R. Woolley

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.

The development and structure of flame instabilities and cellularity at low Markstein numbers in explosions

Abstract Flame instabilities and the formation of cellular structures during spherical gaseous explosions have been studied experimentally using natural light and schlieren high-speed cine photography, as well as single-shot planar laser-induced fluorescence (PLIF) from the OH radical. High-pressure, rich-hydrocarbon and lean-hydrogen flames at low Markstein numbers were employed. Ranges of unstable wavelengths have been identified as a function of Markstein and Peclet numbers. The cine photography enables the dynamics of cell growth and fissioning to be studied and qualitatively interpreted, in terms of flame stretch rates and thermodiffusion. The PLIF technique enabled unstable wavelengths to be measured and flame fracture at negatively stretched cracks to be observed. A cascade of unstable wavelengths terminates in a cellular structure. This structure appears at a second critical Peclet number. The smaller cells are continually destabilizing and restabilizing. As they increase in size, the localized stretch rate on the cell surface decreases and the cell becomes unstable. It restabilizes by fissioning into smaller cells with higher localized stretch rates. The cells are bounded by cracks in regions of negative curvature. At sufficiently small Markstein numbers the cracks are fractured. The results are interpreted within the theoretical framework of the stability analysis of Bechtold and Matalon.

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.

Wrinkling and curvature of laminar and turbulent premixed flames

Abstract Premixed iso-octane and methane-air flames have been ignited in a fan stirred bomb in laminar conditions and turbulent flow fields at 1 and 5 bar. Sheet images of the flames were captured using LIF of OH. In spherically expanding laminar flames, the shape of cusps in the flame surface was shown to change from a dent for flames with positive Markstein numbers to a Huygen type cusp at lower Markstein numbers and finally complete quench was observed at the cusp tip on flames with negative Markstein numbers. The curvatures of turbulent flame edges were calculated and pdf’s generated. The pdf’s were symmetrical about a mean of zero, as the turbulence intensity was increased the pdf’s broadened and became flatter. Turbulent rich iso-octane-air flames (φ = 1.4) exhibited areas of quench in the flame front, the distance between areas of quench was shown to increase as the turbulence intensity was raised. The 5 bar flames exhibited higher curvature than those at 1 bar. The influence of laminar flame and turbulent flow properties on the curvature and hence flame wrinkling were investigated.

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 influence of simulated residual and NO concentrations on knock onset for PRFs and gasolines

Modern engine developments result in very different gas pressure-temperature histories to those in RON/MON determination tests and strain the usefulness of those knock scales and their applicability in SI engine knock and HCCI autoignition onset models. In practice, autoignition times are complex functions of fuel chemistry and burning velocity (which affects pressuretemperature history), residual gas concentration and content of species such as NO. As a result, autoignition expressions prove inadequate for engine conditions straying far from those under which they were derived. The currently reported study was designed to separate some of these effects. Experimental pressure crankangle histories were derived for an engine operated in skip-fire mode to eliminate residuals. The unburned temperature history was derived for each cycle and was used with a number of autoignition/knock models. A simple empirical expression proved no less effective than more complex formulations in predicting knock onset for iso-octane and PRFs over a wide range of residual free operating conditions. Prediction of knock onset for two commercial gasoline fuels proved less reliable, but was improved using an octane index correction method. Computations of knock onset times proved sensitive to simulated residual gas/EGR and NO concentrations. The influence of NO proved variable and contrary for iso-octane, gasolines, primary and toluene reference fuel mixtures.

The reduction of NOx formation in natural gas burner flames

Natural gas is widely used in domestic and commercial central heating units because it is a clean fuel. However, some NOx is produced and because more stringent legislation on NOx emissions has been proposed, there is considerable interest in designing combustion chambers to give even lower NOx emission. The burners used in such units use multi-jets and the combustion zones and resultant temperature, composition and flow fields have complicated structures. NOx can be reduced by their optimization. Another low-NOx strategy involves the utilization of porous radiant surface combustors. In the present work these two systems have been investigated and experimental data are presented as an aid to understanding the controlling factors for thermal and prompt NOx. In both cases the NOx formation is modelled by a chemical reaction scheme for thermal and prompt NO. The gas flow and temperature fields were modelled by a commercial CFD package, FLUENT, and the NOx predicted with a post-processing package. The value of this method as a design technique is demonstrated. It also gives an indication of the strategies required for low-NOx combustion chambers.