276 POWER PLANT ENGINEERINGreaction. The secondary air helps to complete the combustion as well as helps to cool the flame tube.
The remaining 50% air is mixed with burnt gases in the “tertiary zone” to cool the gases down to the
temperature suited to the turbine blade materials.
By inserting a bluff body in mainstream, a low-pressure zone is created downstream side that
causes the reversal of flow along the axis of the combustion chamber to stabilize the flame.
In case of swirl stabilization, the primary air is passed through the swirler, which produces a
vortex motion creating a low-pressure zone along the axis of the chamber to cause the reversal of flow.
Sufficient turbulence must be created in all three zones of combustion and uniform mixing of hot and
cold bases to give uniform temperature gas stream at the outlet of the combustion chamber.9.3.4 Gas Turbines
The common types of turbines, which are in use, are axial flow type. The basic requirements of
the turbines are lightweight, high efficiency; reliability in operation and long working life. Large work
output can be obtained per stage with high blade speeds when the blades are designed to sustain higher
stresses. More stages of the turbine are always preferred in gas turbine power plant because it helps to
reduce the stresses in the blades and increases the overall life of the turbine. More stages are further
preferred with stationary power plants because weight is not the major consideration in the design which
is essential in aircraft turbine-plant.
The cooling of the gas turbine blades is essential for long life as it is continuously subjected to
high temperature gases. There are different methods of cooling the blades. The common method used is
the air-cooling. The air is passed through the holes provided through the blade.
9.4 Regeneration and Reheating
Generally, the thermal efficiency of the simple open cycle is only about 16 to 23% as lot of heat
energy goes waste in the exhaust gases. Moreover the cycle efficiency directly depends upon the tem-
perature of the inlet gases to the turbine. And as the metallurgical limitations do not permit the use of
temperatures higher than about 1000°C, a sizeable increase in efficiency cannot be expected through the
increased temperature of the gases. Of course, this efficiency handicap can be overcome by incorporat-
ing thermal refinements in the simple open cycle e.g. regeneration, reheating. But the plant will become
complex in contrast to the simple open cycle plant which is compact, occupies very little space, does not
need any water and can be quickly run up from cold. The thermal refinements can raise the plant effi-
ciency to over 30% and thereby obliterate the advantage of fuel efficiency possessed by diesel or con-
densing steam power plants. These refinements are discussed below:9.4.1 Regeneration
In regeneration, the heat energy from the exhaust gases is transferred to the compressed air
before it enters the combustion chamber. Therefore, by this process there will be a saving in fuel used in
the combustion chamber if the same final temperature of the combustion gases is to be attained and also
there will be a reduction of waste heat. Fig. 9.10. shows a regenerative cycle.