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:

$w_2 = (w_2 \sqrt{T_3}/P_3) )* (P_3/P_2)* (P_2/P_1)*( P_1/ \sqrt{T_1})/ (\sqrt{T_3/T_2} * \sqrt{T_2/T_1} ),$

where:

hp compressor entry mass flow = $w_2 \,$

core size = $(w_2 \sqrt{T_3}/P_3) \,$

hp compressor total head pressure ratio = $P_3/P_2 \,$

lp compressor total head pressure ratio = $P_2/P_1 \,$

lp compressor entry total pressure = $P_1 \,$

lp compressor entry total temperature = $T_1 \,$

hp compressor total head temperature ratio = $T_3/T_2 \,$

lp compressor total head temperature ratio = $T_2/T_1 \,$ which varies more slowly than $P_2/P_1 \,$

So as $P_2/P_1 \,$ 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