D. Bradley

University of Leeds, Leeds, ENG, United Kingdom

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Publications (9)21.06 Total impact

  • G.E. Andrews, Derek Bradley, S.B. Lwakabamba
    Combustion and Flame 02/1976; 26:271-272. · 3.71 Impact Factor
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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; · 3.71 Impact Factor
  • G.E. Andrews, Derek Bradley, S.B. Lwakabamba
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    ABSTRACT: Turbulent burning velocities have been measured in an explosion vessel equipped with four fans driven by air turbines. This arrangement created a central region of uniform isotropic turbulence in which measurements were made of flame speed, turbulent burning velocity and gas velocity just ahead of the flame front. Two techniques were used for such measurements: one involving anemometer measurements of the gas velocity ahead of the flame and the other involving the creation of two kernels of gas originating from two separated, simultaneous sparks. Measurements were made for a range of methane-air and ethylene-air mixtures at an initial pressure of one atmosphere and at different fan speeds. An increase in fan speed resulted in increases in flame speed and turbulent burning velocity, but the gas velocity ahead of the flame showed little change. This last is explained by significant increase in flame thickness with fan speed. A consideration of current ideas of turbulent structure suggests a rational basis for the correlation of burning velocity data is to plot the ratio of turbulent to laminar burning velocity against the turbulent Reynolds number for the unburnt gas. An increase in fan speed gives an increase in the Reynolds number. The experimental results show there is indeed a primary correlation between this ratio and the turbulent Reynolds number. Reference to results of other workers also gives some confirmation of this.
    Symposium (International) on Combustion 01/1975; 15(1):655–664.
  • 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; · 3.71 Impact Factor
  • 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.
  • 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. · 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. · 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. · 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.