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Conference on Modelling Fluid Flow (CMFF’06)

The
13th International Conference on Fluid Flow Technologies

Budapest, Hungary, September 6
-
9, 2006


D
EVELOPMENT OF IMPROV
ED BLADE TIP END
-
PLATE CONCEPT
S

FOR
LOW
-
NOISE OPERATION IN I
NDUSTRIAL FANS

Alessandro C
ORSINI
1
,
Franco

RISPOLI
2
,
A. Geoff SHEARD
3


1

Corresponding Author.
Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”
.
Via Eudossiana

18
,
I00184




Rome
,
Ital
y. Tel.: +3
9

06

44585231
, Fax: +3
9

06

4881759
,
E
-
mail:

alessandro.cors
ini@uniroma1.it

2

Dipartimento di Meccanica e Aeronautica, University of Rome “La Sapienza”.
E
-
mail:
franco.rispoli
@
uniroma1.it

3

Fläkt

Woods Ltd
.
E
-
mail:
geoff.sheard@
Fläkt
woods.com


ABSTRACT

The use
of improved blade tip geometries is
addressed as an effective design concept for passive
noise control in industrial fans. These concepts
,
based on geometrical modification
s

of datum blade
by means of profiled end
-
plates at the tip, are
shown to reduce fan

noise in its tonal and
broadband components by changing the tip leakage
flow behaviour. The three dimensional structures of
tip vortical flow fields are discussed for a family of
axial fans in fully
-
ducted configuration, to
investigate on the aerodynamics

of the proposed
blade tip concepts. The study has been carried
-
out
using an accurate in
-
house developed parallel finite
element RANS solver, with the adoption of non
-
isotropic two
-
equation turbulence closure. The
nature of the flow mechanisms in the fan t
ip region
is correlated to the specific blade design features
that promote reduced aerodynamic noise. It was
found that the tip geometrical modifications
markedly affect the multiple vortex leakage flow
behaviours, by altering the turbulence and velocity
fluctuations in the near
-
wall region as well along
the blade span. The tip end
-
plates were
demonstrated to influence also the rotor loss
behaviour, in the blade tip region. The improvement
of rotor efficiency curves were assessed and
correlated to the cont
rol of tip leakage flows
exploited by the tip end
-
plates.

Keywords:

industrial

fans
,

end
-
plates, tip
leakage flow, noise

NOMENCLATURE


Latin letters

k

[m
2
/s
2
]

turbulent kinetic energy

l.e.


leading edge

PS


pressure side

P

[Pa]

static pressure

r

[mm]

radi
us

SS


suction side

t.e.


trailing edge

U
c

[m/s]

casing relative peripheral velocity

v, w

[m/s]

absolute and relative velocities

x, y, z


Cartesian coordinates

Greek letters



[deg]

stagger angle



[m
2
/s
3
]

turbulent dissipation rate



[
-
]

total loss coefficient,
2
0.5
in
0
0in
p p w





[
-
]

efficiency



[
-
]

hub
-
to
-
casing diameter ratio


t

[
m

s
-
2
]

turbulent viscosity


i

[s
-
1
]

absolute vorticity vector


s

[
-
]

absolute streamwise vort
icity,
( )/(2 )
s i i
w w
  
 



[
-
]

blade solidity



[
-
]

global flow coefficient (annulus
area
-
averaged axial velocity
normalised by
U
c
)



[
-
]

rotor tip

clearance



[
-
]

pressure rise coefficient (

p/(

0.5
2
c
U
))



[rad/s]

rotor

angular velocity

Subscripts and superscripts

a, p, r

axial, peripheral and radial

c

casing wall

h

hub wall

i

Cartesian component index

in

inlet section

s

streamwise component


pitch
-
averaged value

1.
INTRODUCTION

Often in axial flow fans the design
speci
fications demand large tip gap according to the
requiremen
t
s

of operating with variable stagger or
pitch angles
, e.g. cooling fans
,
or

in some cases for
emergency operation at up to 400°C for two hours
to extract smoke in the event of a fire
, e.g.
ventilat
i
ng fans. As well known, the tip clearance
plays a detrimental role affecting the rotor aero
-
dynamics
[1
-
3]
, and, as a

number of studies pointed
out,
significantly

contribut
ing

to

the
aero
-
acoustic

signature

of

impeller in
low speed ventilating
equipment
s.

In this pictures the tip clearance flow
is recognized to inflence the rotor noise spectra by

discrete frequency noise due to

periodic velocity
fluctuation and a broadband
or high
-
frequency
noise due to velocity fluctuation in the blade
passage

[4
-
6]
.

To
this end t
here is a strong
motivation to look for deliberat
e aerodynamic
design in order to minimize the negative effects of
tip gap and to manage the fan or compressor tip
clearance flow to minimize its impact on
performanc
e.
Thus technique
s

and concepts
that
help to reduce tip

clearance noise without
sacrificing aerodynamic efficiency are highly
desired and needed.

By surveying the
techniques
for noise control
in

fans and compressors,
it wa
s found that
the
solutions
proposed

could be grouped into active a
nd
passive noise control techniques,
conceptually
designed to accomplish this

goal by reducing the
leakage flow rate or by enhancing the primary
-
secondary flow momentum transfer.

I
n the ambit of
act
ive control techniques

for fans
and compressors
, recently
a number of
experimental

studies reconsider the tip clearance flow control by
means of fluid injection on the casing wall in axial
compressor
[7]
, and low
-
speed axial flow fan

[8]
.

As far as the

passive control techniques

are
concerned
, the literature rev
iew
puts in evidence
the
role of t
hree

approaches

respectively focused on
three
-
dimensional blade design and on geometrical
modifications of the equipments in the gap region
.
The first concept makes use
of sweep technique in
blade design, recognized as a r
emedial strategy to
improve the aerodynamic limits in compressor and
low
-
speed axial fan rotors owing to the capability of
affecting the rotor stall margin by reducing
secondary flows effects and the flow leakage over
the blade tip
[9
-
11]
.

A second family
of control technique based on
gap geometrical manipulation, the

use
of casing
treatments in the shroud portion over the blade tip
which is reported since the early 70s to improve the
stable flow range by weakening the tip leakage
vortex. Noticeable contrib
utions deal with the use
of grooves and slots
[12
-
13]
, or stepped tip gaps
[14]
. Furthermore, in the ambit of fan technologies
recirculating vanes, and annular rings have been
proposed as anti
-
stall devices
[15].

A

final tip treatment solution,
appeared
du
ring
the last decade, proposed the blade tip
modifications by means of anti
-
vortex appendices
such as the end
-
plates investigated by Quinlan and
Bent
[6]
, or the solutions recently proposed by
industrial patents for ventilating fans
[16
-
19]
.


In this respe
ct, t
he present paper aims to
investigate
on the use of
profiled end
-
plates at the
blade
tip

[20]
. The study focuses on

a family of
commercially available fans

and compares the
aerodynamic and
a
eroacoustic performance

of the
datum blade

against two improve
d tip geometries,
respectively with constant or variable thickness
end
-
plate
[21].

The objective of the paper i
s to report on the
experimental

and numerical assessment of the pay
-
off derived from the blade tip concept
developed at
Fläkt Woods Ltd

with resp
ect to the aerodynamic
performance of a class of l
ow noise level industrial
fans.

The single rotor investigations are carried out at
design
operating condition

for t
hree

configurations
of the six
-
blade axial flow fan under investigation,
namely: the datum
fan, code
d

AC90/6; t
wo

fan
s

modified
by the adoption of

tip feature
s
,
respectively
code
d

AC90/6/TF

and AC90/6/TFvte
.
The studies have been carried in ducted
configuration, adopting a high tip pitch angle
configuration, i.e. 28 degrees, where the fan
provid
es the higher static pressure and flow rate of
its operational range.

The comparative a
erodynamic performance
experiments have been carried out according to ISO
5801 for type D fully ducted configuration set
-
up.
T
he noise performance test have been carried

out in
accordance with the British Standard BS484
, Part 2
for outlet noise hemispeherical measurement. The
fans have been tested

employing

a type A
configuration, in the Fläkt Woods Ltd anechoic
chamber at the design operating conditions.

The tip f
low cha
racteristics are analysed by
using a three
-
dimensional (3D) steady Reynolds
-
Averaged Navier
-
Stokes (RANS) formulation, with
use of first order non
-
isotropic turbulence closure
successfully validated for fan rotor flows
[22
-
23]
.
Despite the steady
-
state con
dition, the RANS is
considered an effective investigation tool for
vortical structure detection
[24]
. The authors adopt
a parallel multi
-
grid (MG) scheme developed for
the in
-
house finite element method (FEM) code
[25]
. The FEM formulation is based on a hi
ghly
accurate stabilized Petrov
-
Galerkin (PG) scheme,
modified for application to 3D with equal
-
order
spaces of approximation.

By means of such
a numerical investigation
, the
tip leakage flow structures of the fans are analysed
in terms of vortical structu
res detection, leakage
flow energy

and loss behaviours
. Emphasis is laid
on the assessment of the benefits related to the
improved tip geometry in terms of efficiency and
operating margin gains.
The overall objective is to
investigate, via steady computati
onal simulations,
the technical merits of a passive control strategy for
controlling the leakage flow and
reducing tip
clearance vortex/stator interaction noise and rotor
-
tip self noise.

2.
TEST
APPARATUS AND PROCED
URES

2.1. Test fans

The present study was

performed on a family of
commercially available highly efficient cooling
fans. The
i
n 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 si
x
-
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, and tip sections
respectively.

Table 1 AC90/6 f
an family specifications. Blade
profile geometry and rotor specifications.


The studied blade configurations, for datum
and modified rotors, feature a high tip stagger
angle, i.e. 28 deg
rees, 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 perf
ormance and noise characteristics
of the investigated fans.


The
fan

blades

are

drawn

in Figure 1
, together
with a detailed view of blade tip for the datum rotor,
and the improved rotors developed for low noise
emission labeled: AC90/6/TF and AC90/6/TFvte
.

Fig
.

1 compares, in a qualitative view not to scale,
the thickness distributions of the developed
improved tip concepts against the datum base
-
line.

The
improved blade tip geometry, for
AC90/6/TF fans,

was originally inspired by the
technique developed f
or tip vortex control and
induced drag reduction by preventing 3D flows in
aircraft wings, also used as anti
-
vortex devices for
catamaran hulls.









Fig. 1
T
est fans and rotor blades

(not to scale)

The tip blade sect
ion
was

modified by adding
an end
-
plate along the blade pressure surface that
ends on the blade t
railing 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 maxi
mum 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
[26]

and fan
[11]
.

A
recent investigation, carried out by Corsini and co
-
workers
[20]
, assessed the aerodynamics and
aeroacoustics gains
of rotor AC90/6/TF
with
respect to the datum
one
. Nonetheless
,

the
num
erical simulation
also
founded
the evidences of
a tip leakage vortex breakdown
affecting
rotor
AC90/6/TF

at the design operating condition. To
this end
the AC90/6/TFvte blade tip geometry has
been proposed that exploits a variable thickness
distribution of

the end
-
plate according to safe
rotation number chord
-
wise gradient concept
[21]
.

2.2
.

Numerical procedure and axial fan
modeling

The Reynolds
-
averaged Navier
-
Stokes equations
are solved by an original parallel Multi
-
Grid Finite
Element flow solver
[
25
]
.
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
[
27
]
, 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
[1
0, 23
]
.


AC90/6 fans



blade geometry

hub

tip


/ t

1.32

0.31

pitch angle


(deg)

36

28

camber angle (deg)

46

41

fan rotor



blade number

6

blade tip pitch ang
le

(deg)

28

hub
-
to
-


0.22

tip diameter

(mm)

900.0



(% span)

1.0

rated rotational frequency

(rpm)

900


935

AC90/6/TF
vte

datum

AC90/6/TF

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 equations
[
2
2
]
. Equal
-
order linear
interpolation spaces (Q1
-
Q1) are used for primary
-
turbulent and constrained v
ariables, implicitly
eliminating the undesirable pressure
-
checkerboarding effects. Concerning the solution
strategy, a hybrid full linear MG accelerator was
built
-
in the in
-
house made overlapping parallel
solver. The Krylov iterations in the
smoothing/solv
ing 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 sub
-
domain problem
involves an efficient non
-
conventio
nal use of
Krylov sub
-
space methods. The preconditioned
GMRes(5) and GMRes(50) algorithms were
respectively used as smoother and core solver.

















Fig. 2 Computational grid of fan rotor, mesh
details in the tip gap region

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, respec
tively 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
2
, 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 t
o mid
-
span 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 r
ow.

2.3.

Boundary conditions and investigated
flow conditions

Standard boundary condition set has been
adopted, already used in recent numerical studies on
high performance fans
[10
-
11]
.

The Dirichlet conditions for the relative velocity
components are im
posed 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 d
istribution 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
[10]
.
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
basedon the characteristic length scale

l


set to
0.01

of rotor pitch at mid
-
span.
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

numerical investigat
ion compares fan
leakage flow patterns for datum
,

AC90/6/TF
and
AC90/6/TF
vte

fan rotors operated in
near
-
design
condition (
D
) with volume flow rate 7 m
3
/s and
global flow coefficient


= 0.278.
The Reynolds
number based on tip diameter and rotor tip speed
is
8.3


10
5
, for normal air condition.

3.
PERFORMANCE EXPERIME
NTS

The aerodynamic and noise performance tests
were
carried

out at Fläkt Woods Ltd laboratory in
Colchester.

3.1.

Aerodynamic tests

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 pres
sure and the
efficiency. Figure

3

compares the
static pressure
and efficiency characteristic
curves
for datum and
AC90/6/TF rotors
.

The analysis of
static pressure
rotor hub


l.e.


t.e.

curves of Fig.

3
, gives the evidence of a small
performance reduction in rotor AC90/6/TF wit
h
improved tip concept,
e.g.
about 2% at 6m
3
/s
,

according

to
t
he
interaction between the tip
clearance flow and the suction side near surface
fluid
[2
8
]
.
On the other hand, rotor AC90/6/TF
vte
shows a performance recovery
that increases

remarkabl
y

by thrott
ling the rotor toward the peak
pressure.

Concerning the efficiency
curves it

is evident
that both the modified rotors feature

an
efficiency
improvement in the range of volume flow rate
highe
r than the design one.
Moreover the efficiency
curve comparison gi
ves evidence that the adoption
of the improved tip concept
s results in the
appearance of a clear

efficiency

plateau that shift
the peak


volume flow rate towards the rotor stall
margin.


























Fig.
3

Static pressure and efficiency characteristic
curves

(dashed lines: datum fan; solid lines:
AC90/6/TF fan; line
-
symbols: AC90/6/TFvte fan)

The rotor performance were a
ssessed along the
operating line. The predicted overall performance
for 900 rpm rotational frequency are compared in
Table 2 to the experimental data measured at Fl
ä
kt
Woods Ltd measurements according to the fan
performance test standards ISO5801:1997 for
installations type
D

with inlet bell
-
mouth.

Efficiency


is computed in terms of static
pressure rise. The comparison confirms the validity
of the predicted performance at the chosen setting
angle, where the blades of this fan are the most
loaded and more readily prone to flow separation.

It is also worth noti
ng that the prediction of
performance parameters have been referred to axial
sections respectively located at the inlet of the
domain, and 1.2 midspan chord downstream the
blade trailing edge. The comparison confirms the
validity of the predicted performan
ce.


Table 2 Predicted and measured fan overall
performance

@ 7 m
3
/s


Measurements

Predictions



p
stat

(Pa)





p
stat

(Pa)




datum

134.8


0.490

133.3


0.510

AC90/6/TF

126.2


0.
510

126.
1


0.50
4

AC90/6/TFvte

1
29.0


0.51
9

12
8.2


0.516

3.2.

Noise tests

The noise performance test have been carried
out in accordance with the British Standard BS484,
em
ploying a type A testing configuration.


1
0
2
1
0
3
1
0
4
5
0
5
5
6
0
6
5
7
0
7
5
8
0
8
5
9
0
9
5
1
0
0
d
a
t
u
m
A
C
9
0
/
6
/
T
F
A
C
/
9
0
/
6
/
T
F
v
t
e
1
0
2
1
0
3
1
0
4
3
0
3
5
4
0
4
5
5
0
5
5
6
0
6
5
7
0
7
5
8
0
8
5
9
0
9
5
1
0
0
d
a
t
u
m
A
C
9
0
/
6
/
T
F
A
C
/
9
0
/
6
/
T
F
v
t
e

Fig.
4

S
ound power level spectra in one
-
third
-
octave band
. a)
un
-
weight
ed spectra
and b)
A
-
weighted
spectra

(dashed lines: datum fan; solid
lines: AC90/6/TF fan; line
-
symbols: AC90/6/TFvte
fa
n)

In this me
thod the fan is placed
downstream

of
a plenum chamber with a free outlet, in an
arrangement similar to that used for compact
cooling fans.

4
5
6
7
8
3
0
4
0
5
0
6
0
7
0
8
0
4
5
6
7
8
0
5
0
1
0
0
1
5
0
2
0
0
2
5
0
4
5
6
7
8
0
5
0
1
0
0
1
5
0
2
0
0
2
5
0
volume flow (m
3
/s)



D

frequency (Hz)

dB

dB
(A
)

b)

a)

frequency (Hz)

datum

AC90/6/TF

AC90/6/TFvte

datum

AC90/6/TF

AC90/6/TFvte


p(Pa)

Figure 4 compares the measured power spectra
in one
-
third
-
octave band. Fig. 4.a and Fig. 4.b
respectively show the
me
asured
sound power level
and the A
-
weighted
sound power level
spectra

for
the frequency
-
dependent human audition. The noise
tests have been done in order to compare the rotors
aeroacoustic signature for identical static pressure
rise, e.g. 190 Pa close to
the peak pressure
operation.
As shown in Fig. 4, the effectiveness of
the improved tip concept
s, with constant and
variable thickness end
-
plates,

is demonstrated by
the reduction of the rotor aeroacoustic signature
both in terms of
tonal noise
and broad
-
ba
nd noise.
These noise components are related to the main
recognized tip noise generation mechanisms in axial
decelerating turbomachinery. The
convection of the
primary tip
vortex
and its interaction with the
statoric structures produces mainly tonal noise
,
while
the oscillating tip vortex behaviour
could be
linked to the production of broadband self
-
generated noise
[
29
]
.

Finally, Table
3

compares the overall soud
power levels for the family of fans under
investigation, for

Table
3

Overall sound power leve
ls @ 190 Pa


u
n
-
weighted SWL

dB

A
-
weighted SWL

dB(A)

datum


94.6

88.7

AC90/6/TF

93.0

86.3

AC90/6/TFvte

91.5

88.1

4. INNER WORKINGS OF

END
-
PLATES

The improved blade tip concepts,
developed
with constant thickness and variable thickness end
-
plates, has b
een shown to control the aeroacoustic
signature of the family of fans under investigations
and this pay
-
off is correlated to a large efficiency
improvement in the peak pressure operating range.
These experimental evidences prompted a
comparative investigat
ion,
for the datum fan and the
fan with blade tip feature
s, onto the inner workings
of the passive control device used to influence the
structure of the tip leakage vortex and other systems
of secondary vorticities. The effectiveness of the
passive tip dev
ices is analysed by comparing the
normalized streamwise vorticity map evolution
along the chord, and the tip vortex core paths. These
analyses are complemented with the evaluation of
tip leakage flow energy contents, affecting the rotor
aeroacoustic signa
tures, and with the presentation of
loss coefficient map evolution within the blade
passage to assess the influence of the geometrical
modification at the blade tip and the rotor
aerodynamic efficiency along in operating range of
the fans.

4.1.

Helicity di
stributions and vortex core
s


The tip leakage
v
o
rtical

structures are now
investigated by using the normalized helicity
H
n

based on the absolute vorticity
[3, 24]

as the
detection tool.
H
n

is defined and normalized as:





/
n i i
H w w
 
 

with
i

=
1,
3
,
where

i

and
w
i

are the Cartesian components of the absolute
vorticity and relative velocity vectors
,


and
w

their norms
.

Figure
4

shows the normalized helicity
distribution in the blade tip region
by comparing the
contours on cross flow planes in near
-
design
operating condition. The probing plane
s

are located,
respectively, at 0.25, 0.43, 0.65, 0.89 and 1.2 blade
chord



from the tip
section

leading edge.
The
normalized helicity d
istribution is plotted with the
vortex cores colored by the
its l
ocal magnitude.

For the investigated fan rotors, a clear vortex
cores
identification is only observed for the leakage
flow structures emerging in the front portion of the
tip blade sections.
In the multiple vortex behaviour
of

datum fan rotor, Fig.
4
.a, the helicity field shows
that
a

main
clock
-
wise
v
o
rtical

structure
(
TLV1
)
develops through

the passage

with
high skew
ing
angle

with respect to the
blade surface
.






Fig. 4

Normalized helicity
H
n

contours on cross
sections and vortex cores at the tip,
D

operating
point:
a) datum rotor, b) AC90/6/TF rotor, and
c) AC90/6/TFvte

Moreover

Fig. 4.b

shows
the existenc
e of a
weak tip secondary vortex
,
co
-
rotating with
TLV1
,
in the v
icinity of the suction surface
,

as
clearly
shown on 0.65


plane by the
streamwise

vorticity
distribution.

In the front portion of the blade, 0.25


downstream the leading edge, the helicity map
shows also the presence
of a third
v
o
rtical

structure

spread
ing

from the leading edge of the blade. In the
blade aft region, the leakage flow is mainly
characterized by the merging of tip separation
vortex
TLV2

and leading edge vortex with
TLV1

0.25



0.
43



0.
6
5



0.
89



1
.
2



0.25



0.
43



0.
6
5



0.
89



1
.
2



0.25



0.
43



0.
6
5



0.
89



1
.
2



c)

a)

b)

one, resulting in a unique clock
-
wi
se
v
o
rtical

structure able to affect a large share of the blade
pitch on the casing annulus.

As far as the AC90/
6/TF

rotor is concerned, Fig.
4
.b gives evidence of a clear modification in the tip
leakage phenomenon.

First of all,
Fig. 4.b shows
the
existen
ce of
a

vortex limiting the mass leak
a
g
e

along the blade pressure surface
,

and recognized as
the
evidence of the
pressure side leg of a ho
rse
-
shoe
like structure.
In t
he fi
rst quarter of the blade chord,
it is evident the trace of an additional leading edg
e
vortex co
-
rotating with
TLV1
, i
n a similar way to
datum rotor.

Both the
TLV1
and the leading edge vort
ices

feature, since their appearance on 0.2
5


plane, a
smaller in
-
passage extension coupled with a
reduced helicity magnitude when co
mpared to the
datum rotor field. Furthermore, the leading edge
vortex never collapses into the
TLV1
, but it decays
as indicated on 0.43


plane. Moving downstream,
about mid
-
chord, the
TLV1

features a gradual
H
n

reduction owing to the wea
kening of flow vorticity
and to

the deflection of vortex core.

This circumstance agrees with the hypothesis of
mass leaking reduction along the chord, that gives
rise to a leakage flow structures nearly adjacent to
the blade suction surface (i.e.
TLV2

stru
cture).

In
the aft portion of the blade, due to the leakage flow
un
-
feeding the
TLV1

collapses
,

producing a bubble
-
type separation recognized as the evidence of a
vortex breakdown by Corsini and co
-
workers
[20]
.

The separated flow turns into a counter
-
cloc
k
wise vortex under the influence of trailing edge
leakage flow streams, rapidly washing
-
out the
vortex behind the rotor

so that

on 1.2


plane no
coherent
v
o
rtical

structure
is

evident.

Finally, Fig. 4.c shows that the use of variable
t
hickness concept markedly influences the tip flow
features. The origin position of the main vortical
structure
TLV1
moves downstream with respect to
datum and AC90/6/TF blades, e.g. about 0.3

,
and, as shown by the vortex core path, it

develops
closer to the blade suction surface. At mid
-
chord the
weakened
TLV1
vortex
interacts with highly
energetic leakage jet that causes the inversion of
vortex rotation. In the aft chord fraction, the tip
vortex finally merges with a rotating cell and

exits
the blade passage appearing as a coherent clock
-
wise vortical structure.

The helicity maps confirms, for this end
-
plate
configuration, the role that is played
by the pressure
side vortical structures as result from the horse
-
shoe
vortex leg and cor
ner vortex interaction.

Figure

5 compares the TLV1 vortex core
trajectories within the blade passage.

As shown in Fig. 5, the vortex core path analysis
gives further information about the inner working of
the developed end
-
plates for leakage flow control.

Except from the
origin
s location,

both the end
-
plates
result in
TLV1

core trajectories
develop along
paths which are less skewed than the blade tip
section.



Fig. 5
T
ip
leakage
vortex
TLV1
trajectories

(line
-
squares: datum, line
-
circles: AC90/6/TF; line
-
triangles: AC90/6/TFvte)

Moreover, due to the anti
-
vortex action exerted
by the end
-
plates,
both the
improved tip rotors
feature a sudden angle variation
of
TLV1

core
due
to the interaction between the low energy
leakage
vortices
and the
passage flow exit
ing from the rotor
.

4.2. Leakage flow energy

The analysis of primary tip vortex structures,
detected by means of the helicity distributions, is
now completed by investigating on their energy
contents. The
rotational

kinetic energy
, defined on
the basis of
crosswise relative velocity components,

and resolved isotropic turbulence intensity are
considered as the bench
-
mark quantities.

The sound sources
,

tonal in nature, are related
to the energy
contents
of the tip leakage vortices
moving

in th
e axial directio
n and interacting with
any stationary surfaces (i.e. struts or outlet guide
vanes). Figure
6

compar
e
s

the

rotational kinetic
energy isolines within the tip gap,
i.e.
R
= 0.998.
In
Fig.

6 the rotational energy is normalized by the
bulk kinetic energy.

The d
atum rotor, as shown by Fig. 6.a, features
the highest rotational kinetic energy value that are
located at the onset of TLV1 where the vertical
structure origins with an energy content comparable
to the bulk one. Moreover, the datum rotor map
shows the evi
dence of the second highly rotating
core corresponding to the tip separation vortex
TLV2. In view of this picture, both the improved
rotors, Fig. 6.b and Fig. 6.c,
provides the evidence
that the end
-
plate
s

markedly affects the leakage
flow
rotational kinet
ic energy
.
Their

action
s

implements

twofold
beneficial mechanism
s: first,
the
reduc
tion

of
the momentum transfer
via

leakage
jet,
is able to
control the rotational energy at the
TLV1

onset
; second,
as shown by
the reduction of
the vortex energy,
below

the
50% of bulk kinetic
energy

across the blade passage
,
the
developed
anti
-
vortex devices act as rotational energy damper in
that
unfeeding the
mass leaking
they
are able to
weaken
the peripheral
momentum transfer to tip
vortices
.





Fig. 6
Rotational

ki
netic
energy

isolines in the
rotor tip gap @
R
= 0.998,
D

operating point:
a)
datum rotor, b) AC90/6/TF rotor, and c)
AC90/6/TFvte

As
a second energy marker
the
turbulence level
maps

are here discussed by comparing the
3D
chord
-
wise tip flow evolution on p
robing sections at
0.25, 0.43, 0.65, 0.89 and 1.2 blade chord from the
tip
section

leading edge. Figure

7

compare
s

the
turbulence intensity
TI

maps
for the investigated
fans
. These figures also include the simulated tip
leakage stream
-
paths.

As shown in Fi
g.
7
, the chordwise evolution of
turbulence level maps gives evidence of significant
differences between the rotors under investigation,
mainly concentrated in the vicinity of the casing
annulus end
-
walls. As a general observation, in
presence of the anti
-
vortex device the attenuation of
leakage flow results also in an attenuation of the
local
turbulence

level that reduces the peak
turbulent kinetic energy
within the leakage vortex
cores
. This circumstance is particularly evident in
Fig.
7
.b

and Fig. 7.c
, w
here the primary tip vortex
rolls
-
up into a low turbulence level core. While the
peak
TI
values are mainly located in the interaction
cores between the suction side near
-
surface fluid
and the leakage flow. In Fig.
7
.b it is also worth
noting that the AC90/
6/TF rotor streamlines
confirms that the primary tip vortex collapses
giving rise to a separation bubble structure, similar
to the time
-
averaged evidence of leakage flow
vortex breakdown shown by Inoue and Furukawa
[24]
.







Fig. 7 Turbulen
ce intensity
TI

contours on cross
sections and tip vortex streamlines,
D

operating
point:
a) datum rotor, b) AC90/6/TF rotor, and
c) AC90/6/TFvte

While

in
Fig. 7.c
, is clearly

evident that the
end
-
plate
variable thickness distribution
is able to
correct th
e
TLV1

brea
kdown acting locally on the
vort
ex rotation number

[21]
.

4.3.

Loss at the rotor tip

Rotor loss behaviors at the blade tip is
discussed with reference to the local total loss
coefficient defined

.
The loss behaviour is, finally,
investigated with reference to the local total loss
coefficient defined as:


=
2
0.5
in
0
0in
p p w


,
where
p
0

is the local total pressure,
0in
p

and
2
0.5
in
w


are respectively the reference pitch
-
average
d relative total and dynamic pressures
computed at the inlet mid
-
span plane.

Figure

8

s
hows the total loss coefficient
distribution within the blade passage, probing the
flow fields in the vicinity of the blade leading edge
,

about mid
-
chord and in the regi
on behind the

blade
(i.e.
respectively about 0.25 and 0.65 and 1.2 chord
from the leading edge
)
.

In the design operating
condition, Fig.s
8
.a
,

8
.b

and 8.c
, the predicted loss



c)

a)

b)

a)

b)

c)

evolutions agree with the evidences found in
literature for low
-
speed rotors. At
the rotor inlet,
all
rotor distributions
feature

loss
cores
mainly
concentrated on the hub annulus walls. Moving
toward the blade aft, the loss maps are characterized
by loss core directly related to the

development of
primary tip vortices crossing the bla
de vane. As
shown in Fig.
8
.b, the improved tip concept rotor
AC90/6/TF, owing to the vortex breakdown, is
affected by a larger peak loss core
.

Nonetheless, by
comparing the loss map on 1.2


plane with the
datum rotor one, it is clearly

evident that the
improved tip rotor presents a spanwise beneficial
loss distribution, outperforming the datum one
within the wake and on the hub end
-
wall on
pressure and suction side corners.





Fig.
8

Evolution of total pressure loss

coefficient


楮i楤攠i桥h扬b摥d灡ssag攺a摡tm牯to爬戩b
䅃A0⼶⽔牯to爬
a湤
挩䅃A0⼶⽔
vte

As far as the AC90/6/TFvte fan
rotor is
concerned, Fig. 8.c gives the evidence that the end
-
plate design concept
[21],

by controlling the
appearance of leakage vortex brea
kdown is able to
reduce the high loss core at the tip, behind the rotor
and along the blade suction side.
Th
is comparative
behaviour
, in agreement with the aerodynamic test
results, could be considered as a consequence of the
reduced 3
-
D flow re
-
arrangemen
t consequent to the
reduction in the mass leaking through the tip gap.
The limited radial migration of near
-
wall surface
fluid, induces smaller hub loss core and the
contraction of suction/corner stall (shown by the
hub corner stall core in Fig.
8.c
).

5. C
ONCLUSIONS

ON END
-
PLATES NOISE
CONTROL

Sound is a weak by
-
product of a subsonic
turbulent flow and noise generation control in fans
is challenging because turbulence is remarkably
inefficient a
s an acoustic source.
The analysis of
leakage flow
was

intended
to provide
evidences of
the aerodynamic mechanisms

realiz
ed by the
end
-
plate
based
tip concept
s

for passive noise control
.

To this end, t
he
pheno
menological viewpoint
s,
given

in

Section 4, must be complemented by
hints

on

the influence that
end
-
plates exer
t on the rotor
aeroacoustic

by altering
the

turbulen
t flow

in the tip
region.

The end
-
plates work as mixing enhancement
devices, altering by that way the turbulence
statistics and the time
-

and length
-
scales of noise
generating eddies. These flow modificat
ions
directly impact the sound field by modifying the
low
-

and high
-
frequency noise components.

In this
respect

the
effectiveness
of
tip end
-
plates, in AC90/6/TF and AC90/6/TFvte,
is first
assessed by comparing the turbulence level iso
-
surfaces in the vici
nity of the tip. In Figure 9 two
turbulence intensity levels have been taken,
respectively
TI
= 0.3

and
TI
= 0.6
.













Fig. 9 T
urbulence
i
ntensity

iso
-
surfaces
at

the
tip
: a) datum rotor, b) AC90/6/TF rotor, and c)
AC90/6/TFvte

I
t
is worth noting th
at the datum rotor features,
Fig. 9.a, a high turbulence level core concentrated
about the tip blade
that develops

over a

large
share
of the
chord.
Concerning the vortex core path, it is
furthermore evident that it entirely
evolves

within a
conical
iso
-
sur
faces at
TI

= 0.3.

With

respect
to

th
is

baseline,
both the rotors exploit
ing
the
improved tip
concepts share common features

as: the peak
TI

cores
are located near the leading edge at the onset
of the main leakage vortex, e.g.
TLV1
; moreover,
the vortex co
re trajectories rema
in within a large
low
turbulence

volume
and the iso
-
surface

TI

= 0.3
bound
s appear
to be
shifted at the periphery of
vortical regions larger than
that detected in the
datum rotor.

Furthermore,
Figure 10 compares the turbulent
viscosity
(

t

=
c


k
2
/

, normalized by
the molecular
viscosity) distribution in vicinity of the blade tip.

As
shown in Fig. 10.b and 10.c, it is confirmed that the
improved tip rotors handle the leakage flow by
c)

a)

b)

0.6

a)

b)

c)

0.3

0.6

0.3

0.6

0.3

enahncing the turbulent diffusivity within the
leakage
vortices.












Fig. 10
Normalized t
urbulent viscosity

t

iso
-
surfaces
at

the tip
:
a) datum rotor, b) AC90/6/TF
rotor,
and
c) AC90/6/T
f
vte

(

t
_
2
: 10
2

v
mol
;

t
_
1
:
5× 10 v
mol
)

On the contrary in Fig. 10.a, the datum rotor
features lower diffusivity lev
el in correspondence
with the
TLV1

path, and reaches the higher viscosity
value behind the blade where the vo
rtex interacts
with primary and secondary flows.

A
s far as the broadband noise is concerned, in
the subsonic range, it could be related to primary
tip
vortices that once formed are responsible for the
convection of large
-
scale fluctuations downwind the
trailing edge that gives rise to scattering and
broadband noise
[
29
]
.
Moreover, observations
drawn from flow visualization experiments and
fluid dynam
ic measurements
complete this picture
by indicating

that
also
secondary flows
contribute

to
the broadband noise generated
by small cooling
axial flow fans.
















Fig. 11
Normalized h
elicity
H
n

iso
-
surfaces
behind the fan rotors:
a) datum rotor,
b)
AC90/6/TF rotor, and c) AC90/6/TFvte

Figure 11 shows the normalized helicity
distribution behind the rotors under investigation, in
order to compare how the wake behaviour is
affected by the end
-
plates adoption.

As a general comment it could be conclud
ed
that the application of the improved tip concepts
extends

its
beneficial effect on the blade passage
secondary flows.
As clearly shown in Fig. 11.b and
11.c, the helicity maps give the evidence of the
magnitude reduction of the secondary phenomena
relat
ed to the wake and to the hub corner vortices.

6.
SUMMARY

We have carried out a study on the structure of
tip leakage flow and its influence on the fluid
dynamical behaviour in a family of axial flow fans.
The aim of the paper has been to speculate on the
effectiveness

of improved blade tip concept
s

to
control the leakage flow phenomena
and rotor
aeroacoustic signatures
.
Two end
-
plate geometries
have been developed the first one with constant
thickness, and the second one with a variable
thickness distribut
ion accordind to
to safe rotation
number chord
-
wise gradient concept.

The aerodynamic tests have shown that the
improved tip concept
s

are

affected by a small
performance de
-
rating, but
the

efficiency
curves
give evidence of an
improvement with increased
pe
ak performance and wider high
efficiency

plateau
towards the rotor stall margin.
The noise test,
furthermore, demonstrated a reduction of the rotor
aeroacoustic signature both in terms of
tonal noise
and broad
-
band noise.

The investigation was based on an
in
-
house
developed parallel finite element Navier
-
Stokes
solver. The physical interpretation of the detailed
3D flowfield predictions were discussed by means
of streamlines, streamwise vorticity, or leakage
flow kinetic energy and loss maps. The following
conclusions are drawn on the basis of the found
fluid dynamical behaviours.

The comparison of the detected leakage vortical
structure evolutions showed that
the datum rotor
features a multiple vortex behaviour characterized
by a
dominant
leading edge vorti
cal

structure,
highly skewed with respect to the local relative flow
direction, and a weak tip secondary vortex in the
vicinity of the suction surface. The presence of the
end
-
plate
s

influence the leakage flow
s

structure at
the leading edge by changing its

orientation with
respect to the local
relative

streamlines that govern
the
secondary

flow advection
in the rotor frame.

Both t
he

rotors designed with the improved tip
geometries
appear to be
characterized

by two
v
o
rtical

structures

at the tip,

respectivel
y the
trace
of the
pressure side leg of an incoming horse
-
shoe
vortex like structure and the suction side trace of the
main leakage flow. In this viewpoint the
existence

of this pressure side vortex, peculiar of the
improved tip concept
s

rotor,
is recogniz
ed

as one of
the factor
contributing

to the control of the leakage
phenomenon promoting a
vena contracta

effect. By
that way the mass leaking is un
-
fed close to the
leading edge and the gained control on leakage flow
onset turns into potentially improved a
eroacoustic
performance.

The analysis of the leakage flow energy
contents, indirectly related to the recognized tip
leakage noise mechanisms, has given the evidence

t
_
1


t
_
2


t
_
1


t
_
2


t
_
1


t
_
1


t
_
2

a)

b)

c)

c)

b
)

a
)

t.e.

tip

hub

of a reduction in the
rotational

kinetic energy and
turbulence level on the casing owing t
o the presence
of the anti
-
vortex device.

The loss coefficient distributions confirm that
the highest loss regions were always
observed

in
coincidence with the leakage vortex core with a
nearly constant peak loss value. The comparative
analysis of mechanic
al energy loss within the gap
showed that the presence of the anti
-
vortex device
at the tip leads to a reduction of mechanical energy
loss within the gap, suggesting that the loss level
within the tip gap is mainly controlled by the mass
leaking.

ACKNOWLED
GEMENTS

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

. The authors
gratefully acknowledge Mr. I. Kinghorn and Mr. B.
Perugini for
their contribution to the experiments.

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-
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[2]

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