Axial fan design

An axial fan is a type of fan that causes gas to flow through it in an axial direction, parallel to the shaft about which the blades rotate. The flow is axial at entry and exit. The fan is designed to produce a pressure difference, and hence force, to cause a flow through the fan. Factors which determine the performance of the fan include the number and shape of the blades. Fans have many applications including in wind tunnels and cooling towers. Design parameters include power, flow rate, pressure rise and efficiency. Axial fans generally comprise fewer blades (two to six) than centrifugal fans. Axial fans commonly have larger radius and lower speed (ω) than ducted fans (esp. at similar power. Stress proportional to r^2). Since the calculation cannot be done using the inlet and outlet velocity triangles, which is not the case in other turbomachines, calculation is done by considering a mean velocity triangle for flow only through an infinitesimal blade element. The blade is divided into many small elements and various parameters are determined separately for each element. There are two theories that solve the parameters for axial fans: Slipstream Theory Blade Element Theory In the figure, the thickness of the propeller disc is assumed to be negligible. The boundary between the fluid in motion and fluid at rest is shown. Therefore, the flow is assumed to be taking place in an imaginary converging duct where: D = Diameter of the Propeller Disc. Ds = Diameter at the Exit. In the figure, across the propeller disc, velocities (C1 and C2) cannot change abruptly across the propeller disc as that will create a shockwave but the fan creates the pressure difference across the propeller disc. and The area of the propeller disc of diameter D is: The mass flow rate across the propeller is: Since thrust is change in mass multiplied by the velocity of the mass flow i.e., change in momentum, the axial thrust on the propeller disc due to change in momentum of air, which is: Applying Bernoulli's principle upstream and downstream: On subtracting the above equations: Thrust difference due to pressure difference is projected area multiplied by the pressure difference.