T-stages, sometimes called booster stages, are mounted on the low pressure (LP) shaft of some turbofan engines directly behind the fan.[1]
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:
- hp compressor entry mass flow =
- core size =
- hp compressor total head pressure ratio =
- lp compressor total head pressure ratio =
- lp compressor entry total pressure =
- lp compressor entry total temperature =
- hp compressor total head temperature ratio =
- lp compressor total head temperature ratio = which varies more slowly than
So as increases with the addition of T-stages, also increases.
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 another stage to the rear of the LP turbine.
Although T-stages usually only supercharge the core stream, some engines do feature a deliberately oversized intermediate pressure (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.