Turbofan:Alternative Designs

Alternative design for turbofans

All recent jet engines have the same basic architecture: a fan at the front, the turbine engine behind it, and their two flows mix at the exhaust, inside the engine for low bypass ratio engines and outside for high bypass ratio engines. Early designs of turbofans were actually created by adding a ducted fan on the aft part of existing turbojet engines. They were not so bad in terms of efficiency compared to front-mounting engines, but the main issue was that the fan was evolving in a quite hot gas flow, which would eventually corrode or fatigue it more than when it blows fresh air as in a front-mounted fan design.

A second fact is that high-efficiency engines, or modern engines, all use axial-type compressor and axial-type turbine. Some early or less power-requiring designs feature a centrifugal-type compressor, and only one engine to our knowledge had a centrifugal turbine, long ago.

From these two facts, we propose a novel design for turbofans, at least while research or people won't have proven it was wrong: an axial-compressor, hybrid-turbine, aft-mounted ducted fan. The hybrid turbine is a mix of axial and centrifugal designs, in which the hot gas flow would be slightly diverted from its course, while extracting some of its energy for shaft rotation work. The aft-fan would intake the mixed flow of the fresh intake and the turbine discharge, providing higher energy to the fan flow. Properly mixing the two flows would allow the fan to be build with metals supporting low temperatures, like 2000- or 7000- series aluminum alloys. Besides, the resulting design will inevitably be a longer engine.

Full transonic engine design in a single spool with 2.0 virtual BPR

We speak here of virtual BPR because since flows are mixed before the fan intake, there is no clear separation between flows of the fan and the engine's core. However, there is still an inlet area for the compressor and one for the fan, and the ratio between the two mass flow rates going into each is what we call the virtual bypass ratio (VBPR).

Drawings (schematics or 3D CAD models) are coming soon.