Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-03-11
2004-11-16
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S039170, C060S736000, C060S782000, C060S806000
Reexamination Certificate
active
06817187
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to gas turbine engines and working cycles of the type in which fuel and excess air are burnt together in a first combustor, the combustion products are passed through a high pressure turbine, the exhaust of the high pressure turbine is then burnt together with further fuel in a second combustor to consume the excess air, and the exhaust of the second combustor is passed through a low pressure turbine. In particular, the invention relates to improved use of cooling air which has passed through turbine components in such a turbine.
BACKGROUND OF THE INVENTION
Re-fired gas turbine engines commonly find use as prime movers in electrical generation plants. Referring to
FIG. 1
, in general terms an example of such an engine
1
comprises a multi-stage bladed rotary compressor
10
which compresses atmospheric air
12
to a high pressure. This compressed air
14
is then fed to a first combustor
20
, which also receives gaseous and/or liquid fuel
16
, the air and fuel being burnt together in a first stage of combustion. The resulting high-pressure, high-temperature combustion products
22
are used to drive a bladed rotary high pressure turbine
30
, whose work output primarily drives the compressor
10
via transmission shaft
40
. The air
14
supplied to combustor
20
is more than is required for complete combustion of the fuel
16
and therefore the turbine exhaust
34
contains excess air which can then be burnt with further fuel
36
in a reheat combustor
50
. Bladed rotary low pressure turbine
60
receives the reheat combustion products
52
from reheat combustor
50
and uses them to drive electrical generator
70
through shaft
80
. The low pressure and high pressure turbines are optionally connected together by a shaft
90
, shown in dashed lines, shaft
90
being present if it is desired that both turbines
30
and
60
always run at the same speed. The exhaust
62
of the low pressure turbine
60
can be passed to atmosphere, preferably after passing through a heat exchanger for heat recovery (not shown).
To extend the life of components in the turbines, such as rotor blades, stator blades and nozzle guide vanes, it is well known to pass compressed air through them for cooling purposes. Hence, as shown in
FIG. 1
, both the high and low pressure turbines are provided with supplies of cooling air via lines
90
and
92
respectively, these being tapped off from the compressor
10
. Because high pressure turbine
30
requires cooling air
90
which is at the highest available pressure, its supply is taken from at or near the output of the compressor
10
, but low pressure turbine
60
can be supplied with cooling air
92
at a lower pressure, so it is taken from an earlier stage of the compressor. In both cases, the amount and pressure of compressed air taken from the compressor
10
for cooling purposes must be the minimum necessary to provide adequate cooling of the components, because extraction of working fluid from this early part of the engine's working cycle imposes a cycle efficiency penalty which, unless it can be compensated for by the use of higher working fluid temperatures in the turbines, reduces the amount of power available from the low pressure turbine
60
on shaft
80
.
In known engine arrangements of this type, after the cooling air has passed through the hollow interior of a turbine component such as a blade or vane, it is exhausted into the turbine annulus either from the outer tip or shroud of the blade or vane, or via small cooling holes in their flanks or in their leading or trailing edges, the cooling holes being provided in areas particularly exposed to high temperature combustion products. For example, a common cooling technique used in such circumstances is so-called “film cooling”, in which an array of small closely-spaced holes are provided to connect part of the exterior surface of the component to an interior passage through which the cooling air is flowing. Where the cooling air exits from the array of holes onto the component surface, a film of relatively cool air is formed next to the surface, thereby protecting the component from the full effects of the hot combustion gases. However, this practice of exhausting used cooling air into the turbine working passages further complicates the problem of optimizing cycle efficiency, because the cooling flow reduces the mean working fluid temperature in the turbine passage, so reducing turbine power output and efficiency.
SUMMARY OF THE INVENTION
It is an object of the present invention to increase turbine power output and turbine efficiency by reducing the diluting effects of cooling air exhausted from turbine components.
According to the present invention, a gas turbine engine comprises in flow series;
a compressor for compressing air to a high pressure,
a first combustor having fuel injection means for burning fuel together with high pressure air supplied from the compressor, the air supplied to the first combustor being in excess of that required for complete combustion of the fuel,
a high pressure turbine intended to be driven by combustion products from the first combustor,
a second combustor having fuel injection means for burning further fuel together with exhaust gases of the high pressure turbine, thereby to consume the excess air,
a lower pressure turbine having at least a first turbine stage comprising a ring of nozzle guide vanes and a first stage of rotor blades intended to be driven by combustion products from the second combustor, the lower pressure turbine having components of at least the lower pressure turbine which are cooled by cooling air supplied from the compressor,
means for supplying the second combustor with at least a portion of the cooling air after it has passed through the lower pressure turbine components, and
means for supplying sufficient fuel to the second combustor to burn therein with the portion of cooling air, thereby to increase the first stage turbine rotor entry temperature relative to an otherwise similar engine in which the portion of cooling air is exhausted into the lower pressure turbine after it has passed through the lower pressure turbine components.
For a given exit temperature of the combustion gases from the second or reheat combustor, the invention reduces cooling air dilution of the turbine gases relative to the prior art, i.e., the efficiency of the turbine is increased. For example: compare a prior art engine, in which some of the cooling air passing through the nozzle guide vanes (NGV's) is exhausted to the turbine passage through film cooling holes, with an engine according to the present invention (but otherwise identical with the prior art), in which the same amount of cooling air is recycled to the second combustor instead of being exhausted to the turbine passage. Provided that enough fuel is supplied to be burnt with the cooling air in the second combustor to ensure the combustor exit temperature is at least maintained at the same value as in the prior art, the turbine rotor entry temperature will be increased relative to the prior art.
In an preferred embodiment of the invention, the gas turbine engine is provided with a heat exchanger arrangement for cooling the cooling air before it is supplied to the turbine components; thus, the heat exchanger arrangement may put the cooling air in heat exchange relationship with fuel, thereby to heat the fuel before injection of the fuel into the first and/or the second combustor.
The cooled turbine components may comprise stator blades in at least one stage of turbine blading and preferably include at least nozzle guide vanes constituting a first stage of the low pressure turbine.
The invention further provides a gas turbine engine operating cycle, in which fuel and excess air are burnt together in a first combustor, the combustion products are passed through a high pressure turbine, the exhaust of the high pressure turbine is then burnt together with further fuel in a second combustor to consume the excess air, and the exhaust of
Alstom (Switzerland) Ltd.
Casaregola Louis J.
Kirschstein, Ltd.
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