enclosure helmholtz 515

Proc. of Int. Conf. on Recent Trends in Mechanical, Instrumentation and Thermal Engineering 2012

2012 AMAEDOI: 03.MES.2012.2.

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Low Frequency Noise Control of Diesel GeneratorSets using Helmholtz Resonators

S.R.Jagatap1, S.H.Kulkarni1, C.D.Shete2, R.Kollam21Department Of Mechanical Engineering

Veermata Jijabai Technological Institute, Matunga 400016,Mumbai,IndiaPhone: 8446859440; Email: [email protected]

2Power Generation Business UnitCummins Research and Technologies, Pune, India

Abstract: This paper introduces an impedance model ofHelmholtz Resonator for prediction of acoustic response of aPower Generator enclosure. The acoustic characteristics ofthe enclosure are modelled using the BEM method(SYSNOISE). Model describes the contribution of all theboundaries including velocity BC at engine-radiator-alternator surfaces; reflection BC at ground, jump of pressureBC at the surfaces exposed to surrounding.

I. INTRODUCTION

With the growth of standby, prime and peaking powerinstallations in highly populated areas, we have focused ourattention on understanding how generator set noise ispropagated and controlled. Like many types of rotatingmachinery, reciprocating engine-powered generator setsproduce noise and vibration. An untreated generator set noiselevels can approach 100 dB(A) or more.

There are six major noise sources in a Genset ,enginenoise mainly caused by mechanical and combustion forcesand typically ranges from 100 dB(A) to 121 dB(A), coolingfan noise results from the sound of air being moved at highspeed ranges from 100 dB(A) to 105 (A) dB, alternator noiseis caused by cooling air and brush friction and ranges fromapproximately 80 dB(A) to 90 dB(A), Induction noise iscaused by fluctuations in current in the alternator windingsthat give rise to mechanical noise that ranges from 80 dB(A)to 90 dB(A), engine exhaust without an exhaust silencer, thisranges from 120 dB(A) to 130 dB(A) or more, all are measuredat 1 meter distance from enclosure.

Some basic strategies for reducing generator set highfrequency noise are reduce

the sound level of the source, acoustic barriers, acousticinsulation, isolation mounts, cooling air attenuation, exhaustsilencers, efforts to maximize the distance between thegenerator set and the property line (or people). But lowfrequency noise remains untreated due to their longerwavelengths in above practices. ANC (Active Noise Control)[2] has been extensively researched but failed to develop amature technology to be used for complex environments likespower generator acoustic. PNC (Passive Noise Control)system as Helmholtz resonators (HRs) have been used invarious applications for low frequency noise (recent workfrom Esteve et. al. in Virginia for Aerospace) [1]. This paperextends use of HRs for controlling low frequency noisecontrol from genset.

II. THEORY

A HR is an acoustic bandstop filter comprised of a rigidcavity with protruding neck that connects the cavity to thesystem of interest. The behaviour of a Helmholtz resonator isanalogous to that of a vibration absorber. The volume of airin the neck of the Helmholtz resonator behaves much like avibration absorber mass and the volume of air in the cavityacts like compliance (reciprocal stiffness). The excitation isprovided by tonal pressure fluctuations acting over theopening of the neck, resulting in oscillations of the volumeof air in the neck. The pressure increase within the cavityprovides a reacting force analogous to that of spring.Damping appears in the form of radiation losses at the neckends and viscous losses due to friction of the oscillating airin the neck.

Figure 1. Helmholtz resonator and vibration absorber

III. IMPEDANCE CALCULATION FOR HR

For a neck with radius a which is flanged at both ends,effective length (the mass inside the neck and the mass nearthe neck edges) Leff [3] is approximately:

(1)where L is actual neck lengthThe acoustic mass of a Helmholtz resonator is given by

(2)

where is acoustic mass of the resonator, is fluid density,,S is cross section area of the neck.

The stiffness of the resonator is defined as the reciprocalof the compliance, and it is defined as

(3)

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where , F is force applied in the y direction at theresonator neck entrance, P is pressure at the neck entrance.For adiabatic system with air as an ideal gas, thethermodynamic process equation for the resonator is (4)where V is Cavity volume of the Helmholtz resonator.Differentiating this equation gives, (5)The change in the cavity volume is (6)where y is displacement in the direction pointing inward theneck substituting these into differential equation, it can bere-casted as (7)

Or considering and c , resonatorstiffness is then:

, where c is the speed of sound

Two source of damping in the Helmholtz resonator can beconsidered:a. Sound radiation from the neck: Sound radiation resistanceis a function of the outside neck geometry. For a flangedpipe, the radiation resistance is approximately

(8)

where k is the wave number given as (

b. Viscous losses in the neck: The mechanical resistance dueto viscous losses can be considered as

(9)

where Rs for a sufficiently large neck diameter given by

, where f is the excitation frequency..

The mechanical impedance of the mechanical system isdefined as the ratio of the driving force and the velocity ofthe system at the driving point. The mechanical impedanceof a driven mass-springdamper system is

(10)

the mechanical impedance of a Helmholtz resonator isobtained by replacing mass and damping from Helmholtzresonator system in above equation:

(11)

The natural frequency of a Helmholtz resonator,f0, is the fre-quency for which the reactance is zero:

(12)

and the acoustic impedance of the Helmholtz resonator is

(13)

Resonance occurs when the natural frequency of the resona-tor is equal to the excitation frequency. HR are typically usedto attenuate sound pressure when the system is originally atresonance.

IV. ANALYSIS OF GENSET ENCLOSURE

Factors addressed in enclosure analysis are, noisereduction potential, placement strategies (in high potentialzone), HR volume requirement (


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Figure 4: Enclosure cavity response showing potential low frequency modes selected for SYSNOISE Analysis

V. DOUBLE LAYER POTENTIAL PLOTS

Figure 5: DLP Plot for 115 Hz Figure 7: DLP Plots for 140 Hz

Figure 6: Pressure Plot 115 Hz at 1m distance from all sides ofenclosure

Figure 8: Pressure Plot 140 Hz at 1m distance from all sides ofenclosure

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VI. SYSNOISE ANALYSIS WITH IMPEDANCE MODEL OF HR

For each target frequency resonator dimensions [4] andnumber of resonators are calculated (total resonator volume

1% total cavity volume).Based on the DLP plots ofenclosure, resonator locations are chosen (maximum response/anti node locations). Impedance (in terms of admittance)boundary conditions are applied to the identified location ofBEM Mesh of enclosure.

Resonator dimensions are as follows,

Figure 9: Two elements applied with admittance BC for 115 Hz

Figure 10: Two elements applied with admittance BC for 140 Hz

Frequency 115 Hz :- .

VI. RESULTS AND DISCUSSION

1. Neck Diameter (d)=0.102m2. Neck Length (Ln)=0.15m3. Cavity Diameter (D)=0.306m4. Cavity Length (Lc)=0.12m5. Number of Resonators=2Frequency 140 Hz :-1. Neck Diameter (d)=0.102m

2. Neck Length (Ln)=0.15m3. Cavity Diameter (D)=0.306m4. Cavity Length (Lc)=0.0829m5. Number of Resonators=2

Figure 11: SYSNOISE response plot for 115 Hz

Figure 12: SYSNOISE response plot for 140 Hz

SYSNOISE response plots for both frequencies 115 & 140 Hz(Figure 11 & 12) shows substantial reduction in the soundpressure level with application of impedance model of HR.To get potential Sound Pressure Level (SPL) reduction oneneed to target several critical modes of the enclosure, alsodamping in the resonator will further reduce the SPL In practiceadditional damping in resonator can be achieved by addinggrill cloth(e.g loudspeaker cone cloth) at the resonator neckbase.

CONCLUSION

The acoustic performance of the enclosure is quite smoothat targeted frequencies. Helmholtz resonator having less than1% cavity volume at the target frequency (115 & 140 Hz)results into approximately dB reduction in SPL.

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REFERENCES

[1]. Active control of payload fairing noise by Steven A. Lane,Marty Johnson, Chris Fuller, Arnaud Charpentier, Journal ofSound and Vibration Volume 290, Issues 35, 7 March 2006,Pages 794819

[2]. Analysis of low frequency acoustic response in a dampedrectangular enclosure by J. Pan, S. J. Elliott and K.H. Baek,Journal of Sound and Vibration (1999) 223(4)

[3]. On the theory and design of acoustic resonators by Uno Ingard.The Journal of The Acoustical Society of America 25(6):10371061, 1953

[4]. A Note on interaction between Helmholtz Resonator and modesof an Enclosure by F. J. Fahy and C. Schofield, Journal ofSound and Vibration (1980) 72(3), 365-378

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