Axial fan design

For more details on this topic, see Mechanical fan.

An axial fan is a type of a compressor that increases the pressure of the air flowing through it. The blades of the axial flow fans force air to move parallel to the shaft about which the blades rotate. In other words, the flow is axially in and axially out, linearly, hence their name. The design priorities in an axial fan revolve around the design of the propeller that creates the pressure difference and hence the suction force that retains the flow across the fan. The main components that need to be studied in the designing of the propeller include the number of blades and the design of each blade. Their applications include propellers in aircraft, helicopters, hovercrafts, ships and hydrofoils. They are also used in wind tunnels and cooling towers. If the propeller is exercising propulsion, then efficiency is the only parameter of interest and other parameters like power required and flow rate are considered of no interest. In case the propeller is used as a fan, the parameters of interest includes power, flow rate, pressure rise and efficiency.[1]

An axial fan consists of much fewer blades i.e., two to six, as compared to ducted fans. Axial fans operate at high specific speed i.e., high flow rate and low head and hence adding more blades will restrict the high flow rate required for its operation. Due to fewer blades, they are unable to impose their geometry on the flow, making the rotor geometry and the inlet and outlet velocity triangles meaningless. Also the blades are made very long with varying blade sections along the radius.

Calculation of parameters

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.[1] There are two theories that solve the parameters for axial fans:[1]

Slipstream theory

Variation of Pressure and Velocity of Flow through a Propeller disc.[1]

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[1] [2] where:

Parameters at −∞ and +∞ and their relationship
Parameter Pressure Density Velocity Stagnation enthalpy Enthalpy
−∞ Pa ρa Cu (upstream velocity) hou hu
+∞ Pa ρa Cs (slipstream velocity) hod hd
Relationship Equal Equal Unequal Unequal Equal
Comments Pressure will be atmospheric at both −∞ and +∞ Density will be equal at both −∞ and +∞ Velocity will change due to flow across an assumed converging duct Stagnation enthalpy will be different at −∞ and +∞ The Enthalpy will be same at −∞ and +∞ as it depends upon the atmospheric conditions that will be the same

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.[1]

C_{\rm 1} = C_{\rm 2} = C and P_{\rm 1} \ne P_{\rm 2}
A = \frac {\pi D^2}{4}
\dot m = {\rho AC}
F_{\rm x} = {\dot m}{(C_{\rm s} - C_{\rm u})} = {\rho AC}{(C_{\rm s} - C_{\rm u})}
\begin{align} P_a + \frac{1}{2} {\rho C_u^2} &= P_1 + \frac{1}{2} {\rho C^2} \\ P_a + \frac{1}{2} {\rho C_s^2} &= P_2 + \frac{1}{2} {\rho C^2} \end{align}

On subtracting the above equations:[1]

P_2 - P_1 = \frac{1}{2} \rho (C_s^2 - C_u^2)
F_x = A (P_2 - P_1) = \frac{1}{2} \rho A \left(C_s^2 - C_u^2\right)

Comparing this thrust with the axial thrust due to change in momentum of air flow, it is found that:[1]

C = \frac{C_s + C_u}{2}

A parameter 'a' is defined such that[1] -

C = (1 + a) C_u where a = \frac{C}{C_u} - 1

Using the previous equation and "a", an expression for Cs comes out to be:

C_s = (1 + 2 a) C_u
\Delta h_o = \Delta h_od - \Delta h_ou = \left(h_d + \frac{1}{2} C_s^2\right) - \left(h_u + \frac{1}{2} C_u^2\right) = \frac{1}{2} \left(C_s^2 - C_u^2\right)

Now, Ideal Value of Power supplied to the Propeller = Mass flow rate * Change in Stagnation enthalpy;[1]

P_i = \dot m {\Delta h_o} where \dot m = \rho AC

If propeller was employed to propel an aircraft at speed = Cu; then Useful Power = Axial Thrust * Speed of Aircraft;[1]

P = F_x C_u
\eta_p = \frac{\text{Actual Power}(P)}{\text{Ideal Power}(P_i)} = \frac{F_x C_u}{\frac{1}{2} \rho AC \left(C_s^2 - C_u^2\right)} = \frac{C_u}{C} = \frac{1}{1 + a}

Therefore;

D_s^2 = \left(\frac{1 + a}{1 + 2a}\right) D^2

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.[1]

Blade element theory

Long Blade of a Propeller Fan with varying Blade Section.[1]

In this theory, a small element (dr) is taken at a distance r from the root of the blade and all the forces acting on the element are analysed to get a solution. It is assumed that the flow through each section of small radial thickness dr is assumed to be independent of the flow through other elements.[1][3]

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. ΔL = Lift Force (Perpendicular to w) and ΔD = Drag Force (Parallel to 'w'). The Axial and Tangential forces are ΔFx and ΔFy respectively and the Resultant force ΔFr is at an angle Φ to the Lift.[1]

Resolving Forces in the figure[1] -

\Delta F_x = \Delta L \sin(\beta) - \Delta D \cos(\beta)
\Delta F_y = \Delta L \cos(\beta) + \Delta D \sin(\beta)

Lift Coefficient (CL) and Drag Coefficient (CD) are given as -

\mathrm{Lift}(\Delta L) = \frac {1}{2} C_L \rho w^2 (ldr)
\mathrm{Drag}(\Delta D) = \frac {1}{2} C_D \rho w^2 (ldr)

Also from the figure [1]-

\tan(\phi) = \frac {\Delta D}{\Delta L} = \frac {C_D}{C_L}

Now,

\Delta F_x = \Delta L(\cos\phi - \frac{\Delta D}{\Delta L} \sin \phi) = \Delta L(\cos\phi - \tan\phi \sin \phi) = \frac{1}{2} C_L \rho w^2 ldr \frac{\sin(\beta - \phi)}{\cos\phi}

No. of Blades (z) and Spacing (s) are related as,[1] s = \frac {2 \pi r}{z} and the total thrust for the elemental section of the propeller is zΔFx.

Therefore,[1]

\Delta p (2 \pi rdr)= z\Delta F_x
\Rightarrow \Delta p = \frac {1}{2} C_L \rho w^2 (\frac{l}{s}) \frac {\sin(\beta - \phi)}{\cos\phi} = \frac {1}{2} C_D \rho w^2 (\frac{l}{s}) \frac {\sin(\beta - \phi)}{\sin\phi}

Similarly, solving for ΔFy, ΔFy is found out to be[1] -

\Delta F_y = \frac{1}{2} C_L \rho w^2 ldr \frac{\cos(\beta - \phi)}{\cos\phi}

and (\mathrm{Torque})\Delta Q = r \Delta F_y

Finally, thrust and torque can be found out for an elemental section as they are proportional to Fx and Fy respectively.[1]

Performance characteristics

This Figure shows the Performance Curve for Axial Flow Fan.[1]

The relationship between the pressure variation and the volume flow rate are important characteristics of fans. The typical characteristics of axial fans can be studied from the performance curves. The performance curve for the axial fan is shown in the figure. (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 -

  1. As the flow rate increases from zero the efficiency increases to a particular point reaches maximum value and then decreases.
  2. The power output of the fans increases with almost constant positive slope.
  3. The pressure fluctuations are observed at low discharges and at flow rates(as indicated by the point "S" ) the pressure deceases.
  4. The pressure variations to the left of the point "S" causes for unsteady flow which are due to the two effects of Stalling and surging.

Causes of unstable flow

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.

Stalling effect/Stall

The cause for this is the separation of the flow from the blade surfaces. This effect can be explained by the flow over an air foil. When the angle of incidence increases (during the low velocity flow) at the entrance of the air foil, flow pattern changes and separation occurs. This is the first stage of stalling and through this separation point the flow separates leading to the formation of vortices, back flow in the separated region. For further the explanation of stall, rotating stall refer to compressor surge. The stall zone for the single axial fan and axial fans operated in parallel are shown in the figure.[4]

The Figure shows the Stall Prone Areas differently for One fan and Two fans in parallel.[4]

The following can be inferred from the graph :

VFDs are not practical for some Axial fans

Many Axial fan failures have happened after controlled blade axial fans were locked in a fixed position and Variable Frequency Drives (VFDs) were installed. The VFDs are not practical for some Axial fans. Axial fans with severe instability regions should not be operated at blades angles, rotational speeds, mass flow rates, and pressures that expose the fan to stall conditions.[5]

Surging effect/Surge

Surging should not be confused with stalling. Stalling occurs only if there is insufficient air entering into the fan blades causing separation of flow on the blade surface. Surging or the Unstable flow causing complete breakdown in fans is mainly contributed by the three factors

System surge

This situation occurs when the system resistance curve and static pressure curve of the fan intersect have similar slope or parallel to each other. Rather than intersecting at a definite point the curves intersect over certain region reporting system surge. These characteristics are not observed in axial fans.

Fan surge

This unstable operation results from the development of pressure gradients in the opposite direction of the flow. Maximum pressure is observed at the discharge of the impeller blade and minimum pressure on the side opposite to the discharge side. When the impeller blades are not rotating these adverse pressure gradients pump the flow in the direction opposite to the direction of the fan. The result is the oscillation of the fan blades creating vibrations and hence noise.[6]

Paralleling

This effect is seen only in case of multiple fans. 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. To avoid beating use is made of differing inlet conditions, differences in rotational speeds of the fans, etc.

Methods to avoid unsteady flow

By designing the fan blades with proper hub-to-tip ratio and analyzing performance on the number of blades so that the flow doesn't separate on the blade surface these effects can be reduced. 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. For controlling and directing the flow use of guide vanes is suggested. Turbulent flows at the inlet and outlet of the fans cause stalling so the flow should be made laminar to prevent the effect.

Notes

  1. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 S. M. Yahya (10 October 2010). "14". Turbines Compressors And Fans (4th Edition) (4th ed.). McGraw-Hill Education (India) Pvt Limited. pp. 622–9. ISBN 978-0-07-070702-3. Retrieved 13 May 2013.
  2. ↑ POOLE, R (1 Jan 1935). "THE THEORY AND DESIGN OF PROPELLER-TYPE FANS." (PDF). ICE Selected Engineering Papers 1 (178). doi:10.1680/isenp.1935.13442. Retrieved 2013-05-23.
  3. ↑ Marble, Frank E. (1948). "The Flow of a Perfect Fluid Through an Axial Turbomachine with Prescribed Blade Loading". Journal of the Aeronautical Sciences (Institute of the Aeronautical Sciences) 15 (8): 473–485. doi:10.2514/8.11624. Retrieved 2013-05-23.
  4. 1 2 "Stall,problems and solutions" (PDF). Retrieved 2013-05-10.
  5. ↑ https://www1.eere.energy.gov/manufacturing/tech_assistance/pdfs/fan_sourcebook.pdf U.S. Dept. of Energy, Improving Fan System Performance, Page 35 (39/92), Last paragraph
  6. ↑ "System Surge, Fan Surge and Paralleling" (PDF). Retrieved 2013-05-12.

References

See also

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