Actual air flow in the impeller is not uniform in each section but is fast at the backside of the blades and is slow at the front side of the blades.
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Note : "front" and "back" are the words concerning rotating direction.
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So, the pressure at the front side of blades is higher than those at the back sides.
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When average air speed in the impeller is much higher, that is, the air volume through the impeller is much larger, the air flow is normal with small reverse flow area.