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Innovative modification fuel blends based on a dynamic dissolution of combustible gases in the liquid hydrocarbon fuels (part one)

2013-04-20
Andrew (Gabriel) Livshits


In mechanics and physics of continuum theory of combustion is a peculiar place for a number of outstanding features that are, in essence, the fundamental characteristics of this science.

About them mentioned in the introduction, here we discuss simple concrete examples that illustrate the main concepts of the science of combustion and explosion.

In addition, the first chapter provides basic information of the background of chemical kinetics and molecular physics, which will facilitate the reading of those who first decided to get to the subject of the book.

Chemical reactions of burning is extremely sensitive to small changes in initial and boundary conditions of the problem, to the conditions of heat and mass transfer, to the geometry of the combustion chambers, etc.

Often, the transition from one mode to the other is perceived as a gap-jump process. This may be an explosion or fire or flame failure. The first chapter deals with the theory of thermal explosion, proposed by Semenov - about work, which is counting on the history of the modern theory of combustion.

It was clearly stated the condition of thermal explosion as a result of termination of the steady state solution of the exothermic reaction in the heat sink, the critical reaction mode was identified with a branch point solutions, found a small parameter characterizing the asymptotic nature of the solution.

Analytical study of the chemical reaction accompanied by heat, in a laminar gas flow and with vigorous stirring of the reaction products with precursors revealed hyper-sterezisny nature of the solutions of the theory of combustion, increasing, for example, the initial temperature of the fuel gas, it is possible to the point where the gas ignited, but, to extinguish burning gas at high temperature, it is necessary to reduce the initial temperature well below the temperature at which ignition occurred.



This example of a chemical reactor with ideal mixing, discussed in the first chapter, also illustrates a fundamental property of the combustion process - the possibility of the existence of several stationary combustion regimes with the same set of external parameters.

This ambiguity distinguishes stationary processes with a continuous supply of raw material and withdrawal of the reaction products from the static thermodynamic equilibrium situation. As will be shown below, the reacting mixture of ideal gases is characterized by a unique equilibrium state, smoothly depending on parameters.

In the first part of this article also describes the stability of the flame - the problem is closely related to the fundamental concepts of the theory of combustion and very important for the applications of the theory.

The essential features of explosive reactions were observed in the middle of the last century, Bunsen (RW Bunsen) and van't Hoff (IH Van't Hoff). Depending on the temperature of the reaction rate changes very rapidly and, for example, at room temperature and atmospheric pressure of hydrogen and oxygen react in a little over the years.

With increasing temperature, the reaction rate is immeasurably small up to some critical value, which depends on the experimental conditions. For example, for a stoichiometric mixture of hydrogen and oxygen - the so-called "explosive mixture" - at atmospheric pressure is a critical value of about 550 ° C.

At higher temperatures, even if the excess of the critical value is only a few degrees, the explosive mixture reacts very quickly, pressure rises sharply, and a rupture of the vessel. The reaction rate in this explosion is so great that the researchers of the last century could not study in detail its kinetics.


There are two ways to implement the chemical exothermic reaction in the flow of pre-mixed gases. The first is that, say, the cylindrical combustion chamber is fed from the end of a mixture of fuel and oxygen or air in such a way that it moves parallel to the layers, ie, the speed of the gas particles in this section, except tehg that are directly at the camera, almost the same.
If the flow rate is high enough, so that the diffusion and heat transfer in the opposite direction of the flow, do not matter, then each gas behaves the same as in the reaction medium at rest, is kept at a constant pressure.
As the gas flow along the pipe, if the heat output is low, the temperature is always enhanced by the liberated during the reaction heat. Depending on the temperature and concentration of the reactant on the coordinate along the gas flow corresponds to the dependence of temperature and concentration versus time of the reaction in a medium.

Each of the major points ensures the development of the next following sections of the front surface, convex with respect to fuel gas flame areas tend according to Huygens' principle to increase the total surface area of the flame front and help to increase the overall speed of the combustion of fuel gas in a turbulent flow.

Each of the major points ensures the development of the next following sections of the front surface, convex with respect to fuel gas flame areas tend according to Huygens' principle to increase the total surface area of the flame front and help to increase the overall speed of the combustion of fuel gas in a turbulent flow.

Reynolds numbers corresponding to the absence of turbulence, hydrodynamic instability of the flame can lead to distortion of the flame front. The ratio (1.15) remains valid, but / (0) = 1 and (1.16) does not hold.

Pulsations of gas against the direction of propagation of the combustion flame cast back and create a concave to the combustible mixture front. In accordance with the principle of Huygens on concave areas, the formation of fractures of the flame front, which tend to reduce the burning surface.

Finally, the velocity fluctuations directed across the direction of propagation of the combustion causes the asymmetry of the flame front curvature, promote the formation of turns and a complex structure.

The main task of the theory is the study of turbulent combustion in stationary average turbulent flame: in a given turbulent flow field is required to find the time-averaged structure of the combustion zone and to determine the statistical characteristics: the average rate of combustion, the average surface of the flame front, the width of the field, the average processing curved flame front , and other characteristics. Important role in this theory must play a leading point curved front-laminar flame rendered turbulent fluctuations in the direction of the fuel gas, as they ensure the existence of the following after them flame surface and determine the average rate of combustion.

Velocity fluctuations of the turbulent flow of gas at the fully developed turbulence often exceed the normal rate of flame propagation.
Rate of removal of the leading points in the combustible gas is determined mainly by the characteristics of the flow pulsation. Normal flame speed allows for the reduction of the front surface. Concave flame and thus does not allow infinitely increase the burning surface.
If the surface of the flame did not have the ability to navigate through the combustible gas, according to the law of turbulent diffusion is a sprawling space and continuously over time increased its surface.
"Eating" of the total surface of the front on the mechanism of the normal flame propagation on the concave regions of the flame does not allow the combustion zone to grow indefinitely, and it "tends to a fixed width in the middle with a constant on the average of the curved surface of the flame front. Therefore, the relation of turbulent combustion should be present as one of the defining values of the normal flame speed.

These qualitative representation of the distribution of curved laminar flame front in a turbulent flow field, despite their physical transparency and simplicity, still yet to be implemented in the mathematical ratios. This is due largely to the fact that at present there is a fairly complete description of turbulent flows.

Additional complications also arise from the fact that in the flame front is the thermal expansion of the gas, which affects the turbulent flow field of gas, in other words, in addition to the direct effects of turbulence on the combustion, there is a reverse effect on the turbulence combustion. As a first approximation, however, can be carried out theoretical analysis, abstracting from this effect.

The very flame spread on combustible gas may vary due to the effects of high-frequency small-scale (comparable to the thickness of the flame front) part of the turbulent fluctuations on the structure of the laminar flame front.
These pulsations intensify the transfer processes in the flame and thus increase the speed of its movement through the combustible gas. More detail the phenomenon of turbulent combustion can be found in the monographs and reviews



... to be continued ...

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