If the fan air is ducted to the rear, the total fan pressure must be higher than the static gas pressure in the primary engine's exhaust, or air will not flow. By the same token, the static fan discharge pressure must be less than the total pressure in the primary engine's exhaust, or the turbine will not be able to extract the energy required to drive the compressor and fan.
Source: Aircraft Gas Turbine Technology by IRWINE TREAGER, Chapter 2, Section: Turbofan Engine
I am not able to understand:
- Why total pressure of fan is being compared with static pressure of engine?
- Is bypassed air which is ducted to rear is involved in this operation? And how the pressure difference is necessary to prevent backflow when both sections do not meet at all?
- How turbine energy extraction depends on static fan discharge pressure? And is the discharge pressure involves ducted air pressure?