Proceedings of IGTI06
51st International Gas Turbine and Aeroengine Congress and Exhibition
May 8 – 11, 2006, Barcelona, Spain
GT2006-90592
INVESTIGATION ON IMPROVED BLADE TIP CONCEPT FOR AXIAL FLOW FAN
Alessandro CORSINI, Bruno PERUGINI, Franco RISPOLI
Dipartimento di Meccanica e Aeronautica
Università di Roma “La Sapienza”
Via Eudossiana, 18
I-00184 Rome, Italy
Geoff SHEARD, Iain KINGHORN
Flakt Woods Limited
Tufnell Way, Colchester
CO4 5AR UK
ABSTRACT
The three dimensional structures of the blade tip vortical flow
field is herein discussed for a family of axial fans in fully-ducted
configuration, to investigate an improved blade tip concept. This
concept is based on geometrical modification of datum blade by
means of profiled end-plates at the tip. The investigation has
been carried-out using an accurate in-house developed multilevel parallel finite element RANS solver, with the adoption of a
non-isotropic two-equation turbulence closure. Due to the fullyducted configuration, the fans have a complex vortical flow field
near the rotor tip. The nature of the flow mechanisms in the fan
tip region is correlated to the specific blade design features that
promote reduced aerodynamic noise. It was found that the tip
geometrical modification markedly affects the multiple vortex
leakage flow behaviour, by reducing the pressure difference
within the tip gap and by altering the near-wall fluid flow paths
on the blade surfaces. The rotor loss behaviour, in the blade tip
region, was also discussed in order to assess the effect of blade
tip geometry onto the rotor efficiency.
INTRODUCTION
The blade tip aerodynamics is recognized to be governed by
complex flow phenomena, taking origin from the boundary
layers development under the influence of tip leakage and
secondary flows. The tip clearance is known to be detrimental to
the rotor performance establishing the magnitude of inefficiency,
its loading limit (Fukano and Takamatsu, 1986), (Storer and
Cumpsty, 1991), (Furukawa et al., 1999). A number of studies
have also pointed out the influence of tip leakage flow on rotor
aero-acoustic signature in low speed ventilating equipment (???)
as well as in high-bypass-ratio turbofan engines. In this ambit the
most prominent noise source is associated with the fan in both
subsonic and supersonic tip Mach number regimes (Ganz et al,
1998), respectively corresponding to the approach and the takeoff. Unfortunately, control and reduction of noise generated by
the rotor-tip flow field has not received the full attention it
deserves, primarily because of a lack of physical understanding
and the geometrical and flow complexities involved.
Often in axial flow fans and compressors, the design
specifications demand large tip gap according to the requirement
of operating with variable stagger or pitch angles, as well as
owing to manufacturing limitations. Consequently there is a
strong motivation to look for deliberate aerodynamic design to
minimize the deleterious effects of tip gap and to manage the fan
and compressor tip clearance flow to minimize its impact on
stability and performance. Thus technique and concepts that help
to reduce tip-clearance noise without sacrificing aerodynamic
efficiency are highly desired and needed.
A number of techniques have appeared to date for
accomplishing this goal, by reducing the leakage flow rate, or by
enhancing the primary-secondary flow momentum transfer. In
fans and compressors the research efforts to develop improved
tip aerodynamics could be synthetically grouped into three
classes.
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Copyright © 2006 by ASME
The use of casing treatments in the shroud portion over the
blade tip is reported since the early 70s to improve the stable
flow range by weakening the tip leakage vortex. Noticeable
contributions deal with the use of grooves and slots (Takata and
Tsukuda, 1977) (Smith and Cumpsty, 1984), or stepped tip gaps
(Thompson et al., 1998). Furthermore, in the ambit of fan
technologies recirculating vanes, and annular rings have been
proposed as anti-stall devices (Jensen, 1986).
Recently, a number of experimental studies reconsider the
use of active control of tip clearance flow by using fluid
injection on the casing wall in axial compressor (Bae et al.,
2005), and low-speed axial flow fan (Roy, et al., 2005).
As a last route, the use of sweep technique in design
concepts has been investigated as a remedial strategy to control
the aerodynamic limits in compressor and low-speed axial fan
rotors owing to the recognized capability of affecting the rotor
stall margin (Wadia et al., 1997), (Corsini and Rispoli, 2004),
(Corsini et al., 2004).
As a complement to the literature review, the present paper
aims to investigate on improved blade tip concept based by
means of geometrical modifications of datum blade. This
modification is based on the introduction of profiled end-plates
at the tip.
The objective of the paper is to report on the experimental
and numerical assessment of the pay-off derived from the blade
tip concept developed at Flakt Woods Ltd with respect to the
aerodynamic performance of a class of low noise level industrial
fans. The single rotor investigations are carried out at design and
off-design conditions for two configurations of the six-blade
axial flow fan under investigation, namely: the datum fan, code
AC90/6; the fan modified with the tip feature, code AC90/6/TF.
The studies have been carried in ducted configuration, adopting
a high tip pitch angle configuration, i.e. 28 degrees, where the
fan provides the higher static pressure and flow rate of its
operational range.
The aerodynamic performance experiments have been
carried out according to ISO 5801 for type C fully ducted
configuration set-up. The tip flow characteristics are analysed by
using a three-dimensional (3D) steady Reynolds-Averaged
Navier-Stokes (RANS) formulation, with use of first order nonisotropic turbulence closure successfully validated for fan rotor
flows (Corsini et al, 2003), (Corsini and Rispoli, 2005). Despite
the steady-state condition, the RANS is considered an effective
investigation tool for vortical structure detection (Inoue and
Furukawa, 2002). The authors adopt a parallel multi-grid (MG)
scheme developed for the in-house finite element method (FEM)
code (Borello et al., 2002). The FEM formulation is based on a
highly accurate stabilized Petrov-Galerkin (PG) scheme,
modified for application to 3D with equal-order spaces of
approximation.
By means of such tool, the tip leakage flow structures of the
fans are analysed in terms of vortical structures detection, losses
and loading. Emphasis is laid on the assessment of the benefits
related to the improved tip geometry in terms of efficiency and
operating margin gains. The role of the developed tip concept in
the aerodynamic performance of the investigated fan is studied
in terms of leakage vortex detection, losses and blade loading at
the tip region. The overall objective is to investigate, via steady
computational simulations, the technical merits of a passive
control strategy for reducing tip clearance vortex/stator
interaction noise and rotor-tip self noise.
NOMENCLATURE
Latin letters
Cp
static pressure coefficient ( ( p  pin ) / 0.5 U c2 )
dEm
non-dimensional mechanical energy loss through the
gap
Hn
normalized helicity (i wi /|||w|)
k
turbulent kinetic energy
blade chord

l.e.
leading edge
PS
pressure side
p
static pressure
Ptot
total pressure
r
radius
R
non-dimensional radius (r/rc)
SS
suction side
t
blade pitch
t.e.
trailing edge
Uc
casing relative peripheral velocity
v, w
absolute and relative velocities
x, y, z
Cartesian coordinates
Greek letters

turbulent dissipation rate



i





2
total loss coefficient, ( p0in  p0 0.5  win )
efficiency
hub-to-casing diameter ratio
vorticity vector
blade solidity
global flow coefficient (annulus area-averaged axial
velocity normalised by Uc)
rotor tipclearance (% of the span)
pressure rise coefficient (p/(0.5 U c2 ))
rotor angular velocity
Subscripts and superscripts
0
total flow properties
a, p, r
axial, peripheral and radial
c
casing wall
gap
gap quantities
h
hub wall
i
Cartesian component index
in
inlet section
n
normal component
s
streamwise component
area-averaged value
2
Copyright © 2006 by ASME
TEST FANS
The present study was performed on a family of
commercially available highly efficient cooling fans. The in
service experiences indicated that this family of fans gives good
acoustic performance with respect to the state-of-the-art
configurations. The investigated fans have six-blade unswept
rotor, with blade profiles of modified ARA-D geometry type
originally designed for propeller applications. The blade profiles
geometry is given in Table 1, for the datum fan AC90/6 at the
hub, mid-span and tip sections respectively.
Table 1 Blade profile geometry
AC90/6 blade
hub
midspan
tip
1.32
0.52
0.31
(deg)
36
31.2
28
camber angle (deg)
46
44
41
/t
pitch angle
The AC90/6_TF fan rotor blades differ from the datum ones
in the vicinity of the tip, as shown in the meridional view of
Figure 2. The AC90/6_TF blade tip geometry was originally
inspired by the technique developed for tip vortex control and
induced drag reduction by preventing 3D flows in aircraft wings,
also used as anti-vortex devices for catamaran hulls. The tip
blade section is modified by the addition of an end-plate along
the blade pressure surface that ends on the blade trailing edge
with a square tail. By means of the introduction of the end-plate,
the blade section is locally thickened of a factor 3:1 with respect
to the maximum thickness at the tip of datum blade. According
to the theory behind the end-plate design, this dimension was
chosen as the reference radial dimension of leakage vortex to be
controlled that could be estimated in the range 0.2  0.1 blade
span, as shown by former studies on rotors of axial compressor
(Inoue et al., 1986) and fan (Corsini et al., 2004).
Moreover, Figure 1 and Table 2 show the datum fan rotor
and its specifications. The studied blade configurations, for
datum and modified rotors, feature a high tip stagger angle, i.e.
28 degrees, measured, as is customary in industrial fan practice,
from the peripheral direction. This rotor angular setting has been
chosen in order to exploit operating points where the vortical
flow near the rotor tip dramatically affects the aerodynamic
performance and noise characteristics of the investigated fans.
Fig. 2 Rotors blade details: a) datum fan, and b) improved fan.
Fig. 1 Solid model of the test fan rotors
Table 2 AC90/6 and AC90/6_TF fan specifications
blade number
6
blade tip pitch angle (deg)
28
hub-to-casing diameter ratio
0.22
tip diameter (mm)
900.0
clearance (% span)
rated rotational frequency (rpm)
1.0
900 - 935
NUMERICAL METHOD
The Reynolds-averaged Navier-Stokes equations are solved by
an original parallel Multi-Grid Finite Element flow solver
(Borello et al., 2003). The physics involved in the fluid
dynamics of incompressible 3D turbulent flows in rotating frame
of reference, was modelled with a non-linear k- model (Craft et
al., 1996), here used in its topology-free low-Reynolds variant.
This turbulence closure has been successfully validated on
transitional compressor cascade flows, as well as high-pressure
industrial fan rotors (Corsini and Rispoli, 2004 and 2005).
The numerical integration of PDEs is based on a consistent
stabilised Petrov-Galerkin formulation developed and applied to
control the instability origins that affect the advective-diffusive
incompressible flow limits, and the reaction of momentum and
turbulent scale determining equations. The latter ones,
respectively, related to the Coriolis acceleration or to the
dissipation/destruction terms in the turbulent scale determining
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Copyright © 2006 by ASME
equations (Corsini et al., 2004). Equal-order linear interpolation
spaces (Q1-Q1) are used for primary-turbulent and constrained
variables, implicitly eliminating the undesirable pressurecheckerboarding effects. Concerning the solution strategy
(Borello et al., 2001), a hybrid full linear MG accelerator was
built-in the in-house made overlapping parallel solver. The
Krylov iterations in the smoothing/solving MG phases are
parallelized using an original additive domain decomposition
algorithm. The message passing operations were managed using
the MPI libraries. By that way, the fully coupled solution of subdomain problem involves an efficient non-conventional use of
Krylov sub-space methods. The preconditioned GMRes(5) and
GMRes(50) algorithms were respectively used as smoother and
core solver.
Validation analyses
Previous studies, carried out using the current numerical
method with non-isotropic turbulence closure, have shown fair
predicting capabilities of the flow physics pertinent to highly
loaded axial fans (Corsini and Rispoli, 2004 and 2005), and lownoise fan rotor (Corsini et al., 2004).
In particular Corsini and Rispoli (2005) have presented a
numerical assessment of the pay-off derived from the use of an
anisotropic turbulence closure with respect to the prediction of
the blade tip in fan rotors. The study was devoted to axial
ventilating fan rotor designed for non-free vortex operation and
tested by Vad and Bencze (1998). For this purpose, Fig. 3 shows
the pitch-wise mass averaged pressure rise coefficient 
distributions at rotor outlet in near-design (D) and near-peak
pressure operating conditions (P). Fig. 3 compares standard k-
solution with that predicted by the non-linear k- model against
the available LDA data (Vad and Bencze, 1998).
With respect to  profiles, both the computations capture
the slight overturning near the hub, due to the passage vortex
effect on hub boundary layer. According to the spanwise
increasing ideal total head rise, the measured outlet swirl
increases with radius and it decreases near the casing due to the
underturning effect of stationary wall and leakage flow. This
feature, essential for the validity of the numerical solution, is
well resolved only by the non-linear model that is able to
simulate the experimental swirl drop close to the blade tip at the
D point.
Rotor modeling and boundary conditions
The mesh has been built according to a non-orthogonal body
fitted coordinate system, by merging two structured H-type grid
systems.
The mesh in the main flow region, surrounding the blade,
and an embedded mesh in the tip gap region. The mesh has
1546858 nodes, respectively in the axial, pitch, and span wise
directions. In the axial direction the node distribution consists of
20%, 50% and 30% of nodes respectively upstream the leading
edge, in the blade passage and downstream of it. Moreover, there
are 14 grid nodes to model the tip-clearance along the span. The
computational grid is illustrated, in Figure 4, providing detailed
view at the tip of the mesh in meridional and blade-to-blade
surfaces.
The mesh has an adequate stretch toward solid boundaries,
with the ratio of minimum grid spacing on solid walls to midspan blade chord set as 2 10-3 on the blade tip, casing wall, and
blade surfaces. The adopted grid refinement towards the solid
surfaces controls the dimensionless distance + value about 1 on
the first nodes row.
1
r/rc
0.95
0.9
D point
P point
0.85
t.e.
0.8
l.e.
0.75
0.7

0.4
0.6
0.8
1

hub
1.2
Fig. 3 Pitchwise-averaged flow pressure rise coefficient distributions at
rotor exit, in near-design and near-peak pressure operations.
(symbols: experiments; dashed lines: linear k-;
solid lines: non-linear k-)
Fig. 4 Computational grid of fan rotor
Boundary conditions and investigated flow conditions.
Standard boundary condition set has been adopted, already used
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Copyright © 2006 by ASME
in recent numerical studies on high performance fans (Corsini
and Rispoli, 2004) (Corsini et al., 2004).
The Dirichlet conditions for the relative velocity components
are imposed at the inflow section half a mid-span chord far
upstream the leading edge. The velocity profile has been
obtained from flow simulation in an annular passage of identical
hub-to-casing diameter ratio that includes an upstream spinner
cone. The inlet distribution of the turbulent kinetic energy k is
obtained from axi-symmetric turbulence intensity (TI) profile
derived on the basis of former studies on ducted industrial fans
(Corsini and Rispoli, 2004). The TI profile features a nearly
uniform value in the core region (about 6 percent) and it grows
markedly approaching the endwalls (about 10 percent). The inlet
profile of turbulence energy dissipation rate is based on the
characteristic length scale l set to 0.01 of rotor pitch at midspan. Flow periodicity upstream and downstream the blading,
and Neumann outflow conditions (homogeneous for k and  and
non-homogeneous for the static pressure) complete the set of
boundary data.
The comparative fan leakage flow patterns for datum and
AC90/6/TF fan rotors have been investigated for two operating
points: near-design condition (D) with volume flow rate 7 m3/s
and global flow coefficient  = 0.278; near-peak pressure
condition (P) with volume flow rate 6 m3/s and global flow
coefficient  = 0.225. The Reynolds number based on tip
diameter and rotor tip speed is 2.1 106, for normal air
condition.
PERFORMANCE EXPERIMENTS
The aerodynamic and noise performance tests were carried
out at Flakt Woods Ltd laboratory in Colchester. The
aerodynamic tests were conducted according to ISO 5801 set up,
for fully ducted configuration and installation type D. This
installation features ducted inlet and outlet regions and the fan is
supplied with a properly-shaped inlet bell mouth. The primary
performance parameters measured were the fan static pressure
and the efficiency. Fig. 5 compares the static pressure and
efficiency characteristic curves for datum and AC90/6/TF rotors.
The analysis of static pressure curves of Figure 5, gives the
evidence of a small performance reduction in rotor AC90/6/TF
with improved tip concept, e.g. about 2% at 6m3/s. On the other
hand, rotor AC90/6/TF shows an efficiency improvement in the
range of volume flow rate higher than the design one. In terms
of peak value, the AC90/6/TF rotor features a 72.9% efficiency
with a 1.25% of improvement. Moreover the efficiency curve
comparison gives evidence that the adoption of the improved
blade tip concept results in the appearance of an efficiency
plateau by reducing the peak efficiency volume flow rate.
The noise performance test have been carried out in
accordance with the British Standard BS484, employing a type A
testing configuration. In this method the fan is placed
downstream of a plenum chamber with a free outlet, in an
arrangement similar to that used for compact cooling fans.
80

D
P
70
60
250
50
p(Pa)
200
40
P
30
150
D
20
3
100
4
5
6
7
8
9
50
0
3
4
5
6
7
8
9
volume flow (m3/s)
Fig. 5 Static pressure and efficiency characteristic curves and tested
operating conditions
(dashed lines: AC90/6 datum fan; solid lines: AC90/6/TF fan)
Figure 6 compares the measured sound power spectra SWL
(dB) in one-third-octave band.
SWL (dB)
frequency (Hz)
Fig. 6 Static pressure and efficiency characteristic curves and tested
operating conditions
(dashed lines: AC90/6 datum fan; solid lines: AC90/6/TF fan)
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Copyright © 2006 by ASME
As shown in Fig. 6, the effectiveness of the improved tip
concept is demonstrated by the reduction of the rotor
aeroacoustic signature in terms of tonal noise and broad-band
noise. This noise components are broadly related to tip noise
generation mechanisms such as convection of the primary (tip)
vortex and its interaction with the stator vanes produces mainly
tonal noise, or the oscillating tip vortex behaviour which results
in a broadband self-generated noise (Khourrami and Choudari,
2001).
Jang, C., Furukawa, M., Inoue, M., (2001). Analysis of vortical
flow field in a propeller fan by LDV measurements and LES Part I&Part II, J. Fluids Eng., 123, 748-761.
Jang, C., Fukano, T.,Furukawa, M., (2003), Effects of the tip
clearance on vortical flow and its relation to noise in an axial
flow fan, JSME Transaction Series B, 46, 356-365.
ACKNOWLEDGMENTS
The present research was done in the context of the contract
FW-DMA03, between Flakt Woods Ltd and Dipartimento di
Meccanica e Aeronautica University of Rome “La Sapienza”.
Alessandro Corsini and Franco Rispoli would like to
acknowledge MIUR (Italian Ministry for the academic research)
under the projects Ateneo 2004. Moreover, Alessandro Corsini
gratefully acknowledge Carlo Barbetta, at Flakt Woods Spa, for
the long-lasting support.
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