International Journal of Electronics, Communication & Soft Computing Science and Engineering ISSN: 2277-9477, Volume 2, Issue 2

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Manikanda Pirapu P.K., Srinivasa G.R., Sudhakar K.G., MadhuD.

Abstract — Performance of axial fan is found to reducedrastically when instability is encountered during its operation.Performance of an axial fan is severely impaired by manyfactors mostly related to system instabilities due to rotatingstall and surge phenomenon experienced during its operation.The present work involves measuring the sound spectrummeasurements in ducted axial fan under stall conditions atfrequency range from 0 Hz to 500 Hz. Objective of theexperiment is to measure the frequency domain signal andstudy the sound Characteristics in ducted axial fan by usingspectrum analyser. Different types of FFT signals have beenmeasured under stall conditions for the frequency range of 0Hz to 500 Hz with respect to rotor speed and different graphsare plotted for ducted axial fan.

Key Words — Microphone, BNC connector, Data AcquisitionSystem, LABVIEW, Spectrum Measurements, Throttleposition, Rotor speed.

I. INTRODUCTIONMining fans and cooling tower fans normally employ

axial blades and or required to work under adverseenvironmental conditions. They have to operate in a narrowband of speed and throttle positions in order to give bestperformance in terms of pressure rise, high efficiency andalso stable condition. Since the range in which the fan has tooperate under stable condition is very narrow, clearknowledge has to be obtained about the whole range ofoperating conditions if the fan has to be operated usingactive adaptive control devices. The performance of axialfan can be graphically represented as shown in figure 1.

Fig: 1 Graphical representation of Axial Fan performancecurve

II. TEST FACILITY AND INSTRUMENTATION

Experimental setup, fabricated to create stall conditionsand to introduce unstall conditions in an industrial ductedaxial fan is as shown in figure 2 to figure 5.

Fig: 2 Ducted Axial Fan Rig

Fig: 3 Side View of Ducted Axial Fan Rig

Sound Spectrum Measurements in Ducted Axial Fan underStall Conditions at Frequency Range from 0 Hz to 500 Hz

International Journal of Electronics, Communication & Soft Computing Science and Engineering ISSN: 2277-9477, Volume 2, Issue 2

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Fig.4.Variable Frequency Drive for Speed Control

Fig.5 Automatic Throttle ControllerA 2 HP Variable frequency 3-phase induction electrical

drive is coupled to the electrical motor to derive variablespeed ranges. Schematic representation of ducted fan setupis shown in figure 6.

The flow enters the test duct through a bell mouth entryof cubic profile. The bell mouth performs two functions: itprovides a smooth undisturbed flow into the duct and alsoserves the purpose of metering the flow rate.

Fig: 6 Ducted Axial Fan – Schematic

The bell mouth is made of fiber reinforced polyester witha smooth internal finish. The motor is positioned inside a381 mm diameter x457 mm length of fan casing. The aspect(L/D) ratio of the casing is 1.2. The hub with blades, set atthe required angle is mounted on the extended shaft of theelectric motor. The fan hub is made of two identical halves.The surface of the hub is made spherical so that the bladeroot portion with the same contour could be seated perfectlyon this, thus avoiding any gap between these two matingparts. An outlet duct identical in every way with that at inletis used at the down stream of the fan. A flow throttle isplaced at the exit, having sufficient movement to present anexit area greater then that of the duct.

III. BASIC SOUND SPECTRUM ANALYSER SYSTEM

Basic sound Spectrum analyzer schematic diagramconsists of various components as shown in fig.7.Microphone acquires the sound pressure fluctuation andconverts them to an analog signal. BNC connector sendsthe signal to Data acquisition system. Data Acquisitionsystem receive the signal from the BNC connector andsends to LABVIEW software. Once the amplitude of thesignal has been measured, the computer system displaysthe measurement signal of spectrum through LABVIEWsoftware.

FAN

SIGNAL

MICROPHONE BNC CONNECTORDATA AQUISITION

SYSTEM (DAQ)

SOUND SPECTRUMMEASUREMENT

FROM COMPUTER

Fig.7 Schematic diagram of Sound Spectrum Measurement

IV. SOUND SPECTRUM ANALYSER

Experimental setup of Spectrum analyzer consists ofvarious components is shown in fig.8. Microphone acquiresthe sound signals, frequency range from 0 Hertz to 10000 Hzand measure the decibel range from 0 to 130 decibel.Microphone sensitivity is the ratio of its electrical output tothe sound pressure at the diaphragm of the microphone.Since a microphone output is usually measured in millivolts(mv) and sound pressure is measured in Pascal’s. The unit ofsensitivity of microphone is mv/Pa. microphone connects toBNC connector. BNC connector transmits the signal to DAQsystem. DAQ card consists 2 channel input port to acquirethe signal and send the signal to system achieve throughLABVIEW software inbuilt with National Instruments noiseand vibration acquisition system and 2 channel output port toreceive the signal from the system and to make a activefeedback control system in ducted axial fan.

International Journal of Electronics, Communication & Soft Computing Science and Engineering ISSN: 2277-9477, Volume 2, Issue 2

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Fig.8 Experimental Rig for sound Spectrum Analyser

V. SOUND SPECTRUM MEASUREMENTS

Experiments were carried out to examine the nature ofsound pressure variations in a ducted axial fan under a stallcondition for the frequency range from 0 Hz to 500 Hz atthrottle position of 3 cm and varying the rotor speed from2400 rpm to 3600 rpm by using spectrum analyzer. Thevariation in sound pressure amplitude level as a function ofrotor speed for particular throttle position 3 cm from thecasing is shown in fig.9.

In an axial fan setup, eight number of axial fan bladeshave been transferred the energy to the fluid. In one rotationof axial fan rotor, aerofoil section of axial fan bladestransfers the energy to the fluid through lift force. Lift forceof Newton across the fan blade area gives the pressure rise tothe fluid as per the operating condition and design conditionof fan blade. In a single rotation of blade of ducted axial fan,eight times sound pressure amplitude value will raise anddown periodically. For a same rotation, increase the soundpressure amplitude from energy transfer from blade to fluidand two blades between gap fluids enter freely, for that noenergy transfer between fluids to blade. So, decrease thesound pressure amplitude in between the gap of blades.

Every rotation, rotor speed varies from 2400 to 3600 rpmand throttle position of 3 cm from the casing, with respect toblade passing frequency sound pressure amplitude will risesuddenly up and abruptly falls down in ducted axial fanperiodically.

A variation in sound pressure amplitude of air atthrottling positions of 3 cm when the rotor rotates at 2400rpm is shown in fig.9. Maximum sound pressure amplitudeis found to be 105 decibels at stall conditions and theMinimum sound pressure amplitude is found to be 80decibels at stall conditions which is attributable tocombinatorial effects of blockage in mass flow, rotatingstall, periodic vibration due to air flow and excitation of fanblade.

A variation in sound pressure amplitude of air atthrottling positions of 3 cm when the rotor rotates at 2700rpm is shown in fig.10. Maximum sound pressureamplitude is found to be 113decibels at stall condition and

the Minimum sound pressure amplitude is found to be 85decibels at stall conditions.

A variation in sound pressure amplitude of air atthrottling positions of 3 cm when the rotor rotates at 3000rpm is shown in fig.11. Maximum sound pressure amplitudeis found to be 108 decibels at stall condition and theMinimum sound pressure amplitude is found to be 78decibels at stall conditions.

Fig.9 Rotor Speed 2400 Rpm, Throttle Position 3 cm

Fig.10 Rotor Speed 2700 Rpm, Throttle Position 3 cm

Fig.11 Rotor Speed 3000 Rpm, Throttle Position 3 cm

International Journal of Electronics, Communication & Soft Computing Science and Engineering ISSN: 2277-9477, Volume 2, Issue 2

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Fig.12 Rotor Speed 3300 Rpm, Throttle Position 3 cm

Fig.13 Rotor Speed 3600 Rpm, Throttle Position 3 cm

A variation in sound pressure amplitude of air atthrottling positions of 3 cm when the rotor rotates at 3300rpm is shown in fig.12. Maximum sound pressure amplitudeis found to be 105 decibels at stall conditions and theMinimum sound pressure amplitude is found to be 81decibels at stalled conditions.

A variation in sound pressure amplitude of air atthrottling positions of 3 cm when the rotor rotates at 3600rpm is shown in fig.13. Maximum sound pressure amplitudeis found to be 115 decibels at stall condition and theMinimum sound pressure amplitude is found to be 90decibels at stall conditions.

CONCLUSIONIn this paper, an attempt has been made to measure the

sound spectrum in frequency domain for the frequency rangefrom 0 Hz to 500 Hz under stall condition with respect torotor speeds in ducted axial fan by using spectrum analyzer.It is useful to examine the characteristics of stall in ductedaxial fan. Further, this work can be extended by working onthe mathematical model of sound spectrum study in ductedaxial fan. The results so far discussed, indicate that soundspectrum measurements of ducted axial fan is verypromising.

ACKNOWLEDGMENTThe authors gratefully thank AICTE (rps) Grant. for the

financial support of present work .

NOMENCLATURES

= Whirl velocity in m/s = Pressure ratio

N = Tip speed of the blades in rpmp = Pressure rise across the fan in N/m2

d = Diameter of the blade in mρair = Density of air in kg/m3

L p = Sound Pressure Level in db BPF = Blade passing frequency in Hz

L N = Normalized Sound Level in db

REFERENCES[1] Day I J,”Active Suppression of Rotating Stall and Surge in Axial

Compressors”, ASME Journal of Turbo machinery, vol 115, P 40-47,1993

[2] Patrick B Lawlees,”Active Control of Rotating Stall in a Low SpeedCentrifugal Compressors”, Journal of Propulsion and Power, vol 15,No 1, P 38-44, 1999

[3] C A Poensgen ,”Rotating Stall in a Single-Stage Axial Compressor”,Journal of Turbo machinery, vol.118, P 189-196, 1996

[4] J D Paduano,” Modeling for Control of Rotating stall in High SpeedMultistage Axial Compressor” ASME Journal of Turbo machinery,vol 118, P 1-10, 1996

[5] Chang Sik Kang,”Unsteady Pressure Measurements around Rotor ofan Axial Flow Fan Under Stable and Unstable Operating Conditions”,JSME International Journal, Series B, vol 48, No 1, P 56-64, 2005

[6] A H Epstein,”Active Suppression of Aerodynamic instabilities inturbo machines”, Journal of Propulsion, vol 5, No 2, P 204-211, 1989

[7] Bram de Jager,”Rotating stall and surge control: A survey”, IEEEProceedings of 34th Conference on Decision and control, 1993

[8] S Ramamurthy,”Design, Testing and Analysis of Axial Flow Fan,”M E Thesis, Mechanical Engineering Dept, Indian Institute ofScience, 1975

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[10] William W Peng, Fundamentals of Turbo machinery, John Wiley &sons.Inc, 2008

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AUTHOR’S PROFILE

Manikandapirapu P.K. received his B.E degreefrom Mepco Schlenk Engineering college, M.Tech fromP.S.G College of Technology,Anna University,and now ispursuing Ph.D degree in Dayananda Sagar College ofEngineering, Bangalore under VTU University. HisResearch interest include: Turbomachinery, fluidmechanics, Heat transfer and CFD.

Srinivasa G.R. received his Ph.D degree from IndianInstitute of Science, Bangalore. He is currently working asa professor in mechanical engineering department,Dayananda Sagar College of Engineering, Bangalore. HisResearch interest include: Turbomachinery, Aerodynamics,Fluid Mechanics, Gas turbines and Heat transfer.

Sudhakar K.G. received his Ph.D degree fromIndian Institute of Science, Bangalore. He is currentlyworking as a Dean (Research and Development) in CDGI,Indore, Madhyapradesh. His Research interest include:Surface Engineering, Metallurgy, Composite Materials,MEMS and Foundry Technology.

Madhu D. received his Ph.D degree from IndianInstitute of Technology (New Delhi). He is currentlyworking as a Professor and Head in GovernmentEngineering college, KRPET-571426, Karnataka. HisResearch interest include: Refrigeration and AirConditioning, Advanced Heat Transfer Studies, Multiphase flow and IC Engines.