Our interest in this chapter is to look inside the thermochemical reactors discussed in the previous chapter. Inside the reactors, the formation of visible flame is one obvious phenomenon. Flames are of two types; (a) premixed flames and (b) nonpremixed or diffusion flames. A Bunsen burner, shown in Fig. 8.1, is a very good example in which both types of flames are produced. In this burner, air and fuel are mixed in the mixing tube; and this premixed mixture burns forming a conical flame of a finite thickness (typically, blue in color). This is called the premixed flame. It is so called because the oxygen required for combustion is obtained mainly from air, which is mixed with the fuel. This premixed combustion releases a variety of species, stable and unstable. Principally, the carbon monoxide resulting from the fuel-rich combustion burns in the outer diffusion flame. The oxygen required for combustion in this part of the flame is obtained from the surrounding air by diffusion (or by entrainment). The overall flame shape is determined by the magnitude of the mixture velocity and its profile as it escapes the burner tube, coupled with the extent of heat losses from the tube wall.