Bernard Lewis

Stability and Structure of Burner Flames

It has been shown that the condition of equality of gas and burning velocities, required for stabilization of a flame above a burner, is established near the rim of the orifice or an obstruction within the stream by the effects of friction and inhibition of the explosive reaction. This condition is maintained between two critical gradients of the gas velocity at the solid surface, the lower gradient bordering on the flash‐back and the upper gradient on the blow‐off range. Values of the gradients in the range of laminar flow were determined by hydrodynamic equations from gas flow and tube dimensions; and their independence of tube diameter, except for extreme sizes, has been demonstrated both for upright and inverted flames. In the latter the critical velocity gradient for blow‐off was also found to be independent of the diameter of the centrally mounted wire within a considerable range. The effect of the surrounding atmosphere on the critical blow‐off gradient has been shown. The gas‐flow pattern was stud...

Nonisotropic Propagation of Combustion Waves in Explosive Gas Mixtures and the Development of Cellular Flames

The phenomenon of nonisotropic propagation consists in the spontaneous development of blisters or cells on the surface of a combustion wave. The present experiments on spherical flames and the experiments of Markstein on flames in wide tubes show that the phenomenon is characteristic of nonstoichiometric explosive mixtures in which the deficient reactant constituent is also the constituent of largest diffusivity. This suggests that the phenomenon is primarily caused by the effect of diffusion processes on the burning velocity. It is proposed that in curved areas of the wave that are convex with respect to the burned gas, the burning velocity is reduced because the lines of diffusion diverge, and hence the concentration of the faster diffusing constituent decreases; whereas in concave areas the lines of diffusion converge, and hence the concentration of the faster diffusing constituent increases. In rich mixtures of hydrocarbon and oxygen, additional evidence for the effect is furnished by the emergence of carbon streamers and by characteristic changes of light emission from convex wave areas, showing that the oxygen concentration, and probably therefore the burning velocity, is decreased in these areas below the average for the mixture.

On the Theory of Flame Propagation

A new theory of flame propagation in slow inflammation is proposed which considers that the highly energized atoms or radicals formed in the flame front play a more important role than heat conductivity in bringing the unburned gas to the reacting state. A small number of atoms or radicals diffuse over into the unburned phase, there initiating the chemical reaction. This and other considerations allow one to propose that the sum of thermal and chemical energy per unit mass of gas in all layers from the burned phase to the unburned phase is constant. The fundamental equation for the velocity V of the flame is derived simply from the fact that the number of molecules of combustible gas entering the reaction zone (flame front) in unit time equals the number reacting in the zone in the same time. If the flame front extends from xu to xb and the unburned gas enters it at a rate just sufficient to maintain the flame front stationary, then for 1cm¯2 cross section the equation reads: VNc(u)= ∫ TuTb−∂Nc∂tdxdTdT wh...

The Cambridge history of Islam

First published in 1970, The Cambridge History of Islam is the most comprehensive and ambitious collaborative survey of Islamic history and civilization yet to appear in English. On publication it was welcomed as a work useful for both reference and reading, for the general reader, student and specialist alike. It has now been reprinted, with corrections, and for ease of handling the original two hardcover volumes have each been divided into two separate paperbacks.

Determination of the Speed of Flames and the Temperature Distribution in a Spherical Bomb from Time‐Pressure Explosion Records

A method has been developed for determining the velocity of flame relative to the mass movement of the gases, in a closed spherical bomb from an analysis of the time‐pressure record of the explosion. The speed of the flame can be evaluated at any moment during its progress from the center to the periphery of the bomb, as well as the temperature existing in the unburned phase, the temperature immediately behind the flame front, the temperature gradient from the latter point to the center of the bomb, and the pressure in the bomb at the same moment. Given a certain fraction burned of the total amount of gas, the volume occupied by the products can be determined for three conditions: (1) Before it has expanded against the rest of the unburned gas; (2) after it has expanded; and (3) when combustion is complete and it has been compressed by subsequent burning of gas nearer the periphery. Calculations of flame speeds, temperatures, etc., have been made and tabulated for explosions of mixtures of ozone and oxygen. The speed of flame increases from the center of the bomb to the wall. At the same time the pressure and temperature of the gas about to be burned increases. The temperature gradient in the gas from the center to the wall has been calculated when the combustion is complete. A complete diagrammatic description is given for one explosion. It is shown that the temperature gradient actually existing in the bomb does not affect the specific heat results obtained by the usual method of calculating the final temperature from the maximum pressure by means of the gas law. It is pointed out that the practical coincidence of the pressure curve with the zero line in the first part of the time‐pressure record is not due to a time lag between passage of the spark and ignition, but to the small fraction of gas which has burned during this time.

Ignition of Explosive Gas Mixtures by Electric Sparks. I. Minimum Ignition Energies and Quenching Distances of Mixtures of Methane, Oxygen, and Inert Gases

An apparatus and experimental procedure is described for measuring capacitances and gap voltages of condensed electric spark circuits for sparks just powerful enough to ignite explosive gas mixtures. Mixtures of methane, oxygen, and inert gases are investigated. From the measured capacitances and gap voltages the minimum ignition energies are calculated. These energies are found to be independent of gap voltage. With increasing gap length they attain a minimum at critical distances which mark the farthest penetration of the flame‐quenching effect of the electrode material. Above the quenching distances the energies remain constant over some range which is governed by mixture composition and pressure. Energies measured in this range may be regarded as absolute minimum energies, as defined in a subsequent paper. Data of such minimum energies and of quenching distances are presented for mixtures at room temperature and pressures ranging from 0.2 to 1 atmosphere.

Mechanism of the Thermal Reaction Between Hydrogen and Oxygen

Explosion limits and reaction rates of hydrogen and oxygen have been measured in spherical quartz and Pyrex vessels of varying diameter, clean and coated with various substances and for various temperatures, pressures, and mixture compositions, including addition of inert gases. Clean and B2O3‐coated surfaces give rise to rapid and erratic reaction, indicating a surface chain‐breaking efficiency e≪λ/d (ratio of mean free path to vessel diameter); the reaction is self‐accelerating, probably due to poisoning of the surface by H2O, which decreases e. By coating with various salts such as KCl, BaCl2, K2B2O4, K2B4O7, and Na2WO4, the condition e≫λ/d is established in the region between second and third explosion limits. The limits are farther apart, the rates much lower and not accelerating, and both rates and limits are reproducible and identical for the various salts. For K2B4O7, e≃λ/d for small reaction rates. The chain‐breaking mechanism on clean and salt‐coated surfaces is discussed. With the elimination o...

Anomalous Pressures and Vibrations in Gas Explosions. Determination of the Dissociation Energy 2H2O⇌2OH+H2

Using thermodynamic functions of gases derived from band spectra, calculations have been made of theoretical explosion pressures in mixtures of hydrogen and oxygen containing various inert gases which are compared with explosion pressures observed experimentally by different investigators. Details of methods of calculation are given. In carefully dried mixtures of hydrogen and oxygen with excess hydrogen, the observed pressures are too low. This effect is eliminated by the addition of small amounts of water vapor to the original mixture which, according to an hypothesis advanced by Wohl and von Elbe, quenches the luminescence from OH radicals formed in the flame front. In mixtures containing excess oxygen or nitrogen the observed pressures are too high. The existence of a lag in the excitation of the vibrational energy levels of oxygen and nitrogen during the explosion period, as advanced by Wohl and Magat, is offered as explanation. The excitation lag is greater in oxygen than in nitrogen. Explosions of ...

Ignition of explosive gas mixtures by electric sparks: III. Minimum ignition energies and quenching distances ofmixtures of hydrocarbons and ether with oxygen and inert gases

Summary Minimum ignition energies have been determined for various mixtures of paraffin hydrocarbons from ethane to heptane (excluding pentane) with oxygen-nitrogen atmospheres from air to 100 percent oxygen and for cyclopropane, diethyl-ether, cyclohexane and benzene in oxygennitrogen atmospheres. Values in oxygen-helium atmosphere have been determined for diethylether. In the homologous series of paraffins the minimum energies shift toward the rich side with increasing number of carbon atoms. In this respect the effect of molecular structure for the same number of carbon atmos is small. The smallest minimum energy for ignition (minima of the curves) is practically identical for all the compounds studied.

Heat Generation by Electric Sparks and Rate of Heat Loss to the Spark Electrodes

In a capacitance spark the gas between the electrodes is heated almost instantaneously; subsequently, the spark‐generated heat flows from the gas to the material of the electrodes. In the present experiments sparks of 0.1 to 2 millijoules were passed between Pt electrodes in a bulb containing helium or argon or xenon, and either the pressure change, Δp, at constant volume v or the volume change, Δv, at constant pressure p was recorded, the former by means of a sensitive diaphragm and the latter by the movement of a droplet in a capillary tube attached to the bulb. The spark‐generated heat H residing in the gas at any instant was computed from the equations H=1.5vΔp and H=2.5pΔv, which apply to constant volume and pressure, respectively, and are derived from the gas law and the energy equation, using the heat capacity of monoatomic gases. From the values of H and the discharge energy corresponding to the measured capacitance and breakdown voltage, the percentage of spark‐generated heat residing in the gas ...