Axial flow fan design
sjvdspuy@sun.ac.za
Today
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Background
Dimensionless numbers
Hub-to-tip ratio
Velocity diagrams
Free vortex velocity distribution
Blade element theory
Testing of axial flow fans
The majority of this presentation comes from:
• Dixon & Hall (2010)
• Wilkinson (2017)
• Bruneau (1994)
Turbomachinery
Axial flow fans
(incompressible)
Axial flow cooling fans
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Axial flow fans are used to convey
air for HVAC, ventilation or cooling.
Howden.com
Axial flow fans
Matimba power plant air-cooled condenser (ACC).
– 288, 9.1 m diameter, axial flow
fans
– installed 45 m above ground level
– Eskom currently building two new
4800 MW direct dry-cooled
plants, Medupi and Kusile.
Axial flow fans
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Can have IGVs and or outlet stator
High flow, low pressure
4 to 8 blades
Hub-tip ratio of 0.1 to 0.6
Flow axial in meridional plane
Inlet area = outlet area
Axial flow fans
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Significant dimensionless numbers, derived using
Buckingham PI – theorem.
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For incompressible flow:
Axial flow fans
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Significant dimensionless numbers, derived using
Buckingham PI – theorem.
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Power coefficient
P
P=
ρN 3 D 5
Flow coefficient
Q
φ=
ND 3
Pressure coefficient
ψ=
Reynolds number
ρND 2
Re =
μ
gH
N 2D2
Use fluid density
to calculate
Incompressible flow machines
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The dimensionless numbers are applicable to:
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A specific machine (scale the effect of speed and density).
And for a geometric similar one.
The requirements for scale models tests are:
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Geometric similarity (lengths).
Kinematic similarity (velocity).
Dynamic similarity (forces).
Dimensional Analysis
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Efficiency:
ρgHQ
η=
P
ρN 3 D 5
Q
gH
=
×
× 2 2
3
P
ND N D
φψ
=
P
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Since Φ, ψ and P are dimensionless, efficiency is also
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dimensionless.
The same for similar machines
Dimensional Analysis
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Specific speed:
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Start with:
φ=
Q
ND 3
ψ=
gH
N 2D2
Eliminate D. We then get specific speed:
radians
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Specific speed enables us to arrange similar machines in
terms of speed - , flow – and pressure capacity. If we
therefore have the pressure, flow rate and speed, we can get
an indication of machine type and efficiency.
Dimensional Analysis
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Specific speed (Csanady, 1964)
Dimensional Analysis
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Specific diameter:
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Eliminate N. We get specific diameter:
Dimensional Analysis
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The Cordier diagram (1953) is widely used:
Dimensional Analysis
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The Lewis diagram (1996) is more explanatory:
Axial flow cooling fan
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Typical design specification
Determined by
cooling requirements
of heat exchanger
installation
Determined by
efficiency, structural
and noise limitations
on fan installation
Determined by
location of heat
exchanger
installation
Axial flow cooling fan
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Typical fan curves
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Fan static pressure
Axial flow cooling fan
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Typical fan curves
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Fan shaft power
Axial flow cooling fan
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Typical fan curves
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Fan static efficiency
Axial flow cooling fan
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Consider the use of the term fan “static pressure”
There are two terms used for fans, namely fan total pressure and fan
static pressure:
Fan static pressure is actually fan total-to-static pressure. It is
typically used in installations where the outlet kinetic energy of the fan
is dissipated. Typical for ACCs where outlet air is dissipated.
Fan total pressure is actually fan total-to-total pressure and is used
where the outlet kinetic energy of the fan plays a significant role in the
system design. Typical for ducted fans where outlet energy can be
recovered.
Note that efficiency is defined in the same manner
Axial flow fan
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Velocity triangles
IGVs can lead to excessive inlet losses but does give
a well conditioned outlet flow.
Note the condition
of the inlet and
outlet flow
Axial flow fan
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Velocity triangles for rotor-only fan
Axial flow fan
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Design process:
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The most important part is to
start right to finish right.
The first step is to specify the
hub-to-tip ratio correctly.
Hub-to-tip ratio selected
based on the method of Van
Niekerk (1958).
Axial flow fan
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Hub-to-tip ratio
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Based on the work of Van Niekerk (1958) with modifications by
Bruneau (1994)
Calculates total-to-total and total-to-static efficiencies for different
hub-to-tip ratios at design point
The derivation of this equation is shown in the work of Bruneau
(1994).
Isolated foil theory is assumed, which requires the blade solidity to
be less than 0.7. If this is not the case, cascade theory has to be
considered.
Axial flow fan
Tip flow coefficient
Hub swirl factor
Hub-to-tip ratio
Axial flow fan
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The following information is required in order to perform
the calculation:
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Range of hub-to-tip ratios to be used, in this case 0.1 to 0.9
Design flow rate
Design fan total-to-static pressure
Tip radius
Fan rotational speed
Estimate of fan annulus efficiency
Estimate of aerofoil section drag to lift ratio
Axial flow fan
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The calculation assumes a free vortex velocity profile
downstream of the fan. This means that:
Assuming zero inlet swirl
A first approximation for k is obtained by assuming ts efficiency of 60% and
assuming that the static pressure rise at the average radius is equal to the
fan static pressure.
Axial flow fan
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Fan annulus efficiency:
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Spanwise average airfoil lift-to-drag profile
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Simultaneously the hub swirl factor is limited to prevent
backflow at the hub:
Axial flow fan
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Vortex design
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The vortex design is by designing on an outlet swirl velocity distribution and
then applying simple radial equilibrium in order to determine the axial
velocity distribution. The SREE is basically a balance of the radial forces
due to curvature of the streamlines, with zero radial flow.
Axial flow fan
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Vortex design (repeated from Dixon & Hall)
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Stagnation enthalpy can be written as:
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From thermodynamics
Axial flow fan
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Vortex design (repeated from Dixon & Hall)
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The SREE now becomes:
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Assuming that the stagnation enthalpy and entropy stays constant w. radius
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Assuming incompressible flow
Axial flow fan
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Vortex design (repeated from Dixon & Hall)
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Combining the previous equation and the SREE:
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Assuming constant total pressure distribution, we again get the previous
equation:
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This last equation gives us the ability to derive an axial velocity distribution
of a swirl distribution is given.
Axial flow fan
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Vortex design
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It should be apparent that the vortex design is multidimensional. The decision for swirl distribution is affected by
many factors.
The free- vortex distribution gives close to the optimum
efficiency.
The free vortex distribution might however result in
excessive blade twist.
At this stage it is required to sub-divide the blade span into a
set number of radial sections.
Axial flow fan
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Example
Axial flow fan
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Blading design
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Once the vortex distribution is fixed, the blading design can be completed.
The vortex design gives the required velocity triangles downstream of the
blade, at the different radial locations.
It is advisable to select a base profile for the design at this stage. This can
however be omitted and made part of the design question.
Isolated airfoil theory is used.
Blade profile properties (at the different radii) assume the average flow
conditions to prevail over the blade.
Axial flow fan
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Blading design
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The blade loading factor is derived from Dixon by assuming the drag
contribution to be negligible:
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The loading factor therefore gives a balance between the chord length and
blade section lift coefficient required to satisfy the velocity requirements.
Five factors have to be considered:
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Efficiency – higher speeds and chords lead to higher Re number, giving
higher resistance against stall. Centrifugal load should however be
considered.
Noise – higher solidity means possible lower running speed, less blades
lead to less noise sources.
Manufacture - shorter chords lead to easier manufacture.
Stability – extreme blade setting angles, close to stall should be avoided.
Cost – less blades
Axial flow fan
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Blading design
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The design solution is therefore to have the minimum number of blades
(cost) at the highest possible speed (efficiency) with the largest
manufacturable chord (manufacturing) far from stall (stability) at the
minimum possible noise. The last requirements is often ignored.
The chord distribution is therefore selected to be in the order ~1 m. The
number of blades is then manipulated to give Lift coefficient values between
0.5 and 1.0.
The fan efficiency is then estimated from:
Axial flow fan
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Blading design
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Blade sectional properties can be obtained from published data or XFOIL:
Axial flow fan
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Blading design
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For the efficiency estimation, the losses are given as:
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The drag coefficients are estimated from airfoil profiles that correlate to the
calculated lift coefficient. The blade stagger angle is finaly defined from the
sectional lift curve, as:
Axial flow fan
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Typical results
Axial flow fan testing
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Testing are performed using standardised airways.
According to ISO 5801 part a, there are four standard
test configurations.
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Type A: free inlet-outlet
Type B: Free inlet, ducted outlet
Type C: Ducted inlet, free outlet
Type D: Ducted inlet, ducted outlet
The test configuration selected based on the eventual
system configuration.
Axial flow fan testing
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For large diameter cooling fans, we use a Type A test
facility:
Axial flow fan testing
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Fan is tested to scale at the same tip speed to maintain
kinematic similarity.
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The flowrate is calculated as:
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The fan static pressure is calculated as:
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Fan shaft power
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and static efficiency is calculated as:
Axial flow fan testing
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Test results are scaled to standard speeds and density.
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Scaling is generally accurate if in fully turbulent region
Axial flow fan testing
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Scaled fan installed.
Attention is paid to repeatable accurate setting of the
blade angle.
Axial flow fan testing
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Fan test results: pressure curve
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When matching a system to an experimental fan, make sure that the
matching is done correctly.
Axial flow fan testing
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Fan test results: efficiency
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The correct efficiency can only be obtained if matching is done correctly.
Conclusion
1.
2.
3.
4.
5.
6.
Dimensionless numbers
Hub-to-tip ratio
Velocity diagrams
Free vortex velocity distribution
Blade element theory
Testing of axial flow fans