flame

Flame
1- Introduction
The main aim of combustion study is the acquisition of a thorough understanding of the mechanisms of ignition, species distribution, flame propagation and energy release of combustible mixtures. The practical results of such knowledge are evidently the control of the combustion process, both from the point of view of safety and its utilization as a source of energy.
Burning velocity is a physical constant for a given combustible mixture. It is the velocity relative to the unburned gas; with which a plane, one-dimensional flame front travels along normal to its surface. When a combustible mixture, formed by mixing a fuel with an Oxidant in proper proportion prior to ignition, is ignited by a suitable external source a flame will be generated by a self-propagation exothermic reaction.
The reaction zone is often called the (Flame Zone, Flame Front or Reaction Wave). Within the flame zone, rapid reactions take place and light is usually (but not always) emitted from the flame. This phenomenon depends upon the role of physical and chemical procedures
2- Flame Speed and Burning Velocity
One of the distinguished features of laminar premixed flames is that has a characteristic propagation rate; commonly called the burning velocity;
……(1)
If the flame speed is in the same direction as the flow of the unburned gas the minus sign (-); if the flame speed is against the unburned gas flow, the plus sign (+) is valid. The equation is useful to deduce several important characteristics of premixed flame. If the fresh mixture speed is so adjusted that the flame is fixed at a station (Sf=0), the burning velocity (Su) is equal in magnitude to the supply velocity.
The burning velocity, Su, is more precisely defined as the velocity at which unburned gases move through the combustion wave in the direction normal to wave surface. It is used in many areas of combustion science as in designing burners and predicting explosions.
There are several methods for determining the laminar burning velocity. These methods are divided in two classes (stationary flame (counter flow-burner method) and nonstationary flame (constant volume method))
2.1 Stationary Flames Methods
A flame to remain stationary, the flame speed must be equal to the speed of the normal component of unburned gas at each location.
2.1.1 Bunsen Burner Method:
The Bunsen burner flame provides an interesting example of laminar premixed flames with which most students have some familiarity and can be easily used in classroom.
The main advantage of this method is the equipment are simple, flexible and easily adapted for measurement. There are many disadvantages:
1. The diffusion interchange with the surrounding atmosphere alters the fuel-oxidant ratio; so that the burning velocity observed may not represent the measured fuel-air ratio
2. One can never completely eliminate wall-quenching effects
3. The flame cone can act as a lens in a shadow measurement; this causes uncertainties in the proper cone size.