T-stage

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:

${\displaystyle w_{2}=(w_{2}{\sqrt {T_{3}}}/P_{3}))*(P_{3}/P_{2})*(P_{2}/P_{1})*(P_{1}/{\sqrt {T_{1}}})/({\sqrt {T_{3}/T_{2}}}*{\sqrt {T_{2}/T_{1}}}),}$

where:

hp compressor entry mass flow = $w_{2}\,$

core size = ${\displaystyle (w_{2}{\sqrt {T_{3}}}/P_{3})\,}$

hp compressor total head pressure ratio = ${\displaystyle P_{3}/P_{2}\,}$

lp compressor total head pressure ratio = ${\displaystyle P_{2}/P_{1}\,}$

lp compressor entry total pressure = ${\displaystyle P_{1}\,}$

lp compressor entry total temperature = ${\displaystyle T_{1}\,}$

hp compressor total head temperature ratio = ${\displaystyle T_{3}/T_{2}\,}$

lp compressor total head temperature ratio = ${\displaystyle T_{2}/T_{1}\,}$ which varies more slowly than ${\displaystyle P_{2}/P_{1}\,}$

So as ${\displaystyle P_{2}/P_{1}\,}$ increases with the addition of T-stages, $w_{2}\,$ 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.

References

1. http://www.flightglobal.com/pdfarchive/view/1969/1969%20-%201658.html?search=%22thrust%20growth%22