T-stage

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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.