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					  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                       Maximum
                                                      max
      SM           Surge margin                                     Operating point
                                                      op
      V            Volume flow rate                   sp            Surge point




1- INTRODUCTION
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 cross-

section 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   Prandtl probe                   Spherical
           Centrifugal fan                        Static taps
                                                                      tube                                       valve

                                      Flow
Static taps


                                              Pitot tube

Straight                                                                                          Multi-channel Static and/or
                                                        Computer             Micromanometer
 blade                                                                                              Switch      total pressure
                                                Sound level
                                                  meter         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
                                                                                                            D
                          45

                             Fig. (1) test rig and measuring devices layout.
                                                                                                                      L




                                                                                                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.
                                                                  Air
                                                                  inlet
                                                                  b) Setting location of straighteners.
                                                      Bellmouth


                                                          Fig. (1) Test rig and measuring devices layout.


                                             160 mm
                                                                                                                        D
                                                45




                                                                                                                                 L
                                                                                                              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.




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




                               0.00
Total suction head (m water)




                               -0.05

                               -0.10                                                                                             Free
                                                                                                                                 d=2.5 mm
                               -0.15
                                                                                                                                 d=4 mm

                               -0.20                                                                                             d=15 mm


                               -0.25
                                       0.0                0.2                       0.4                 0.6             0.8               1.0
                                                                                               r/r o.
                                  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 I   Impeller II   Impeller III   Impeller
                             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
                                                                                                              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
                                                                          V (m 3.
                                                                                /s)
                                      40.0
Static Efficiency %




                                      30.0

                                      20.0

                                      10.0

                                       0.0
                                         0.00       0.05      0.10     0.15      0.20     0.25      0.30   0.35
                                                                          V (m 3.
                                                                                /s)

                                       2.0
                   Shaft Power (kW)




                                       1.5

                                       1.0

                                       0.5

                                       0.0
                                         0.00       0.05      0.10     0.15      0.20     0.25      0.30   0.35
                                                                           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
                                                                                                              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
                                                                         V (m3.
                                                                              /s)
                                      40.0
Static Efficiency %




                                      30.0

                                      20.0

                                      10.0

                                       0.0
                                             0.00    0.05     0.10    0.15     0.20     0.25     0.30       0.35
                                                                         V (m3./s)

                                       2.0
                   Shaft Power (kW)




                                       1.5

                                       1.0

                                       0.5

                                       0.0
                                         0.00       0.05     0.10     0.15     0.20     0.25     0.30       0.35
                                                                             V (m 3.
                                                                                   /s)
                                              Fig. (5) Fan performance for backward impeller ( 2 = 60 )
                                                         with different for backward impeller (2 = 60 )
                                               Fig. (5) Fan performancestraighteners.
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
                                                                        V (m3.
                                                                             /s)
                                      40.0
Static Efficiency %




                                      30.0

                                      20.0

                                      10.0

                                       0.0
                                         0.00      0.05     0.10     0.15     0.20     0.25     0.30        0.35
                                                                        V (m 3.
                                                                              /s)

                                       2.0
                   Shaft Power (kW)




                                       1.5

                                       1.0

                                       0.5

                                       0.0
                                         0.00      0.05     0.10     0.15     0.20     0.25     0.30        0.35
                                                                            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 )
                                                       with different straighteners.
Delivery Static Head (m water)
                                      0.30                                                            2.=105
                                                                                                            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
                                                                     V (m 3.
                                                                           /s)
                                      40.0
Static Efficiency %




                                      30.0

                                      20.0

                                      10.0

                                       0.0
                                         0.00   0.05    0.10      0.15      0.20       0.25   0.30   0.35
                                                                     V (m 3.
                                                                           /s)

                                       2.0
                   Shaft Power (kW)




                                       1.5

                                       1.0

                                       0.5

                                       0.0
                                         0.00   0.05    0.10      0.15      0.20       0.25   0.30   0.35
                                                                     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
                                       0.25                                                                        Free
                                                                                                                   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
                                                                             V (m 3.
                                                                                   /s)
                                       30.0
Static Efficiency %




                                       20.0


                                       10.0


                                        0.0
                                              0.00    0.05      0.10      0.15      0.20       0.25      0.30   0.35
                                                                             V (m 3.
                                                                                   /s)

                                        2.0
                    Shaft Power (kW)




                                        1.5

                                        1.0

                                        0.5

                                        0.0
                                              0.00    0.05      0.10      0.15      0.20       0.25      0.30   0.35
                                                                             V (m 3.
                                                                                   /s)


                                                Fig. (8) Fan performance for forward impeller (2 = 120 )
                                     with different straighteners.




             90
                                                  2. =90
                                                                                         Free
Noise (dB)




                                                                                         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
                                                    V (m 3.
                                                          /s)
             90


                                                  2. =60
Noise (dB)




             85




             80
                  0.00       0.05     0.10       0.15      0.20      0.25    0.30      0.35
                                                    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  
                                                         
                                                              X 100%
                                                             
                                                      Vmax 
                                                              
  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.
                                        Circular type (d= 15 mm)                   Zigzag type                          Zigzag type
                                0.30              2.=90         0.30                   =60
                                                                                      2.                0.30              2.=75



                                0.25                               0.25                                 0.25
Deivery Static Head (m Water)




                                0.20                               0.20                                 0.20


                                0.15                               0.15                                 0.15


                                0.10                               0.10                                 0.10
                                                Free                                     Free                             Free

                                0.05            L/D=1.4            0.05                  L/D=1.4        0.05              L/D=1.4
                                                L/D=1.125                                L/D=1.125                        L/D=1.125

                                0.00                               0.00                                 0.00
                                       0.0    0.1    0.2         0.3         0.0   0.1   0.2          0.3         0.0   0.1   0.2         0.3
                                               V (m 3.
                                                     /s)                           V (m3./s)                            V (m3./s)

                                40.0                               40.0                                 40.0




                                30.0                               30.0                                 30.0
Static Efficiency %




                                20.0                               20.0                                 20.0




                                10.0                   Free        10.0                     Free        10.0                  Free
                                                       L/D=1.4                              L/D=1.4
                                                                                                                              L/D=1.4
                                                       L/D=1.125                            L/D=1.125
                                                                                                                              L/D=1.125
                                 0.0                                   0.0                                  0.0
                                       0.0    0.1      0.2       0.3         0.0   0.1   0.2          0.3         0.0   0.1      0.2      0.3
                                               V (m3./s)                           V (m3./s)                            V (m3./s)

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


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




References




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[9] Chen, P., Soundra-Nayagam, M., Bolton, A. N. and Simpson, H. C. "Unstable Flow

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[10] Montazerin, N. and Damangir, A. and Mirian, S., "A New Concept for Squirrel-

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[11] Kassens, L and Rautenberg, M., "Flow Measurements behind the Inlet Guide

Vane of a Centrifugal Compressor," ASME paper 98-GT-86, 1998.
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pp. 153-164.


[13] Jack, B. E., "Fan Selection and to Reduce Inefficiency and Low Frequency Noise

Generation," Fan Noise 2003, International Symposium Senlis, 23-25, September.


[14] Bhope, D. V. and Padole, P. M. "Experimental and Theoretical Analysis of

Stresses, Noise and Flow in Centrifugal Fan Impeller," Mechanism and Machine

Theory, Volume 39, Issue 12, December 2004, Pages 1257-1271


[15] Liberman, M. Yu., "Investigation of Noise Characteristics for Centrifugal Ship

Fans," XI Session of the Russian Acoustical Society, Moscow, November 19-23, 2001,

pp. 572-575.


[16] Cumpsty, N. A., "Compressor Aerodynamics," John Wiley & Sones, Inc. New

York, 1989.

				
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