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Fan picture

Image source

I understand part of the reasoning for minimal clearance between the casing and the fan blades, is to have the larger fan blade possible, which generates a larger low pressure area to increase air intake.

However I don't understand why, if leaking occurred, it would reduce the overall efficiency.

If it was to leak, it's still going through the intake to the compressor or bypass duct.

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  • $\begingroup$ "If it was to leak, it's still going through the intake to the compressor" this sentence is not clear and seems contradictory, unless you define "leak" (the fan wants air to go to the low pressure compressor, leak is supposed to mean air going from behind the fan back to the intake where it comes from). Perhaps you mean air entering the engine by bypassing the fan (this would not be called leak, and would be welcome, but this is likely not possible, as @Federico explained). $\endgroup$ – mins May 13 '17 at 18:57
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If it was to leak, it's still going through the intake to the compressor or bypass duct.

This is incorrect. A "leak" would cause the compressed air to circle around the fan and exit again from the front, reducing the efficiency of the fan, because part of the power would be used to compress air that does not do any useful work in the engine.

The circling around is a consequence of the work the fan itself does: after the fan the air is at a higher pressure than the air in front of the fan, this means that the air outside is pushed back by the air inside, and if there is a leak, the higher pressure air (inside) leaks out from the front.

Minimizing this "leak" helps maximising the efficiency of the fan, and of the engine as a consequence.

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