Introduction

In terms of mixing, there are two extremes: 1. premixed combustion where fuel and oxidizer are completely mixed prior to their entering the combustion chamber and 2. nonpremixed combustion where fuel and oxidizer enter separately. We have treated these two cases in the preceding two chapters. In technical applications, however, the optimum often lies somewhere between the extremes, trying to profit from advantageous features of both while avoiding their adverse effects. If fuel and oxidizer enter separately, but partially mix by turbulence, combustion takes place in a stratified medium, once the mixture is ignited. Such a mode of combustion has traditionally been called partially premixed combustion.

Turbulent flame propagation in a stratified mixture occurs, for instance, in aircraft gas turbines. Liquid kerosene is fed by an air-blast injector into the gas turbine combustion chamber, where it is mixed with the compressed air. There is typically a pilot injector for idling and a main injector for part load and full load operations. When the main injector is started, an inhomogeneous ignitable mixture is formed at its inlet. When this comes into contact with the hot exhaust gases from the pilot injector, flame propagation takes place from the pilot burner to the main burner through a stratified mixture. This process is difficult to control and modeling of the mixing and of partially premixed combustion poses a main challenge to CFD simulations of gas turbine combustion.

Another example is partially premixed flame propagation in direct injection gasoline engines, where a spray of liquid gasoline is injected directly into the cylinder rather than into the intake manifold as in conventional homogeneous charge spark-ignition engines.