Premixed gases

Chomiak, ]., Dissipation fluctuations and the structure and propagation of turbulent flames in premixed gases at high Reynolds numbers. Proceedings of the Combustion Institute, 16, 1665-1673,1977. 

G. Searby and P. Clavin. Weakly turbulent wrinkled flames in premixed gases. Combustion Science and Technology, 46 167-193, 1986. 

In this method premixed gases flow up a jacketed cylindrical tube long enough to ensure streamline flow at the mouth. The gas bums at the mouth of the tube, and the shape of the Bunsen cone is recorded and measured by various means and in various ways. When shaped nozzles are used instead of long tubes, the flow is uniform instead of parabolic and the cone has straight edges. Because of the complicated flame surface, the different procedures used for measuring the flame cone have led to different results. 

In the introduction to this chapter a combustion wave was considered to be propagating in a tube. When the cold premixed gases flow in a direction opposite to the wave propagation and travel at a velocity equal to the propagation velocity (i.e., the laminar flame speed), the wave (flame) becomes stationary with respect to the containing tube. Such a flame would possess only neutral stability, and its actual position would drift [1], If the velocity of the unbumed mixture is increased, the flame will leave the tube and, in most cases, fix itself... 

The topic of concern here is the stability of laminar flames fixed to burner tubes. The flow profile of the premixed gases flowing up the tube in such a system must be parabolic that is, Poiseuille flow exists. The gas velocity along any streamline is given by... 

Generally, the gas-phase reactions in both flame models for premixed gases and the burning of energetic materials are assumed to be bimolecular and hence of second order. Eq. (3.54) can then be expressed as... 

In this section we consider the combustion of premixed gaseous fuel and air mixtures. Consider first the laboratory Bunsen burner, shown in Figure 10-1 1. Natural gas from the gas supply system enters the bottom of the burner, where it is mixed with air, with flow rates adjusted by the gas valve and holes in the bottom of the burner, where air is sucked in by natural convection. The premixed gases travel up the barrel of the burner (a tubular reactor), and, if flows are suitably adjusted and a match has been used to ignite the mixture, a stable flame forms at the top of the tube. 

Flames are characterized by aflame velocity Uf, which is the velocity with which a flame front will move down a tube containing stagnant premixed gases. The upper and lower compositions where the flame velocity goes to zero are called the flammability limits, and the flame velocity is usually its maximum near the stoichiometric composition for CO2 and H2O formation. 

In practice there is only a certain range of compositions where premixed gases will bum There is obviously a great deal of interest in this composition because it sets limits of operation of many processes. The ignition limit is only loosely defined because it depends sensitively on temperature, flow conditions, and the presence of trace chemicals in the fuel, which can act as flame initiators or inhibitors by enhancing or slowing either r or rt. 

In the previous section we described one type of reaction that can cause an explosion, namely, the situation where fuel and oxidant are mixed in solids or liquids that can release large amounts of energy very quickly. These processes can also occur in premixed gases. An example of useful explosions in gases is the repeated explosions that occur in the automobile engine. However, most examples of explosions in gases are in fact disasters in which products burst walls of containers and cause havoc. 

This non-uniformity in the distribution of the atoms in the flame arises because the flame has a distinct structure. Figure 2.4 shows the structure of a typical premixed flame. Premixed gases are heated in the preheating zone, where their temperature is raised exponentially until it reaches the ignition temperature. Surrounding the preheating zone is the primary reaction zone, where the most energetic reactions take place. 

In general, the stable thermodynamic products of ordinary flames have little worth, but many of the uncommon flames have products of value. The chlorination of hydrocarbons may be carried out in a flame process which was recently announced (A4). A most fascinating example is the formation of boron nitride from the flame reaction between diborane and hydrazine, two compounds which are ordinarily thought of as fuels (B2, VI). The stabilization of this flame depends upon the proper preparation of the premixed gases, since a solid adduct between the reactants prevents flame stabilization if the preflame residence time is too great. 

Thus, for rapid combustion of non-premixed gases, in the reaction zone we obtain exactly the same concentration of combustion products as if we had mixed the burning gases in a stoichiometric ratio and carried out the chemical combustion reaction without any diffusion exchange. 

However this theory of the effect of external thermal losses is inapplicable to the combustion of non-premixed gases. The reason is that lowering the combustion temperature in this case does not lead to a change in the amount of gas which burns per unit surface of the flame since the combustion rate is determined here exclusively by the rate of diffusion supply of oxygen and fuel to the flame surface and not by the propagation rate (which depends on the reaction rate) as for a ready mixture. 

The combustion limit of non-premixed gases is determined by the temperature decrease, which depends on the final chemical reaction rate. 

The limit of combustion intensity found above for non-premixed gases explains, at least qualitatively, the fact that in the flow of a fast jet out of a tube the flame is completely located at some distance from the mouth of... 

We considered the distribution of the concentration of the combustion products and temperature in the combustion of non-premixed gases. It is shown that under the simplest assumptions these concentrations and temperature at the flame surface are the same as in the combustion of a premixed stoichiometric mixture of the gases considered. 

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