Fan efficiency: curved versus straight blades
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Davies Craig product story
Edited by the Engineeringtalk editorial team
Dec 7, 2005
The relative merits and technology of curved versus straight blades has been debated for some time.
Davies Craig has conducted an extensive research programme to formulate the design of fans in which several design types including curved blades with and without a ring were considered.
Curved blades generated less noise but at the expense of a significant loss in performance caused by the fact that curved blades stall at a lower static pressure than straight blades.
And if a fan moves less air, then you would expect less noise anyhow.
Noise is generated mainly from a blade passing a shroud strut.
The noise level is higher when the whole length of a blade overlaps/passes a shroud strut and if all the blades pass all the struts of a shroud at the same time, noise is at its worst.
Some manufacturers have chosen to curve the blades and keep the struts straight.
As this approach significantly affects performance, Davies, Craig has proved it is best to adopt the reverse approach of having straight blades and curving the struts.
The investigations also found that when air flowed in layers (laminar flow), it breaks up into counter rotating airflow vortices.
These vortices consume energy and therefore reduce the airflow.
It was found that by inserting a triangular protrusions near the leading edge of the blade, the air is tripped into going into vortices earlier but they were much smaller and removed much less energy therefore making the blade more efficient.
The idea of a ring around the blade tips was also researched but rejected as the performance suffered due to the increased energy required to rotate a blade with so much weight at the tips.
The purpose of a ring is to reduce losses from air passing from the high-pressure side of the blade to the low-pressure side as shown.
However, Davies, Craig achieved this by adding winglets to the blade tips without the energy loss of a ring.
When all aspects of blade design are considered, straight blades with trips, winglets and the right shroud is by far the best combination.
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