Axial fan design and its features

An axial fan is a fan that causes gas to flow through it in an axial direction,parallel to the axis of rotation of the blades.Flow is axial at the inlet and outlet.Fans are designed to create a pressure differential which creates a force to induce flow through the fan.Factors that determine fan performance include the number and shape of the blades.Fans have many applications, including wind tunnels and cooling towers.Design parameters include power,flow,pressure rise and efficiency.Axial fans typically contain fewer blades (2 to 6) than ducted fans.Axial fans generally have a larger radius and lower speed (ω) than ducted fans (especially at similar power.Stress is proportional to r^2).

Features: 

1.The relationship between pressure change and volume flow is an important characteristic of fans.Typical characteristics of axial flow fans can be studied from the performance curves.The performance curve of the axial flow fan is shown in the figure.(The vertical line connecting the points of maximum efficiency intersects the pressure curve at point "S").From the curve the following can be deduced.

As the flow rate increases from zero,the efficiency increases to a certain point where it reaches a maximum value and then decreases.

2.Fan output power increases with an almost constant positive slope.

3.Pressure fluctuations are observed at low discharge and flow rates (as indicated by point “S”) when pressure drops.

4.Pressure changes to the left of point "S" lead to unstable flow due to the two effects of stall and surge.

Causes of unstable flow

Stall and surge affect fan performance,blades,and output and are therefore undesirable.They occur due to poor design,improper fan physics,and are often accompanied by noise production.

Stall effect/stall:

The reason for this is the separation of the airflow from the surface of the blade.This effect can be explained by the flow over the airfoil. When the angle of incidence at the inlet of the airfoil increases (during low velocity flow),the flow pattern changes and separation occurs.This is the first stage of stall,through which point of separation flow separation leads to the formation of vortices,backflow in the separation region.See Compressor Surge for further explanation of stall and rotating stall.The stall area of a single axial flow fan and a parallel axial flow fan.

The following can be inferred from the figure:

• For fans operating in parallel, the performance is lower compared to a single fan.

• Fans should be operated in a safe operating area to avoid stall effects.

• VFD not practical for some axial fans.

• Many axial fan failures have occurred after locking controlled blade axial fans in a fixed position and installing variable frequency drives (VFDs).VFDs are not practical for some axial fans.Axial fans with severely unstable regions should not be operated at blade angle,speed,mass flow and pressure,which would put the fan in a stalled condition.

Surge effect/surge:

• Surge should not be confused with stall. A stall occurs only when insufficient air enters the fan blades causing the airflow at the blade surfaces to separate.Surge or unstable airflow that can lead to complete fan failure is mainly caused by these three factors

• System surge.

• Fan surge.

• Parallel.

System surge:

This occurs when the system resistance curve intersects the fan static pressure curve with similar slopes or is parallel to each other. The curves intersect not at defined points,but in specific areas where systemic surges are reported.These characteristics are not observed in axial fans.

Fan surge:

This unstable operation is caused by the development of a pressure gradient opposite the flow direction.The maximum pressure is observed at the outlet of the impeller blades and the minimum pressure is observed on the side opposite to the outlet side.When the impeller blades are not rotating, these unfavorable pressure gradients pump airflow in the opposite direction of the fan.The result is that the flapping of the fan blades vibrates, which creates noise.

Parallel:

This effect only occurs with multiple fans.The air volume of the fans is compared and connected under the same outlet or the same inlet conditions.This creates noise,especially in the case of paralleled fans called Beating.To avoid bounce,use different inlet conditions,differences in fan speed,etc.