Gas Turbine Handbook : Principles and Practices

Detectable Problems 189
Note: Accurate airflow measurement requires a calibrated
bellmouth at the inlet of the gas generator supported by
temperature readings at this location. A bellmouth, when provided,
is calibrated at the factory, usually with the gas generator.
Since calibrated bellmouths are not normally provided,
airflow measurements are rarely taken in the field. However,
relative changes in airflow can be obtained by instrumenting
the inlet bellmouth and establishing a baseline. Therefore,
changes in airflow are indicated in this section in the hope
that this parameter may eventually be utilized to evaluate
engine health.
turbine efficiency and an increase in N 2
stall margin. More readily
seen parameter changes (at constant power) are increases in compressor
pressure ratio of the low pressure compressor (P 3
/P 2
), exhaust gas
temperature, and fuel flow, and decreases in compressor pressure
ratio of the high pressure compressor (P 4
/P 2
), compressor discharge
temperature, and N 2
rotor speed (Figure 12-2). Decreases in N 1
rotor
speed and air flow also occur, but these changes are not easily detectable,
especially with small changes in nozzle area.
Considering the low pressure turbine on dual-spool machines,
turbine blade and vane erosion result in a decrease in turbine efficiency
and an increase in N 1
stall margin. More readily seen parameter
changes (at constant power) are increases in N 2
rotor speed,
fuel flow, exhaust gas temperature, and high pressure compressor
discharge temperature (T 4
), and decreases in N 1
rotor speed, low
pressure compressor discharge pressure ratio (P 3
/P 2
), and low pressure
compressor discharge temperature (T 3
), as shown in Figure 12-3.
Although a decrease in airflow also occurs, the change is not easily
detectable.
To improve gas turbine package performance and increase overall
power output, manufacturers have continually increased turbine
inlet temperatures. While much of this increase has resulted from
improvements in turbine materials, the implementation of turbine
cooling has been a major contributing factor.
Turbine blade and nozzle cooling is accomplished by routing
relatively cool air from the gas turbine-compressor back to the hot
turbine components. This relatively cool (400°F-600°F) air is ported