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:
- where:
- core mass flow =
- core size =
- core total head pressure ratio =
- inverse of core total head temperature ratio =
i.e. f(
)
- core entry total pressure =
- core entry total temperature =
- 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.
