C. G. W. Sheppard

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 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.

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.

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.

On the Nature of Autoignition Leading to Knock in HCCI Engines

ABSTRACT There is worldwide interest in developing HomogeneousCharge Compression Ignition concepts for lean burn/heavily diluted, low emissions, combustion engines.Although there has been much work and considerableprogress relating to autoignition onset in such engines,there has been relatively little study of the reasons for theknock which limits their operation at high power. Buildingon earlier work on end gas autoignition development andknock in spark ignition engines, it is suggested that theproblem is rooted in the mode of autoignition. Theconditions supporting the most violent ‚™developingdetonation™™ mode, with increasing charge dilution, bulkunburned gas temperature level and local temperaturegradient are explored in relation to HCCI engines. INTRODUCTION In spark ignition (SI) engines, satisfactory turbulent flamepropagation can only be achieved over a narrow band ofequivalence ratio (φ) close to stoichiometric [1]. Thisleads to the need to modulate power output using athrottle, with consequent inefficiency due to fithrottlinglossfl. Gasoline direct injection engines are beingdeveloped in an attempt to extend the lean limit (andhence range of power output requiring a throttle), byproviding a local rich region to promote flamepropagation of overall lean conditions. Similarly, cylinderdisablement, energy storage and hybrid concepts arebeing adopted to circumvent this fundamental problem offlammability limits in SI engines. Diesel, compressionignition (CI), engines are not constrained by flamepropagation limitations and are stratified by nature [1];however, the existence of local rich and lean regionsleads to emissions problems (particularly at low load,when the engines operate very lean). In recent years,there has been an explosion of interest in HomogeneousChange Compression Ignition (HCCI) engine concepts[2]. These, ideally, operate with uniform andsimultaneous autoignition and chemical reactionthroughout the charge and are thus theoretically immuneto flame propagation and charge stratification problems.In this paper, the acronym HCCI is used in a genericsense to cover the many different concepts advanced inthe literature [3-6]. However, in HCCI engines, the rangeof equivalence ratio (or equivalent dilution with exhaustgas recirculation, EGR) is also restricted to a ratherlimited, lean, range of equivalence ratio. For φ ~ 0.4, noisy and potentially damagingfiknockfl is encountered. Accordingly, hybrid SI/HCCI andCI/HCCI concepts are generally proposed, with HCCIcovering the low load part of the engine operating rangeand SI/CI the higher engine output. There is a gapbetween the upper φ for HCCI operation and the lowest φfor throttle-less SI running, with consequent loss ofefficiency and introducing switchover control anddriveability problems. It would be beneficial to extend theupper φ limit for HCCI operation. There is a furtherconstraint imposed on this by NO

Burn Rate Implications of Alternative Knock Reduction Strategies for Turbocharged SI Engines

This work is concerned with the analysis of different charge dilution strategies employed with the intention of inhibiting knock in a high output turbocharged gasoline engine. The dilution approaches considered include excess fuel, excess air and cooled external exhaust gas re-circulation (stoichiometric fuelling). Analysis was performed using a quasi-dimensional combustion model which was implemented in GT-Power as a user-defined routine. This model has been developed to provide a means of correctly predicting trends in engine performance over a range of operating conditions and providing insight into the combustion phenomena controlling these trends. From the modelling and experimental data presented, it would appear that the use of cooled externally re-circulated exhaust gases allowed fuel savings near to those achieved via excess air, but with improved combustion stability and combustion phasing closer to the optimum position.

The Influence of Equivalence Ratio Variation on Pollutant Formation in a Gas Turbine Type Combustor

Abstract A research gas turbine type combustoi has been used to examine the effect on performance of variation in primary zone equivalence ratio. The chamber, which comprised a conventional tubular primary zone followed by a lengthy plug flow section, was fuelled by propane vapour and operated over the equivalence ratio range 0.25-0.77 at atmospheric pressure. Comprehensive internal gas analysis surveys were undertaken and contour maps of gas concentration and derived information have been compiled. Substantially super-equilibrium CO levels persisted throughout the burner, although a closer approach to equilibrium was obtained in the plug flow section at the higher equivalence ratios. This was qualitatively in accord with the predictions of a simple computer model presented previously. The data are being used for the verification and development of mathematical models of varying degrees of complexity.

Measurement of temperature PDFS in turbulent flames by the CARS technique

Accurate temperature measurement with the CARS technique is difficult when appreciable temperature gradients occur within the measurement volume. The problem is aggravated by the non-linearity of the spectral intensity with temperature. The paper reports on the results of steps taken partially to ameliorate the problem, in the context of attempts to measure temperature pdfs in turbulent combustion. These involve a two beam system for the emitted nitrogen spectra, one beam of which is attenuated to reduce the intensity. By this means the full range of intensities that arise in combustion can be accommodated by the diode array camera and accuracy is preserved over a wide range. Furthermore, the intensity of the spectrum is measured and a calibration relates this to the proportion of hot gas in the control volume, although this strategy is only partially successful because of the variation of laser intensity from shot to shot. Within the spectrum a ‘hot’ band and a ‘cold’ band are used separately for theoretical spectral matching and consequent temperature measurement. This is relevant to the case where a thin flame lies across the measurement volume. Further refinements make some allowance for flame thickness, but even with these measures some ambiguities remain. The refinements are discussed in relation to measurements of temperature probability density functions through a premixed methane-air turbulent flame of relatively low Karlovitz number on a V burner. Expected bimodal distributions are confirmed.

A Note on Carbon Monoxide Oxidation with Particular Reference to Gas Turbine Combustion

A simple model of CO oxidation is used to show that there is an optimum temperature for the attainment of low CO emissions from a gas turbine combustor. Experimental measurements support the view that minimum CO emissions at idling conditions are likely to be achieved when the primary zone exit temperature is between 1600 and 1800/sup 0/K.