Showing papers in "Combustion Science and Technology in 1993"

A Review of NOx Formation Under Gas-Turbine Combustion Conditions

Abstract: Gas turbines offer very high cycle efficiency, exceeding 50% in modern 250 MW-class combined-cycle units for power generation, as well as very low NOv in the lean premixed combustion mode with natural gas fuel. They also account for virtually all commercial aeropropulsion systems, in which case kerosene-based fuel is used. To meet future NOv and CO regulations, a higher level of understanding of turbulence, chemical kinetics and their interactions is required. NOv in particular has become a pacing consideration, although other constraints are present. Selected NOv data obtained at laboratory and machine conditions with gaseous fuel are reviewed here. Although the important chemical reactions cover a wide range in effective Damkohler number, the measure of turbulence-chemistry interactions, it appears that NOv is formed in a distributed zone manner. Equilibrium and superequilibrium effects can broaden the NOv-forming zones beyond the fine scales of turbulence, even in non-premixed flames. Pressure...

Using CSP to Understand Complex Chemical Kinetics

Abstract: The conventional methods of simplified kinetics modeling through the use of partial-equilibrium and quasi-steady approximations are reviewed and critiqued. The method of computational singular pertutbation (CSP) is then presented with special emphasis on the interpretation of CSP data to obtain physical insights on massively complex reaction systems. A simple example is used to demonstrate how CSP deals with complex chemical kinetics problems without the benefits of intuition and experience.

High Pressure Oxidation of Liquid Fuels From Low to High Temperature. 1. n-Heptane and iso-Octane.

Abstract: Abstract–Normal heptane and iso-octane oxidations in a high-pressure jet-stirred reactor have been investigated experimentally in a wide range of conditions covering the low and high temperature oxidation regimes (550· 1150K, 10atm, 0.3 ≪Φ ≪ 1.5). Reactants, intermediates and final products have been measured providing a useful picture of n-heptane oxidation. The relatively high level of oxygenated compounds formed in the low temperature oxidation regime of n-heptane contrasts with the results obtained during iso-octane oxidation in the same conditions. The results are interpreted in terms of knocking and non-knocking tendencies related to fuel structure and low temperature oxidation mechanism.

Analysis of Convection and Secondary Reaction Effects Within Porous Solid Fuels Undergoing Pyrolysis

Abstract: A mathematical model of transport phenomena (heat, momentum and mass transfer) and chemical processes (primary and secondary reactions) of the thermal degradation of wood is presented. Implicit finite difference equations for energy, momentum and chemical species mass balances are formulated according to an operator-splitting technique and are numerically solved. The progress of the pyrolysis process along a wooden slab, radiatively heated on one side, is characterized by the following main processes: 1( a virgin wood region, crossed by a slow flow of pyrolysis products, where temperature and pressure values decrease as the non-irradiated boundary is approached; 2( a primary pyrolysis region where, due to the relatively low temperatures, secondary reactions are not active and 3( a char layer where volatile products of primary pyrolysis mainly flow and, temperature being higher, undergo secondary reactions. For low medium permeabilities, a peak in the gas overpressure is observed, separating the v...

Stabilization of Adiabatic Premixed Laminar Flames on a Flat Flame Burner

Abstract: A simple analysis and measurements are presented, which show that adiabatic premixed laminar flames can be stabilized on a flat flame burner, especially designed for this purpose. The physical properties of these flames are identical to those of flat freely propagating flames. The adiabatic state can be accomplished in practice when the burner plate temperature is well above the temperature of the unburnt mixture. The net heat loss of the flame to the burner is zero (i.e. the flame is adiabatic) when the measured radial temperature profile of the burner plate is uniform. These flames are particularly suitable for comparison with theoretical or numerical flat flame studies.

Experimental and Numerical Study of Premixed Combustion Within Nonhomogeneous Porous Ceramics

Abstract: An experimental and numerical investigation of premixed methane combustion within a nonhomogeneous porous ceramic was performed. The burner consisted of two porous ceramic cylinders of equal length and diameter that were stacked together and insulated around the circumference. Four series of experiments were carried out to determine the lean limit using three different pore sizes in the downstream ceramic cylinder (SBR). The pore size in the upstream ceramic cylinder was constant in all four cases. A new definition of the lean limit was introduced to account for the effects of the porous ceramic. The burners were tested over the range of lean limit 0.41 < φ≤ 0.68 and the numerical simulations were performed over 0.43 < φ ≤ 1.0. The results demonstrated that porous ceramic burners provide a range of stable burning rates at a constant φ, The maximum flame speed inside the burners was much higher than the premixed, freely burning adiabatic laminar flame (free flame) speed. The lean limits in the por...

Thermophoretic Effects on Particles in Counterflow Laminar Diffusion Flames

Abstract: Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, αT D, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH4/ O2Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this ...

Opposed Forced Flow Smoldering of Polyurethane Foam

Abstract: An experimental study is carried out of the effect on the propagation of a smolder reaction through the interior of a porous fuel of a forced flow of oxidizer opposing the direction of smolder propagation. The potential effect of buoyancy in the process is also analyzed by conducting the experiments in the upward and downward propagation, and comparing the respective results. The experiments are conducted with a high void fraction flexible polyurethane foam as fuel and air as oxidizer, in a geometry that approximately produces a one-dimensional smolder propagation. Measurements are performed of the smolder reaction propagation velocity and temperature as a function of the location in the sample interior, the foam and air initial temperature, the direction of propagation, and the air flow velocity. For both downward and upward smoldering three zones with distinct smolder characteristics are identified along the foam sample. An initial zone near the igniter were the smolder process is influenced by...

Minimum Explosible Dust Concentrations Measured in 20-L and 1-M3 Chambers

Abstract: Minimum explosible concentrations (MEC) of dusts were measured in the Bureau of Mines 20-L chamber and in the Fike L-m3 (1000-L) chamber. The MEC values for gilsonite dust and bituminous coal dust were measured in each chamber at several ignition energies. The explosibility of anthracite coal was also studied in the two chambers. Strong chemical ignitors with energies of 500 to 10 000 J were used in the tests. The uniformity of the dust dispersions in each of the chambers was studied by using optical dust probes. One purpose of the research was to determine if the 20-L chamber was “overdriven” at high ignition energies. The MEC-values measured in the 20-L chamber with 2500-J ignitors were comparable to those measured in the 1-m3 chamber with 10 000 J ignitors. At higher ignition energies in the 20-L chamber, there was evidence of overdriving.

Comparison of Experimental and Computed Species Concentration and Temperature Profiles in Laminar, Two-Dimensional Methane/Air Diffusion Flames

Abstract: Experimental concentration measurements of the major stable species and five radical species (OH., H atom, O atom, CH., and CH3) obtained on a rectangular Wolfhard-Parker slot burner are compared with a detailed computation of the chemical structure of an axisymmctric laminar, CH4/air diffusion flame burning at atmospheric pressure. In order to examine these CH4/air flames with different geometries and different sizes, the species profiles are plotted as functions of the local mixture fraction, and the scalar dissipation rate has been matched in a region around the stoichiometric surface. The overall agreement in the absolute concentrations, the shape of the profiles, and their location in terms of the local mixture fraction is good to excellent for the stable species (except for O2) and for the most abundant radicals OH, H atom, and O atom. For example, the calculated OH- maximum concentration is in much better agreement with the experimental results than are full equilibrium and partial equilib...

Implementation of the Finite Volume Method for Calculating Radiative Transfer in a Pulverized Fuel Flame

Abstract: The newly developed Finite Volume Method (FVM) was incorporated into a general pulverized fuel (PF) flame model to predict radiative heat transfer in furnaces. The code was applied to a benchmark test case where exact radiative heat transfer solutions were available and to a pilot scale front-firing tunnel furnace where flame measurements had been taken. The FVM predictions were found to approach the exact solutions in the benchmark case and compare well to the measured data in the pilot scale furnace. The results demonstrate the feasibility and potential of the FVM for radiation modelling in PF flames.

Fundamental Kinetics and Mechanisms of Hydrogen Oxidation in Supercritical Water

Abstract: The kinetics of hydrogen oxidation in supercritical water were determined in an hermal, ug-flow reactor at temperatures between 495 and 600°C and at a pressure of 246 bars. Inlet hydrogen and oxygen concentrations ranged from 0.4 × 10−6to 4.6 × 10−6 mol/cm3, with fuel equivalence ratios varying from 0.04 to 4.0. Over the range of conditions studied, the kinetics were independent of oxygen concentration and exhibited a first-order dependence on hydrogen concentration, with an activation energy of 372±34 kJ/mol and a pronounced induction time. An elementary reaction model, based on existing gas-phase models and modified to account for the high-pressure environment, was able to reproduce closely the experimental results. including the overall concentration dependencies (reaction orders) and activation energy. Based on the model. important elementary reactions were identified and details of the oxidation mechanism were inferred.

Laser-Saturated Fluorescence Measurements of Nitric Oxide in Laminar, Flat, C2H6/O2/N2 Flames at Atmospheric Pressure

Abstract: We have performed both laser-saturated fluorescence (LSF) and linear laser-induced fluorescence (LIF) measurements of NO in lean and rich atmospheric-pressure C2H6/O2/N2 flames. Unlike previous LSF measurements of OH, NH, and CH, the LSF measurements of NO require a broadband detection scheme, and thus we include a comprehensive theory for broadband LSF. Saturation of NO is found to be easily attainable at atmospheric pressure. When high laser energies are used to insure saturation of NO, background fluorescence often occurs from additional flame species; hence, a subtraction technique is introduced to eliminate this fluorescence from the NO signal. Calibration of both the LSF and LIF techniques was accomplished by doping lean flames with known quantities of NO. A comparison of the LSF and LIF signals from postflame gases at ~ 1700 K as a function of equivalence ratio suggests that the influence of stoichiometry on fluorescence quenching is nearly negligible. Finally, we discuss the relative meri...

Material Pyrolysis Properties, Part I: An Integral Model for One-Dimensional Transient Pyrolysis of Charring and Non-Charring Materials

Abstract: A new integral thermal pyrolysis model for the transient pyrolysis of charring and on-charring materials has been developed and evaluated by comparison of the results with exact solutions. The purpose for the development of this simple model has been the desire for predicting pyrolysis histories of materials exposed to pre heat fluxes by using “equivalent” properties tailored to the present model and common flammability test measurements. The pyrolyzing material is divided into a char layer and a unpyrolyzed (virgin) layer where the material has not yet pyrolyzed. These two layers are separated by an isothermal interface which is at a pyrolysis temperature (characteristic of the material). At this interface, heat is transferred to the virgin layer, causing further pyrolysis of the material (namely a thermal pyrolysis model is used). A one-dimensional transient heat conduction model is used to predict the heat transfer within the material. Exponential temperature profiles were assumed for the heat conducti...

Prediction of Kinetic Parameters for Hydrogen Abstraction Reactions

Abstract: Abstrad–This paper deals with an effective method for the prediction of kinetic parameters of H-abstraction reactions from hydrocarbon species. The prediction of the reactivity of complex molecules is obtained starting from a large set of well defined experimental rate values for small radicals and molecules as a basis. making use of proper analogy rules and a limited number of reference kinetic parameters. This empirical method allows generalization and extension of previous work. Most of the predicted values agree reasonably well with literature rate constants, when available. These values can be useful when modeling large kinetic schemes with thousands of reactions (like pyrolysis. oxidation and chlorination of higher hydrocarbons), where rather than defining the real values for all the kinetic parameters, it is more important to properly define the relative importance of different reaction channels.

Nonlinear Analysis and Modelling of Combustion Instabilities in a Laboratory Combustor

Abstract: The spectra of pressure oscillations in combustion chambers often contain large peaks at frequencies corresponding to chamber acoustic modes. Pulsed combustors are designed to operate with fixed amplitude oscillations but in many systems the oscillations have undesirable consequences. An understanding of the nonlinear mechanisms responsible for the limiting-amplitude behavior is therefore desired. This paper is divided into two parts. First, characterization of the oscillations in terms of attractors in mathematical phase space has been performed on pressure signals measured in a laboratory combustor of premixed gases. The results for one set of operating conditions show a quasiperiodic attractor of dimension two over an order of magnitude of scales. Next, the nonlinear combustion response to oscillations of a single acoustic mode are used to model autonomous or 'self-excited' behavior. Two simple models of nonlinear combustion processes observed in the laboratory combustor result in unstable oscillations that reach limiting-amplitudes. With the variation of model parameters, the periodic limit cycles undergo subharmonic bifurcations and transition to chaos.

Thermal Pulse Combustion

Abstract: This paper describes theoretical and experimental observations of combustion oscillations produced in a continuously mixed, jet-stirred combustion system. This work is distinct from other investigations of pulse combustion, because it is shown both theoretically and experimentally that combustion oscillations can be produced with a steady supply of fuel and air, requiring no mechanical or aerodynamic valves. The theory is a direct extension of thermal theories of combustion in back-mixed reactors, extended to include the unsteady behavior of the combustor and tailpipe. Because of the demonstrated effect of heat transfer on the oscillations, the name thermal pulse combustion is chosen to describe these oscillations. Effects of friction in the combustor tailpipe, heat loss from the combustion zone, and flow rate, are investigated theoretically. Depending on operating parameters, oscillating combustion, steady flames, or blow-out are all predicted. The effects of finite mixing rate are investigated ...

Self-Propagating High-Temperature Synthesis of Ni3AI

Abstract: –Combustion synthesis of the Ni,AI intcrmetallic was studied using the self-propagating hightemperature synthesis (SHS) mode. Since the reaction wave propagates rather quickly through the sample in SHS, a successful application of this process requires an understanding of the influence of processing variables on its dynamics, and on the phase composition and microstructure of the product. Thus, the effects of green density, ignition power, preheating temperature, and the particle sizes of aluminum and nickel were investigated. In addition, nickel particles with different morphology were used. It was found that higher green density, lower ignition power and higher preheating temperature lead to fully reacted product with a well-developed microstructure. The reactant particle sizes and their morphology also significantly influence the product properties. The obtained results arc helpful in understanding the mechanism of for mation of Ni3Al.

Lewis number effects on turbulent premixed flame structure

Abstract: The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H2/Air and C3H8/Air mixtures with similar laminar burning rates were investigated with Lewis numbers of 0·33 and 1·85 respectively. At the conditions studied the laminar H2/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3 mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame f...

Numerical investigations of the tulip flame instability : comparisons with experimental results

Abstract: A two-dimensional adaptive finite-element code is used to numerically investigate the propagation of a laminar premixed flame in a closed rectangular chamber giving rise to the so-called tulip instability. The physical model includes a single one-step chemical reaction where the physical parameters involved in the model are chosen in order to adequately represent a stoichiometric methane-air flame. Attention is focused on the shape of the flame and the flowfield generated by the combustion process. A detailed comparison between the numerical results and available experimental data shows a very good agreement, for various sizes of the combustion chamber.

The Properties of Elliptical Wildfire Growth for Time Dependent Fuel and Meteorological Conditions

Abstract: Abstract— This work considers an existing system of differential equations based on Huygens' Principle that can, under some assumptions, trace the position in time of the perimeter of a large wildland fire. These equations in general require a sophisticated numerical solution, however it is demonstrated that if fuel and meteorological conditions are functions of time only, then an analytic solution can be obtained to the equations. The solution is in the form of a definite integral involving the affecting variables expressed as functions of time. For the special case of only time dependent wind velocity, the solution is presented in terms of probability distributions. The solution allows an analysis of its properties giving an insight into the behaviour of wildland fire. A number of general properties are derived, such as how the history of the fire can be traced back in time and how fires from finite length ignition lines can be simulated. For the case of a point source ignition the solution has...

Three-dimensional discrete-ordinates modeling of radiative transfer in a geometrically complex furnace

Abstract: Discrete·ordinates S2' 54, 56 and 58 approximations arc used to study radiative transfer in athree-dimensional furnace with complex geometry including cooling-pipes in the combustion chamber. Theability of the discrete-ordinates method to model highly-directional shadowing effects caused by the internalpipes is illustrated. Predicted values for incident wall flux and net radiative transfer to the pipes are shown tocompare well with experimental data. The large energy sink produced by the pipes minimizes the sensitivityof results to variations in wall emissivity. The 54 approximation is shown to produce results of adequateaccuracy except in cases where detailed modeling of the pipe shadows requires the higher order 56 and 58approximations. Results from mesh densities of 76.000, 388,000 and 815,000 cells show different mesh sizesdo not significantly alter incident flux or net heat transfer predictions,

Adsorption of Mercuric Chloride Vapours from Incinerator Flue Gases on Calcium Hydroxide Particles

Abstract: The adsorption of mercuric chloride vapours on a fixed bed of calcium hydroxide fine particles has been experimentally studied. The study has been conducted at laboratory scale using simulated flue gases, constituted of mercuric chloride vapours in nitrogen, which have been in contact with a Ca(OH)2 fixed bed. The effect of the bed temperature, the inlet HgCl2 concentration, and the relative gas-solid velocity on the HgCl2 removal efficiency has been studied. The experimental results indicated that high removal efficiencies of up to 95% can be obtained. The bed temperature has been found to be the most relevant parameter; particularly as the experimental results show that the lower the bed temperature, the higher the removal efficiency. Moreover, the removal efficiency increases when the HgCl2 concentration in the inlet gas increases and when the relative gas-solid velocity decreases. A model based on a simplified expression of the Freundlich adsorption isotherm has been proposed, which is able t...

Application of Continuation Methods to Plane Premixed Laminar Flames

Abstract: The composition flammability limits of freely propagating premixed hydrogen-air and methane-air flames are investigated. A modified formulation of the plane premixed laminar flame problem is first derived. The resulting parameterized nonlinear two-point boundary value problem is then solved by phase-space, pseudo arclength, continuation methods that employ Euler predictors, Newton-like iterations and adaptive gridding techniques. The efficiency of the method is illustrated by studying the dependence of the peak temperature and the adiabatic flame speed on the equivalence ratio of hydrogen-air and methane-air mixtures. In particular, we discuss the calculation of lean and rich extinction limits in the absence of heat losses. These composition limits for plane adiabatic flames are shown to be physically irrelevant turning points due to flame thickening in finite length computational domains. These artificial turning points are also shown to be dependent on the length of the computational domain.

Primitive Variable Modeling of Multidimensional Laminar Flames

Abstract: We employ an accurate numerical algorithm to simulate two model flames—an unconfined lifted and a confined, coflowing, methane-air jet diffusion flame using detailed chemistry and complex transport models. The algorithm employs Newton's method to obtain the primitive variable solution of the large system of strongly coupled elliptic governing equations. The Newton equations are solved by a block-line tridiagonal method. We employ a global grid refinement technique which equidistributes meshes according to the gradients and curvatures of the solution obtained on the previous mesh and bounds the ratio of adjacent grid step size. The algorithm can be applied to problems ranging from non-reacting flows to reacting flows in two- or three-dimensional configurations. In the unconfined case, the lifted flame and the “triple flame” are both predicted in the numerical solution. The computed solutions agree well with the experimental results. The comparison of the present solutions with the previously repor...

Experimental and Numerical Investigation of NOx Formation and its Basic Interdependences on Pulverized Coal Flame Characteristics

Abstract: The formation of nitrogen oxide in pulverized coal combustion is analysed by experiments in a 500 kW test rig and by numerical predictions. This study extracts some of the dozens of parameters that were investigated during the trials. It turned out that the huge number of process variables that have an impact on NOx formation can be reduced to a smaller figure by an appropriate data analysis. Major parameters presented in this contribution are the fineness of the coal dust and the amount of swirl. Special emphasis is put on the fact that the level of NOx concentration must necessarily be monitored together with the degree of bumout of the coal dust

Pulse Combustion: Impinging Jet Heat Transfer Enhancement1

Abstract: A new method for convective heat transfer enhancement is described. The technique involves the use of a pulse combustor to generate a transient jet that impinges on a flat plate. Enhancements in convective heat transfer of a factor of up to 2.5, compared to a steady impinging jet at approximately the same Reynolds number, have been obtained. Heat transfer data for several experimental configurations are presented. Flow field visualizations and theoretical calculations are used to suggest an enhancement mechanism.

The Linearised Response of the Mass Burning Rate of a Premixed Flame to Rapid Pressure Changes

Abstract: An investigation is made into the response of the mass burning rate of a premixed flame to small but sharp changes in pressure. In this paper a fast time scale is considered such that (K1 = thermal diffusivity, u01 1 = initial burning velocity, l1 a = a typical acoustic length, a01 1 = frozen sound speed, θ = nondimensional activation energy). Previous studies by the author (Mclntosh 1990)and by other researchers (e.g. Ledder and Kapila 1991) have considered the case of r = 1 which is the usual value corresponding to audible resonance of flames in tubes, burner ports etc. However the case r = Θ2 (Mclntosh 1991)is a most interesting and distinct case since then the fast time scale alters the inner reaction zone which consequently obeys a different and essentially unsteady diffusion-reaction equation. The mass burning rate is then on an appreciably larger scale. The time response of the mass burning rate to pressure fluctuations at this fast time scale is governed in general by a non-linear partial...

Autoignition of Hydrocarbon/Air Mixtures in a CFR Engine: Experimental and Modeling Study

Abstract: In order to see the accurate role of low temperature reactions in autoignition nd nock phenomena, oxidation of stoichiometric hydrocarbons/air mixtures has been performed in a CFR engine and in a flow system. The results show that reactions of isomerization of R02 radicals areimponant for hydrocarbons containing five or more carbon atoms. The pernxidic compounds formed through these isomerization reactions are alkylketohydroperoxides. A correlation is made between fuel structure, octane number. isomerization reactions and autoignition. A chemical kinetic model. created to interpret autoignition and knock. reproduces the results obtained with n-heptane and the influence of engine parameters as speed, intake temperature and pressure.

Combustion of Binary and Ternary Silicon/Oxidant Pyrotechnic Systems, Part I: Binary Systems with Fe203 and Sn02 as Oxidants

Abstract: Abstract–Studies of binary Si/oxidant pyrotechnic systems have been extended by examination of the temperature profiles of Si/Sb2O3 and Si/KNO3 compositions. The burning rates of the Si/2O3 compositions (1.6 10 8.5 mm s-1) showed a maximum at ≃40% Si. Maximum heat of reaction was measured at ≃25% silicon. The temperature profiles showed a smooth rise to maximum temperatures of ≃1000 to 1300°C, except for low (<30%) Si contents. Maximum reaction temperatures for the Si/KNO3 compositions were beyond the range of the noble metal thermocouples for all compositions except for the 90% Si mixture. Burning rates (1.7 to 34.5 mm s-1) increased with silicon content (30 to 85%). The results for all four binary systems are compared and the effects of composition, compaction, surface areas of fuel and oxides and additives are discussed.