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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: firstname.lastname@example.org 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 = VHst/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.  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 , Greitzer  and Baghdadi and Lueke . Similarly for centrifugal fans, Wright et al.  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. , Montazerin et al. , Kassens and Rautenberg  and Coppinger and Swain . 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. . Also, Jack  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  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, . 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, . 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  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. 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