T-stage

T-stages are the small compressor stages, featured on some turbofan engines, that are mounted on the LP shaft directly behind the fan.

Although T-stages usually only supercharge the core stream, some engines do feature a deliberately oversized IP compressor, which compresses both the core flow and a proportion of the bypass flow. This enhances the stability of the T-stages during throttling. Where necessary, the alternative is to employ blow-off valves.

T-stages are used to increase overall pressure ratio and, for a given core size, the core mass flow. This is demonstrated by the following relationship:

:$w\_2\; =\; (w\_2\; sqrt\{T\_3\}/P\_3)\; *\; (P\_3/P\_2)\; *\; (sqrt\{T\_2/T\_3\})\; *\; (P\_2/sqrt\{T\_2\})\; ,$

:where:

:core mass flow = $w\_2\; ,$

:core size =$(w\_2\; sqrt\{T\_3\}/P\_3)\; ,$

:core total head pressure ratio = $(P\_3/P\_2)\; ,$ :inverse of core total head temperature ratio = $T\_2/T\_3\; ,$ i.e. f($P\_3/P\_2\; ,$)

:core entry total pressure = $P\_2\; ,$

:core entry total temperature = $T\_2\; ,$

:Note::in this case, the core refers to the whole of the gas generator compression, not just that of the HP compressor.

T-stages are a popular method for uprating the thrust of an engine (see, for example the Pratt & Whitney Canada PW500).

The alternative is to place a zero-stage, mounted on the HP shaft, at the front of the HP compressor. This approach requires a significant change in the HP turbine, whereas a T-stage can, if necessary, be accommodated by simply adding an additional stage to the rear of the LP turbine.