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Recirculation Margin and Engine Performance
Inlet Recirculation
In order to prevent recirculation, it was shown that FVABI control is necessary to accelerate the fan flow to reduce the static pressure, thereupon, there was no sign of fbw reversal towards MSV, as it is gradually opened. To estimate how the opening area of the F/R-VABI affects the ft>w pattern, especially recirculation, CFD calculation was employed to simulate such a condition that the opening areas of F/R-VABI are the same as those in CASE1, whilst the inlet and outlet thermo-cycle parameters are the same as those in CASE5. The
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Figure 5. Mach number (Left) and static pressure (Right) contours |
fbw pattern thus obtained is shown in the upper part of Figure 6. It is apparent that the recirculation does occur unless FBAVI is narrowed. The fan bypass air pressure becomes much higher than the total pressure of the air at MSV inlet. If the static pressure in the bypass duct be not reduced to yield negative gradient along the ram inlet duct, there occurs a back few towards MSV as it opens gradually, therefore, the present control sequence is necessary.
