Gas Turbine Cooling
The OEMs may represent the cooling in a diagram often referred to
as the worm chart. Figure 1 shows a schematic of the turbine and
the various amounts of flow as they are reintroduced to the gas path.
Actual representation of the cooling flows can be modeled using
a more sophisticated 3-D characterization of the flow as shown in
Effect of Gas Turbine Cooling
To demonstrate the effect of cooling flow on the performance of a
gas turbine typical of a simple cycle or combined cycle operation
CPS produced a theoretical model in a commercially available code
(Gas Turb) and changed cooling flows to see the impact on operating
point (the model uses a generic compressor and turbine map).
Gas Turb allows changing the amount of cooling flow and the energy
of cooling flow.
Cooling Flows Overview
As discussed, the main purposes of cooling flows are to maintain safe
vane and blade metal temperatures during gas turbine operation.
Cooling flows have a net positive result on gas turbine performance.
Since turbine cooling flows do not go through the combustor, some
of the work is lost. However, cooling flows allow for a higher firing
temperature which leads to a net higher gas turbine power output
and better efficiency.
Engine Operation Overview
At base-load, most gas turbine engines are operated on an exhaust
temperature control curve. A sample exhaust temperature control
curve is presented in Figure 4.
The base-load exhaust temperature control curve is usually
created by running the engine model through a range of ambient
temperatures at some given design firing temperature. Based on the
model output and site specific conditions, a control curve for base-load operation is created. The above control curve was created using
an F-frame model developed in Gas Turb.
The main disadvantage of the curve is that there is no
awareness of what the actual firing temperature of an engine is. So if
cooling flow increases, firing temperature increases as well in order
to maintain the same exhaust temperature output for a certain shell
pressure. On the other hand, if cooling flows decrease the exhaust
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Figure 1 – A “worm chart” showing cooling flows in the first two
stages of a gas turbine. (Courtesy VKI lecture series LS1999-02)
Figure 2 – The flow circuit
diagram (in this case
between the first nozzle
and the second stator).
Figure 3 – Computational fluid
dynamics (CFD) model of cooling
flow emerging on the surface of a
turbine blade and its influence by
the main stream flow. (Courtesy
of B&B-AGEMA GmbH)
Figure 4 – Sample