We will approach the off-design analysis problem of separate-flow turbofan engines the same way we did in the turbojet problem, that is, we assume the first nozzle choking stations in the turbine as well as the exhaust nozzle throat in off-design engine operation. Then we set the mass flow rates between the choked stations equal to each other and establish the constants of gas turbine engine operation.
Figure 11.12 shows a definition sketch of a two-spool separate-flow turbofan engine. The fan is driven by the low-pressure turbine (LPT), and the high-pressure compressor (HPC) is driven by the high-pressure turbine (HPT). Again, we assume that the first turbine nozzles of the HPT and LPT are choked at design point and remain choked in off-design operation, that is,
The second assumption deals with the exhaust nozzle throats, that is,
The known design parameters for this turbofan engine are
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