition for oxidizer.

The influence of ammonia on the laminar burning velocities of methylcyclohexane and toluene: An experimental and kinetic modeling study

Laminar burning velocities of methylcyclohexane and toluene blended with ammonia have been determined using the heat flux method at atmospheric pressure and initial temperature of 338 K, over equivalence ratios ranging from 0.7 to 1.3 and ammonia blending fractions in the binary fuel mixtures from 0 to 90%. It was observed that the addition of ammonia to methylcyclohexane and toluene leads to a decrease in laminar burning velocity that is not proportional to the ammonia mole fraction. Such a burning velocity reduction is due to synergistic thermal and kinetic effects. In addition, ammonia has a slightly higher impact on the burning velocities of toluene due to fuel structure effects. The CRECK detailed kinetic model has been used to interpret the experimental measurements and minor modifications on methylcyclohexane, toluene, and methyl-phenoxy radical chemistry allowed even improved agreement. New experimental results have been compared with predictions of this refined kinetic mechanism. The model provided good predictions of the measurements capturing the effect of equivalence ratio and ammonia fraction ranges investigated. Finally, a mass fraction-based mixing rule was shown to be predictive for binary blends of NH3 with methane and several hydrocarbons typically used to formulate surrogates for practical fuels.

Comparative Effect of Ammonia Addition on the Laminar Burning Velocities of Methane, n-Heptane, and Iso-octane

Adiabatic laminar burning velocities for methane, n-heptane, and iso-octane blended with ammonia were experimentally determined using the heat flux method. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.7 to 1.4 and ammonia blending fractions in the binary fuel mixtures from 0 to 90%. These experiments are essential for the development, validation, and optimization of chemical kinetic models, e.g., for the combustion of gasoline-ammonia fuel mixtures. It was observed that the addition of ammonia to methane, n-heptane, and iso-octane leads to a decrease in the laminar burning velocity that is not proportional to the ammonia mole fraction. In addition, ammonia has the same impact on the burning velocities of n-heptane and iso-octane but a slightly higher effect on those of methane. Such a burning velocity reduction is due to synergistic thermal, kinetic, and indirect transport effects. New experimental results were compared to predictions of the POLIMI detailed chemical kinetic mechanism. An overall good agreement between the measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio and ammonia fraction ranges investigated. (Less)

A Comparison of EGR Correction Factor Models Based on SI Engine Data

The article compares the accuracy of different exhaust gas recirculation (EGR) correction factor models under engine conditions. The effect of EGR on the laminar burning velocity of a EURO VI E10 specification gasoline (10% Ethanol content by volume) has been back calculated from engine pressure trace data, using the Leeds University Spark Ignition Engine Data Analysis (LUSIEDA) reverse thermodynamic code. The engine pressure data ranges from 5% to 25% EGR (by mass) with the running conditions, such as spark advance and pressure at intake valve closure, changed to maintain a constant engine load of 0.79 MPa gross mean effective pressure (GMEP). Based on the experimental data, a correlation is suggested on how the laminar burning velocity reduces with increasing EGR mass fraction. This correlation, together with existing models, was then implemented into the quasi-dimensional Leeds University Spark Ignition Engine (LUSIE) predictive engine code and resulting predictions are compared against measurements. It was found that the new correlation is in good agreement with experimental data for a diluent range of 5%-25%, providing the best fit for both engine loads investigated, whereas existing models tend to overpredict the reduction of burning velocity due to EGR.

Investigation on dilution effect on laminar burning velocity of syngas premixed flames

To provide more insight into the laminar burning velocity reduction of diluted syngas premixed flames, the laminar burning velocities of diluted syngas premixed (H2/CO/N2/air and H2/CO/CO2/air) laminar spherical flames were systematically and thoroughly investigated experimentally via Schlieren technology. In the present investigation, the volume fraction of H2 was varied from 30% to 70%, the volume fraction of CO was varied from 30% to 70%, the volume fraction of CO2 was varied from 0% to 60%, and the equivalence ratio was varied from 0.7 to 1.0. In the present investigation, the effects of equivalence ratio, hydrogen fraction, and dilution fraction on the laminar burning velocities were studied. Compared to N2 dilution, the laminar burning velocity reduction with CO2 dilution was analysed through the thermal, transport, and chemical effects. In addition, the contributions of third-body and direction effects were also studied.

A parametric study of lycopodium dust flame

Dust flames are associated with two-phase combustion phenomena where flame characteristics depend on interactions between solid and gas phases. Since organic dust particles can be effectively utilized in energy production systems, investigation of this phenomenon is essential. In this study, an analytical model is presented to simulate the combustion process of moist organic dust. The flame structure is divided into three zones: preheat zone, reaction zone, and postflame zone. To determine the effects of moisture content and volatile evaporation, the preheat zone is also divided into four subzones: first heating subzone and drying subzone, second heating subzone, and volatile evaporation subzone. The results obtained from the presented model are in reasonable agreement with experimental data for lycopodium particles. An increase in moisture content causes a reduction in burning velocity owing to moisture evaporation resistance. Consequently, the effects of some important parameters, like volatilization temperature, volatilization Damköhler number and drying Damköhler number are investigated. In special cases, like high moisture content, low volatilization temperature, and high drying resistance, the second heating subzone is omitted.

Effect of N2/CO2 dilution on laminar burning velocity of H2–CO–O2 oxy-fuel premixed flame

The dilution effect of N2/CO2 on the laminar burning velocity of H2–CO–O2 mixtures was investigated. The dilution fraction of N2 and CO2 in the unburned mixtures varied from 0% to 70% and 0%–50%, respectively, and H2 content in H2–CO fuels altered from 5% to 100%. All the studies were carried out at standard laboratory conditions (1 atm, 298 K) with equivalence ratio changing from 0.6 to 2.0. The Heat flux method and OH-PLIF (Planar Laser-Induced Fluorescence) based Bunsen flame method were employed to measure the laminar burning velocities. The Li mechanism was used in simulations, due to its good prediction of laminar burning velocities. Based on extensive experimental results, the correlations between dilution fraction and laminar burning velocity reduction rate were analyzed. It was found that, for a given dilution fraction, the reduction in laminar burning velocity is largely independent equivalence ratio and fuel H2–CO mole fraction. This behavior does not extend to all fuels, e.g. methane. Exploiting the lack of dependence on equivalence ratio and fuel composition, a unified correlation equation was proposed which can be used to predict the laminar burning velocities of H2–CO fuels for given fuel component, dilution rate and equivalence ratio.

Effect of H2/CO ratio and N2/CO2 dilution rate on laminar burning velocity of syngas investigated by direct measurement and simulation

Laminar burning velocities of syngas/air premixed flames, varying with H2/CO ratio (from 5/95 to 75/25) and N2 or CO2 dilution rate (from 0% to 60%), were accurately measured using a Teflon coated Heat Flux burner and OH-PLIF based Bunsen flame method. Experiments were carried out at atmospheric pressure and room temperature, with fuel/air equivalence ratios ranging from fuel-lean to fuel-rich. Coupled with experimental data, three chemical kinetic mechanisms, namely GRI-Mech 3.0, USC Mech II and Davis H2–CO mechanism, were validated. All of them can provide good prediction for the laminar burning velocity. The laminar burning velocity variations with H2 and dilution gas contents were systematically investigated. For given dilution gas fraction, the laminar burning velocity reduction rate was enhanced as H2/CO ratio increasing. Effects of the syngas components and equivalence ratio variation on the concentrations of radical H and OH were also studied. It appears that there is a strong linear correlation between the laminar burning velocity and the maximum concentration of the H radical in the reaction zone for syngas. This characteristic is exclusively different from that in methane air premixed flame. These findings indicated that the high thermal diffusivity of the H radical played an important role in the laminar burning velocity enhancement and affected the laminar burning velocity reduction rate under dilution condition.

Laminar burning velocity measurements of methane and carbon dioxide mixtures (biogas) over wide ranging temperatures and pressures

Biogas is an important renewable energy resource that is mostly methane but contains up to 40% carbon dioxide by volume. As the carbon dioxide is a diluent, it reduces the laminar burning velocity, the flame stability and the flammability range, all of which can have adverse effects on combustion system performance. When used in gas turbines, spark ignited gas engines and dual fuel engines it is important to have laminar burning velocity data for high temperatures and pressures. These data have been obtained by using the pressure rise data from constant volume combustion, so that a single experiment yields burning velocity data (linked by the isentropic compression) for a range of pressures and temperatures. The pressure/temperature dependency has been decoupled by conducting experiments with different initial temperatures and pressures. The results from experiments with different carbon dioxide fractions and equivalence ratios have been fitted to a correlation with twelve coefficients. Data have been obtained with up to 40% carbon dioxide, pressures of up to 18bar, temperatures of up to 660K and equivalence ratios in the range 0.7–1.4. Results showed a decrease in burning velocity with pressure and an increase with temperature. The effect of dilution with carbon dioxide was a reduction in burning velocity, to around 65% of that of pure methane when the carbon dioxide fraction was 40% for stoichiometric mixtures. There is only a very limited amount of prior burning velocity data for high temperatures and pressures, but where possible, comparisons have been made with published data.

Suppression of hydrogen–oxygen–nitrogen explosions by fine water mist: Part 1. Burning velocity

A work programme has been undertaken to investigate the practical viability of using fine water mists to mitigate or suppress hydrogen explosions during nuclear decommissioning operations. In the first part of this study measurements of burning velocity, required primarily for the development of explosion models, are presented. Burning velocity measurements were made with the introduction of ultrasonically generated fine water mists. With water mist, the burning velocity was reduced over a wide range of equivalence ratios for fuel-free oxygen fractions of 0.1–0.21. Flame instability increased substantially with increasing water mist density especially with lean hydrogen mixtures. The experimental results obtained for the reduction of burning velocity with entrained water mist in hydrogen–air systems are compared to computer simulated values reported earlier in the literature. An important overall objective of the work programme was to provide information to plant engineers to assess the effectiveness of using water mists for the suppression or mitigation of explosion.

The effect of mono-dispersed water mist on the suppression of laminar premixed hydrogen–, methane–, and propane–air flames

Computational simulations are used to predict and understand the influence of fine water mists on the suppression of laminar, freely propagating methane–, propane–, and hydrogen–air atmospheric-pressure premixed flames. The model solves a coupled, chemically reacting, two-phase-flow problem. Flame suppression is measured in terms of a reduction in burning velocity. The effects of droplet diameter, net water loading, and fuel–air stoichiometry are reported. The results show similar qualitative features for all the flames. Generally speaking, smaller droplets are more effective than the larger droplets. Sufficiently small droplets (approximately 10 μm diameter for methane–air flames) are in a small-droplet limit, where even smaller droplets have the same suppression characteristics for the same net mass loading. Droplets above a certain diameter (approximately 30 μm for methane–air flames) lead to a turning-point extinction, where the burning velocity at the turning point is approximately half of the unperturbed burning velocity without any water-mist loading.

Inhibition of premixed methane/air flames by water mist

The effect of submicron water drops on the buring velocity of methane/air mixtures was investigated. Results are compared to the suppression effect of water vapor, gaseous thermal agents N2 and CF4, and chemical agent CF3Br to determine if the theoretical thermal suppression effect expected for water mist can be experimentally achieved. These studies lay the groundwork (both experimental and modeling) for the effects of larger drops as well as for drops with solutes. Stoichiometric mixtures were stabilized at atmospheric pressure on one of two nozzle-type burners, producingcone-shaped flames. Burning velocities were determined using the total area method. An atomizer was used to produce water mist with a mean drop diameter of less than 1 μm, which was delivered as part of the reactant stream. Studies were carried out using both dry and humidified air. On a mass basis, the burning velocity of N2- and CF4-inhibited flames exhibited similar characteristics. Water vapor was observed to be more effective than N2 or CF4, but less effective than the same mass of water mist, consistent with thermodynamic analyses. Water mist inhibition results were well predicted by a recently developed multiphase flame model. Under these conditions, the burning velocity reduction effectiveness of water mist is approximately 3.5 times greater on a mass basis than for N2 and CF4 and is comparable to modeled data for CF3Br-inhibited flames.

Flame inhibition by ferrocene and blends of inert and catalytic agents

The production of the fire suppressant CF3Br has been banned, and finding a replacement with all of its desirable properties is proving difficult. Iron pentacarbonyl has been found to be up to several orders of magnitude more effective than CF3Br, but it is flammable and highly toxic. Ferrocene [Fe(C5H5)2], which is much less toxic and flammable than Fe(CO)5, can also be used to introduce iron into a flame. We present the first experimental data and numerical modeling for flame inhibition by ferrocene and find it to behave similarly to Fe(CO)5. A ferrocene mole fraction of 200 ppm reduced the burning velocity of slightly preheated premixed methane/air flames by a factor of two, and the effectiveness dropped off sharply at higher mole fractions. For air with a higher oxygen mole fraction, the burning velocity reduction was less. We also present experimental data and modeling for flames with ferrocene blended with CO2 or CF3H. The combination of the thermally acting agent CO2 with ferrocene mitigated the loss of effectiveness experienced by ferrocene alone at higher mole fractions. An agent consisting of 1.5% ferrocene in 98.5% CO2 performed as effectively as CF3Br in achieving a 50% reduction in burning velocity. Likewise, four times less CO2 was required to achieve the 50% reduction if 0.35% ferrocene was added to the CO2. In contrast, addition of 0.35% ferrocene to the hydrofluorocarbon CF3H reduced the CF3H required to achieve the 50% reduction in burning velocity by only about 25%. Thermodynamic equilibrium calculations predict that the formation of iron/fluoride compounds can reduce the concentrations of the iron-species oxide and hydroxide intermediates which are believed to be responsible for the catalytic radical recombination cycles.

Laminar Burning Velocities of Trifluoromethane–Methane Mixtures: Experiment and Numerical Simulation

The laminar burning velocities of trifluoromethane(CHF 3)–methane(CH 4)–oxygen(O 2)–diluent mixtures were determined over extensive fuel concentration ranges using the counterflow flame technique. Numerical simulation was performed by employing a detailed kinetic model compiled on the bases of the GRI-Mech for methane combustion and a recent CHF 3 reaction kinetic model. Comparisons between the experimental data and numerical results indicate that while the qualitative experimental trends are well predicted by the model, there exist significant numerical disagreements between model and experiment. Through sensitivity and flux analyses, we identified several rate parameters which are influential to burning velocity predictions, and proposed reasonable adjustments to these parameters either based on new experimental measurements or by considering their associated uncertainties. The effect of CHF 3 addition on the reduction of CH 4 burning velocities was also experimentally and numerically examined. By substituting the inert gas in the unburned CH 4–O 2–inert mixture with CHF 3 while maintaining a constant adiabatic flame temperature, both burning velocities and mass burning rates decrease with an increase in CHF 3 substitution, thus demonstrating positively the combined kinetic and transport effects of CHF 3 on burning velocity reduction and in flame inhibition.

Inhibition of premixed methane-air flames by fluoromethanes

This paper presents the first calculations and measurements of the burning velocity of premixed hydrocarbon flames inhibited by the three one-carbon fluorinated species CH 2F 2, CF 3H, and CF 4. The chemistry of these agents is expected to be similar to that of some agents that may be used as replacements for CF 3Br, so that studying their behavior in methane flames provides an important first step towards understanding the suppression mechanism of hydrocarbon fires by fluorinated compounds. The burning velocity of premixed methane-air flames stabilized on a Mache-Hebra nozzle burner is determined using the total area method from a schlieren image of the flame. The inhibitors are tested over a range of concentration and fuel-air equivalence ratio, φ. The measured burning velocity reduction caused by addition of the inhibitor is compared with that predicted by numerical solution of the species and energy conservation equations employing a detailed chemical kinetic mechanism recently developed at the National Institute of Standards and Technology (NIST). Even in this first test of the kinetic mechanism on inhibited hydrocarbon flames, the numerically predicted burning velocity reductions for methane-air flames with values of φ of 0.9, 1.0, and 1.1 and inhibitor mole fractions in the unburned gases up to 0.08, are in excellent agreement for CH 2F 2 and CF 4 and within 35% for CF 3H. The numerical results indicate that the agents CF 3H and CH 2F 2 are totally consumed in the flame and the burning velocity is reduced primarily by a reduction in the H-atom concentration through reactions leading to HF formation. In contrast, only about 10% of the CF 4 is consumed in the main reaction zone and it reduces the burning velocity primarily by lowering the final temperature of the burned gases.

Markstein lengths of CO/H 2/air flames, using expanding spherical flames

Markstein lengths (defined as the reduction in burning velocity per unit stretch) have been measured for a series of CO/H2/air flames at atmospheric pressure. Values as a function of stoichiometry are reported for three fuels (95%/5% CO/H2, 50%/50% CO/H2, and 100% H2). In addition, the dependence of Markstein length on H2 content has been measured for stoichiometric CO/H2 mixtures.

Inhibition of hydrogen-air and hydrogen-nitrous oxide flames by halogen compounds

The paper describes an experimental and theoretical study of the inhibitory effects of halogen compounds on hydrogen-nitrogen-oxygen and hydrogen-nitrogen-nitrous oxide flames. Studies were made of the effects of methyl bromide, ethylene, and hydrogen bromide on the rich flammability limits of hydrogen-nitrogen-nitrous oxide mixtures, and the results were compared with published data for the hydrogen-air system. The effects of CH 3 Br and HBr on the limit were considerably less in the nitrous oxide supported flame than in the air flame; but, when ethylene was the additive, the difference was not so great. The effect of trace additions of HBr on the burning velocities of some near-limit hydrogen-nitrogen-oxygen flames showed a considerable degree of inhibition, e.g., addition of 0.027% HBr to a flame containing 4.60% oxygen reduced the burning velocity from 9.2 to 6.4 cm sec −1 . The measured reduction in burning velocity has been used as an experimental criterion of the success or otherwise of possible reaction mechanisms which were the subject of parallel computer studies. The computer studies involved the solution of the flame-conservation equations to obtain burning velocities and temperature and composition profiles. It was concluded that most of the added HBr is removed by Reaction (xxiii) in the early stages of the flame to give an equilibrium mixture of H, Br, H 2 , and HBr over most of the reaction region. The temporary removal of H atoms in this way is not sufficient, however, to explain H+HO 2 =H 2 +O 2 (xii) H+HBr⇌H 2 +Br (xiii) H+HBr⇌H+Br 2 (xxiv) Br+Br+M=Br 2 +M (xxv) H+Br+M=HBr+M (xxvi) Br+HO 2 =HBr+O 2 (xxvii) the observed inhibition. For this, new chain-breaking steps such as Reactions (xxv)–(xxvii) must be added to the reaction mechanism for the uninhibited flame. The rate constants which must be assumed for Reactions (xxv)–(xxvii) are not unreasonable, e.g., 0.25≤ k 27 / k 12 ≤0.5.