Pre-mixed flame

The above analysis, which is exceedingly brief and simplified is designed to demonstrate how, even in a pre-mixed flame, the question arises as to what is the appropriate reaction volume (i.e. the flame thickness). In heterogeneous reactions, this is a question that will recur again and ain and the designer of reactors must not attempt to avoid it. It is interesting to note that, in the next but one example to be treated, the overall reaction rate (a flame speed cm s in the above) becomes a mass transfer coefficient (also cms" ) when considering the absorption of gas into a liquid with which it reacts quickly. Furthermore, exactly the same sort of analysis as the above leads to the dependence of the mass transfer coefficient fej on the reaction rate coefficient and the diffusivity, D, in the liquid phase, of ki o. (rD), cf. z a RKY above. 

There are a great many published reports describing studies of carbon black formation in flames. Many of these deal with gaseous fuels in either pre-mixed flames or diffusion flames. The principal objectives are to develop a better knowledge of combustion through an understanding of the kinetics and mechanism of carbon formation,... 

Amos M. D. and Willis J. B. (1966) Use of high-temperature pre-mixed flames in atomic absorption spectroscopy, Spectrochim Acta 22 1325— 1343. 

Libby PA, Bray KNC (1981) Countergradient diffusion in pre-mixed flames. AlAA J 19 205-213... 

Basically we distinguish between pre-mixed or non pre-mixed (formerly called diffusion flames ) flames. With pre-mixed flames the mixing between fuel and oxidant occurs before combustion with non pre-mixed flames mixing and combustion are simultaneous. A pre-mixed flame is obtained, for example, if the air supply of a Bunsen burner is opened if it is closed the flame becomes non premixed. Another example of a non pre-mixed flame is a burning candle. 

For pre-mixed flames the velocity of combustion is limited by the kinetics of the combustion process. In case of a laminar non pre-mixed flame the limitation usually stems from the diffusion velocity of air into the fuel, with turbulent non pre-mixed flames on the other hand the kinetics becomes more determining. 

The transition from laminar to turbulent flames occurs for Re 2,000 with the Reynolds number referring to the flame. It is smaller than in the unbumt mixture because the viscosity of gases rises with increasing flame temperature. The combustion process of a turbulent pre-mixed flame can be controlled well. However, for safety reasons it is not readily applied because flammable mixtures may accumulate and hence explode. 

Non pre-mixed flames are characterized by more complex chemical processes than pre-mixed ones and may comprise the entire spectrum 0 < (() < 00. They occur if a pure fuel flows from an outlet opening and is then mixed with the surrounding air and thus with oxygen by difihision and entrainment. Contrary to pre-mixed flames non pre-mixed flames do not propagate and hence cannot be characterized by the laminar burning velocity. For the flame length we have [16]... 

With increasing velocity at the outlet the laminar flame becomes turbulent. The transition between the two regimes occurs at Re 2,000. Contrary to laminar non pre-mixed flames its length does not depend on the velocity at the outlet. 

The adiabatic flame temperature applies to pre-mixed flames, usually in stoichiometric mixtures with oxygen from air. Non pre-mixed flames reach temperatures of appr. 1,500 K. If the oxidant is pure oxygen instead of oxygen from the air the occurring temperatures are 700-800 K higher. 

Flames are of two sorts diffusion flames where the flammable substance has first to form a flammable mixture with air and pre-mixed flames where... 

Those theories were based on the assumption that the flame stmcture in the stabilization zone corresponded to either a completely pre-mixed flame [18] or a flame controlled by diffusion [20]. In the first case, the stabilization criterion is the balance of the velocities at the near stoichiometric composition line and turbulent combustion at the flame lift-off height. In the second case - it is the totality of the conditions imposed on the scalar dissociation rate of the flame extinguishing process. Later, an up-dated model combining both approaches was developed [21], it put emphasis on the necessity of taking into account both mixed flame... 

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