3 - Nonpremixed turbulent combustion

Summary

Introduction

In many combustion applications fuel and oxidizer enter separately into the combustion chamber where they mix and burn during continuous interdiffusion. This process is called nonpremixed combustion.

A typical example is combustion in furnaces, which are operated under nonpremixed conditions mainly for safety reasons. Fuel is supplied, for instance, by jets of gaseous fuel, which entrain enough air from the surroundings so that all the fuel can be burned within a certain distance from the nozzle. That distance is called the flame length. Other fuels used in furnaces are coal dust injected with air as a carrier gas, or liquid fuel that is injected as a spray. Since mixing and combustion in jets and sprays occur simultaneously, the formation of large volumes of unburnt flammable mixture can be avoided. In a practical application this requires a control system to make sure that each of the flames in a furnace is burning as long as fuel is supplied.

Other applications of nonpremixed combustion include diesel engines and gas turbines. In diesel engines the air is compressed by the piston before a liquid fuel spray is injected into the combustion chamber. The hot compressed air is entrained into the spray, leading to liquid fuel breakup, evaporation, and finally to auto-ignition. During the combustion phase, at first the premixed fraction of the gas is rapidly consumed, but then combustion takes place under nonpremixed conditions. During this phase most of the formation of NO_x and soot is taking place, but it also provides the necessary conditions for soot oxidation.

In aircraft gas turbine engines nonpremixed combustion occurs in the swirlstabilized combustion zone downstream of the spray injector.