Showing papers in "Combustion Science and Technology in 1994"

Measurement of flame temperature and adiabatic burning velocity of methane/air mixtures

Abstract: A simple and accurate method is presented to determine the flame temperature and the adiabatic burning velocity of laminar premixed flat flames, using a specially constructed flat flame burner. The heat loss of the flat flame is determined from measurement of the temperature profile of the burner plate. The adiabatic burning velocity is found when the burner plate temperature profile is uniform, implying that the net heat loss of the flame is zero. Several methods are presented to determine the thermal conductivity of the burner plate. The results for methane/air mixtures are compared with experimental data found in the literature and one-dimensional flame calculations, and close agreement is found. The method is particularly useful in providing an accurate reference for other temperature measurement techniques (e.g. spectroscopic laser diagnostics), and to determine adiabatic burning velocities over the entire range of flamm ability

Sources of Combustion Irreversibility

Abstract: Approximately 1/3 of the useful energy of the fuel is destroyed during the combustion process used in electrical power generation. This study is an attempt to clarify and categorize the reasons for the exergy destruction taking place in combustion processes. The entropy production is separated into three subprocesses: (1) combined diffusion/fuel oxidation, (2) “internal thermal energy exchange” (heat transfer), and (3) the product constituent mixing process. Four plausible process paths are proposed and analyzed. The analyses are performed for two fuels: hydrogen and methane. The results disclose that the majority (about 3/4) of the exergy destruction occurs during the internal thermal energy exchange. The fuel oxidation, by itself, is relatively efficient, having an exergetic efficiency of typically 94% to 97%.

Solid Flames: Discoveries, Concepts, and Horizons of Cognition

Abstract: This paper is devoted to the solid flame phenomenon and, at least extent, to self-propagating high-temperature synthesis: to the discoveries in the field, modern concepts of SF, the mechanism of the main and concomitant processes, and the routes of chemical transformation in SHS

Heat Feedback to the Fuel Surface in Pool Fires

Abstract: A series of measurements designed to investigate the heat feedback in pool fires burning liquid fuels are reported. Such measurements are essential for the development and validation of detailed models which predict the burning rate of liquid hydrocarbons and solid polymers. The radial variation of the local radiative and local net heat flux incident on the surface of 0.30 m diameter pool fires were measured. A water-cooled, nitrogen purged, narrow view-angle gauge was developed to measure the radiative flux incident on the fuel surface. Measurements of the mass burning rate in a burner composed of annular rings was used to estimate the local heat feedback. A number of different fuels were studied, yielding flames with a wide range of heat release rates and luminosities. Consideration of the heat balance for a control volume enclosing the liquid PPOI indicated that radiation was an important component of the heat feedback for non-luminous fires and a dominant component in luminous fires.

Detailed Kinetic Modelling of Chemistry and Temperature Effects on Ammonia Oxidation

Abstract: An extensive set offlat laminar premixedNH3/H2/O2,NH3,/NO/H2/O2 and NH3O2 flames have been investigated by detailed chemical kinetic modelling to facilitate the construction of a reaction mechanism capable of satisfactory predictions for a wide range of flames. Available information for the rate coefficients of all the reactions in the detailed mechanism has been reviewed. An extensive sensitivity analysis has been performed to distinguish the reactions of greatest importance to the formation and destruction of nitric oxide. The relative significance of the different NO formation channels is found to depend entirely on the flame conditions: (i) for all flames the reaction of NH2 with the O radical is found to be significant; (ii) in pure ammonia flames the reaction of the NH radical with OH becomes important; (iii) in hydrogen flames with ammonia and ammonia with nitric oxide dopants the Zel'dovich mechanism becomes increasingly significant with increasing fuel concentrations. The conversion of N...

Measurements of Emissions and Radiation for Methane Combustion within a Porous Medium Burner

Abstract: In this paper, we report the results of an experimental investigation of methane-air combustion within a porous medium burner for various equivalence ratios and flow rates. Measurements of emissions and radiant output are presented. The results indicate that CO and NOx, emissions increase with flame speed. For a given equivalence ratio, however, NOx, emissions are fairly constant over the range of flame speed studied. The radiant output increases but the radiant thermal efficiency decreases with flame speed.

Investigation of the Kinetics of Pyrolysis of PVC by TG-MS-Analysis

Abstract: Pyrolysis or combustion of plastics are of interest from different points of view: on the one hand polymers from which harmful substances may arise are involved in fires or are fed to incinerators; on the other hand plastic refuses are an alternative resource of energy or chemical raw materials. For designing recycling procedures the behaviour of the decomposition of polymers with regard to the decomposition products and the kinetics of decomposition must be known. Plastic refuses encompass chlorine- and/or nitrogen-containing polymers which may form toxic or corrosive products under pyrolytic conditions or in combustion. These components must be separated before processing or the troublesome elements have to be eliminated by pretreatment. The aim of this work was to investigate the kinetics of pyrolysis of polyfvinyl chloride) and mixtures of poly(vinyl chloride) with polyethylene, polypropylene, polystyrene and polyamide 6. By means of simultaneous thermogravimetry-mass spectrometry the kinetic...

Vaporization of liquid oxygen (LOX) droplets in supercritical hydrogen environments

Abstract: A comprehensive theoretical analysis has been developed to study vaporization of liquid oxygen (LOX) droplets in hydrogen over a wide range of pressure The formulation accommodates complete sets of conservation equations for both gas and liquid phases, and accounts for variable properties, thermodynamic non-idealities, and vapor-liquid phase equilibria The model is capable of treating the entire history of a vaporizing LOX droplet, including the thermodynamic phase transition through the critical mixing point Various distinct high-pressure effects on the droplet behavior were investigated in depth In particular, a parametric study of the droplet lifetime as a function of the ambient pressure, temperature, and initial droplet diameter has been conducted The droplet lifetime exhibits a strong pressure dependence and can be correlated well with the square of the initial diameter

Acetylene Oxidation in a JSR From 1 to 10 Atm and Comprehensive Kinetic Modeling

Abstract: Acetylene oxidation in a jet-stirred reactor has been investigated at high temperature (800-1100 K) in the pressure range 1 to l0atm. Molecular species concentration profiles of H2, CO, CO2, CH4, C2H2, C2H4, C2H6 were obtained by probe sampling and GC analysis. Acetylene oxidation was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C1,—C6 hydrocarbons. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet-stirred reactor, ignition delay times measured in shock tube, atmospheric pressure C2H2/Air flame speeds, and C2H2/O2/Ar premixed flame structures obtained at low pressure on flat flame burners. The mechanism has also been used successfully to represent the kinetics of the oxidation of CH4, C2H4, C2H6, C3H6, C3H3, mixtures of CH4 with C2H6 and/or C3,H8 in the same conditions

Thermophoretic Effects on Seeding Particles in LDV Measurements of Flames

Abstract: The motion of LDV seeding particles under the influence of viscous and thermophoretic forces in the rapidly-accelerating, high-temperature-gradient flame environment was studied via the counterfiow premixed twin-flame configuration Results demonstrate that thermophoretic force can induce significant lag between the fluid and,particle velocities in the active preheat zone of a flame, and suggest that caution should be exercised when interpreting LDV data obtained in this region A thermophoretic velocity correction to the LDV-determined velocity, with known experimental or computational temperature profile, is proposed Additional considerations of LDV diagnoslics and its determination of laminar flame speed are also presented

High Pressure Oxidation of Liquid Fuels from Low to High Temperature. 2. Mixtures of n-Heptane and iso-Octane

Abstract: The oxidation of mixtures of n-heptane and iso-octane in a high-pressure jet-stirred reactor has been investigated experimentally in a wide range of conditions covering the low and high temperature oxidation regimes (550-1150K, 10 atm, o=1). The cool flame and negative temperature coefficient regimes were observed below 750K, Above 750 K, the normal high temperature oxidation of both fuels was observed. Reactants, intermediates and final products have been measured providing a useful picture of n-heptane-iso-octane mixtures oxidation. Cyclic ethers resulting from the low-temperature oxidation chemistry of n-heptane and iso-octane were identified. The evolution of their concentration as a function of RON have been measured. These results are interpreted in terms of reaction mechanism. Tentative correlations between RON and CO, CO2 and C2H4 in the cool flame region and for the high temperature oxidation (normal combustion) are presented.

A Wide Range Modeling Study of Methane Oxidation

Abstract: Goal of this article is to present a detailed kinetic scheme able to describe a large variety of macroscopic experimental data related to methane pyrolysis, partial oxidation and combustion. Due to the importance of C2 chemistry, detailed comparisons for pyrolysis and oxidation of ethane, ethylene and acetaldehyde are also reported. The main effort has been put in collecting and unifying these experimental knowledge into a single and comprehensive kinetic scheme. The result of this analysis is to enlarge the simulation capabilities of a single and general kinetic scheme; such a model, expensive in terms of species and reactions (more than 70 real and ‘;equivalent’ species involved in more than 1600 elementary reactions), maintains the typical advantages of mechanistic kinetic models. Therefore, once the submechanisms of the simpler species are validated and tested on the basis of a large set of experimental data, it becomes feasible to analyze the oxidation and decomposition of larger hydrocarbon...

Combustion Characteristics of Biomass Fuels

Abstract: Seeking to systemalically identify and organize the physical and chemical properties of biomass particles for combustion in furnaces, an attempt is made here to theoretically describe the temporal change in mass of a burning particle of an organic solid. The development involves a series of simple models to highlight the mechanisms involved, the physical and chemical properties of concern and the expected results. Although many simplifying assumptions are made, the properties identified, the parameters which evolved and the final functional form of the results are expected to becorrect.

The Strain Exerted by a Vortex on a Flame—Determined from Velocity Field Images

Abstract: Velocity field imaging techniques were used to observe how a single toroidal vortex, which represents one eddy in a turbulent flow, exerts aerodynamic strain on a premised flame. By achieving dense seeding of the flow, the spatial derivatives of velocity were determined accurately, which allows the following to be measured as a function of space and time: the aerodynamic strain rate that is tangential to the flame, the rate of flame stretch, the vorticity field, the shear strain rate field and the flow pattern near the forward stagnation point. The vortex strength was sufficient to cause quenching of the flame. An unexpected result is that the maximum strain on the flame does not occur on the centerline near the forward stagnation point. Instead the strain rate distribution is significantly different from numerical simulations of Poinsot, et al., for which strain is maximum on centerline. The difference is due to the different vortex sizes considered, which indicates that small vortices exert a d...

A Model of Low-Speed Concurrent Flow Flame Spread Over a Thin Fuel

Abstract: A numerical model is developed to examine steady, laminar flame spread and extinction over a thin solid fuel in low-speed concurrent flow. The model incorporates an elliptic treatment of the upstream flame stabilization zone near the fuel burnout point, and a parabolic treatment of the downstream flame, which has a higher flow Reynolds number. This provides a more precise fluid-mechanical description of the flame than using parabolic equations throughout, and is the first time such an approach has been used in concurrent flame spread modeling. The parabolic and elliptic regions are coupled smoothly by matching boundary conditions. The solid phase consists of an energy equation with surface radiative loss and a surface pyrolysis relation. Calculations (with the flame spread rate being an eigenvalue) are performed for forced flow without gravitational influences in a range of velocities which are lower than those induced in a normal gravity buoyant environment. Steady spread with constant flame and...

Turbulence-Induced Noise Effects on High-Frequency Combustion Instabilities

Abstract: A statistical approach is introduced for describing influences of turbulence on high-frequency combustion instability in liquid-propellent rocket motors. The Rayleigh criterion is generalized to account for turbulence-related spatial variations of the combustion response. Special attention is given to the distinguished limit in which the acoustic oscillations are rapid compared with the turbulent fluctuations, which in turn are rapid compared with the linear growth rate of the instability. It is shown that nonlinear acoustics of the instabilities need to be considered to describe the influences of turbulence. When this nonlinearity involves supercritical bifurcation, the turbulence lends to decrease the most probable intensity of the instability. When it involves subcritical bifurcation, the turbulence can produce a bimodal probability density function for the intensity of the instability, with appreciable probabilities of high-amplitude and low-amplitude acoustic oscillations but small probabili...

On the response of premixed flames to time-dependent stretch and curvature

Abstract: In the framework of a constant-density model, we study the linear response of a premixed flame to large scale, but time dependent, curvature and stretch of given strengths and frequency. It is analytically suggested that: i) as the frequency of forcing increases above the reciprocal transit time across the flame, the local instantaneous burning speed gets less and less sensitive to hydrodynamical stretch. ii) the influence of differential diffusion of heat and deficient reactant is milder and milder when the high-frequency limit is approached, so that the Lewis-number-effects tend to disappear. The predicted trends are put in perspective with the results of recent measurements on acoustically destabilized flames.

Stretch and the adiabatic burning velocity of methane- and propane-air flames

Abstract: Results of measured adiabatic burning velocities of methane- and propane-air premixed laminar flames are presented, as obtained by the Heat Flux method. A comparison is made with results of the Counterflow method. The effect on the Counterflow method results of a non-linear instead of a linear correction for stretch is estimated. With a non-linear correction the agreement between both methods is good. It can be concluded that non-linear correction for stretch on the results of the Counterflow method is necessary in general. However, its effect may vary with varying fuel gas.

Monitoring PAH-Emissions from Combustion Processes by Photoelectric Charging

Abstract: Polycyclic aromatic hydrocarbons (PAH) are important in combustion processes from two points of view: As precursors for soot formation and as pollutants, if they are emitted. Usually PAHs are measured by GC-MS or HPLC analysis of filter extracts, which is a time and cost intensive procedure and does not allow on-line measurements. An alternative is photoelectric charging of particles by irradiation with ultraviolet light. If an appropriate wavelength is chosen, the emission probability of electrons mainly depends on the amount of particle bound PAHs. This allows a simple and fast monitoring of PAHs in emission and ambient air measurements. Here the application of this method to measure particle bound PAHs in the exhaust of different combustion systems (oil burner, combustion engines) is discussed.

A Phenomenological Model for Smoke-Point and Soot Formation in Laminar Flames

Abstract: Aglobal soot formation model is proposed, validated and applied based on the postulate that soot formation in laminar diffusion flames is controlled by gaseous reaction processes and not the available soot surface area. Although this model was not derived from detailed soot formation mechanisms, it is consistent with such mechanisms if the active sites for soot growth are so many that soot growth depends on the availability of gaseous species (e.g., C2H2 ). Global soot formation models in laminar diffusion models based on a surface growth process can not be reconciled with experiments. For the present model, a soot formation time in axisymmetric laminar diffusion flames below the smoke-point is shown to beproportional to the smoke-point height. Moreover the present model implies that smoke-point heights are proportional to pressure in agreement with experiments. Predictions of the model for flow. dilution and pressure effects on soot concentrations agree with experiments. both in laminar jet flow...

SHORT COMMUNICATION High Pressure Oxidation of Liquid Fuels from Low to High Temperature. 3.n-Decane

Abstract: The oxidation of n-decane in a high-pressure jet-stirred reactor (JSR) has been investigated experimentally in a wide range of conditions covering the low and high temperature oxidation regimes (550-1150K, lOatm, O = 0.1 to 1.5), Reactants, intermediates and final products have been measured providing a detailed picture of n-decane oxidation. Cyclic ethers resulting from the so-called Iwo-temperature oxidation chemistry of n-decane were identified. The results are interpreted in terms of reaction mechanism. A detailed chemical kinetic modeling of the high-temperature oxidation of n-decane is used to assess the influence of low temperature chemistry in the intermediate temperature range.

Structure of a Turbulent Reacting Mixing Layer

Abstract: Results are presented of direct numerical simulations (DNS) of an unsteady, three-dimensional, temporally developing, compressible mixing layer under both non-reacting and reacting non-premixed conditions. In the reacting case, a simple chemistry model of the type A + rB -»(1 + r)Products is considered. Based on simulated results, it is shown that at sufficiently large Reynolds numbers the global and statistical features of mixing transitions are similar to those observed experimentally. At sufficiently large Mach numbers, it is shown that eddy shockletsdo indeed exist in three-dimensional (3D) flow. However, the strength of these shocks is less than that in two-dimensional (2D) layers of the same compressibility level. Aided by the analysis of the DNS data, the extent of validity of the “Steady Laminar, Diffusion Flamelet Model” (SLDFM) and the “Conditional Moment Method“ (CMM) are assessed. In the evaluation of the SLDFM, DNS results for different stoichiometric coefficients and reaction types ...

Laser Doppler Thermometry in Flat Flames

Abstract: Laser Doppler Velocimetry measurements are performed in flat flames, stabilized on a newly developed flat-flame burner. It is shown that the velocity component perpendicular to the main flow direction, induced by expansion in the reaction zone and buoyancy in the burnt gas, is significant. A method is derived with which the temperature profile of flat flames can be determined from two-dimensional Laser Doppler Velocimetry measurements. Good agreement is found when compared to one-dimensional flame calculations and experimental data obtained from the literature. A method is presented to describe the influence of expansion and buoyancy on the burnt gas flow.

Length-Scales of Wrinkling of Weakly-Forced, Unstable Premixed Flames

Abstract: In the framework of a nonlinear model which incorporales the Landau-Darrieus instability mechanism and curvature effects, we study thin premixed flames subjected to incoming velocity fluctuations of known r.m.s, intensity (∼ µ') and length-scale ( ∼ Lint). Specifically, we numerically integrate a forced evolution equation of Michelson-Sivashinsky type, with periodic boundary conditions and focus attention on the mean spacing 'erul(t)between front crests. The forcing mimics the one-component, near Lorentzian spectrum of moderate, isotropic turbulence; the phases and turnover times are randomly sampled. Processing the results of many runs suggests that: i) in the long-time limit /(:resl fluctuates around a value Ll;lest which, once ensemble-averaged to give the quantity 1/' only feebly depends on the lateral flame extent (if the latter is large enough); instead, AcreI, mainly scales like the marginally-stable wavelength Aneutral of the linearized. unforced evolution equation. ii) oActnJou' 0 and, if u'/SL i...

Oxidation and Ignition of Methane-Propane and Methane-Ethane-Propane Mixtures: Experiments and Modeling

Abstract: The kinetics of the oxidation of natural gas blends (CH4/C3H8 and CH4/C2H6/C3H8) has been studied in a jet stirred reactor (900 < T/K < 1230, 1 < P/atm < 10, 0.1 < equivalence ratio < 1.5). Mole fractions of reactants, intermediates and products have been measured in a JSR as a function of temperature and mean residence time. These results have been used to validate a detailed kinetic reaction mechanism. Literature ignition delay times of CH4/C3H8/O2 mixtures measured in shock tube have also been modeled. A general good agreement between the data and the model is found. The same mechanism has also been used to sucessfully represent the oxidation of methane, ethyne, ethene, ethane, propene, and propane in various conditions including JSR, shock tube and flame. The present study clearly shows the importance of traces of ethane and propane on the oxidation of methane. The computations indicate that the oxidation of methane is initiated by its reaction with O2 and by thermal dissociation when no othe...

PAH and High Molecular Weight Species Formed in a Premixed Methane Flame

Abstract: The concentration profiles of CO, CO2,02 light hydrocarbons, condensed species and soot have been measured in thesoot inception region of a slightly-sooting premixed methane flame. Condensed species (CS) are early detected within the main oxidation zone ofthe flame and their concentration increases rapidly reaching a maximum at 4.5mm together with the maximum formation of acetylene. Sootbegins to appear just in correspondence ofthe CS decrease at5 mm, thereafter both soot and CS concentration rise in the post oxidation region of the flame. Polycyclic aromatic hydrocarbons (PAH) up to coronene havebeen easily identified in the condensed species and their absorption and fluorescence strongly contribute to the absorption and fluorescence of the condensed species in the ultraviolet, but do not justify the noteworthy absorption and fluorescence in the visible. By extraction and high pressure liquid chromatography, other aromatic species, more absorbing and fluorescent in the visible than light PAH, ha...

Laser-Induced Fluorescence Measurements and Modeling of Nitric Oxide Formation in High-Pressure Flames

Abstract: We have obtained quantitative LIF measurements of NO concentration in the postflame zone of a series of flat, laminar, premixed C2H6/O2/N2 flames at pressures ranging from 3.05 to 14.6 atm. The temperatures of the flames were 1600-1850K, indicating that most of the NO produced in the flames is prompt NO. We have found that the equivalence ratio corresponding to the peak NO concentration at a given pressure shifts towards leaner conditions with increasing pressure. In addition, we have modeled the flames using two chemical kinetics models: the Glarborg-Miller-Kee mechanism as modified by Drake and Blint (GMK-DB), and the Miller-Bowman mechanism (MB). Both models accurately predict the pressure trends of NO formation, while the quantitalive agreement with measurements is, in general, good for the GM K-DB model and poor for the M B model. Explanations are provided both for the poor agreement found with the MB model and for the shift in peak NO concentration with pressure.

Detailed Modeling of the Pyrolysis of Trichloroethene: Formation of Chlorinated Aromatic Species

Abstract: Comprehensive product yield determinations from the high-temperature, gas-phase pyrolysis of trichloroethene (C 2 HCl 3 ) using two fused silica tubular flow reactors coupled to in-line gas chromatographic-mass spectrometric analyses are reported. Initial decomposition was observed at 1000 K with formation of HCl and C 2 Cl 2 . Pronounced molecular growth was observed at higher temperatures as evidenced by the formation of C 2 Cl 4 , C 4 Cl 4 , and C 6 Cl 6 (cy) as major (≤5 mole%) products and C 4 Cl 2 , C 4 Cl 6 , C 6 HCl 5 (cy), C 8 Cl 6 (cy), C 8 Cl 8 (cy), C 10 Cl 8 (cy), and C 12 Cl 8 (cy) as minor (≤5 mole %) products. The effects of reactor surface area to volume (S/V) ratio were evaluated by conducting detailed product analyses with 0.1 cm i.d. and 1.0 cm i.d. reactors. Under the higher S/V ratio, C 2 HCl 3 decomposition was increased by an order of magnitude and product distributions suggested that radical-radical and radical-atom recombination rates were enhanced. Product yields under reduced S/V ratio indicated that yields of perchlorinated aromatic and perchlorinated PAH species were a factor of 10 larger than observed for higher S/V ratios. A detailed reaction mechanism is presented for the 1 cm i.d. reactor data describing molecular growth up to the formation of C 8 Cl 6 (cy) and C 8 Cl 8 (cy). Comparison of predicted versus experimental major and minor species profiles are presented, with generally good agreement. Important radical-molecule addition reactions leading to molecular growth are identified using sensitivity analysis and production rate calculations

The Effects of Load Height on the Emissions from a Natural Gas-Fired Domestic Cooktop Burner

Abstract: A single production cooktop burner was studied at three levels of thermal input, to determine the effects of load height on its efficiency and gaseous emissions. In this work, load height was defined as the vertical distance from the center of the base of the load vessel to the top of the burner ports. To simulate practical operation as closely as possible, the production gas injector was used, so that primary aeration varied with thermal input in these experiments. The Australian Gas Association regulations limit the operating range of load height for this burner to values between 7mm (CO/CO2 40%). Minimum NO2 emission rates of 8.7,8.0 and 8.8 ng/J were observed at load heights of 25, 40 and 48 mm respectively, at thermal inputs of 0.49, 0.83 and 1.65 kW respectively. A new measure of pollutant emission was proposed, to provide a means of assessing the “balance” between the requirements for lower emission rate and higher thermal efficiency. The total emission of a ...

Rotating and Modulated Rotating States of Cellular Flames

Abstract: Ordered states of cellular flames on circular porous plug burners consist of concentric rings of cells. At certain values of the flow rate and equivalence ratio a transition is made to a state in which entire rings of ceils rotate. The direction of rotation depends only on the initial conditions. Our observations of rotating cellular flames include a single rotating ring, an outer ring rotating about a single inner cell, a rotating inner ring surrounded by a fixed outer ring, and two corotating or counterrotaling concentric rings. A rotating ring of cells can also make a transition to a modulated rotating state in which the shapes of the cells and the speed of rotation periodically change. In another rotating mode, a single central cell takes the shape of a spiral which rotates inside a fixed outer ring of cells. The physical characteristics of these modes are described and comparisons with relevant theoretical studies are made.