EFFECT OF INLET STRAIGHTENERS ON CENTRIFUGAL FAN
PERFORMANCE
A. ABDEL HAFIZ1,N. N. BAYOMI1, and A. M. OSMAN2
ABSTRACT
The use of straighteners in the inlet duct of centrifugal fans are suggested for
eliminating any inlet distortion. An experimental investigation was performed to
study the effect of inlet straighteners on the performance characteristics of
centrifugal fans. Two types of straighteners were used, circular tubes and zigzag
cross-section with different lengths. Circular tubes with different diameters have
been investigated. The study was carried out on three types of fans, namely
radial, backward with exit blade angles 60 and 75 and forward with 105 and
120. The results confirm that the inlet straighteners exhibit different effect on
the fan performance for the different blade angles. Accordingly, the results
implement selection of long circular tube straighteners with large diameter for
radial blades, long zigzag type for backward 60 blade angle whereas short zigzag
type for blade angle 75. Generally, good improvements in efficiency are
observed for radial and backward blade on account of a slight drop in static head.
In addition, an increase in the flow margin up to 12% and a decrease in the noise
level from 3 to 5 dB are indicted compared to free inlet condition. On the
contrary, unfavorable influences are exerted on the forward fan performance.
_________________________________________________________________
1
Faculty of Engineering, Mataria, Helwan University, 11718 Masaken
El-Helmia,Cairo, EGYPT, E-Mail: nnbayomi@hotmail.com
2
Faculty of Engineering, Shoubra, Zagazig University, Cairo, EGYPT.
Nomenclature
D
Inlet duct diameter
1
Inlet blade angle
d
Straighteners tube diameter
2
Exit blade angle
FM
Flow margin
Hst
Static head difference
h
Straightener zigzag height

Specific weight
L
Straightener length

Efficiency = VHst/Psh
Psh
Shaft power

Static pressure ratio
r
Radius
Subscript
ro
Outer radius of inlet duct
SM
Surge margin
V
Volume flow rate
1- INTRODUCTION
max
op
sp
Maximum
Operating point
Surge point
In the traditional market of centrifugal fans for industrial, commercial, and
utility applications, strong emphasis has long been placed on the initial cost of
these fans. Considerations of ease of manufacturing and installation and
maintenance of the equipment in the field have tempered any improvements
in performance. The growing width of fan applications causes variation in
inlet duct configuration due to spatial restrictions. The flow non-uniformity is
frequently generated at the impeller inlet and consequently, deterioration of
fan performance is expected. Generally this is known as the inlet distortion.
Deviations from a steady uniform distribution of the flow properties can
include variations in swirl, velocity, turbulence, total and static pressures,
velocity, temperature, flow angle and fluid density. Non-uniform inlet profiles
are created in industrial fans or in ventilation systems using a 90 bend
directly, upstream of the inlet due to mechanical limitations which dictate the
radial enter to the machine. Generally, the air separates at the top surface of
the bend and generates secondary flow within the cross sections of the inlet.
The swirl generated by the secondary flow and the separation results in a
distortion of the flowfield at the fan entry. Ariga et al. [1] divided the inlet
distortion for compressors into two dominant forms radial and
circumferential distortions. The former one is subdivided into tip and hub
distortion. Hub distortion occurs when an axisymmetric obstacle is used at the
center portion of the inlet fan such as a tachometer pick up and hub cover.
The tip distortion is happened when axisymmetric boundary layers of an inlet
duct exist or axisymmetric obstacles such as an orifice plate are used. The
circumferential distortion happens from non-axisymmetric obstacles such as
struts or bending duct.
Although the non-uniformity of inlet flow appears frequently in centrifugal fan,
only few data about the inlet distortions are found in the literature most of
them for centrifugal compressors. Field measurements of compressor
performance indicated that both efficiency and pressure rise were several
percentage points lower than expected performance, Ariga et al. [1&2]. Onset
of stall was influence on magnified by severely distorted inflows, by Graber
and Braithwaite [3], Greitzer [4] and Baghdadi and Lueke [5]. Similarly for
centrifugal fans, Wright et al. [6] showed significant degradation in efficiency
and pressure rise as much as 10% to 15% resulting from moderately to
severely distorted inflow patterns. The existence of inlet distortion is
considered to cause partial flow separation at the entrance of the fan
compared to non-distorted conditions. Moreover the flow range becomes
narrower due to the fact that the begging of the instability of the flow such as
the rotating stall and surge in the centrifugal fans is affected by seriously
distorted inflows.
Consequently, it is necessary that the distorted flow have to be rectified
before entering to the impeller. This can be done by different ways based on a
mechanism in which secondary vortices are counteracted by the vortices
generated in the opposite sense of the secondary flow by additional vortex
generators. Inlet guide vanes are employed by Madhavan and Wright [7 & 8],
Chen et al. [9], Montazerin et al. [10], Kassens and Rautenberg [11] and
Coppinger and Swain [12]. Unfortunately, additional inlet vortex occurs in fans
with inlet vane control causing unstable flow at entrance of the impeller, which
further complicates the situation. This unstability causes unfavorable effect on
the stall point, increased noise and vibration levels which can lead to fatigue
cracks in inlet ducts as well as in the rotor, Chen et al. [9]. Also, Jack [13]
cleared that centrifugal fans that operate at inefficient lower volumes are subject
to rotating stall or surge, which wastes power and generates excessive low
frequency noise. Bhope & Padole [14] investigated the noise level and fluid flow
in centrifugal fan impeller.
The present paper suggests the fitting of annular straighteners at the
entrance of the impeller in order to rectify the non-uniformity of the flow and to
eliminate the vortices generated caused by the existence of inlet distortion. The
main objective of this work is to assess the use of these straighteners on the fan
performance. For this purpose two different types of straighteners, circular and
corrugated (zigzag) are considered with different size. The investigation is conducted
on five different impellers with different exit blade angles. Comparisons with free
inlet fans are performed. Measurements of static head, shaft power and noise levels
at different loads are conducted for the different cases. The analysis of these
measurements gives some information concerning operating range and surge margin
for these types of fans.
2- TEST FACILITIES AND INSTRUMENTATION
Experimental investigation was carried out in the Turbomachine Lab at
Mataria Faculty. The test rig consists of a low-pressure commercial centrifugal fan of
the radial type, a test inlet duct and a delivery duct. The fan wheel comprises 16
straight blades of 3 mm thickness with constant blade width of 60 mm welded to a
back plate and a shroud. The impeller inner and outer diameters are 215 mm and
394 mm, respectively. The scroll casing is of constant rectangular width. The fan is
driven by an electric motor of shaft power 3 hp at constant speed of 2800 rpm. The
test inlet duct is of 160 mm diameter and 300 mm length. The exit circular duct of
100 mm diameter is connected to the rectangular outlet of the fan through a conical
connection and fitted at the end with a throttle valve. Figure (1) illustrates the test
rig layout equipped with the measuring devices.
In this investigation, the suggested straighteners for overcoming any tip or
circumferential distortion are located in the inlet duct at a distance of 30 mm from
the impeller entrance. Two types of straighteners are designed both with constant
annular cross-section of inner and exit diameters 45 mm and 160 mm, respectively.
One type consists of annular bundles of plastic or PVC tubes with different
diameters, 2.5, 4 and 15 mm. The other type (the zigzag type) is manufactured as the
same process for catalytic converters for car exhaust. Hardened paper foil is
corrugated and wound up together with non-corrugated foil making a triangle crosssection of height 10 mm. The various layers of corrugated and non-corrugated foils
are glued to each other making the annular shape. The length of the straighteners
considered as a parameter, has been varied from 225 mm to 180 mm making a
length to duct diameter ratio, L/D, 1.4 and 1.125. Schematic drawings show the
different shapes of straighteners in Fig. (2).
The average static pressures at inlet and exit of the fan are measured through
four taps equally distributed circumferentially, Fig. (1). The flow velocity distribution
across the delivery duct diameter was measured using a standard cylindrical Prandtl
probe with inner diameter 2 mm mounted on a traverse mechanism with accuracy
0.1%. The probe is located at ten diameters from the delivery duct inlet to ensure
uniformity of the flow. The flow through the fan is controlled by a spherical regulator
valve located at the end of the delivery pipe.
In order to check the flow uniformity at the fan inlet downstream the
straighteners, the total pressure distribution is measured by a shielded Pitot tube
using a traversing mechanism with accuracy 0.1%. All the pressures were measured
through a multi channel switch by a digital micro-manometer model Yokogawa 2655,
with resolution of 0.1 Pa and updating of the reading every 0.4 sec. An average of
the readings is computed every 5 sec using an A/D converter and a PC. Figure (3)
shows the total suction head distribution of the radial blade impeller with and
without straighteners at design point. From this figure it can be seen that the total
head at the straighteners exit is approximately constant. Compared with the free
inlet, a drop in the suction head is detected by the presence of the straighteners that
increases as the diameter of tubes decreases.
Circular
tube
Static taps
Centrifugal fan
Prandtl probe
Spherical
valve
Flow
Static taps
Pitot tube
Straight
blade
Computer
Sound level
meter
Micromanometer
Multi-channel Static and/or
total pressure
Switch
Wattmeter
a) Test rig.
Impeller
Straightener
Impeller
Air
inlet
b) Setting location of straighteners.
Bellmouth
Fig. (1) Test rig and measuring devices layout.
160 mm
45
D
L
Fig. (1) test rig and measuring devices layout.
h=10 mm
d=2.5 mm
d=4 mm
d=15 mm
2640 tube
1030 tube
74 tube
Circular type
Zigzag type
Fig. (2) A schematic drawing of the different shape of straighteners.
inlet
b) Setting location of straighteners.
Bellmouth
Fig. (1) Test rig and measuring devices layout.
160 mm
45
L
D
h=10 mm
d=2.5 mm
d=4 mm
d=15 mm
2640 tube
1030 tube
74 tube
Zigzag type
Circular type
Fig. (2) A schematic drawing of the different shape of straighteners.
Total suction head (m water)
Fig. (2) A schematic drawing of the different shape of straighteners.
0.00
-0.05
-0.10
Free
d=2.5 mm
-0.15
d=4 mm
d=15 mm
-0.20
-0.25
0.0
0.2
0.4
0.6
0.8
1.0
r/ro.
Fig. (3) Total suction head distribution at the inlet of the radial fan
with and without straighteners at design point.
Fig. (3) Total suction head distribution at the inlet of the radial fan
with and without straighteners at design point.
The shaft power of the fan is measured by a digital wattmeter with accuracy
0.09%, while the rotational speed by a digital tachometer model Lutron Dt-2236
with accuracy 0.05%. The noise level in dB, measured by sound pressure level of
the fans with different inlet configuration, is determined. A portable sound level
meter equipped with a special stand and set to A-weighting (slow response) is used.
Three different near field measuring locations have been chosen at a standard
distance equal to twice the impeller-housing diameter in accordance with DIN
45635: at fan inlet, near the delivery duct exit and behind the fan motor. The noise
level was always found maximum near the exit of the delivery duct, therefore
measurements were recorded and presented only at this station.
The effect of the straighteners on fans with different exit blade angles has
been also investigates. Accordingly, four new impellers, two backward and two
forward with different exit blade angles have been constructed using the original
scroll housing. More details about the different impellers are tabulated in Table (1).
Table (1) Characteristics of the different impellers.
Parameter
Original
impeller
Impeller I
Impeller II
Impeller III
Impeller
IV
Outlet angle, 2
90
60
75
105
120
Inlet angle, 1
90
25
60
125
150
Blade length (mm)
80
86
84
84
86
3- EXPERIMENTAL RESULTS AND DISCUSSION
The characteristic curves of the five different tested impellers are
shown in Figs. (4-8). The measured delivery static head, together with the
calculated static efficiency and the shaft power are plotted versus the
volume flow rate for each fan with the different types of inlet straighteners
of L/D=1.4. Comparisons with free inlet condition were performed on the
same plots.
The performance of the radial fan using different straighteners is shown in Fig.
(4). At large flow rate a remarkable increase in static head due to straighteners
can be noticed until 0.15 m3/s. This is accompanied by appreciable
improvement in the fan efficiency. By decreasing the flow rate the effect of the
straighteners is vanished. As the diameter of the straightener tubes increases
more flattened efficiency curve is observed. An improvement of 5 points in
efficiency corresponding to a relative increase of 18% is obtained with
straighteners of 15 mm diameter and corresponding decrease in shaft power is
noticeable. This is due to the good guidance of the flow provided by the
straighteners at impeller inlet. Furthermore, the unstable operating range of the
fan extends farther out due to the straighteners. It is useful to note that during
experiments the surge point was detected by fluctuations in pressure readings
in addition to high audible noise. The results of the noise test in Fig. (9)
expressed in sound pressure levels in dB (A) show that the use of straighteners
decreases the noise level along the operating range by approximately 5 dB at
high flow rate. In practice an overall sound power increase of 3 dB is just
perceptible to the human ear and 5 dB is clearly louder.
The performance of the backward fans with 60 and 75 exit blade angle is
shown in Figs. (5&6), respectively. The first impeller exhibits improvement
that could reach about 6 points in efficiencies employing the zigzag
straighteners. This represents a relative increase of 21% in efficiency.
However, a small drop in the static head associated with an increase in shaft
power is observed. A reduction of about 3 dB in noise level is resulted, Fig. (9).
It worth to note that the noise level increases as the blade angle increases, this
was also detected by Liberman, [15]. As the blade angle increases to 75 lower
efficiency is obtained allover the operating range at free inlet condition. This is
due to the high incidence losses resulting from the corresponding large inlet
blade angle. Using the inlet straighteners leads to further drop in static head
as well as in efficiency. However, the use of straightener with very small tube
diameter increases obviously the efficiency but on account of low delivery
head. This is associated with a noticeable decrease in the maximum flow rate,
choke point. However, the onset of the surge point shifts to lower flow rate.
Figures (7&8) indicate the performance of the forward fans with exit blade angle
105 and 120. The use of inlet straighteners result in small increases in static
efficiency for 105 on account of appreciable drop in static head. Whereas, for
impeller with 120 deterioration in efficiency as well as in delivery head are
noticed. In this case, it worth to note that at free inlet condition the fan
efficiency is already very low. This is due to probable separation of the flow
inside the impeller passages as the number of blades is much lower than
usually for forward blades. This is associated with higher noise level
compared to radial and backward blades. This is in agreement with the results
of Liberman, [15].
Delivery Static Head (m water)
0.30
2.=90
0.25
Free
0.20
d=2.5 mm
0.15
d=4 mm
d=15 mm
0.10
Zigzag
0.05
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Static Efficiency %
40.0
30.0
20.0
10.0
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Shaft Power (kW)
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (4) Fan performance for radial impeller ( 2.= 90 )
with different straighteners.
Fig. (4) Fan performance for radial impeller (2 = 90 )
with different straighteners.
Delivery Static Head (m water)
0.30
2.=60
0.25
Free
0.20
d=2.5 mm
0.15
d=4 mm
d=15 mm
0.10
Zigzag
0.05
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Static Efficiency %
40.0
30.0
20.0
10.0
0.0
0.00
0.05
0.10
0.15
0.20
V (m3./s)
Shaft Power (kW)
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (5) Fan performance for backward impeller ( 2 = 60 )
Fig. (5) Fan
performance
for backward impeller (2 = 60 )
with
different straighteners.
with different straighteners.
Delivery Static Head (m water)
0.30
0.25
2.=75
Free
0.20
d=2.5 mm
0.15
d=4 mm
0.10
d=15 mm
0.05
Zigzag
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Static Efficiency %
40.0
30.0
20.0
10.0
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Shaft Power (kW)
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (6) Fan performance for backward impeller ( 2.= 75 )
with different straighteners.
Fig. (6) Fan performance for backward impeller (2 = 75 )
Delivery Static Head (m water)
with different straighteners.
2.=105
0.30
Free
0.25
d=2.5 mm
0.20
d=4 mm
0.15
d=15 mm
0.10
Zigzag
0.05
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Static Efficiency %
40.0
30.0
20.0
10.0
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Shaft Power (kW)
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (7) Fan performance for forward impeller (2 = 105 )
with different straighteners.
Delivery Static Head (m water)
0.30
2.=120
Free
0.25
d=2.5 mm
0.20
d=4 mm
0.15
d=15 mm
0.10
Zigzag
0.05
0.00
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Static Efficiency %
30.0
20.0
10.0
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Shaft Power (kW)
2.0
1.5
1.0
0.5
0.0
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (8) Fan performance for forward impeller (2 = 120 )
with different straighteners.
90
2.=90
Noise (dB)
Free
d=2.5 mm
85
d=4 mm
d=15 mm
Zigzag
80
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.25
0.30
0.35
V (m 3./s)
Noise (dB)
90
2. =60
85
80
0.00
0.05
0.10
0.15
0.20
V (m 3./s)
Fig. (9) Noise level for different straighteners for radial
and backward (60 ) impellers.
Fig. (9) Noise level for different straighteners for radil
and backward (60) impellers.
To assess the effect of the straighteners on the fan operation some parameters
should be taken into consideration. These parameters are the flow margin and
the surge margin. The flow margin is defined as
  Vsp
Flow margin (FM)  1  
  Vmax

 X 100%


Where Vsp and Vmax are the volume flow rate at surge point and the maximum
flow rate, respectively. The surge margin is calculated from the definition
 Π


 V )sp
Surge margin (SM)  
 1
 Π )op 
 V

given by Cumpsty [16] as
Where  is the static pressure ratio and the suffix op indicates operating
point corresponding to the condition at maximum efficiency.
Figures (10&11) show the calculated flow margin and the surge margin,
respectively, for the different fans with different straighteners compared to
free inlet condition. The results show that the flow margin may be arbitrary
increased up to 12% by using inlet straighteners for backward and radial
impellers. For forward blade, this reduces the flow margin especially with
small diameter. Great improvements in the surge margin are depicted for the
different blade angles when using the different straighteners, Fig. (11).
From the previous results, it can be deduced that the effect of the straighteners
on the fan performance varies according to the exit blade angle. Accordingly,
the most suitable straightener for each impeller type can be selected.
Straighteners with tube diameter 15 mm conform well to the radial blades,
whereas the zigzag type is advisable to be used with backward blades. It
follows from the result analysis that inlet straighteners are not convenient for
forward blades.
The effect of straighteners length on the fan performance has been studied.
Samples of the results obtained using short straighteners of L/D=1.125 are
presented in Fig. (12) compared to the longer one taking into consideration the
most efficient inlet configuration for each fan. It can be noted that for radial
and backward impeller with blade angle 60 decreasing the length of the
straighteners weakened the performance of the fan. Whereas for blade angle
75 the shorter zigzag type of straighteners improves the fan performance.
Fig. (10) Comparison of the flow margin for different
blade angles with different straighteners.
Fig. (11) Comparison of the surge margin for different
blade angles with different straighteners.
Deivery Static Head (m Water)
0.30
Circular type (d= 15 mm)
0.30
2.=90
0.25
0.25
0.25
0.20
0.20
0.20
0.15
0.15
0.15
0.10
0.10
0.10
Zigzag type
2.=60
Free
0.05
0.05
Free
0.05
L/D=1.4
L/D=1.125
L/D=1.125
0.00
0.1
0.2
V (m3./s)
0.3
0.00
0.0
0.1
0.2
V (m3./s)
0.3
0.0
40.0
40.0
40.0
30.0
30.0
30.0
20.0
20.0
20.0
Free
10.0
Free
10.0
L/D=1.4
L/D=1.125
L/D=1.125
0.2
V (m3./s)
0.3
0.2
V (m3./s)
0.3
Free
L/D=1.4
0.0
0.1
0.1
10.0
L/D=1.4
0.0
0.0
L/D=1.4
L/D=1.125
0.00
Static Efficiency %
0.30
Free
L/D=1.4
0.0
Zigzag type
2.=75
L/D=1.125
0.0
0.0
0.1
0.2
V (m3./s)
0.3
0.0
0.1
0.2
V (m3./s)
Fig. (12) The effect of straighteners length on fan performance.
Fig. (12) The effect of straighteners length on fan performance.
0.3
Conclusion
The present paper investigates the effects of inlet straighteners on the
performance, operating range and instantaneous surge of a centrifugal fan.
Experimental investigations concerning different types and sizes of inlet
straighteners for radial, backward and forward fans were conducted. The following
conclusion can be drawn:
1- The effect of straighteners on the fan performance depends mainly on the exit
blade angle. More flattened efficiency curve is obtained by increasing the
straightener tube diameters. An improvement of 5 points in efficiency corresponding
to 18% relative increase in efficiency is obtained using circular tube straighteners
with 15 mm diameter and L/D=1.4 for radial impeller. A relative increase of 21% in
the efficiency of backward fan of 60 blade angle associated with a small drop in
delivery head is obtained when using straighteners of zigzag type. A bad effect for
the different straighteners is observed on the fan performance for forward impeller.
Seldom effect is noted on the maximum permissible flow rate (choke point) for radial
fan, while for backward and forward blades it decreases by using straighteners.
2- The flow margin increases up to 12% for backward and radial impellers.
3- Improvements of surge margin are depicted for the different blade angles.
4- The use of straighteners decreases the noise level by approximately 3 to 5 dB at
high flow rate compared with free inlet condition for radial and backward impellers.
5- The effect of straighteners length varies with exit blade angles. For backward
impeller with blade angle 60 as well as for radial fans the longer zigzag and
circular straighteners, respectively, give better performance. Whereas for blade
angle 75 the shorter zigzag is the best.
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