Page 428 - Advanced thermodynamics for engineers

418 CHAPTER 17 GAS TURBINES

airflow through an atomiser. There are many types of atomiser design and these will not be discussed
here. However, the atomiser should possess the following characteristics:

• good atomisation over the entire range of fuel flows,
• rapid response to changes in throttle setting,
• freedom from flow instabilities,
• low susceptibility to blockage by contaminants,
• low susceptibility to gum formation by heat soakage,
• capability for scaling, to provide design flexibility,
• low cost, light weight, ease of manufacture and ease of removal,
• low susceptibility to damage during manufacture and installation.
It should also provide:
• an easily ignitable mixture,
• a ratio of maximum to minimum fuel flow that exceeds the ratio of maximum to minimum
combustor airflow,
• controlled dispersion of the fuel throughout the primary combustion zone,
• an exit gas temperature distribution that is insensitive to variations in fuel flow rate.

The combustion volume shown in the annular chamber in Fig. 17.25 can be divided into three
zones: primary, secondary and dilution. These zones will now be discussed.
Primary zone: the functions of the primary zone are to anchor the flame and to provide sufficient
time, temperature and turbulence to achieve essentially complete combustion of the fuel. The snout
imparts a high swirl into the flow to induce a strong recirculation region in the primary zone; this swirl
enables the residence time in the primary zone to be lengthened without making this region excessively
long. It also promotes good mixing through turbulence. And this mixing promotes efficient combustion
and minimum pollutant formation. The fuel injector will be either a high-pressure spray atomiser or an
air-blast injector. Sometimes a smaller pilot injector is used to assist ignition. Ignition is achieved by
means of a powerful plasma jet igniter. In the primary combustion zone the air–fuel ratio will be
approximately stoichiometric (15:1), and the adiabatic temperature rise will take the temperature up to
about 2000 K, at which temperature NO x will be formed. Jet-mixing processes play an important role
in achieving satisfactory combustion performance. In the intermediate zone, rapid mixing of the
injected air with the hot gases from the primary zone is needed to accelerate soot oxidation and to
convert any dissociated species into normal products of combustion. Finally, the attainment of a
satisfactory pattern factor at the combustor exit is dependent on thorough mixing of air and combustion
products in the dilution zone.
Secondary zone: The gas enters here from the primary zone and is diluted with secondary air
bleeding through the side walls of the chamber. This air will induce further turbulence, and supply
more oxygen to complete the burning of the soot formed during the rich primary combustion. The
secondary air will also lower the gas temperature to about 1800 K, and reduce the amount of NO x
produced. This zone must have sufficient length for the two main functions that it performs. At low
altitudes it serves as a region in which dissociation losses in the gas flow can be recovered, and the
burning of any imperfectly mixed fuel-rich pockets of gas can be completed. Dissociation has been
discussed in Chapter 12, and this results in carbon dioxide (CO 2 ) dissociating to carbon monoxide
(CO) and oxygen: to a lesser extent, there is also dissociation of water vapour. Reducing the gas