Expert: Pat March - 9/12/2007

Question
I am working on axial fan design- i want to design propeller fan which will suck heat generated by motor and through outside the approximately size will be 2" in diameter.
can you help me understand & how it affects in designing fan.
like hub tip ratio, no of blades, blade angle, inlet and outlet velocity triangle. etc.
is there any methodology for this?  

Answer
Hub-tip ratio is usually a function of specific speed. Take specific speed= rpm x (cubic feet per minute)squared/ (Head in feet of air)^.75. Hub ratio is then= 2000/specific speed plus or minus about 10 per cent.

Once you have hub ratio you can determine a mean diameter= ([Douter^2   Dhub^2]/2)^0.5. This is the diameter that divides the flow into equal inner and outer parts.

Use this diameter to determine the rotational velocity at this diameter. um = Dm(in feet) x rpm/229.  Then calculate the head coefficient Psi = gh/(um)^2. For "solid rotation" (recommended) Psi is constant at all radii.

This is getting difficult! Proper expression of math is impossible in ascii. Please E-mail me at insideman@worldnet.att.net So will be able to respond with a Word document that corrects this flaw.  

This only begins the design. Whole books been written on this topic.