0146-1

Case Acoustic Resonance Induced by Karman Vortex Plant

Heat exchanger for the heat recovery

Severe noise of 130dB can be observed near the heat exchanger for the exhaust heat recovery when opening of the upstream valve is about 30%. Fluid velocity through a tube array in the heat exchanger is 6.5m/s under this operation condition. On the other hand, any noise cannot be observed when the valve opening is 40%. The fluid velocity through the tube array is 8.5m/s under this operation condition.

It is presumed that the acoustic resonance occurs between the excitation by vortex shedding at the tube array and the acoustic natural vibration in the heat exchanger since the noise can be observed at only limited range of the flow rates.

1) Sound pressure distribution along the casing width direction is measured by moving amicroscope when severe noise can be observed. Results of this measurement is shown inFig.2. Fig.2 shows that the 1st acoustic natural mode with half wave length is excited inthe casing.

2) Pressure inside the casing and vibration displacement at the casing wall are also measuredsimultaneously when the heat exchanger emanates the severe noise. A dominantfrequency of 68 Hz is clearly found in both frequency spectrum of the pressure in thecasing and the vibration displacement at the casing wall as shown in Fig. 3.

3) Karman vortex frequency at a tube array shown in Fig.4 fk can be calculated by thefollowing equation:

fk=St U/D= 63 Hz, where St: Strauhal Number = 0.31 (for this tube array) U: Fluid Velocity = 6.5m/s D: Tube outside diameter = 0.0318m

4) Acoustic natural frequency in width direction of the heat exchanger casing fn can becalculated by the following equation:

fn=na/2L= 70 Hz, where n: Mode number = 1 a: Sound Speed of Fluid = 417mm/s at 200 deg.C L: Casing Width = 2.985m

Since both fk and fn exist close to the measured dominant frequency of 68Hz, it can be considered that periodical pressure pulsation by vortex shedding excites the 1st acoustic vibration mode in the casing and the pressure pulsation amplitude is considerably amplified by the acoustic resonance. Steel partition plates are inserted into the casing as shown in Fig.5 in order to avoid the acoustic resonance by shifting the acoustic natural frequency to higher frequency. The noise is completely suppressed by this countermeasure.

It is important to know that severe noise and vibration by the acoustic resonance between excitation by vortex shedding and acoustic mode vibration can be induced in heat exchangers

Blevins, R. D., “Flow-Induced Vibration, Second Edition”, 1990 Van Nostrand Reinhold Co Nothing in particular.

Flow-Induced Vibration, Acoustic Resonance, Karman Vortex Keyword

References

Lesson

Countermeasures

and Results

Analysis and Data

Processing

Cause Presumed

Observed

Phenomena

Object Machine

The

Japa

n Soc

iety o

f Mech

anica

l Eng

ineers

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE

0146-2

The Ja

pan S

ociet

y of M

echan

ical E

ngine

ers

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE

v_BASE