M. Lawes

Laminar burning velocity and Markstein lengths of methane–air mixtures

Abstract Spherically expanding flames propagating at constant pressure are employed to determine the unstretched laminar burning velocity and the effect of flame stretch as quantified by the associated Markstein lengths. Methane–air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa are studied at equivalence ratios of 0.8, 1.0, and 1.2. This is accomplished by photographic observation of flames in a spherical vessel. Power law correlations are suggested for the unstretched laminar burning velocity as a function of pressure, temperature, and equivalence ratio. Zeldovich numbers are derived to express the effect of temperature on the mass burning rate and from this, a more general correlation of burning velocity, based on theoretical arguments, is presented for methane–air mixtures. Flame instability is observed for mixtures at high pressure, and the critical radius for the onset of cellularity is correlated with Markstein number. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism, and the second considers a one-dimensional flame using a full kinetic scheme. The results are compared with those of other researchers. Comparison also is made with iso-octane–air mixtures, reported elsewhere, to emphasize the contrast in the burning of lighter and heavier hydrocarbon fuels.

The measurement of laminar burning velocities and Markstein numbers for iso-octane-air and iso-octane-n-heptane-air mixtures at elevated temperatures and pressures in an explosion bomb

Abstract Spherically expanding flames have been employed to measure flame speeds, from which have been derived corresponding laminar burning velocities at zero stretch rate. Two burning velocities are defined, one based upon the rate of propagation of the flame front, the other on the rate of formation of burned gas. To express the effects of flame stretch upon burning velocity, Markstein lengths and numbers for both strain and curvature also have been obtained from the same measurements of flame speed. The effects of the initial mixture temperature and pressure on these parameters also have been examined and data have been obtained for iso-octane–air mixtures at initial temperatures between 358 K and 450 K, at pressures between 1 and 10 bar, and equivalence ratios, φ, of 0.8 and 1.0. Burning velocities and Markstein numbers also are reported for a fuel comprised of 90% iso-octane and 10% n-heptane, with air, for the same range of pressures, temperatures, and equivalence ratios. An important observation is that, as the pressure increases, a cellular flame structure develops earlier during flame propagation. The reasons for this are discussed. As the flame surface becomes completely cellular there is an increase in flame speed and this continues as the flame propagates. The increase in the rate of flame propagation due to flame cellularity has been carefully charted. General expressions are presented for the increase in stretch-free burning velocity with initial temperature and its decrease with pressure. The measured burning velocities are compared with those of other researchers and reasons for the differences discussed.

Turbulent burning velocities: a general correlation in terms of straining rates

All known experimental values of turbulent burning velocity have been scrutinized. These number 1650, a significant proportion of which at the higher turbulent Reynolds numbers we measured in a fan-stirred bomb. Dimensionless correlations which have a theoretical basis are presented. These are in terms of flame straining rates and the effective r. m. s. turbulent velocity, as well as the laminar burning velocity of the mixture. When a flame develops from an ignition source it is not initially exposed to the lower frequencies of the turbulent spectrum. As the kernel grows the flame is affected by ever-lower frequencies and the turbulent burning velocity increases towards a fully developed value. An experimental dimensionless power spectral density function is presented, and used to show how both effective r. m. s. turbulent velocity and flame straining rate develop in an explosion. The results are relevant to a variety of practical devices, including gasoline engines, as well as atmospheric explosions.

Lewis number effects on turbulent burning velocity

Experimental values of turbulent burning velocities for propane, hydrogen and iso-octane mixtures with air are reported under conditions of high turbulence and high turbulent Reynolds number. The measurements were made by the double kernel method during explosions in a fan-stirred bomb, with four fans, capable of speeds of up to 10,000 rpm. The ratio of turbulent to laminar burning velocity, u t /u l , is correlated primarily with the ratio of r.m.s. turbulent velocity to laminar burning velocity, u′/u l and a Karlovitz stretch factor given by the ratio of a strain rate u′/λ to a flame gradient given by u l /δ l , where λ is the Taylor microscale and δ l the laminar flame thickness. Asymptotic analyses of strained laminar flames, together with the two-eddy theory of turbulent burning, show the additional importance of Lewis number effects. These result in lean hydrocarbon mixtures being quenched more readily than rich ones, with an opposite effect for H 2 mixtures. This was observed in the experiments. However, full quantitative agreement between theory and experiment was not achieved, due to the inherent limitations of the two theories, which are discussed.

Turbulent burning velocity, burned gas distribution, and associated flame surface definition

Abstract Experimental studies of premixed, turbulent, gaseous explosion flames in a fan-stirred bomb are reported. The turbulence was uniform and isotropic, while changes in the rms turbulent velocity were achieved by changes in the speed of the fans. Central spark ignitions created mean spherical flame propagation. The spatial distributions of burned and unburned gases during the propagation were measured from the Mie scattering of tobacco smoke in a thin planar laser sheet. The plane was located just in front of the central spark gap and was generated by a copper vapor laser operating at a pulse rate of 4.5 kHz. High-speed schlieren images also were captured simultaneously. The distributions of the proportions of burned and unburned gases around circumferences were found for all radii at all stages of the explosion, and mean values of these proportions were derived as a function of the mean flame radius. The flame brush thickness increased with flame radius. The way the turbulent burning velocity is defined depends on the chosen associated flame radius. Various definitions are scrutinized and different flame radii presented, along with the associated turbulent burning velocities. Engulfment and mass turbulent burning velocities are compared. It is shown how the latter might conveniently be obtained from schlieren cine images. In a given explosion, the burning velocity increased with time and radius, as a consequence of the continual broadening of the effective spectrum of turbulence to which the flame was subjected. A decrease in the Markstein number of the mixture increased the turbulent burning velocity.

Premixed turbulent flame instability and NO formation in a lean-burn swirl burner

Abstract Computed results are presented from a Reynolds stress, stretched laminar flamelet model, for premixed swirling combustion in a rotating matrix burner. These are in good agreement with flame photography and coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements. The principal variable is the equivalence ratio, φ, while the swirl number and mean axial entry velocity remain constant. At higher equivalence ratios, the flame is stabilized by hot gas in both the inner and outer recirculation zones. As φ is reduced below 0.6 another steady-state solution appears, in which the flame is stabilized only by the hot gas in the inner zone. In this regime, the experiments show unstable combustion with low-frequency oscillations between the two states. This instability is shown to originate in flame quench in the regime between the two recirculations. At the values of φ below 0.6, most of the NO produced originates as prompt NO in the reaction zone. When φ is reduced to 0.56, there is only one steady-state solution. While the investigations were of a laboratory scale burner, some of the general findings are relevant to practical combustors.

The formation of NOx in surface burners

Abstract Surface combustion of premixed methane/air mixtures within and near the downstream surface of a porous matrix burner were experimentally investigated. The experiments included measurements of radiant flux, surface temperature, gas temperature, and stable species concentrations. Particular attention was given to the burned gas temperature profiles in order to define the flame zone, and compare its position with that predicted by a theoretical model that utilizes large-activation-energy asymptotic methods. Although most of the methane is combusted (∼ 90%) within the porous medium, the maximum rate of heat release was found to occur at, or just outside, the surface. Prompt and thermal NO formation was modeled, and the vast majority of the NO was found to be formed by the prompt-NO mechanism.

VARIATION OF TURBULENT BURNING RATE OF METHANE, METHANOL, AND ISO-OCTANE AIR MIXTURES WITH EQUIVALENCE RATIO AT ELEVATED PRESSURE

ABSTRACT Turbulent burning velocities for premixed methane, methanol, and iso-octane/air mixtures have been experimentally determined for an rms turbulent velocity of 2 m/s and pressure of 0.5 MPa for a wide range of equivalence ratios. Turbulent burning velocity data were derived using high-speed schlieren photography and transient pressure recording; measurements were processed to yield a turbulent mass rate burning velocity, u tr. The consistency between the values derived using the two techniques, for all fuels for both fuel-lean and fuel-rich mixtures, was good. Laminar burning measurements were made at the same pressure, temperature, and equivalence ratios as the turbulent cases and laminar burning velocities and Markstein numbers were determined. The equivalence ratio (φ) for peak turbulent burning velocity proved not always coincident with that for laminar burning velocity for the same fuel; for iso-octane, the turbulent burning velocity unexpectedly remained high over the range φ = 1 to 2. The ratio of turbulent to laminar burning velocity proved remarkably high for very rich iso-octane/air and lean methane/air mixtures.

Darrieus-landau and thermo-acoustic instabilities in closed vessel explosions

Experiments involving a spherical explosion bomb are reported, in which Darrieus–Landau thermo-diffusive, D-L,T-D, flame instabilities interacted with primary and secondary, self-excited, thermo-acoustic oscillations. Explosions with central ignition demonstrated that rich i-octane and lean hydrogen-air mixtures generated strong pressure oscillations, a consequence of their negative Markstein numbers. Utilizing dual wall ignitions, the structures of high pressure flames were studied using appropriate optical techniques. The conditions that gave rise to the greatest increase in the rate of combustion were strong initial D-L,T-D, flame instabilities and a high rate of change of the heat release rate, sufficient to generate strong secondary pressure oscillations. These, in turn, generated Rayleigh-Taylor instabilities that further wrinkled the flames. The bomb was equipped with four fans which showed that an rms turbulent velocity in excess of about 0.6 m/s was sufficient to reduce, and almost eradicate, the effect of these instabilities on the flame speed.

The structure of coal-air-ch4 laminar flames in a low-pressure burner: cars measurements and modeling studies

Abstract An experimental study is described of the structure of a flat, premixed, laminar, coal-air flame, with some methane added for flame stability. A low-pressure burner, at a combustion pressure of 30.4 kPa, was employed, in order to extend the reaction zone. Gas temperatures were measured by the CARS technique and the C 2 emissions observed with the laser diagnostics were found to depend upon the laser power. Concentration profiles of permanent species also were measured over a range of equivalence ratios. Measured values are compared with those predicted by a mathematical model, which assumes that CH 4 and HCN devolatilize from the coal and react in the gas phase. Allowance also is made for reactions of char and radiative heat transfer. The model gives good predictions of the temperature and oxygen concentration profiles, while predictions of NO are somewhat higher than those measured. Formation of NO is favored by OH and removal of it by NH 2 and NH. The sensitivity of the modeled results to various activation energies and pre-Arrhenius constants is examined and optimal values of these are in line with other values in the literature. The principal limitation in the model is the overprediction of CO concentration. An explanation of this lies in the formation, neglected in the model, of tarry structures of high molecular mass, followed by the generation of soot. This interpretation is supported by the measured profiles of C 2 emission intensity and their dependence upon the laser power, in contrast to the weaker emissions from rich CH 4 -air flames, which show no such dependence and are less persistent.