Combustion Science and Technology

SAITO, M.; SADAKATA, M.; SAKA, T.

doi: 10.1080/00102208708960319pmid: N/A

Abstract Study of The pulverised coal combustion in air was performed using a laminar flow furnace at gas temperatures between 800 and 1200° C, particle size between 44 and 359 μ m and oxygen level of 21 percent. The time sequential variation of the fractional burn-off, the particle size distribution and the particle density during the pulverized coal combustion were measured by using two different type coals. The curve of the fractional bum-off indicated that the combustion of the pulverized coal is a two-staged combustion — the rapid volatile matter combustion followed by the slow char combustion. However, it was observed that char combustion occured simultaneously in the volatile matter combustion region. From the measurements of the combustion rate based on the unit external surface area of the particle, the rate of large coal particle was higher than that of small ones under the same gas temperature within the range of particle size used in this experiment. The fact could be explained by the temperature difference between the particle surface and the surrounding gas

CHELLIAH, H. K.; WILLIAMS, F. A.

doi: 10.1080/00102208708960320pmid: N/A

Abstract Two-term expansions for burning velocity in activation-energy asymptotics are developed for four-species, two-reactant, steady, planar, adiabatic laminar flames with irreversible one-step chemistry. General Stefan-Maxwell transport is included with Soret and Dufour effects, pressure gradients, body forces and radiant transport neglected. The results lead to identification of effective Lewis numbers (combinations of Lewis numbers for different pairs of species) that affect the burning velocity, and by exhibiting influences of fully variable transport and thermodynamic properties they provide a basis for achieving improved accuracy in asymptotic analysis of flame propagation in two-reactant systems such as hydrogen-halogen mixtures. When suitably specialized to the more restrictive conditions considered in earlier work, the burning velocities obtained here agree with those derived previously

BIRKAN, M. A.; LAW, C. K.

doi: 10.1080/00102208708960321pmid: N/A

Abstract The complete evolution of the one-dimensional, steady, convective-reactive combustion process of hydrocarbon/ air mixtures with the four-step semi-global kinetic mechanism of Hautman, Schug, Dryer, and Glassman, is simulated by using large activation energy asymptotic analysis. The analysis identifies four major regimes, which respectively represent the initial fuel pyrolysis and the subsequent oxidation of ethylene, hydrogen and carbon monoxide, as well as fifteen sub-regimes embedded within them. Transition between different sub-regimes is controlled by temperature variation as well as the generation and thereby availability of the reactants relevant to the specific sub-regimes. The structure is typical of that of thermal explosion, with the fuel pyrolysis regime dominating the physical extent of the process, and the highly-exothermic oxidative reactions being localized downstream of it. Reasonable agreements exist with the numerically-computed results

SENSER, D. W.; CUNDY, V. A.; MORSE, J. S.

doi: 10.1080/00102208708960322pmid: N/A

Abstract Gas samples have been extracted from laminar atmospheric pressure CH2 CI2-CH4 -air flat flames using uncooled quartz probes with 0.05 to 0.10 mm orifices. The samples ( 50torr total pressure) were analyzed by gas chromatography for a number of major C1 and C2 hydrocarbons, chlorinated hydrocarbons and CO, CO2, O2 CH4 N2and H2 Temperatures were obtained throughout the reaction zone and into the postflame region using Pt-Pt (13 percent Rh) uncoated thermocouples with typical bead diameters of 0.15 mm. Measurements of species concentrations (major and minor stable products) along with temperatures are provided as a function of vertical distance above the burner for three separate flames with Cl/ H ratios of 0.060, 0.33 and 0.72 for φ 0.80. All intermediate species (with the exception of CHC13) are observed to fall below detectable limits when the CH2 CI2 concentration is less than 1 percent of its initial value. Peak values of major intermediates are presented as a function of Cl/ H ratio. In addition, plots of major intermediates are provided as a function of Cl/ H ratios to observe the effect of chlorine loading. The results provide a better understanding of the fundamental combustion processes which occur during the incineration of CH2 C12 and similar chlorinated methanes.

HODGES, JOSEPH; FOSTER, DAVID

doi: 10.1080/00102208708960323pmid: N/A

Abstract A generalized system of rate equations governing simultaneous coagulation, surface growth and nucleation of a homogeneous soot dispersion is presented. Coagulation is modeled by free molecule Brownian collision kinetics and surface growth is treated in terms of a global chemical reaction scheme. Particle nucleation is neglected and the equations are numerically integrated to yield the time dependence of particle number density, mean size and size distribution standard deviation. A time dependent lognormal particle volume distribution function (PVDF) is assumed. For coagulation only, the asymptotic lognormal PVDF (i.e., standard deviation = 0.912) is shown to be almost identical to the self-preserving distribution function based on comparison of their respective moments. Particle coagulation is found to strongly influence net particle growth via its effect on particle surface area. The major effect of surface growth on coagulation dynamics is a tendency to narrow the particle size distribution

ADAMCZYK, ANDREW A.; ROTHSCHILD, WALTER G.

doi: 10.1080/00102208708960324pmid: N/A

Abstract The amount of fuel stored and the ensuing increase in HC emissions from a plasma-sprayed zirconia ceramic coating and a densified alpha-alumina coating have been determined in a combustion bomb (static reactor) for four fuels (methane, ethane, propane and n-butane) at a fuel-lean equivalence ratio of 0.9, at initial pressures between 200 and 500 kPa, and for several positions of the ceramic in the reactor. The experiments were performed by removing samples of the combusted gas mixture from the reactor, with or without the ceramic in place during combustion, and analyzing the samples by gas chromatography. The principal exhaust effluent is unburnt fuel, related to the amount of pore volume and the pressure at the time the flame overrides the boundary of the ceramic, because the combustible mixture convects into the porous recesses on a time scale of the combustion process ( μ 24 msec) in the reactor. When the ceramic was located near the spark, the amount of stored fuel was 3 times lower than when it was located near a position equivalent to the end of combustion. The emission data can be used to estimate the connected pore volume of the ceramic, with results agreeing well with values estimated from measurements of the bulk density

FRENKLACH, MICHAEL; WARNATZ, JüRGEN

doi: 10.1080/00102208708960325pmid: N/A

Abstract A detailed modeling study of the formation of polycyclic aromatic hydrocarbons in a burner-stabilized low-pressure sooting 23.6 % C2 H2-21.4% 02-Ar flame of Bockhorn and co-workers is reported. The model predicts the correct orders of magnitude and relative appearances of the concentration peaks, but overstates the decline of the species concentrations in the post-flame zone. Imprecise knowledge of the thermochemical data and unknown details of the oxidation of hydrocarbon radicals are the reasons identifed for the latter. The main reaction pathways for cyclization and growth of polycyclic aromatics and the results of the sensitivity tests are in close agreement with those of the previous modeling study of acetylene oxidation under shock-tube conditions. An additional factor that is important in the. flame environment is the diffusion of hydrogen atoms from the main reaction zone into a cooler preflame region

SAITO, K.; WILLIAMS, F. A.; GORDON, A. S.

doi: 10.1080/00102208708960326pmid: N/A

Abstract A small orange emission region, separated by a sharp boundary from a yellow one, is reported for hydrocarbon-air diffusion flames. The phenomenon, which has not been previously reported, occurs only in very small flames, only over a small range of fuel flow rates, and it has been demonstrated on a cylindrical as well as a slot burner. It has been shown to occur for the ten different aliphatic fuels tested and appears to be a general phenomenon. Thermocouple measurements showed that the gas temperature of the flame interior increases monotonically along the axis; the yellow color is in a lower temperature gas than the orange. Since color temperatures can be deceptive, temperatures in each zone were surveyed by a Kurlbaum method, as well as by a spectral measurement in the two regions. Both methods gave a higher soot particle temperature in the yellow zone than in the orange. Furthermore, measured composition profiles showed that the olefinic concentration approaches zero at the interface between the yellow and orange regions. On the basis of these experimental results and of theoretical estimates of soot-particle temperatures, it is suggested that the orange corresponds to radiant emission from soot particles whose temperatures are equilibrated with those of the gas, while in the yellow region the soot particles are hotter because of the energy release associated with their rapid growth by reaction with gas-phase unsaturates, primarily acetylene

SIRIGNANO, W. A.

doi: 10.1080/00102208708960327pmid: N/A

Abstract A transport or evolution equation for the joint PDF of a reacting scalar and the gradient of that scalar is developed and presented. The derivation clearly indicates the relationship between the transport equation for the PDF of a reacting scalar and the transport equation for the joint PDF. It also demonstrates why molecular mixing cannot be modelled exactly in the simpler equation governing the PDF. Transport in scalar space due to chemical reaction and molecular mixing and transport in scalar gradient space due to chemical reaction are represented exactly. The molecular mixing results in an inverse diffusion process in time. Transport in scalar gradient space due to molecular mixing and vortex dynamics is identified and modelled via physical arguments. Nondimensionalization of the equation yields the important nondimensional groupings and demonstrates the multi-time scale nature of the problem

TSURUDA, TAKASHI; HIRANO, TOSHISUKE

doi: 10.1080/00102208708960328pmid: N/A

Abstract The growth of flame front turbulence, when a flame was propagating through a methane-air mixture acceleratedly flowing across a block, was examined by high-speed schlieren photography. Three types of flame front turbulence induced by different mechanisms were observed to appear. The turbulence induced by the Taylor-Markstein mechanism appeared at the leading flame front normal to the direction of mixture acceleration and became of a needle-like structure over the block. In the present particular situation, the turbulence that appeared at the flame front parallel to the direction of acceleration and that near the walls grew later and their rales of growth were much smaller than that which appeared at the leading flame front