3.7 Axial and Mixed-Flow Fans
143
With the Stokes circulation theorem follows that rotation
ω in the axial direction is
created downstream of the rotor. Streamwise rotation is normally zero with constant
work. The flow downstream of the rotor always features swirl due to the presence
of the tangential velocity component, but, with a free vortex blade, the rotation ( )
ω
locally equals zero. This is the meaning of the term free vortex flow: fluid particles
nearer to the shaft have higher tangential velocity, keeping a direction linked to the
flow parallel to itself during the vortex motion. With non-constant work, the consequence is that a streamsurface that is cylindrical upstream of the rotor, does not remain so downstream. Figure 3.30 (top, right) shows the twisted streamsurface where
fluid at the pressure side migrates to the tip and fluid at the suction side migrates to
the hub. Local vortex motions created in the blade wake take a part of the kinetic
energy from the core flow and dissipate it downstream of the rotor. This generates a
loss mechanism associated to the twist of the streamsurfaces. Figure 3.30 (bottom)
demonstrates that streamsurface twist may be compensated by advancing the chord
at the tip. The relative velocity at the leading edge has a component parallel to the
leading edge and a component perpendicular to it. The parallel component remains
almost unaffected with flow through the rotor. The perpendicular component increases at the suction side and decreases at the pressure side. As a consequence, the
relative streamlines deviate as drawn. This generates twist on the streamsurfaces,
which compensates the twist by the inhomogeneous work distribution.
3.7.3 Axial Fan Characteristics; Adjustable Rotor Blades
Figure 3.31 is a sketch of the variation of the total pressure rise as a function of the
flow rate. The characteristic curve has a maximum and a minimum. Figure 3.28
demonstrates that, with flow rate decrease (with same direction of v 1 ), the angle of
attack of the flow entering the rotor increases. This increases the rotor deflection
and so the rotor work. The angle of attack on the stator, if downstream, increases as
well (with same direction of w 2 ). Flow rate decrease thus increases rotor work, but
boundary layer separation occurs below a certain flow rate, within the rotor or the
stator. This diminishes the total pressure rise. The total pressure rise is higher however at a very low flow rate. Recirculation flows are generated, causing a significant
radius increase for the through-flow (Fig. 3.31). Due to the centrifugal force, pressure build-up arises in the flow.
The flow separation with reduced flow rate at rotor entrance and at stator entrance
with a downstream stator can be removed with adjustable rotor blades. Figure 3.32
shows how by turning the rotor blades into a more tangential position and by lowering the angle of attack of the rotor blades, high incidences can be avoided at reduced
flow rate. This means that by turning the rotor blades into a more tangential direction (closing the rotor) the fan is adjusted to operation at lower flow rate and lower
pressure rise. In practise, with this turning some loss of efficiency occurs because
the blade twist is not fully adapted anymore to the flow so that design incidence
angles cannot be reached at all radii.
143
With the Stokes circulation theorem follows that rotation
ω in the axial direction is
created downstream of the rotor. Streamwise rotation is normally zero with constant
work. The flow downstream of the rotor always features swirl due to the presence
of the tangential velocity component, but, with a free vortex blade, the rotation ( )
ω
locally equals zero. This is the meaning of the term free vortex flow: fluid particles
nearer to the shaft have higher tangential velocity, keeping a direction linked to the
flow parallel to itself during the vortex motion. With non-constant work, the consequence is that a streamsurface that is cylindrical upstream of the rotor, does not remain so downstream. Figure 3.30 (top, right) shows the twisted streamsurface where
fluid at the pressure side migrates to the tip and fluid at the suction side migrates to
the hub. Local vortex motions created in the blade wake take a part of the kinetic
energy from the core flow and dissipate it downstream of the rotor. This generates a
loss mechanism associated to the twist of the streamsurfaces. Figure 3.30 (bottom)
demonstrates that streamsurface twist may be compensated by advancing the chord
at the tip. The relative velocity at the leading edge has a component parallel to the
leading edge and a component perpendicular to it. The parallel component remains
almost unaffected with flow through the rotor. The perpendicular component increases at the suction side and decreases at the pressure side. As a consequence, the
relative streamlines deviate as drawn. This generates twist on the streamsurfaces,
which compensates the twist by the inhomogeneous work distribution.
3.7.3 Axial Fan Characteristics; Adjustable Rotor Blades
Figure 3.31 is a sketch of the variation of the total pressure rise as a function of the
flow rate. The characteristic curve has a maximum and a minimum. Figure 3.28
demonstrates that, with flow rate decrease (with same direction of v 1 ), the angle of
attack of the flow entering the rotor increases. This increases the rotor deflection
and so the rotor work. The angle of attack on the stator, if downstream, increases as
well (with same direction of w 2 ). Flow rate decrease thus increases rotor work, but
boundary layer separation occurs below a certain flow rate, within the rotor or the
stator. This diminishes the total pressure rise. The total pressure rise is higher however at a very low flow rate. Recirculation flows are generated, causing a significant
radius increase for the through-flow (Fig. 3.31). Due to the centrifugal force, pressure build-up arises in the flow.
The flow separation with reduced flow rate at rotor entrance and at stator entrance
with a downstream stator can be removed with adjustable rotor blades. Figure 3.32
shows how by turning the rotor blades into a more tangential position and by lowering the angle of attack of the rotor blades, high incidences can be avoided at reduced
flow rate. This means that by turning the rotor blades into a more tangential direction (closing the rotor) the fan is adjusted to operation at lower flow rate and lower
pressure rise. In practise, with this turning some loss of efficiency occurs because
the blade twist is not fully adapted anymore to the flow so that design incidence
angles cannot be reached at all radii.