Showing papers in "Combustion and Flame in 1978"

The venting of gaseous explosions in spherical vessels. I—Theory

Abstract: A study is made of the required vent areas for pressure relief during explosions with normal flame propagation. Two idealised analyses are presented for what is shown to be the worst case of central ignition in a spherical vessel with venting of unburnt gas: one is simple and for a particular condition, whilst the other is based on a comprehensive computer model. Initial internal and external pressure are taken to be atmospheric. Results are presented in a recommended form of values of maximum pressure rise plotted against AS0, where A is the product of vent area and coefficient of discharge divided by the total sphere area, and S0 is the ratio of gas velocity just ahead of the flame front to the acoustic velocity in the unburnt gas just after ignition. Comparisons are made on this basis with the results of previous workers. Solutions are also presented for burnt gas venting. The gas velocity ahead of the flame front, during venting, is computed and the generation of pressure waves discussed.

Entrainment, momentum flux and temperature in vertical free turbulent diffusion flames

Abstract: A systematic study has been made of jet spread rate, mass entrainment rate, and momentum growth rate in vertical free turbulent diffusion flames, and general correlations have been developed for predicting these quantities. The principal dimensionless parameter geverning the system behavior is the Richardson ratio which determines the transition between forced and natural convection. There is evidence for a marked change in large eddy structure in this transition; the trend is towards increased large-eddy size and accentuated meandering and break-up of flames in the natural convection limit. The result is a dramatic drop in maximum temperature levels and a large increase in entrainment rate.

Visible length of vertical free turbulent diffusion flames

Abstract: An experimental study has been made, which together with literature results covers the practically accessible range of operation from forced convection to natural convection. General correlations are given for predicting flame length over this range. The lower Reynolds ratio limit of the correlations has been determined. Under natural convection conditions flames of large base diameter are found to be rather short, and in these an effect of length to diameter ratio is apparent. Explanations have thus been found for the behavior of the pan fires of Blinov and Khudiakov. It has also been shown that flames in the forced convection limit are around 50% longer than was previously suspected.

Prediction of propagating laminar flames in methane, oxygen, nitrogen mixtures

Abstract: An unsteady-state, one-dimensional, flame propagation model was used to study the characteristics of methane oxygen and methane air flames. The transformed species and energy concervation equations were solved with a numerical computer solution proposed by Spalding [1]. The diffusion model employed is based on the Chapman-Enskog kinetic theory and the development of Mason, Monchick and co-workers [2–6] for polyatomic gas mixtures containing one polar component. A methane oxygen reaction mechanism consisting of 29 elementary reactions was used in the flame model. The effects of equivalence ratio, pressure, initial temperature, and transport coefficients on the flame velocity, flame thickness, temperature profile, and concentration profiles were investigated for a series of methane air flames. Many of the model predictions were compared with experimental data. Some of the chemical kinetic data were selected or slightly modified in order to improve the agreement between calculated and measured values, which was good, when the final set of rate coefficients was used. The relative importance of each of the 29 elementary reactions was examined. The concentrations of the radicals H and OH were major factors in determination of flame characteristics.

Reaction rate coefficients for flame calculations

Abstract: A list of recommended rate coefficients for chemical reactions occurring in flames is given. Rate coefficients, expressed as functions of temperature for the range 1000 ⩽ T ⩽ 3000 K, are either taken from experiments described in the scientific literature or estimated by comparison with rate coefficients for analogous reactions. Brief notes on the origins of recommended coefficients are included and rough uncertainties are attached to the listed values. A table showing reaction equilibrium constants as functions of temperature is also provided.

The venting of gaseous explosions in spherical vessels. II—Theory and experiment

Abstract: The computer solutions of Part I [Combust. Flame 32, 221 (1978)] are compared with available experimental data, in terms of the suggested parameters. The use of the vent area to vessel volume ratio is criticised. The reasons for the differences between theoretical and experimental data are discussed. In the light of this evidence, recommendations are made for safe values of the dimensionless ratio, A S 0 , introduced in Part I, for both initially uncovered and covered vents. It is concluded that uncertainty exists about the effects of turbulence and pressure wave generation.

Review of flashback reported in prevaporizing/premixing combustors☆

Abstract: Flame flashback into the fuel preparation tube of advanced prevaporizing/premixing gas turbine combustors is considered. Evidence in the literature indicates that classical flashback, which results from the flame velocity exceeding the fuel/air velocity in either the tube boundary layer or in the free stream, has not been observed in noncatalytic combustion systems burning hydrocarbon fuels. Instead, autoignition of the air stream, flame propagation through reversed flow fields, and preignition of the fuel/air mixture in separated flow regions of the mixing tube are responsible for the reported phenomena. The latter two mechanisms may be caused by either small details in the geometry of the premixing tube such as steps, diverging sections, or surface discontinuities, or by methods of fuel injection, such as contraflow injection. Only catalytic combustors operating at very low reference velocities appear capable of experiencing classical flashback; however, flashback and autoignition must be differentiated even under these conditions.

The size and optical properties of soot particles

Abstract: The emissivity of soot particles makes an important contribution to heat transfer by radiation from industrial flames. This investigation has involved the measurement of the size distribution of aggregates of soot particles from both a soot generator and a flame. The techniques involved sampling by electrostatic precipitation, filtration and impingement. Subsequent measurements of particle size were carried out with the aid of an electron microscope and an image analysing microscope (Quantimet). Spectral transmission measurements in the visible range were also made on both the flame and the generator. Calculations, based on the Mie theory, were made for the measured volume-equivalent particle size distributions to predict the variation of the spectral transmission with wavelength and the ratio of polarised components of scattered light with angle. It was concluded that spectral transmission measurements are more sensitive to particle size distribution than measurements of angular scattering. The measured particle sizes were found to follow a log-normal distribution and a complex refractive index for soot of 1.9-0.35 i gave the closest agreement between experimental and calculated results.

Near-limit downward propagation of hydrogen and methane flames in oxygennitrogen mixtures

Abstract: Measurements are reported on nitrogen concentrations required to prevent downward propagation of hydrogen oxygen and methane oxygen flames in open tubes of 2.5 and 5.1 cm diameter. Limits plotted as functions of the fuel oxygen ratio show that quenching is most difficult the fuel-lean side for hydrogen flames and near stoichiometric for methane flames. Cellular patterns were observed near flammability limits, and near-limit flame speeds were measured. Results are interpreted mainly in terms of conductive losses from cellular flames since alternative mechanisms appear to produce qualitatively inconsistent predictions.

Combustion of methane in fuel-rich mixtures☆

Abstract: The combustion of CH 4 in fuel-rich, CH 4 /O 2 /Ar = 9/1/90, mixtures was studied by infrared (IR) laser kinetic absorption spectroscopy behind incident shock waves with 1800 T K at total densities of ∼1.2 × 10 −6 mol cm −3 . Computer simulations using a 63-reaction mechanism were used to identify the elementary reactions that determined the data parameters and to investigate the consequences of various rate-constant assumptions. An experimental data base from the literature was also used to test the mechanism and rate constants over a wide variety of conditions of temperature, pressure, and equivalence ratio. Rate-constant expressions are suggested for CH 2 + CH 3 = C 2 H 4 + H and CH 3 + O 2 = CH 2 O + OH.

A theoretical model for the upward laminar spread of flames over vertical fuel surfaces

Abstract: A theoretical model was developed for the upward laminar propagation of flames over vertical fuel surfaces. The model, which applies to small scale fires, makes use of a boundary layer approximation to describe the flow and of an ignition temperature to define the rate of flame spread. Expressions for the rate of flame spread along thermally thick and thin fuels are obtained as functions of the thermophysical properties of the fuel and ambient oxidizer. Experimental measurements are reported of the dependence on time of the height of the pyrolysis front and of the flame height for upward laminar propagation of flames over polymethylemethacrylate (PMMA) sheets. It is shown that the theoretical predictions agree qualitatively with the experimental measurements.

Ignition of liquid fuel sprays at subatmospheric pressures

Abstract: Experiments were carried out to determine the influence of ambient pressure and temperature on the amount of spark energy needed to ignite flowing kerosine-air mixtures. All the measurements of minimum ignition energy were performed using predetermined optimum values of electrode spacing and spark duration. Three different types of fuel atomizer were employed in order to produce a wide range of drop sizes in the ignition zone. Ignition was accomplished using capacitance sparks whose energy could be varied in steps of 2mJ from 2 to 800mJ. The results obtained show that the breakdown voltage for liquid sprays is considerably higher than for corresponding gaseous mixtures, and that the optimum spark duration for minimum ignition energy increases with increase in spray SMD (Sauter mean diameter) and with reductions in air pressure and/or velocity. The strong influence of SMD on minimum ignition energy, as previously observed at atmospheric pressure, was confirmed as all levels of subatmospheric pressure down to 0.2 atmos. The energy required to effect ignition was also found to rise appreciably with any reduction in air pressure or temperature. It was noted that the ignition energy requirements for well-atomized liquid kerosine-air mixtures (SMD

A shock tube study of the H2/O2/CO/Ar and H2/N2O/CO/Ar Systems: Measurement of the rate constant for H + N2O = N2 + OH

Abstract: Emissions at 450 mm and 4.27 μm have been measured when a variety of mixtures containing H 2 , CO, either O 2 or N 2 O, and Ar were heated behind reflected shock waves to temperatures of 2000–2850 K and total concentrations near 5 × 10 18 molecule/cm 3 . These emissions were used to obtain absolute concentration-time data for both oxygen atoms and carbon dioxide. The data were then compared to the results of numerical integrations of the likely mechanisms. It was observed that quantitative agreement between calculations and observations were obtained for the H 2 /CO/O 2 /Ar system using recent high temperature literature rate constants. For the H 2 /CO/N 2 O/Ar system, the rate constant for the reaction H + N 2 O = N 2 + OH was adjusted to fit the data. Here it was found that a good fit to both [O] and [CO 2 ] profiles could be achieved with k = 3.0 × 10 −9 exp (−113 kJ/RT ) cm 3 molecule −1 s −1 . Comparison to data at lower temperatures suggests that this might be another example of a “non-Arrhenius” rate constant. The implications of these results to studies of hydrocarbon oxidation are discussed.

Theory of thermal ignition in fuel droplet burning

Abstract: A theory for the thermal ignition of a fuel droplet in a hot oxidizing atmosphere is formulated using an existing gas-phase ignition criterion and an analysis for the transient droplet heating process. Limits on the ignitability of a system, either spontaneous or delayed, are identified. A kinetically controlled ignition regime and a droplet-heating-controlled ignition regime are shown to exist in cold and hot environments respectively, each exhibiting distinctively different ignition characteristics. Theoretical results yield reasonable agreement with existing experimental data.

Laminar burning velocity of stoichiometric methane-air: pressure and temperature dependence

Abstract: Refinements to the spherical constant volume bomb method are reported which enable more accurate measurements of laminar burning velocity to be made than previously possible. Data is presented for stoichiometric methane-air mxitures in the ranges of unburnt gas pressures and temperatures of 0.06 ⩽ p ⩽ 0.228 MPa and 290 ⩽ T u ⩽ 525 K respectively. These include an assessment of flame front thickness effects and a calculation of instantaneous spatially averaged burnt gas density. Over most of the range of values reported the error in burning velocity is less than 5%. A suggested simplification removes the need for much of the sophistication which has previously deterred many from using this method of measurement.

The positive and negative ions in oxy-acetylene flames

Abstract: The positive and negative ions in atmospheric pressure flames of C 2 H 2 + O 2 + N 2 have been examined mass spectrometrically with the much improved sampling systems now available. There is clear evidence that some (but not all) ionic reactions are fast enough to proceed in the time of around 1 μs taken by the sample to enter the instrument, leading to a falsification of the observed spectra. Even so, it is possible to conclude that there is strong evidence for C 3 H 3 + being a primary ion in C 2 H 2 flames, in addition to CHO + . The first negative ion to be formed in the reaction zone is O 2 − as a result of electron attachment to O 2 , but there is a possibility that C 2 H − is also formed from electrons by dissociative attachment to C 2 H 2 . Some of the subsequent reactions of both positive and negative ions are discussed. The mass spectra for positive and negative ions reveal the presence of large neutral hydrocarbon species, such as C n , C n H 2 and C n H y with molecular weights of at least 100; these are presumably associated with carbon formation. In addition, positive and negative nitrogenous ions reflect the formation of NO, CN, NCO, NH 3 , and in particular, the production of NO by the ‘prompt’ mechanism. The major positive ion in the burnt gases is NO + , and because recombination in NO + + e − → N + O becomes the principal reaction for charged species, it has proved possible to measure its rate coefficient as 4.2 ± 2.5 × 10 −7 ions −1 ml s −1 at 2600 K.

Combustion and extinction in the stagnation-point boundary layer of a condensed fuel

Abstract: The combustion and extinction phenomena in the stagnation point boundary layer of a condensed fuel is studied experimentally and theoretically with emphasis on the near-limit flame. The numerical analysis assumes a second-order forward overall chemical reaction in the gas phase, with gas-phase activation energy and modified frequency factor, determined by comparison with the experimental results. The effect of external radiation on the extinction limit is computed using a simplified model. Burning rates and extinction data are determined from measurements taken on polymethylmethacrylate samples in an opposed-jet diffusion flame apparatus. Favorable agreement between experimental extinction data and theoretical predictions is obtained for a gas-phase activation energy of 30 kcal/mole and a modified frequency factor of 5.2 × 107 sec−1.

Rotational Raman scattering from premixed and diffusion flames

Abstract: Use of rotational Raman spectroscopy as a flame probe is examined. Results of rotational and vibrational Raman scattering from hydrogen oxygen, methane-air and propane-air flames are presented. Time-averaged temperatures are measured, both below and above the inner cones of the premixed flames, based upon rotational Raman scattering from the major species (N2, O2, H2 and CO2). Corrections for vibrational-rotational interactions and for the effects of rotational transitions from vibratinally excited molecules are included in the temperature calculations. For hydrogen or methane flames, temperatures based upon rotational Raman scattering from N2 or O2 have lower uncertainties (1–4%) than those based upon vibrational Raman scattering (3–9%) because rotational Raman transitions are generally more intense and give rise to many more transitions. Scattering from CO2, H2O, or C3H8 does not seriously interfere with rotational Raman temperature measurements in the flames studied. It is concluded that rotational Raman spectroscopy can be a useful nonperturbing probe of concentrations and temperatures in atmospheric pressure flames. Rotational Raman intensity factors relative to N2, are obtained from room temperature pure rotational Raman spectra to be 2.61 ± 0.13 for O2 and 0.20 ± 0.05 for HCl.

Structure and extinction of laminar diffusion flames above condensed fuels with water and nitrogen

Abstract: The objective of this study was to develop an improved understanding of mechanisms of fire suppression by studying diffusion-flame extinction in the presence of suppressive agents. Fuels studied were methanol, haptane, and wood. Suppressants considered were water and nitrogen. The configuration studied was the counterflow diffusion flame produced by directing an oxidizing gas stream, containing the suppressant, downward onto the burning surface of a condensed fuel. Data include concentration profiles (obtained by sampling and gas chromatographic analysis), temperature profiles (measured with thermocouples), and extinction conditions (in terms of suppressant content and flow rate of oxidizer). Overall rate parameters for the gas-phase combustion of these fuels were obtained through evaluation of a critical Damko¨hler number for extinction. The influence of water on flame extinction was observed to be thermal. It is suggested that the extinction mechanisms for gas-phase diffusion flames above cellulosic fuels are similar to those observed for liquid fuels.

Theoretical examination of assumptions commonly used for the gas phase surrounding a burning droplet

Abstract: A finite reaction-rate model is compared to three commonly used flame-sheet models. The latter differ in their treatment of the evaporation from the surface and the value used for the molecular weights in the evaporation law. All four models are applicable to both steady and unsteady burning of droplets. Further, they account for variations of droplet radii and allow for differences in ambient conditions. Numerical results (obtained forn-decane) show that if the radius of the droplet is 10^(−2) cm the thin-flame approximation is excellent at 10 atm if the droplet surface temperature is not close to either the boiling point or the ambient temperature. However, this approximation is unacceptable at 1 atm. Among the three flame-sheet models, the one using non equilibrium evaporation at the surface and individual molecular weights best approximates the finite reaction-rate theory. However, this agreement breaks down for smaller droplets with lower surface temperatures, or for air with a larger oxygen content. These conclusions are independent of the chosen kinetics. The Clausius-Clapeyron approximation is shown to be excellent away from the boiling point for R = 10^(−2) cm. However, as the droplet surface temperature approaches the boiling point, or the droplet radius decreases, this assumption leads to considerable errors in the evaporation rate and also distortion of the thermal layer. Even larger errors are obtained when an average molecular weight is used. Here, large underestimates of the evaporation rate and great distortions of the thermal layer of the droplet are obtained. In spite of these errors, all models agree well at wet-bulb conditions.

A thermophysical mathematical model of steady-draw smoking and predictions of overall cigarette behavior

Abstract: Manipulation of the effeluent smoke composition from a cigarette will be facilitated by a working model of the combustion and smoke formation process. A first step toward such a model is presented. For simplicity, the situation considered here is steady-draw smoking from ignition. A one-dimensional model of the actual two-dimensional burning process is derived from the usual conservation laws (mass, momentum, energy, species). The chemical processes are simplified by the concept of grouping of reactions. The model thus includes only a one-step char oxidation reaction and a one-step pyrolysis reaction; the kinetic parameters of these are obtained by thermal analysis of tobacco. Other input parameters, notably convective heat transfer coefficient, are measured values. Model predictions of the effects of flow rate and oxygen concentration on burning rate and pressure drop are compared with experiment. Reasonable agreement is found for these overall behavior parameters.

The effects of a coaxial spark igniter on the performance of and the emissions from an internal combustion engine

Abstract: A comparison between a conventional ignition system and one employing a plasma jet igniter has been performed on two different internal combustion, single cylinder engines burning either gasoline or propane. For measurements made at constant timing, the plasma jet ignition greatly extended the lean limit ( air fuel ratios of 25 were obtained for gasoline fuel) and engine performance, for example output power or specific fuel consumption, were improved always but were greatly improved for lean running conditions. The CO and hydrocarbon concentrations were not materially affected but the NO concentration was higher. For measurements made at mean best torque (MBT) timing the engine performance and pollutant production were essentially independent of which ignition system was used for air fuel ratios between about 10 to 17. However, for mixtures leaner than this there were significant improvements in engine performance and pollutant concentrations were low (as would be expected for these lean conditions). Engine performance was quite insensitive to timing with plasma jet ignition, which was not the case for conventional ignition.

Laser-induced fluorescence measurement of sodium in flames

Abstract: In the present work the use of Saturated Laser-Induced Fluorescence Spectroscopy to measure sodiumatom concentrations in flames is described. A flashlamp pumped-dye laser is used to excite the sodium, and the fluorescence signal is observed at 90° forming a focal volume about 27 μm by 1 mm. A linear curve of growth is obtained in an atmospheric flame for concentration in the range 0.008–0.2 PPM.

The reduction of nitric oxide during the combustion of hydrocarbons: Methodology for a rational mechanism

Abstract: Mixtures of nitric oxide, oxygen and acetylene were heated to temperatures in the range 1400–2600°K in a single-pulse shock tube. Reaction temperatures were estimated from measured incident shock velocities. The final product concentrations were measured via gas chromatographic techniques. The analytical data were then reduced to mole fraction profiles of products vs. reaction temperature for dwell times (0.35–1.25 ms) under reflected shock conditions. A systematic procedure was developed for listing all possible reactions which could occur between pairs of reagents included within a specified list of molecules and molecular fragments. A set of criteria was formulated for eliminating from this very large set of possible reactions all those which were significantly less important than the dominant steps retained in the final mechanism. Computer modeling with the proposed mechanism led to good agreement with the experimental concentration profiles. This was done first for acetylene oxygen mixtures, and then extended to the acetylene nitric oxide oxygen system. That the computer generated mole fraction profiles for mixtures containing NO compared well with the experimental results serves to establish the key reactions which reduce the pollutant NO when the exhaust gases from internal combustion engines are mixed (at about 1500°K) with hydro-carbons and air. The systematic procedure proposed in this report for developing mechanisms for complex systems is applicable to most gaseous reactions.

Combustion in spouted beds

Abstract: The use of spouted beds for burning poor fuels is explored both experimentally and theoretically, following previous work on burners with massive enthalpy recirculation. The spout passes through the combustion zone and becomes part of a toroidal vortex of the bed material which acts as an effective mobile heat exchanger between hot products and cold reactants. The high-temperature attrition in a flowing particulate medium appears ideally suited for coping with “fuels” in any state of aggregate, as well as for rapid heat transfer to cold surfaces placed within it. It is shown that reactants of heat content equivalent to less than half the normal limit of flammability can be burned in such systems. Comparison of an approximate theory with practical performance and measured temperature distributions suggests possible refinements in design.

Mechanism of catalytic activity of transition metal oxides on solid propellant burning rate

Abstract: Transition metal oxides are known to promote the thermal decomposition of ammonium perchlorate (AP) [1, 2], AP deflagration [3-5], AP-binder sandwich burning [6, 7], and the combustion of AP based on composite solid propellants [8-10]. Many attempts have been made in the past to understand the mechanism of the action of these catalysts on decomposition and combustion, but no clearcut picture has emerged so far particularly on the latter process. The effect of the oxide catalyst on the thermal decomposition has been explained as follows [12, 13].

Spectroscopic study of prompt nitrogen oxide formation mechanism in hydrocarbon-air flames

Abstract: Total emission intensities of OH∗, CH∗, C2∗, NH∗, and CN∗ in the reaction zone, saturation currents, unburned hydrocarbons, and NOx in the exhaust gases were measured in atmospheric pressure, hydrocarbon-air flames with and without added fuel N (NO and NH3). It has been found that the overall temperature dependence of prompt NO is about 10 kcal/mol for any mixture strength and that NO concentrations are not very dependent on the kind of fuel (CH4, C3H8, and C4H10). The total amount of CH radicals produced in the reaction zone is estimated by comparing NH∗ emission intensity with NO for the flame of added fuel nitrogen, and it is shown that the amount is uniquely determined by the mixture strength without depending on the flame temperature and is also scarcely influenced by fuel. Furthermore, the amount of prompt NO is proportional to that of total CH radicals for fuel-rich, stoichiometric, and lean flames of three different fuels. The above two experimental facts strongly suggest the CH-origin prompt NO formation mechanism (i.e., CH + N2 → HCN + N), and the rate constant is estimated to be k = 4 × 1011 exp (−13,600/RT) cm3/mol s−1 by comparing the amount of prompt NO with that of saturation current.

Flame profiles and combustion mechanisms of methanol-air flames under reduced pressure

Abstract: If methanol is to be used as a fuel or as an additive to hydrocarbons, more information is required on its combustion kinetics. Three methanol-air flames (9.3%, 12.6%, and 16.9% by volume) have been stabilized on a flat-flame burner and their concentration, and temperature profiles have been measured. Over all rates for stable species have been deduced from mass and species conservation equations. Despite different experimental conditions (atmospheric pressure versus shock tube conditions) results are in agreement with those published by Bowman [8].

Experimental study on the effect of ventilation on the burning of piles of solid fuels

Abstract: The rate of burning (pyrolysis) of cribs of wood and non-dripping synthetic polymers has been studied at various levels of forced ventilation. It has been found that the burning of non-charring fuels is virtually unaffected by the ventilation level, whereas charring materials burn faster up to a maximum as the flow rate of air increases.