D. Bradley

University of Leeds, Leeds, ENG, United Kingdom

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Publications (13)32.19 Total impact

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    ABSTRACT: Current theories of turbulence that seem relevant to the structure of turbulent flames are reviewed. The compatibility of such theories with different turbulent flame models is discussed. It is suggested that the turbulent Reynolds number, Rλ, of the reactants is an important controlling parameter in turbulent flame propagation. When Rλ>100, a wrinkled laminar flame structure is unlikely and the turbulent flame propagation is probably associated with small dissipative eddies. It is proposed that the ratio of turbulent burning velocity to laminar burning velocity can be correlated with Rλ.
    Combustion and Flame 02/1975; DOI:10.1016/0010-2180(75)90163-7 · 3.71 Impact Factor
  • D BRADLEY, Said M.A. Ibrahim
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    ABSTRACT: Rates of electron energy gain from applied electrical fields and rates of energy loss in molecular collisions have been computed. To do this, electron mobilities and rate constants for the excitation of the different molecular energy levels have been computed from electron collisional cross-sections. Gases covered in this way are CO, CO2, H2O, H2O, N2, and O2 and these results are applied to the products of combustion of methane-air. Rates of ionization by electron collisions have been calculated for different values of electrical field. Comparisons are made of the different rates at which the separate energy levels of the different chemical species gain energy from the field. The influence of electrical fields on the combustion process is discussed on the basis of such data. An increased chemical rate of reaction may arise from a general ohmic heating of the gas and also from preferential excitation of certain molecular energy levels.
    Combustion and Flame 02/1974; 22(1):43-52. DOI:10.1016/0010-2180(74)90008-X · 3.71 Impact Factor
  • G.E. Andrews, D. Bradley
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    ABSTRACT: The use of the double kernel method of obtaining burning velocities is described and discussed. Experimental results are presented for the variations of methane-air burning velocity with equivalence ratio and initial pressure. Measurement of the higher burning velocities of some hydrogen-air mixtures is difficult with this technique, but some values are presented showing the variation of burning velocity with mixture strength. Values of burning velocity determined in this way are in good agreement with those obtained using other reasonably reliable techniques.
    Combustion and Flame 02/1973; DOI:10.1016/S0010-2180(73)81259-3 · 3.71 Impact Factor
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    ABSTRACT: When a spherical electrostatic probe is immersed in ionized flame gases, the probe floatingelectrical potential, measured relatively to a fixed electrical earth potential, varies with position. Results are presented for a stoichiometric premixed methane-air flame at a pressure of 38 torr in an axial traverse. The range of variation of floating potential was 1.3 volts and cannot be explained in terms of the calorelectric effect. It is proposed that the generated voltage is a consequence of the diffusion of positive ions and electrons from the region of ion formation. A theoretical expression is presented for the spatial variation of plasma potential. This variation is derived from experimental values of positive ion concentration. Floating potentials are in turn obtained from these values of plasma potential using electrostatic probe theory. Theoretical and experimental values of floating potential are in fair agreement. Differences in floating potential can yield an open-circuit voltage that is able to supply current through an external resistance. A generator working on these principles is termed a diffusion generator. It is shown how current-voltage characteristics for both diffusion and calorelectric generators may be obtained from theoretical dimensionless probe characteristics and a knowledge of the differences of plasma potential and temperature between electrodes. Experimental diffusion generator characteristics are in fair agreement with those of theory.
    Symposium (International) on Combustion 01/1973; 14(1):383–389. DOI:10.1016/S0082-0784(73)80037-2
  • G.E. Andrews, D. Bradley
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    ABSTRACT: The existing methods of measuring the limits of flammability are critically reviewed. Experimental results are presented that were obtained with a cylindrical vessel equipped with windows. Flame propagation was recorded using a laser source, schlieren-interferometric techniques, and a high-speed camera. Gas velocities ahead of the flame front were measured with a hot-wire anemometer. These techniques also provided information on hot-gas kernels produced by the spark, but with no flame propagation. Limits of flammability were observed for upward and downward propagation, and burning velocities in near limit flames were measured, together with hot-gas convective rise velocities.A theory is developed for the effects of natural convection, in which the buoyancy force acting on the hot kernel is equated to the kernel's rate of change of momentum. The reasons for the neglect of drag in the early stages are discussed. The theory gives the time for the top of the flame to move a given distance, and the convective rise velocity. There is fair agreement with the experimental results.The role of natural convection in determining the limit for downward propagation is discussed. The limit for upward propagation is discussed in terms of wall quenching, gas-phase quenching, and initial failure to ignite the mixture.
    Symposium (International) on Combustion 01/1973; 14(1):1119-1128. DOI:10.1016/S0082-0784(73)80101-8
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    ABSTRACT: The paper presents a semitheoretical, quantitative treatment of the interrelationship between extra equilibrium excitation of some species and elevation of electron temperature above gas temperature, in hydrocarbon-air flames. Certain assumptions are necessary in order to overcome the incompleteness of available data. It is suggested that electron temperatures may be elevated by some hundreds, but not thousands, of degrees as a consequence of collisions of the second kind, possibly with vibrationally “warm” OH (X2π).
    Combustion and Flame 10/1972; 19(2):237-247. DOI:10.1016/S0010-2180(72)80214-1 · 3.71 Impact Factor
  • G.E Andrews, D BRADLEY, G.F Hundy
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    ABSTRACT: Hot wires have been calibrated in the régime 0 < Re < 20 at significant values of Kn, up to 0.12, different temperature loadings, and wire aspect ratios. Calibrations were carried out in a wind tunnel and in a variable pressure rig, in which gas composition could also be varied. The influences of the different variables are shown and discussed. The recommended calibration law is Nc = 0.34 + 0.65 Re0.45 where for l/d > 400, 0.02 < Re < 20 and with property values taken for the mean gas temperature. The importance of an accurate knowledge of thermal accommodation coefficient at larger values of Kn is discussed.RésuméDes fils chauds ont été étalonnés dans le domaine 0 < Re < 20 pour des valeurs significatives de Kn atteignant 0,12 différentes températures de chauffage et différents rapports de forme du fil. Les étalonnages ont été effectués dans une, soufflerie et dans une enceinte à pression variable dans laquelle la composition du gaz pouvait être modifiée. Les influences des différents paramètres sont dégagées et discutées. La relation recommandée est : Nc = 0,34 + 0,65 Re0,45 où pour 1/d > 400, 0.02 < Re< 20 et avec les valeurs des propriétés prises pour la température moyenne du gaz. On discute l'importance d'une connaissance précise du coefficient d'accommodation thermique à des valeurs plus grandes de Kn.ZusammenfassungEs wurden Hitzdrähte im Bereich 0 < Re< 20 bei ausgezeichneten Werten von Kn bis 0,12, verschiedenen Temperaturbelastungen und Durchmesser-Längen-Verhältnissen des Drahtes geeicht. Die Eichungen wurden in einem Windkanal und in einer Anordnung für veränderlichen Druck durchgeführt, in der auch die Gaszusammensetzung geändert werden konnte. Die Einflüsse der verschiedenen Veränderlichen werden gezeigt und erörtert. Das empfohlene Eichgesetz laulet : Nc = 0,34 + 0,65 Re0,45 wobei bei e/d > 400, 0,02 < Re < 20 und den Stoffeigenschaften bei der mittleren Gastemperatur. Die Bedeutung einer genauen Kenntnis des thermischen Anpassungskoeffizienten bei grösseren Werten von Kn wird erörtert.
    International Journal of Heat and Mass Transfer 10/1972; 15(10):1765-1786. DOI:10.1016/0017-9310(72)90053-1 · 2.52 Impact Factor
  • G.E. Andrews, D. Bradley
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    ABSTRACT: Results are presented for the variation of burning velocity with equivalence ratio for methane-air mixtures at one atmosphere pressure. Values were determined by the bomb-hot wire and corrected density ratio techniques, for combustion during the prepressure period. The former of these methods gives a maximum burning velocity of 45 + 2 cm/see, at an equivalence ration of 1.07. Results are compared with those of other workers and the reasons for discrepancies are discussed. The influence of pressure and unburnt gas temperature upon burning velocity are discussed also.
    Combustion and Flame 10/1972; 19(2):275-288. DOI:10.1016/S0010-2180(72)80218-9 · 3.71 Impact Factor
  • G.E. Andrews, D. Bradley
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    ABSTRACT: A critical survey is presented of the different experimental techniques for the measurement of burning velocity. Where possible, correction factors are derived to compensate for errors. The survey is carried out with particular reference to the maximum burning velocity of methane-air mixtures. Recommendations are made as to the most suitable methods of measuring burning velocity for both closed vessels and burners. The recommended value of the maximum burning velocity of methane-air is 45 ± 2 cm/sec at 1 atm and 298°K.
    Combustion and Flame 02/1972; 18(1):133-153. DOI:10.1016/S0010-2180(72)80234-7 · 3.71 Impact Factor
  • D. Bradley, G.F. Hundy
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    ABSTRACT: A new technique is described for the measurement of burning velocity in closed-vessel explosions. It entails the use of a hot-wire anemometer to measure the gas velocity ahead of the flame front. The calibration of the hot wire is described. The flame propagation is recorded using a laser source, a reflection-plate interferometer, and a high-speed camera.The technique has been used to measure CH4-air burning velocities at different equivalence ratios and over a range of pressures. The measured burning velocities are higher than most previous values, and the reasons for this are discussed. It is found that Su ∝ P−0.5.Chemical rate expressions have been used in attempts to evaluate Su on the basis of Spalding's expression for Su. The results suggest that the significant rate-determining reactions are associated with the breakdown of the hydrocarbon molecule and not with the oxidation of CO.
    Symposium (International) on Combustion 01/1971; 13(1):575-583. DOI:10.1016/S0082-0784(71)80059-0
  • J.C. Bell, D BRADLEY, L.F. Jesch
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    ABSTRACT: Electrostatic probes have been used to obtain positive-ion concentrations and electron temperatures in methane-air and ethylene-air flames on a low-pressure flat-flame burner. The following measurements were also taken: gas temperatures, using thermocouples; gas velocities, using the particle-tracking technique; emission intensities from excited OH, C2, CH, and CO2, using a monochromator. It was found that electron temperatures were higher than gas temperatures in the reaction zone and the peak value of the former coincided with the peak in OH emission. This elevation of electron temperature is possibly attributable to electron collisions with excited OH and C2. Theoretical values of the rate constant for electron ionizing collisions are presented for CO, O2, NO, and Na, and it is concluded that this type of collision does not seem to be significant in methane-air and ethylene-air flames. It might be more significant in higher-temperature flames and in flames seeded with alkali. For the stoichiometric methane-air flame, the maximum rate of ion production occurred downstream of the peak electron temperature and was 1.9×1012 ions cm−3 sec−1. The value of the ion-recombination coefficient was (1.1.±0.2)×10−7 cm3 sec−1.
    Symposium (International) on Combustion 01/1971; 13(1):345-352. DOI:10.1016/S0082-0784(71)80037-1
  • D. Bradley, C. G. W. Sheppard
    Combustion and Flame 12/1970; 15(3):323-324. DOI:10.1016/0010-2180(70)90015-5 · 3.71 Impact Factor
  • J.C. Bell, D. Bradley
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    ABSTRACT: Data on collisions of electrons with molecules of carbon dioxide, carbon monoxide, and nitrogen are used to compute the elevation of electron temprature above gas temperature for different degrees of excitation of carbon dioxide beyond that for equilibrium. Extra-equilibrium excitations of vibrational and electronic levels are considered. Under certain conditions the electron temperature closely follows the carbon dioxide vibrational temperature. Numerical results are presented to show the effects of the different gases. The rates for ionization of molecules by collisions with electrons are calculated for different values of electron temperature, and the possibility of electron ionizing collisions in flame gases is discussed.
    Combustion and Flame 04/1970; 14(2):225-236. DOI:10.1016/S0010-2180(70)80034-7 · 3.71 Impact Factor