Bypass duct

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A bypass duct is an annular passage that allows some of a turbofan's airflow to bypass the engine core, or gas generator. If the turbofan is unmixed, the bypass duct will terminate at the bypass nozzle. In mixed turbofans, the bypass duct leads to the cold chute of a forced mixer or cold mixer of an annular mixer.

Some bypass ducts are unlined, so the bypass air must pass over a series of flanges, pipes, etc associated with the engine core, which increases the bypass duct pressure loss. A lined bypass duct has a lower pressure loss, due to the streamlining effect of the inner wall introduced.