Andrew (Gabriel) Livshits
Consider the surface of the flame front as the surface of a gas-dynamic discontinuity and analyze the basic conditions of preservation on this; break.
This relation shows that in the case P = sopz1 same stock total internal energy, as in the formula (2. 1) is spent to increase the internal energy of the combustion products (the first term) and the work of expansion of the gas (the second term), which always positive, since U2 Wow. Therefore always less than Tb Tr.
Current average temperature in the vessel coincides with the true, if a chemical reaction, and therefore, the heat is the same throughout the entire volume of the vessel. But more often burning vsosude initiated locally.
For real fuel mixtures temperature difference is much less, because even at a temperature of about one thousand degrees significantly increases heat, especially in triatomic combustion products, H20 and C02. At even higher temperature plays an important role dissociation products of combustion, which requires the cost of heat.
With equal initial pressure combustion in a closed vessel is accompanied by an increase in pressure, which inhibits the dissociation, and this fact also leads to a larger value than you Tb.
The most preferred sites for the pilot study are spherical flames propagating from the source igniting small size, located in the center of the vessel, - such of the experiment eliminates heat loss from the thermal conductivity, friction against the walls of the vessel and other factors that may distort the phenomenon, and is characterized by simple geometry.
Experiments with spherical laminar flame carried by Zel'dovich and A. Rozlovskiy, J. and K. Troshin Shchelkin colleagues, Yu X. Shaulova with employees and other researchers.
In all the papers with increasing radius of the spherical flame manifested hydrodynamic instability, manifested in the sudden acceleration of the flame.
However, the hydrodynamic instability occurs with large delay: Reynolds, built by the normal velocity of the flame, the flame radius and the viscosity of the cold gas, were the manifestation of instability in several tens of thousands, and many times higher than the critical value (the order of one to ten), following the theory of stability flat flame.
The diffusion coefficients and diffusivity increase with temperature approximately as T1 b-2, while the gas velocity is proportional to T. Therefore, the Reynolds number, calculated from the constants of the products of combustion, is 6-10 times smaller, but still of the order of thousands or more, and a value needs no explanation.
In studies it has been noted that before the sharp acceleration of the flame it formed cells - the flame surface becomes like a soccer ball.
Recent volatility in spherical flame has its own specifics related to the fact that it is continuously growing area of the surface front.
Therefore, for a spherical flame to a different definition of an unstable state. A significant change in the shape of the flame, ie, a marked contrast from the disturbed surface of combustion smoother, and the consequent increase in the rate of flame propagation occur when the amplitude of the curved surface is growing faster than the increase in the size of the sphere of flame.
If the protrusions and depressions on the sphere of flame increases more slowly than linearly, in which the radius of the sphere increases in general, the flame eventually smoothed, despite the increase in the absolute value of the amplitude distortion, flame properties with close to those of the unperturbed spherical front.
Therefore, the criterion of instability for spherical flame must accept the terms of increase in time is not absolute (for plane flame) and relative (divided by the radius of the flame) perturbation amplitude surface.
Another important fact is that the propagation of a spherical flame inside the combustion flame spheres "repulsive" cold gas - an action similar to the action of the flame source mass.
In the resulting distribution of cold gas velocity and pressure, which are different from the existing near a flat laminar flame front. Distribution of velocity and pressure unperturbed flow field significantly affect the manifestation of instability.
Finally, this perturbation with a given wavelength over time not only changes in amplitude, but the wavelength - stretched in the same ratio as the radius.
For a flat flame perturbation wavelength const and above relations after integration leads to an exponential function.
For a spherical flame disturbance should not degrade the normal Fourier series, and a series of spherical harmonics, and the perturbation wavelength corresponding to a certain spherical harmonic increases in proportion to the radius of the sphere, and therefore proportional to time.
As a result of (3. 112) turns the power-law variation of the amplitude perturbation in time = [3r0 (t) = ubt ((3 - proportionality factor depending on the number of harmonics, r0 (t) - the current radius of the spherical flame ux - flame speed on the products of combustion), drf! dt - rfun / ubt, rf - tu ub.
Thus, the spherical flame disturbances grow in time slower than the flat flame.
AG Istratov and VB Librovich using this fact to explain the discrepancy between theoretical and experimental results. Consider, following their work, the hydrodynamics of the spherical flame propagation.
Chemical composition R. gas well defined and its physical and chemical properties - is a colorless gas, lighter than air, it is difficult to liquefy, and slightly soluble in water.
It is not poisonous, but if a large admixture of it to the air and cause suffocation, it depends only on a small percentage of oxygen in the mixture.
Some gas mines has a slight ethereal odor, depending, probably from negligible impurities (see above). This scent gives the opportunity to learn about the presence of gas in the mine.
R. rule the gas has no smell (more on the physical and chemical properties, see Swamp Gas). The chemical properties necessary to examine in some detail the combustion reaction is of direct relevance to the explosions R. gas and methods for the quantitative determination of it in mine air.
Complete combustion reaction of pure methane by the following equation: CH4 +2 O2 = CO 2 +2 H 2 O, which shows that the volume of one of its demands for the complete combustion of the two volumes of oxygen or 9.5 volumes of air.
A mixture of methane and air in the said proportion (9.5% methane + 90.5% air) most explosive, its combustion temperature at a constant volume equals 2150 °, and at a constant pressure of 1850 °; amount of heat released by the complete combustion of one gram- molecule (16 gr.) methane = 188 more calories.
Relationship between the pressure experienced by the walls of a closed container in the explosion, and quantitative composition of the mixture experiments Mallyara and Le Chatelier, is expressed by the following figures:
Flash point and flammability limits. In the experience of the same scientists, slow combustion (no flame) explosive mixture becomes noticeable at 450 °, it is the inflammation occurs at 650 °, and between the time when the gas will this temperature, and the moment of the explosion takes quite a considerable period of time, namely about 10 seconds. (At 650 °).
With the rise of temperature of the gap decreases and at 1000 ° C does not exceed 1 second. This attitude to the heat is a characteristic feature of methane, as other explosive mixture, such as a mixture of air with hydrogen or carbon monoxide, explode immediately on reaching the flash point.
Thus, inflammation detonating R. air requires not only a certain temperature, but also a certain duration of heating, and these two values are to each other in the inverse ratio.
Ignition of the explosive mixture in any one of its points is transferred itself to neighboring units and the entire mixture is only in those cases where the composition of its not out of some certain limits, the latter depends on the temperature, extending its increase.
When normal conditions are actually explosive mixtures with the volume of 6% to 16% R. gas. Above 650 ° flammable mixtures of any composition.
Flame spread. Flame spread in the environment explosive mixture depends on many factors: the percentage composition of the mixture, temperature, quality of indifferent gas, Neighborhood cold solid, etc., to a very large extent on whether the mixture is at rest or in motion.
... to be continued ...