Energy Conversion and Management 47 (2006) 3307–3318

www.elsevier.com/locate/enconman

Effect of inlet straighteners on centrifugal fan performance

N.N. Bayomi a,*, A. Abdel Hafiz a, A.M. Osman b

a Faculty of Engineering, Mataria, Helwan University, 11718 Masaken, El-Helmia, Cairo, Egyptb Faculty of Engineering, Shoubra, Zagazig University, Cairo, Egypt

Received 31 July 2005; accepted 30 January 2006Available online 3 April 2006

Abstract

The use of straighteners in the inlet duct of centrifugal fans is suggested for eliminating any inlet distortion. An exper-imental investigation was performed to study the effect of inlet straighteners on the performance characteristics of centrif-ugal fans. Two types of straighteners were used, circular tubes and zigzag cross section, with different lengths. Circulartubes with different diameters have been investigated. The study was conducted on three types of fans, namely radial, back-ward with exit blade angles 60� and 75� and forward with 105� and 120�. The results confirm that the inlet straightenersexhibit different effects on the fan performance for the different blade angles. Accordingly, the results indicate the selectionof long circular tube straighteners with large diameter for radial blades, long zigzag type for backward 60� blade angle andshort zigzag type for backward 75� blade angle. Generally, good improvements in efficiency are observed for radial andbackward blades on account of a slight drop in static head. In addition, an increase in the flow margin up to 12% anda decrease in the noise level from 3 to 5 dB are indicated compared to the free inlet condition. On the contrary, unfavorableinfluences are exerted on the forward fan performance.� 2006 Published by Elsevier Ltd.

Keywords: Centrifugal fan; Straighteners; Noise; Distortion

1. Introduction

In the traditional market of centrifugal fans for industrial, commercial and utility applications, strongemphasis has long been placed on the initial cost of these fans. Considerations of ease of manufacturingand installation and maintenance of the equipment in the field have tempered any improvements in perfor-mance. The growing breadth of fan applications causes variations in inlet duct configuration due to spatialrestrictions. Flow non-uniformity is frequently generated at the impeller inlet, and consequently, deteriorationof fan performance is expected. Generally, this is known as the inlet distortion. Deviations from a steady uni-form distribution of the flow properties can include variations in swirl, velocity, turbulence, total and staticpressures, velocity, temperature, flow angle and fluid density. Non-uniform inlet profiles are created in indus-trial fans or in ventilation systems using a 90� bend directly upstream of the inlet due to mechanical

0196-8904/$ - see front matter � 2006 Published by Elsevier Ltd.

doi:10.1016/j.enconman.2006.01.003

* Corresponding author. Tel.: +20 2 4838988; fax: +20 2 2735437.E-mail addresses: nnmbayomi@yahoo.com, mmbayomi@hotmail.com (N.N. Bayomi).

Nomenclature

D inlet duct diameterD straighteners tube diameterFM flow marginH straightener zigzag heightL straightener lengthPsh shaft powerR radiusro outer radius of inlet ductSM surge marginV volume flow rateb1 inlet blade angleb2 exit blade angleDHst static head differencec specific weightg efficiency = cVDHst/Psh

P static pressure ratio

Subscripts

Max maximumOp operating pointSp surge point

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limitations, which dictate the radial entry to the machine. Generally, the air separates at the top surface of thebend and generates secondary flow within the cross sections of the inlet. The swirl generated by the secondaryflow and the separation results in a distortion of the flow field at the fan entry. Ariga et al. [1] divided the inletdistortion for compressors into two dominant forms, radial and circumferential distortions. The former one issubdivided into tip and hub distortion. Hub distortion occurs when an axisymmetric obstacle is used at thecenter portion of the inlet fan such as a tachometer pick up and hub cover. Tip distortion happens when axi-symmetric boundary layers of an inlet duct exist or axisymmetric obstacles such as an orifice plate are used.Circumferential distortion happens from non-axisymmetric obstacles such as struts or a bending duct.

Although the non-uniformity of inlet flow appears frequently in centrifugal fans, only a few data about inletdistortions are found in the literature, and most of them are for centrifugal compressors. Field measurementsof compressor performance indicated that both efficiency and pressure rise were several percentage pointslower than the expected performance [1,2]. The onset of stall was influenced or magnified by severely distortedinflows [3–5]. Similarly, for centrifugal fans, Wright et al. [6] showed significant degradation in efficiency andpressure rise, as much as 10–15%, resulting from moderately to severely distorted inflow patterns. The exis-tence of inlet distortion is considered to cause partial flow separation at the entrance of the fan comparedto non-distorted conditions. Moreover, the flow range becomes narrower due to the fact that the beginningof the instability of the flow, such as rotating stall and surge, in centrifugal fans is affected by seriously dis-torted inflows.

Consequently, it is necessary that the distorted flow be rectified before it enters the impeller. This can bedone by different ways based on a mechanism in which secondary vortices are counteracted by vortices gen-erated in the opposite sense of the secondary flow by additional vortex generators. Inlet guide vanes wereemployed 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 vortices occur in fans with inlet vane con-trol, causing unstable flow at the entrance of the impeller, which further complicates the situation. This insta-bility causes unfavorable effects on the stall point and increases noise and vibration levels, which can lead tofatigue cracks in inlet ducts as well as in the rotor [9]. Also, Jack [13] found that centrifugal fans that operate at

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inefficient lower volumes are subject to rotating stall or surge, which wastes power and generates excessive lowfrequency noise. Bhope and Padole [14] investigated the noise level and fluid flow in a centrifugal fan impeller.

The present paper suggests the fitting of annular straighteners at the entrance of the impeller in order torectify the non-uniformity of the flow and to eliminate the vortices that are generated by the existence of inletdistortion. 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 withdifferent sizes. The investigation is conducted on five different impellers with different exit blade angles. Com-parisons with free inlet fans are performed. Measurements of static head, shaft power and noise levels at dif-ferent loads are conducted for the different cases. The analysis of these measurements gives some informationconcerning the operating range and surge margin for these types of fans.

2. Test facilities and instrumentation

Experimental investigation were conducted in the Turbomachine Laboratory of the Mataria Faculty. Thetest rig consists of a low pressure commercial centrifugal fan of the radial type, a test inlet duct and a deliveryduct. The fan wheel is comprised of 16 straight blades of 3 mm thickness with constant blade width of 60 mmwelded to a back plate and a shroud. The impeller inner and outer diameters are 215 mm and 394 mm, respec-tively. The scroll casing is of constant rectangular width. The fan is driven by an electric motor of shaft power3 hp at constant speed of 2800 rpm. The test inlet duct is 160 mm in diameter and 300 mm long. The exit cir-cular duct of 100 mm diameter is connected to the rectangular outlet of the fan through a conical connectionand fitted at the end with a throttle valve. Fig. 1 illustrates the test rig layout equipped with the measuringdevices.

In this investigation, the suggested straighteners for overcoming any tip or circumferential distortion arelocated in the inlet duct at a distance of 30 mm from the impeller entrance. Two types of straighteners aredesigned, both with constant annular cross section of inner and exit diameters 45 mm and 160 mm, respec-tively. One type consists of annular bundles of plastic or PVC tubes with different diameters, 2.5, 4 and15 mm. The other type (the zigzag type) is manufactured with the same process used for catalytic convertersfor car exhaust. Hardened paper foil is corrugated and wound together with non-corrugated foil, making atriangular cross section of height 10 mm. The various layers of corrugated and non-corrugated foils are gluedto each other, making the annular shape. The length of the straighteners, considered as a parameter, has been

Setting location of straighteners.

Wattmeter

Computer

Straightener

Centrifugal fan

Straight blade

Impeller

Static taps

Bellmouth

inlet Air

Impeller

Sound level meter

Flow

Pitot tube

Static taps

Multi-channel Switch

Micromanometer

Test rig.

Prandtl probeCircular tube

Static and/ortotal pressure

Sphericalvalve

(b) Setting location of straighteners.

Wattmeter

Computer

Straightener

Centrifugal fan

Straight blade

Impeller

Static taps

Bellmouth

inlet Air

Impeller

Sound level meter

Flow

Pitot tube

Static taps

Multi-channel Switch

Micromanometer

(a) Test rig.

Prandtl probeCircular tube

Static and/ortotal pressure

Sphericalvalve

Fig. 1. Test rig and measuring devices layout.

160 mm

2640 tube

d=2.5 mm

1030 tube

Circular type

d=4 mm

45

Zigzag type

h=10 mm

74 tube

d=15 mm

L

D160 mm

2640 tube

d=2.5 mm

1030 tube

Circular type

d=4 mm

45

Zigzag type

h=10 mm

74 tube

d=15 mm

L

D

Fig. 2. A schematic drawing of the different shape of straighteners.

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varied from 225 mm to 180 mm, making a length to duct diameter ratio, L/D, of 1.4 and 1.125. Schematicdrawings show the different shapes of the straighteners in Fig. 2.

The average static pressures at the inlet and exit of the fan are measured through four taps equally distrib-uted circumferentially, Fig. 1. The flow velocity distribution across the delivery duct diameter was measuredusing a standard cylindrical Prandtl probe with inner diameter 2 mm mounted on a traverse mechanism withaccuracy ±0.1%. The probe is located at ten diameters from the delivery duct inlet to ensure uniformity of theflow. The flow through the fan is controlled by a spherical regulator valve located at the end of the deliverypipe.

In order to check the flow uniformity at the fan inlet downstream of the straighteners, the total pressuredistribution is measured by a shielded Pitot tube using a traversing mechanism with accuracy ±0.1%. All thepressures were measured through a multi-channel switch by a digital micro-manometer, model Yokogawa2655, with resolution of 0.1 Pa and updating of the reading every 0.4 s. An average of the readings is com-puted every 5 s using an A/D converter and a PC. Fig. 3 shows the total suction head distribution of theradial blade impeller with and without straighteners at the design point. From this figure, it can be seen thatthe total head at the straighteners exit is approximately constant. Compared with the free inlet, a drop inthe suction head is detected by the presence of the straighteners that increases as the diameter of tubesdecreases.

0.0 0.2 0.4 0.6 0.8 1.0r/ro

-0.25

-0.20

-0.15

-0.10

-0.05

0.00

Tot

al s

ucti

on h

ead

(m w

ater

)

Free

d=2.5 mm

d=4 mm

d=15 mm

Fig. 3. Total suction head distribution at the inlet of the radial fan with and without straighteners at design point.

Table 1Characteristics of the different impellers

Parameter Original impeller Impeller I Impeller II Impeller III Impeller IV

Outlet angle, b2 (deg) 90 60 75 105 120Inlet angle, b1 (deg) 90 25 60 125 150Blade length (mm) 80 86 84 84 86

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The shaft power of the fan is measured by a digital wattmeter with accuracy ±0.09%, while the rotationalspeed is measured by a digital tachometer, model Lutron Dt-2236, with accuracy ±0.05%. The noise level indB, measured by sound pressure level of the fans with different inlet configurations, is determined. A portablesound level meter equipped with a special stand and set to A-weighting (slow response) is used. Three differentnear field measuring locations have been chosen at a standard distance equal to twice the impeller housingdiameter in accordance with DIN 45635: at fan inlet, near the delivery duct exit and behind the fan motor.The noise level was always found to be maximum near the exit of the delivery duct, and therefore, measure-ments were recorded and are presented only at this station.

The effect of the straighteners on fans with different exit blade angles has also been investigated. Accord-ingly, four new impellers, two backward and two forward facing, with different exit blade angles have been

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iver

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Hea

d (m

wat

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ienc

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1.5

2.0

Shaf

t P

ower

(kW

)

Free

d=2.5 mm

d=4 mm

d=15mm

Zigzag

β2=90º

Fig. 4. Fan performance for radial impeller (b2 = 90�) with different straighteners.

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constructed using the original scroll housing. More details about the different impellers are tabulated inTable 1.

3. Experimental results and discussion

The characteristic curves of the five different tested impellers are shown in Figs. 4–8. The measured deliverystatic head, together with the calculated static efficiency and the shaft power, are plotted versus the volumeflow rate for each fan with the different types of inlet straighteners of L/D = 1.4. Comparisons with the freeinlet 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, aremarkable increase in static head due to the straighteners can be noticed after 0.15 m3/s. This is accompaniedby an appreciable improvement in the fan efficiency. By decreasing the flow rate, the effect of the straightenersvanishes. As the diameter of the straightener tubes increases, a more flattened efficiency curve is observed. Animprovement of 5 points in efficiency, corresponding to a relative increase of 18%, is obtained with straight-eners of 15 mm diameter, and a corresponding decrease in shaft power is noticeable. This is due to the goodguidance of the flow provided by the straighteners at the impeller inlet. Furthermore, the unstable operatingrange of the fan extends farther due to the straighteners. It is useful to note that during the experiments, thesurge point was detected by fluctuations in the pressure readings in addition to the high audible noise. The

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ffic

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1.5

2.0

Shaf

t P

ower

(kW

)

Free

d=2.5 mm

d=4 mm

d=15 mm

Zigzag

β2=60º

Fig. 5. Fan performance for backward impeller (b2 = 60�) with different straighteners.

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results of the noise test in Fig. 9, expressed in sound pressure levels in dB, (A) show that the use of straight-eners decreases the noise level along the operating range by approximately 5 dB at the high flow rate. In prac-tice, 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 and 6, respec-tively. The first impeller exhibits improvement that could reach about 6 points in efficiencies employing thezigzag straighteners. This represents a relative increase of 21% in efficiency. However, a small drop in the statichead associated with an increase in shaft power is observed. A reduction of about 3 dB in noise level is theresult, Fig. 9. It worth noting that the noise level increases as the blade angle increases, which was alsodetected by Liberman [15]. As the blade angle increases to 75�, a lower efficiency is obtained all over the oper-ating range at the free inlet condition. This is due to the high incidence losses resulting from the correspondinglarge inlet blade angle. Using the inlet straighteners leads to a further drop in static head as well as in effi-ciency. However, the use of straighteners with very small tube diameter increases, obviously, the efficiencybut on account of the low delivery head. This is associated with a noticeable decrease in the maximum flowrate, choke point. However, the onset of the surge point shifts to a lower flow rate.

Figs. 7 and 8 indicate the performance of the forward fans with exit blade angle 105� and 120�. The use ofinlet straighteners result in small increases in static efficiency for 105� on account of the appreciable drop instatic head, whereas, for the impeller with 120�, a deterioration in efficiency as well as in delivery head isnoticed. In this case, it is worth noting that at the free inlet condition, the fan efficiency is already very

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1.0

1.5

2.0

Shaf

t P

ower

(kW

)

Free

d=2.5 mm

d=4 mm

d=15 mm

Zigzag

β2=75º

Fig. 6. Fan performance for backward impeller (b2 = 75�) with different straighteners.

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0.0

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2.0

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t P

ower

(kW

)

Free

d=2.5 mm

d=4 mm

d=15 mm

Zigzag

β2=105

Fig. 7. Fan performance for forward impeller (b2 = 105�) with different straighteners.

3314 N.N. Bayomi et al. / Energy Conversion and Management 47 (2006) 3307–3318

low. This is probably due to separation of the flow inside the impeller passages as the number of blades ismuch lower than usual for forward facing blades. This is associated with a higher noise level compared toradial and backward facing blades. This is in agreement with the results of Liberman [15].

To assess the effect of the straighteners on the fan operation, some parameters should be taken into con-sideration. These parameters are the flow margin and the surge margin. The flow margin is defined as

Flow margin ðFMÞ ¼ 1� V sp

V max

� �� �� 100%

where Vsp and Vmax are the volume flow rate at the surge point and the maximum flow rate, respectively. Thesurge margin is calculated from the definition given by Cumpsty [16] as

Surge margin ðSMÞ ¼PV

� �sp

PV

� �op

� 1

!" #

where P is the static pressure ratio and the suffix op indicates operating point corresponding to the conditionat maximum efficiency.

Figs. 10 and 11 show the calculated flow margin and the surge margin, respectively, for the different fanswith different straighteners compared to the free inlet condition. The results show that the flow margin may be

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t P

ower

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)

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d=2.5 mm

d=4 mm

d=15 mm

Zigzag

β2=120º

Fig. 8. Fan performance for forward impeller (b2 = 120�) with different straighteners.

N.N. Bayomi et al. / Energy Conversion and Management 47 (2006) 3307–3318 3315

arbitrarily increased up to 12% by using inlet straighteners for the backward and radial impellers. For the for-ward blade, this reduces the flow margin, especially with small diameter. Great improvements in the surgemargin 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 var-ies according to the exit blade angle. Accordingly, the most suitable straightener for each impeller type can beselected. Straighteners with tube diameter 15 mm conform well to the radial blades, whereas the zigzag type isadvisable to be used with backward blades. It follows from the results analysis that inlet straighteners are notconvenient for forward blades.

The effect of straightener length on the fan performance has been studied. Samples of the results obtainedusing short straighteners of L/D = 1.125 are presented in Fig. 12 compared to the longer one taking into con-sideration the most efficient inlet configuration for each fan. It can be noted that for the radial and backwardimpeller 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.

4. Conclusion

The present paper investigates the effects of inlet straighteners on the performance, operating range andinstantaneous surge of a centrifugal fan. Experimental investigations concerning different types and sizes of

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V (m3/s)

80

85

90

Noi

se (

dB)

80

85

90

Noi

se (

dB)

Free

d=2.5 mm

d=4 mm

d=15 mm

Zigzag

β2=60º

β2=60º

Fig. 9. Noise level for different straighteners for radial and backward (60�) impellers.

Fig. 10. Comparison of the flow margin for different blade angles with different straighteners.

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inlet straighteners for radial, backward and forward fans were conducted. The following conclusions can bedrawn:

1. The effect of straighteners on the fan performance depends mainly on the exit blade angle. More flattenedefficiency curves are obtained by increasing the straightener tube diameters. An improvement of 5 points inefficiency corresponding to 18% relative increase in efficiency is obtained using circular tube straightenerswith 15 mm diameter and L/D = 1.4 for a radial impeller. A relative increase of 21% in the efficiency of abackward fan of 60� blade angle associated with a small drop in delivery head is obtained when usingstraighteners of zigzag type. A bad effect for the different straighteners is observed on the fan performancefor the forward impeller. A slight effect is noted on the maximum permissible flow rate (choke point) for theradial 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.

Fig. 11. Comparison of the surge margin for different blade angles with different straighteners.

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β2=75ºZigzag typeCircular type (d=15 mm)

β2=90º

Zigzag type

β2=60º

Free

L/D=1.4

L/D=1.125

Free

L/D=1.4

L/D=1.125

Free

L/D=1.4

L/D=1.125

Free

L/D=1.4

L/D=1.125

Free

L/D=1.4

L/D=1.125

Free

L/D=1.4

L/D=1.125

Fig. 12. The effect of straighteners length on fan performance.

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4. The use of straighteners decreases the noise level by approximately 3–5 dB at high flow rate compared withthe free inlet condition for radial and backward impellers.

5. The effect of straighteners length varies with exit blade angles. For the backward impeller with blade angle60�, as well as for radial fans, the longer zigzag and circular straighteners, respectively, give better perfor-mance, whereas for blade angle 75�, the shorter zigzag is the best.

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