Wide Range Of Equivalence Ratios Research Articles

95/06387 The impacts of large scale use of electric vehicles in southern California

We present estimates for the effective oxidation rates of methane over a wide range of equivalence ratios using Ar as primary diluent and CO or H2O as additives. The rate of disappearance of methane has been monitored in a fuel-rich CH4O2Ar mixture by direct measurements of the change in transmission for He-Ne laser radiation at 3.392 μ behind reflected shock waves. The rate of formation of water with time has been obtained in independent absolute i.r. emission measurements near 6.75 μ on stoichiometric CH4O2Ar mixtures with and without added water. We have found empirically that a number of data correlations may be used and that the overall rates of disappearance of CH4 and of formation of H2O were the same during the post-induction period behind reflected shock waves. The rates of concentration changes with time to which we refer are average rates observed during the post-induction periods. The temperatures (Tmax) for which the oxygen-atom concentrations are a relative maximum were found to be close to the arithmetic means between the initial temperatures (T5) behind reflected shock fronts and the final equilibrium temperatures. The numerical values found for the correlation parameters as functions of the temperature during the post-induction oxidation are not sensitive to the prior admixture of CO or H2O but vary with mixture ratio.The overall methane conversion rates are directly useful in defining burner designs for the utilization of highly diluted mixtures (i.e. low-Btu gas systems) containing primarily CH4. Our results are not applicable to low-Btu gas mixtures containing such reactive species as H2 or NH3.

A lagrangian model for predicting turbulent diffusion flames with chemical kinetic effects

The present study is a new step in the development of a lagrangian turbulent combustion model devotedto the prediction of non-premixed flames in which kinetic effects occur. Taking into account, in a refined manner, the interaction between chemical reaction and turbulence, the model includes a spectrum of turbulent timescales instead of a single mean timescale. The more recent improvement of the model consists in including a library of chemical delay times derived from a six-step reduced reaction mechanism for methane-air combustion, which is valid over a wide range of equivalence ratio. The predictions of the combustion model in which the library was implemented were compared with experimental data on turbulent jet diffusion flames near extinction. These calculations were performed with a finite volume technique devoted to variable density Navier-Stokes equations. A standard two-equations turbulence model was used and provided reliable results for dynamic variables (mean velocity and kinetic energy of turbulence). Comparisons between computed and measured mean mixture fraction and mixture fraction root mean square (rms) are quite satisfactory, proving that scalar transport is correctly modeled. The more salient point is the prediction of partial extinction and reignition phenomena, in agreement with experiments. This result clearly demonstrates the ability of the turbulent combustion model to simulate the interaction between turbulence and chemical reactions, which occurs in flames displaying kinetic effects.

Comparison of Minimum Ignition Energy for Four Alkanes: Effects of Ignition Kernel Size and Equivalence Ratio

Abstract Minimum ignition energies have been calculated for alkane-air mixtures at standard ambient conditions. The governing conservation equations have been solved using approximate transport properties and chemistry. A six-species, four-step global reaction scheme for combustion of alkane hydrocarbons up to butane (Jones and Lindstedt, 1988) was employed. Radiative heat loss due to band emission from the ignition kernel which is included in the calculations was found to be negligible at equivalence ratios where ignition energy is a minimum. The important effect of energy input radius on minimum ignition energy is discussed in some detail for a propane-air mixture. After establishing a methodology for finding the minimum ignition energy for a specific fuel and equivalence ratio, results are presented for methane through butane over a wide range of equivalence ratios. A comparison is made with experimental results, and the similarities and differences are discussed.

Effects Of Equivalence Ratio On the Extinction Stretch Rate Of Cylindrical Premixed Flames

Abstract The actual flame stretch rates of the stretched cylindrical premixed flame and counterflow twin flames were measured with LDV for the methane-air and propane-air mixtures over a wide range of equivalence ratios. The stretch rate at the flame center at the point of flame extinction was considered to be the maximum stretch rate for the existence of cylindrical and plane ftamelets. It was shown that the actual flame stretch rate at the point of flame extinction for the stretched cylindrical flame was smaller than that for the counterflow twin flames except for the case of very rich propane-air mixtures. The conditions for the existence of the cylindrical flamelet in turbulent premixed flames, which has a similarity to the fine structure of turbulent premixed flames proposed by Chomiak (1970, 1972, 1977), were discussed using the stretch rate data at the extinction point and the turbulence parameters. It was found that the cylindrical flame let could not exist in highly turbulent flames where the dis...

OH Vibrational Thermally-Assisted Laser Induced Fluorescence Temperature Measurements in Flames

Abstract Thermally-assisted laser induced fluorescence temperature measurements using upward vibrational transfer from υ' = 0 to υ' = I in A2 Σ+ OH have been demonstrated in premixed nitrogen diluted flames of acetylene, ethylene, and methane over a wide range of equivalence ratios. Detection of the (0-0) and (1-0) vibrational bands was used to determine the relative populations of υ' =0 and I from which temperature was calculated assuming a simple Iwo level model incorporating the effects of vibrational energy transfer and quenching. It was found that collisional effects that are a source of systematic error can be accounted for through a calibration procedure for each fuel type. The resulting accuracy of the measurement is better than I00K as determined through comparison with sodium line reversal temperature measurements in flames that ranged in temperature from 1700 K to 2700 K. Similar results using detection of the (0-1) and (1-0) vibrational bands in order to minimize self-absorption effects were a...

Effects of Spark Plug Number and Location in Natural Gas Engines

Combustion experiments were conducted on a spark-ignited single-cylinder engine operating on natural gas. A special open chamber cylinder head was designed to accept as many as four spark plugs. Data were obtained to investigate the effects of spark plug quantity and location on NOx, HC, CO emissions, brake and indicated thermal efficiency, MBT timing, combustion duration, ignition delay, peak cylinder pressure, peak cylinder temperature, and heat release over a wide range of equivalence ratios.

The influence of catalytic activity on the ignition of boundary layer flows. Part III: Hydroxyl radical measurements in low-pressure boundary layer flows

The boundary layer ignition of fuel/air mixtures by heated catalytic (platinum) and noncatalytic (quartz) surfaces was studied through spatially resolved measurement of hydroxyl radicals at total pressures from 40 to 175 torr. Subatmospheric pressure operation increases both length scales and the ratio of surface/gas-phase reaction contribution to ignition, facilitating the study of the effect of surface catalytic activity on the initiation of gas-phase combustion. Fuels studied include ethane, acetylene, hydrogen, and methane, usually premixed in air. Ethane ignition was promoted by catalytic reaction at all equivalence ratios studied. This was a wider range of equivalence ratios evidencing catalytic enhancement than noted at atmospheric pressure in previous studies. At 70 torr, this enhancement was observed from 0.015 < Φ ≤ 0.5. Surface hydroxyl radical production rates were estimated for preignition conditions in the gas phase for ethane in air over both quartz and platinum surfaces. Combustion of hydrogen or acetylene in air at these conditions was not significantly affected by the catalytic activity of the surface, because for the reactions studied, gas-phase reaction was significant near the leading edge of the plate. N 2O substitution for O 2 severely inhibited the gaseous ignition of the ethane, hydrogen, and acetylene over platinum surfaces, and suppressed preignition OH concentrations to below our detection limits.

Detailed Mechanism for Oxidation of Benzene

Abstract A detailed mechanism for the oxidation of benzene is presented and used to compute experimentally obtained concentration profiles and ignition delay limes over a wide range of equivalence ratio and temperature. The computed results agree qualitatively with all the experimental trends, and quantitative agreement is obtained with several of the composition profiles and ignition delay limes. There are indications, however, that some important reactions are as yet undiscovered, because of differences between experimental and computed results. Among these is a weaker computed temperature dependence of the computed ignition delay times than that observed experimentally. Available literature expressions have been used for the rate coefficients of all reactions, except for two involving phenol. The phenol pyrolysis rate coefficient used in this work is 25% lower than a recent literature expression and that for the hydroxyl radical attack on phenol had to be increased significantly.

Premixed combustion at a fiber mat

The combustion of a premixed gas passing through a highly permeable metal fiber mat forms a high temperature, stable reaction region in the vicinity of the fiber mat surface for a wide range of equivalence ratio. Using stainless steel fiber mat burner (porosity 93%), measurements were made of the temperature and composition of the combustion gas near the mat surface, the surface and internal temperature of the mat, and the radiation energy from the mat to determine if the combustion region near the mat surface or inside the mat was influenced by heat loss or heat gain caused by the fiber mat. Within the range of mixture velocities (10 cm/s–25 cm/s) and near the stoichiometric mixture condition, the mat emits intense radiation energy more than 6 W/cm2. The temperature distribution inside the mat and near the mat surface as a function of the mixture velocity or equivalence ratio can be reasonably interpreted by the relation of the heatbalance in the mat and movement of the reaction zone. The gas analyses near the mat surface verify that for a combustion condition making the mat radiant a part of raction takes place in the mat but the reaction is considerably controlled in the downstream region of the mat due to heat loss to the mat.

Mechanism of Combustion Reactions in Hydrogen–Air Premixed Flames

Abstract Simulation was performed by use of a chemical kinetic model built for flat hydrogen–air premixed flames in order to investigate the combustion mechanism taking place in flames through a wide range of equivalence ratio. The following results were obtained: the flame zone of the stoichiometric flame can be considered as having two different regions; the low-temperature region has, furthermore, three parts differing in reaction mechanism; since the reactions cannot proliferate hydrogen atoms by themselves in the low-temperature region, the acceleration of the reactions is necessarily governed by the hydrogen atoms diffused from the flame front; in the high-temperature region, the reactions produce an excess of hydrogen atoms which activate the reactions in this region or diffuse toward the low-temperature region; lean hydrogen flames possess smaller burning velocity than that of rich flames because the reactions in the low-temperature region have less reproducibility of hydrogen atoms.

Extinction characteristics of a stretched cylindrical premixed flame

An axially stretched, cylindrical premixed flame was formed by using a radial-flow nozzle burner, which made it possible to produce a highly stretched flow field in laminar conditions and to investigate the extinction characteristics of the flame over a wide range of equivalence ratios. Using this burner, extinction limits and flame diameters at the limits were measured for both methane-air and propane-air mixtures, and the data were compared with those of the counterflow twin flames reported before. The results are discussed from the viewpoint of the investigation of the flame curvature effects on the extinction characteristics of the stretched flame.

Nitrogen dioxide formation by the mixing of hot combustion gas with cold air

NO, NO2, stable specirs concentrations and temperature have been measured in a double concentric jet facility to study the mechanism of NO2 formation by the mixing of hot combustion gases with cold air. The double concentric jets are designed to madel the mixing region in practical combustion systems and consist of a central hot combustion gas jet from a propane/air burner and an annular cold air jet confined in a cylindrical duct. The main objective of this study is to determine the effect of equivalence ratio, on NO2 formation. The burner was operated under a wide range of equivalence ratios from 0.62 to 1.60. NO was found to convert to NO2 in the mixing region of the jets through the HO2 mechanism (oxidation of NO mainly by HO2 radical). The degree of NO2 formation can be quantified from the NO2/NOx ratio measured at the exit of the duct. Under fuel-lean conditions, with decreasing the equivalence ratio, the NO2/NOx ratio increases up to 0.81 though the total NOx level quickly decreases. The effect of initial NO level on the, NO2 formation seems to be dominant in this case, which explains higher NO2/NOx ratio with lowering the initial NO levels. Under fuel-rich conditions, the NO2/NOx ratio is about 0.90 at equivalence ratios above 1.2. The additional supply of radicals from the diffusion flame, and the presence of unburned species in relatively cool regions probably yield the high NO2/NOx ratio under the fuel-rich condition.

Measurement of the burning velocity of propane-air mixtures using soap bubbles.

Experiments have been performed to determine the burning velocity of propane-air mixtures over a wide range of equivalence ratios using soap bubbles and a hot-wire anemometer. This method gives a maximum burning velocity of 50 cm/s at an equivalence ratio of 1.1. The present maximum burning velocity agrees well with the theoretical result predicted by Warnatz. The values of the burning velocity determined in these experiments are also in relatively good agreement with those of other accurate methods using a burner. The flame-front instabilities of a hydrodynamic or diffusional-thermal origin are also discussed.

An Analysis of a Stretched Cylindrical Premixed Flame

Abstract For the basic investigation of a premixed flame subjected to the effects of both flame stretch and flame curvature, the characteristics of the outward propagating cylindrical flame formed in an axially stretching flow were theoretically analyzed and compared with those of counter-flow twin flames, which have no curvature, and other types of non-stretched flames. The flame sheet model was first adopted for the parametrical investigation of the effects of Lewis number and preferential diffusion. Then, the finite reaction rate model was used and the extinction characteristics were analyzed over a wide range of equivalence ratios, resulting in the agreement of the extinction limit curves with the experimental data reported before.

Combustion of Coal/Water Mixtures With Thermal Preconditioning

Thermal preconditioning is a process in which coal/water mixtures are vaporized to produce coal/steam suspensions, and then superheated to allow the coal to devolatilize producing suspensions of char particles in hydrocarbon gases and steam. This final product of the process can be injected without atomization, and burned directly in a gas turbine combustor. This paper reports on the results of an experimental program in which thermally preconditioned coal/water mixture was successfully burned with a stable flame in a gas turbine combustor test rig. Tests were performed at a mixture flowrate of 300 lb/hr and combustor pressure of 8 atm. The coal/water mixture was thermally preconditioned and injected into the combustor over a temperature range from 350°F to 600°F, and combustion air was supplied at between 600°F to 725°F. Test durations varied between 10 and 20 min. Major results of the combustion testing were that: A stable flame was maintained over a wide equivalence ratio range, between φ = 2.2 (rich) and 0.2 (lean); and combustion efficiency of over 99 percent was achieved when the mixture was preconditioned to 600°F and the combustion air preheated to 725°F. Measurements of ash particulates, captured in the exhaust sampling probe located 20 in. from the injector face, show typical sizes collected to be about 1 μm, with agglomerates of these particulates to be not more than 8 μm. The original mean coal particle size for these tests, prior to preconditioning, was 25 μm. Results of additional tests showed that one third of the sulfur contained in the solids of a coal/water mixture with 3 percent sulfur was evolved in gaseous form (under mild thermolized conditions) mainly as H2S with the remainder as light mercaptans.

Determination of burning velocity of methane-air mixtures using soap bubbles and a hot-wire anemometer.

A technique is developed to measure burning velocity using soap bubbles and a hot-wire anemometer. This method is a modified version of that mode by Andrews and Bradley. The principle of this method is the simultaneous measurement of flame speed and unburned gas velocity ahead of the flame in a soap bubble. The flame propagation is recorded using a schlieren system with chopped light source. The method has been used to measure the burning velocity of methane-air mixtures over a wide range of equivalence ratios. This method gives a maximum burning velocity of 45 cm/s, at an equivalence ratio of 1.08. Values of burning velocity determined in this way are in good agreement with those of Andrews and Bradley and those of other accurate methods, but in moderately rich mixtures, the present data are slightly higher than those obtained by other workers using burner methods. The influence of flame curvature upon the burning velocity is discussed.

Some interpretive aspects of elementary sensitivity gradients in combustion kinetics modeling

The applications of sensitivity analysis by gradient techniques to study detailed combustion chemistry will soon become more common as a result of the recent availability of increasingly efficient computer codes. This paper provides a compendium of the various uses and interpretive aspects of sensitivity gradient techniques applied to chemical kinetics. Illustrations are provided through analysis of the frequently studied elementary kinetic mechanism for the CO/H 2O/O 2 system. Elementary first and second order sensitivity coefficients, obtained by the Green's function method, are interpreted and discussed in detail for a fuel lean and dilute system reacting in N 2 at 1100K and 1 atm. From the results of linear sensitivities, the effects of variations in initial conditions and uncertainties in rate constants and equilibrium constants on the model predictions are determined. Second order sensitivities are shown to yield valuable information on how these linear sensitivities vary when an initial condition or rate constant is varied. Elements of the Green's function matrix are shown to have a memory function interpretation, and results of small perturbations in each species concentration at various times during the kinetics on all other species concentrations at later times are presented. These latter sensitivities are unique in that they, in principle, are measurable in the laboratory. The results of feature sensitivity analysis (i.e., the effects of variations in input parameters on specific kinetic predictions, such as induction and kinetic periods) are presented for a wide range of equivalence ratios. Finally, elementary sensitivity analysis is shown to be extremely useful in experimental design as well as model development and evaluation.

Determination of burning velocity using counterflow flames

A technique has been developed to measure burning velocity using counterflow double flames established in the forward stagnation region of the porous cylinder. In this system, two different regimes occur for the inner premixed flame depending on whether the mixtureejection velocity is more or less than the burning velocity. A plot of the flame position versus the mixture-ejection velocity is linear, in the absence of interaction between the flame and the cylinder surface, and this linearity is destroyed suddenly and discontinuously at the critical ejection velocity, i.e., the burning velocity. A major advantage of this new method is that, unlike in burners using non-divergent flows, the flame does not blow off or change its geometry radically when the flow velocity reaches the burning velocity. The method has been used to determine the burning velocity of methane-air mixture and propane-air mixture over a wide range of equivalence ratios. Results are in good agreement with those of other accurate methods using burners. This method appears to offer a wider range, in terms of equivalence ratio, than that available with any other burner.

Low-Btu gas mixtures—I. Methane oxidation rates for a wide range of equivalence ratios

We present estimates for the effective oxidation rates of methane over a wide range of equivalence ratios using Ar as primary diluent and CO or H 2O as additives. The rate of disappearance of methane has been monitored in a fuel-rich CH 4O 2Ar mixture by direct measurements of the change in transmission for He-Ne laser radiation at 3.392 μ behind reflected shock waves. The rate of formation of water with time has been obtained in independent absolute i.r. emission measurements near 6.75 μ on stoichiometric CH 4O 2Ar mixtures with and without added water. We have found empirically that a number of data correlations may be used and that the overall rates of disappearance of CH 4 and of formation of H 2O were the same during the post-induction period behind reflected shock waves. The rates of concentration changes with time to which we refer are average rates observed during the post-induction periods. The temperatures ( T max ) for which the oxygen-atom concentrations are a relative maximum were found to be close to the arithmetic means between the initial temperatures ( T 5) behind reflected shock fronts and the final equilibrium temperatures. The numerical values found for the correlation parameters as functions of the temperature during the post-induction oxidation are not sensitive to the prior admixture of CO or H 2O but vary with mixture ratio. The overall methane conversion rates are directly useful in defining burner designs for the utilization of highly diluted mixtures (i.e. low-Btu gas systems) containing primarily CH 4. Our results are not applicable to low-Btu gas mixtures containing such reactive species as H 2 or NH 3.

Photographic studies of cool flames and knock in an engine

Large Eddy Simulation of knocking in piston engines requires high-fidelity physical models and numerical techniques. The need to capture temperature fields with high precision to predict autoignition is an additional critical constraint compared to existing LES in engines. The present work presents advances for LES of knocking in two fields: (1) a Conjugate Heat Transfer (CHT) technique is implemented to compute the flow within the engine over successive cycles with LES together with the temperature field within the cylinder head walls and the valves and (2) a reduced two-step scheme is used to predict both propagating premixed flames as well as autoignition times over a wide range of equivalence ratios, pressures and temperatures. The paper focuses on CHT which is critical for knocking because the gas temperature field is controlled by the wall temperature field and knocking is sensitive to small temperature changes. The CHT LES is compared to classical LES where the temperatures of the head and the valves are supposed to be homogeneous and imposed empirically. Results show that the skin temperature field (which is a result of the CHT LES while it is a user input for classical LES) is complex and controls knocking events. While the results of the CHT LES are obviously better because they suppress a large part of the empirical specification of the wall temperatures, this study also reveals a difficult and crucial element of the CHT approach: the description of exhaust valves cooling which are in contact with the engine head for part of the cycle and not in the rest of the cycle, leading to difficulties for heat transfer descriptions between valves and head. The CHT method is successfully applied to an engine studied at IFP Energies Nouvelles where knocking characteristics have been studied over a wide range of conditions.