Design and Development
The JT15D is rare among modern turbofans in that it uses a centrifugal compressor as its main high-pressure system. This was a common feature of early jet engines, but was quickly replaced by axial compressors in most roles due to its large frontal size. In the turbofan role most of the jet thrust is generated by the cold air blown past the engine, and the internal "jet" portion is quite small. In this role the high single-stage compression of the centrifugal design has advantages, and the main reason most small turbofans don't use them is that they are often developments of previous turbojet designs.
In the JT15D the fan blows about 70% of the air into the bypass duct, producing most of the overall thrust. On JT15D-4 models and above there is a small "booster" axial stage just behind the fan which is running at the same speed as the fan and directing the remaining 30% of the airflow into the engine core. This air is further compressed by the centrifugal stage, and burned in a reverse-flow annular combustor. The hot gases flow through a "high-pressure" turbine that drives the centrifugal stage, and then two more turbines driving the fan and booster.
Read more about this topic: Pratt & Whitney Canada JT15D
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