Page 429 - Advanced thermodynamics for engineers

17.4 COMBUSTION IN GAS TURBINES 419

temperature to an intermediate level by the addition of a small amount of air allows the combustion of
CO and partly burned fuel to proceed to completion. At high altitudes, the secondary zone serves
principally as an extension of the primary zone.
Dilution zone: the dilution zone is the region where the air remaining after the combustion and
wall cooling requirements have been met is admitted to the main chamber. This then provides an outlet
stream with a mean temperature and a temperature distribution that is acceptable to the turbine. Finally
the cooling air which has been passed down the outside of the combustion chamber will be mixed in
with the hot gases, and the products will be passed to the turbine through the nozzle, which matches the
flow from the combustion chamber to the requirements of the turbine.
A major requirement in the design of a gas turbine combustor is that combustion can be sustained
over the entire range of operating conditions including the transient states of rapid acceleration and
deceleration. For industrial engine combustors this presents no special problems, since the low ve-
locities and high-pressure-loss factors employed in these systems are very conducive to stable com-
bustion. The aircraft chamber, however, is called upon to operate at very low inlet temperatures and
pressures and at fuel–air ratios that lie well outside the normal burning limits of hydrocarbon–air
mixtures. Efficient combustion must be maintained in highly turbulent airstreams flowing at velocities
that greatly exceed the normal burning velocity of the fuel.
The basic principle involved in flame stabilization is quite simple. If combustion is initiated in a
flowing stream, and if the gas velocity U is higher than the flame speed S, the flame will move
downstream at a speed of U–S (flame blowout). If the burning velocity is higher, then the flame will
move upstream with a speed of S–U. Only if U ¼ S will the flame be stationary. The function of the
flameholder is to create a region in a high-speed gas stream whose velocity is lower than the burning
velocity of the mixture. The primary design objective for good stability in any practical combustion
device is to maximise the ratio of burning velocity to flow velocity. This, incidentally, is also a key
requirement for high combustion efficiency. It is normally achieved by creating, at the upstream end of
the liner, a sheltered zone of low velocity in which flame speeds are greatly enhanced by imparting a
high level of turbulence to the primary air jets and by arranging for hot combustion products to
recirculate and mix with the incoming air and fuel (see Fig. 17.25).
The stability performance of a combustor is usually expressed in the form of a stability plot that
separates the regions of stable and unstable combustion. The traditional plot has equivalence ratio or
fuel–air ratio as the ordinate, and some loading parameter, such as velocity or mass flow through the
combustor, as the abscissa. These loops provide two basic kinds of information. First, for any given
fuel–air ratio, they indicate the blowout velocity. Second, for any given combustor loading, they show
the range of fuel–air ratios over which stable combustion can be achieved.
There are many reasons why combustion efficiency is of paramount importance for gas turbine
combustors. First and foremost, combustion inefficiency represents a waste of fuel, which is clearly un-
acceptable in view of the world’s dwindling oil supply and the rapid escalation of fuel costs. Also com-
bustion inefficiency is manifested in the form of undesirable or harmful pollutant emissions, notably
unburned hydrocarbons and carbon monoxide. Combustion efficiencies in excess of 99% at all operating
conditions are demanded to meet emission regulations. For the aircraft engine, an additional requirement
isthatthelevelofcombustionefficiencybefairlyhigh,from75%to80%,whenthecombustionchamberis
being use to accelerate the engine to its normal rotational speed after a flameout at high altitude. A high
combustion efficiency is necessary when the engine is windmilling because the pressure and temperature
of the air flowing through the combustor are close to the ambient values; at high altitudes, these are so low