Introduction

Diffusion flames were introduced in the previous chapter (see Figure 8.2) as flames in which fuel and air are physically separated. Diffusion flames occur in the presence of flowing or stagnant air such that the fuel moves outward and air moves inward to create a reaction zone at the flame periphery. Figure 9.1 shows a typical laminar diffusion flame configuration in stagnant surroundings.

Fuel issues from a round nozzle (diameter D) with axial velocity U0 into stagnant surroundings. As there are no effects present to impart three-dimensionality to the flow, the flow is two-dimensional and axisymmetric. The fuel burns by drawing air from the surroundings forming a flame that is visible to the eye. The flame develops to length Lf and assumes a shape characterized by flame radius rf(x), which is a function of axial distance x.

The energy release is thus principally governed by the mixing process between air and fuel. As such, the chemical kinetics are somewhat less important, being very fast. For this reason, diffusion flames are often termed physically controlled flames. Flames formed around burning volatile liquid droplets or solid particles (whose volatiles burn in the gaseous phase) are thus physically controlled diffusion flames.

The two main objectives of developing a theory of diffusion flames are to predict

1. The flame length Lf and

2. The shape of the flame, that is, the flame radius rf(x).