### THEORY OF THE ABSORPTION STRUCTURES OF MICRO-HOLE PLATES IN MUfflERS

#### 2017-02-10503

I. THEORY OF THE ABSORPTION STRUCTURES OF MICROPOROUS PLATES

When the thickness of the plate is less than 1.0 mm, the perforation rate is 1-5% when the diameter of the hole is less than or equal to 1.0 mm.

There is a certain thickness (5-20cm) of air layer left behind, which does not fill any sound absorbing material, thus constituting the sound absorbing structure of Micro-Perforated plate. It is a resonant sound absorption structure with low sound quality and high sound resistance. The research shows that the sound absorption coefficient and frequency bandwidth which characterize the sound absorption characteristics of Micro-Perforated plates are mainly determined by the sound quality m and the sound resistance R of Micro-Perforated plates, which are related to the diameter D and the perforation rate P of micro-perforated plates. Relative acoustic impedance Z (in unit of characteristic impedance P C of air) of microperforated panel sound absorption structure is calculated by formula (1):

Z = R + JWM = jctg (WD / C) (1)

Formula:

P-air density (kg/cm3);

C - Sound velocity in air (m/s);

D - cavity depth (mm);

M - Relative sound quality;

R - Relative acoustic resistance;

W - angular frequency, W = 2 pi f (f is frequency);

R and m are expressed by formula (2) (3):

R = atkr / DZP (2)

M=(0.294)*10-3 tkm/p(3)

Formula:

T -- Plate Thickness (mm)

D -- Aperture (mm)

P - Perforation rate (%)

Kr-acoustic resistance coefficient kr=(1+x2/32)1/2+(2x)1/2/8*d/t

Km -- Acoustic mass coefficient km = 1+{1+[1/(9+(x2/2)]}+0.85d/t

Where x = abf, A and B are constants, for insulation plate a = 0.147, B = 0.32; for heat conduction plate a = 0.235, B = 0.21. The angular bandwidth of sound absorption is approximately determined by r/m. The larger the value, the wider the absorption bandwidth. R/m=(l/d2)*kr/km(4)

For the metal plate l = 1140, and for the heat insulation plate l = 500. The above formula can also be expressed in Formula (5):

R/m = 50F ((kr/km)/x2) (5)

The approximate formula of kr/km is:

Kr/km=0.5+0.1x+0.005x2(6)

The parameters of x, d, T and P of the sound absorption structure of the microperforated plate can be obtained from the required r, m and F. Because the aperture of the microperforated plate is small and sparse, the fundamental acoustic resistance R value is much larger than that of the ordinary perforated plate, and the sound quality m is very small, the sound absorption band is much larger than that of the ordinary perforated plate resonant sound absorption structure. The sound absorption bandwidth of the monolayer or double-layer microperforated plate sound absorption structure with better general performance can reach 6-10 1/3 of the signal range. This is the biggest characteristic of the sound absorption structure of the microperforated plate.

The maximum absorption coefficient at resonance is alpha 0=4r/(1+r)2(7)

When designing the sound absorption structure of Micro-Perforated plate, it can be calculated and charts can be consulted. The calculated results are close to the measured ones. In order to enlarge the width of sound absorption band in practical engineering, double or multi-layer micro-perforated plate composite structures with different aperture and different perforation rates are often adopted.

2. The Application of Micro-perforated Plate Theory in the Design of Anti-blowout Muffler

There are many kinds of mufflers manufactured by using the acoustic structure design of Micro-Perforated plate, the main type is anti-blowout muffler. This type of muffler is made of stainless steel perforated sheet. Because the nine mufflers are used in petrochemical units and have high air corrosiveness, the sound absorbing material filled in the air layer behind the perforated sheet is corrosion-resistant metal soft silk cloth. Based on the principle of resistive sound absorption of sound-absorbing materials, the effect of noise reduction and noise elimination is further achieved.

Its sound absorption coefficient is high, absorption frequency bandwidth is wide, pressure loss is small, air regeneration noise is low, and it is easy to control. In order to achieve wide bandwidth and high absorption, three-stage micro-perforated plate structure is generally used. The size of the cavity between the micro-perforated plate and the outer shell and between the micro-perforated plate varies according to the frequency band that needs to be absorbed. The low frequency cavity is larger (150-200 mm), the intermediate frequency is smaller (80-120 mm), and the high frequency is smaller (30-50 mm). The front cavity depth of the double-layer structure should generally be less than the back cavity. The ratio of the front cavity depth to the back cavity depth is less than 1:3. The perforation rate of the front layer of the micro-perforated plate close to the airflow should be higher than that of the front layer. In the rear layer, in order to reduce the influence of axial sound propagation, a transverse partition can be added to the cavity of the microperforated panel muffler at about 500 mm intervals. Experiments show that the measured noise attenuation performance of the microperforated panel muffler is better than that of the theoretical estimation. The muffler has nothing to do with the temperature rise of the muffler. When the flow rate reaches 70 m/s, other mufflers can not solve the problem of noise. The microporous muffler can withstand the impact of 70 m/s air velocity, and there are still mufflers with more than 15 dBA. This is also an important characteristic of microporous muffler which is superior to general muffler.

3. Characteristics of anti-blowout muffler

This type of muffler synthesizes the most reasonable muffling principle of Micro-Perforated plate. It makes the high pressure steam in the muffler enter the pressure-reducing chamber after a flow control, and then forms the low pressure steam to eject from the orifice. Finally, the residual noise emitted by the pressure-reducing body is effectively absorbed by the sound-resisting cover outside the pressure-reducing body. Finally, the muffler reaches 35 dB(A) or above (between the first and third mufflers). The proportion was 35%, 50%, 15%.

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