Axial fan design

Factors which determine the performance of the fan include the number and shape of the blades.

Design parameters include power, flow rate, pressure rise and efficiency.

[1] There are two theories that solve the parameters for axial fans:[1] In the figure, the thickness of the propeller disc is assumed to be negligible.

[1] On subtracting the above equations:[1] Comparing this thrust with the axial thrust due to change in momentum of air flow, it is found that:[1] A parameter 'a' is defined such that[1] - Using the previous equation and "a", an expression for Cs comes out to be: Now, Ideal Value of Power supplied to the Propeller = Mass flow rate * Change in Stagnation enthalpy;[1] If propeller was employed to propel an aircraft at speed = Cu; then Useful Power = Axial Thrust * Speed of Aircraft;[1] Therefore; Hence the flow can be modeled where the air flows through an imaginary diverging duct, where diameter of propeller disc and diameter of the outlet are related.

Finally, thrust and torque can be found out for an elemental section as they are proportional to Fx and Fy respectively.

[1] The relationship between the pressure variation and the volume flow rate are important characteristics of fans.

(The vertical line joining the maximum efficiency point is drawn which meets the Pressure curve at point "S")[1] The following can be inferred from the curve - Stalling and surging affects the fan performance, blades, as well as output and are thus undesirable.

They occur because of the improper design, fan physical properties and are generally accompanied by noise generation.

Surging or the Unstable flow causing complete breakdown in fans is mainly contributed by the three factors This situation occurs when the system resistance curve and static pressure curve of the fan intersect have similar slope or parallel to each other.

This unstable operation results from the development of pressure gradients in the opposite direction of the flow.

The air flow capacities of the fans are compared and connected in same outlet or same inlet conditions.

This causes noise, specifically referred to as Beating in case of fans in parallel.

Some of the methods to overcome these effects are re-circulation of excess air through the fan, axial fans are high specific speed devices operating them at high efficiency and to minimize the effects they have to be operated at low speeds.

Variation of Pressure and Velocity of Flow through a Propeller disc. [ 1 ]
Long Blade of a Propeller Fan with varying Blade Section. [ 1 ]
The figure shows the Velocities and Blade Forces for the Flow across the element dr , where w is the Mean Velocity at a direction β from Axial direction. Δ L = Lift Force (Perpendicular to w ) and Δ D = Drag Force (Parallel to 'w'). The Axial and Tangential forces are Δ Fx and Δ Fy respectively and the Resultant force Δ Fr is at an angle Φ to the Lift. [ 1 ]
This Figure shows the Performance Curve for Axial Flow Fan. [ 1 ]
The Figure shows the Stall Prone Areas differently for One fan and Two fans in parallel. [ 4 ]