Acoustic performance of a heat exchange silencer for marine diesel engine

Lianghu Meng 1

Henan Diesel Engine Industry Co., Ltd, China 173 Zhongzhou West Rd, Luoyang, Henan, P. R. China 471039 College of Power and Energy Engineering, Harbin Engineering University, China 145 Nantong St. Nangang Dis. Harbin, Heilongjiang, P. R. China 150001 ABSTRACT For the noise contribution of a marine diesel engine, exhaust noise plays an important role, especially considering its low frequency characteristics. The marine exhaust silencer is an effective device to attenuate the engine exhaust noise. Expansion chamber or resonators have been the backbone of almost all marine exhaust silencers, but issue of bulkiness remains. It is well known that the passband for an expansion chamber is related to its geometric length. For a fixed firing frequency, if the exhaust gas is cooled, then the wavelength of the firing frequency can be reduced. Inspired by this principle, the heat exchanger and silencer are combined to improve the low frequency effect of the device. Various reactive configurations are tried to get a well-balanced silencing effect. The acoustic finite element method is adopted to calculate the noise reduction effect with or without exhaust gas cooling. It is concluded that that the heat exchange silencer has satisfactory cooling effect on the exhaust gas, but its contribution to noise attenuation is only reflected in the frequency band. Since the reduction of the exhaust gas temperature, low-frequency performance of the silencer is enhanced.

1. INTRODUCTION

At present, energy saving and emission reduction have become the vane of the development of the shipbuilding industry. How to reduce energy consumption and emissions and meet the requirements of the International Maritime Organization has always been one of the research hotspots in the shipping industry. With the full implementation of energy consumption and emission regulations, energy saving, emission reduction and noise reduction devices are one of the key devices affecting the low-speed engine supporting market. As the main power of the ship, the thermal efficiency of the diesel engine is close to 50%, but 50% of the energy is still taken away by the exhaust gas and cooling medium. If this part of the waste heat can be fully utilized, the thermal efficiency of the engine can be significantly improved. At the same time, the diesel engine will produce strong vibration and noise when it is working. Exhaust noise is one of the main noise sources of diesel engines, and the silencer is the main equipment to weaken the exhaust noise of diesel engines [1]. The heat exchange silencer is an important part in the utilization of the waste heat of the exhaust gas of the low-speed engine, which combines the heat exchanger and the silencer [2]. The cooling water collects the waste heat in the exhaust gas in the heat exchange section, which can be used as a low-quality heat source to promote the organic working fluid generator to generate electricity again. Both the purposes of waste heat utilization and noise reduction have been achieved.

1 mlh@hnd.com.cn

This paper studies the acoustic performance of an integrated arrangement of a marine diesel exhaust cooling silencer. By optimizing the design, the matching between the acoustic performance of the silencer and the noise source is improved. At the same time, theoretical analysis and experimental research were carried out. 2. RESEARCH OBJECT

Generally, exhaust silencers can be divided into three main categories: resistive, reactive and hybrid silencers. Due to different working principles, various types of silencers have different muffling effects on noise in different frequency ranges [3].

The reactive structure is generally used to eliminate low-frequency noise, and the design should focus on the low-frequency line spectrum noise attenuation effect of the noise source. Reactive structures generally include expansion chambers, Helmholtz resonators, perforated tube structures, etc. Reactive silencers are the main means to control the low-frequency line spectrum noise of exhaust gas. The principle of reactive structures is to use sudden changes in cross-section or destructive interference mechanisms to create impedance mismatch conditions, which in turn reflect noise upstream. Reactive silencers generally have the advantage of simple structure, but their frequency selectivity is strong, especially in the low frequency region. Usually, measures such as reducing the perforation rate and increasing the length of the expansion chamber are taken to obtain better low-frequency effects, which will cause the size of the silencer to be too large. The effective space in the engine room or chimney of the ship is very limited, which requires the exhaust silencer to accurately and effectively reduce the low-frequency line spectrum noise of the diesel engine under the limited size.

If a heat exchanger can be placed upstream of the silencer, the temperature of the exhaust gas can be lowered. As we all know, when the frequency of the exhaust gas is constant, the wavelength of the exhaust noise is related to the exhaust gas temperature. When the exhaust gas temperature is lowered, the wavelength of the noise is lowered. This will facilitate the compact design of the silencer.

The structure size of the heat exchange silencer studied in this paper is shown in Figure 1, which consists of an inlet section, a heat exchange section, a silencer section and an outlet section. Among them, the inlet section is provided with four guide plates, and the middle part of the heat exchange section is provided with a soot blower.

The thermal power of the heat exchange silencer is 0.42MW, and the exhaust gas enters the heat exchange silencer from a pipe with a diameter of 350mm. The section size of the exhaust gas in the heat exchange section is about 0.78m × 0.89m, and the length of the heat exchange section is about 1.4m.

For a well-designed silencer, its acoustic attenuation characteristics must match the target noise source. In this paper, the small-scale prototype used in the laboratory is studied, but the final adaptation target model is a MAN 6S50ME-8.2 diesel engine with a rated power of 9960kW, a speed of 127rpm, and an exhaust temperature of 250 ℃ . The sound power level of exhaust noise is shown in Figure 2 below.

Figure 1 Structure diagram of heat exchange silencer

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Figure 2 Noise source of target diesel engine The gray histogram in Figure 2 is the measured linear sound power level of the exhaust port without the silencer. It can be seen from the figure that the sound pressure amplitude of the low- frequency noise is large, because the strong pulsating pressure formed after the valve is opened makes the sound power level in this frequency band very high. The black histogram is the sound power level of exhaust noise after A-weighting. It can be seen from Figure 2 that after A-weighting, the noise level in the center frequency band of 31.5Hz is greatly reduced and its contribution to the total noise level is very small. After A-weighting, the noise power level in the center band of 63Hz- 1000Hz dominates. Therefore, in the design of the silencer, attention should be paid to the noise in this frequency band.

Due to the non-uniform distribution of the temperature field in the heat exchange silencer, the impedance characteristics of the internal acoustic medium vary with space, which in turn causes its acoustic performance to be different from that of conventional silencers. Therefore, it is necessary to calculate the temperature field of this type of silencer, and to analyze the acoustic performance based on the calculated temperature field. The Efficiency-Heat Transfer Unit method [4] was used to calculate the heat exchange section and the results showed that the exhaust gas temperature could be reduced by 140°C. The calculated temperature of each point is drawn into a curve, and the temperature change curve of the exhaust gas in the heat exchange section is obtained, as shown in Figure 3.

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T ( ° C)

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-1.6 -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 100

Figure 3 Exhaust gas temperature curve in heat exchange section 3. ACOUSTIC CALCULATION AND OPTIMIZATION

x (m)

Our initial scheme is shown in Figure 1. The downstream of the heat exchanger is followed by a resistive silencer. In order to reduce the loss of flow resistance, the upstream surface of the sound absorbing material is designed to have a certain inclination angle.

First of all, we know that there is a cross-sectional abrupt structure in the heat exchanger. Therefore, the heat exchanger can also have a reflective effect similar to the expansion chamber. The low frequency acoustic performance of the silencer can be given by the one-dimensional plane wave transmission loss formula of the expansion chamber [3]

( ) 2 2 0 10lg 1 0.25 1 sin TL m m k l   = + −   (1)

In the above formula, l is the length of the expansion cavity. m is the expansion ratio of the

silencer, where the wave number 0 0 =2 / k f c π 。

The acoustic calculation methods of the silencer include transfer matrix method, modal expansion method, distribution method, boundary element and finite element method. Considering the change of temperature field and the complexity of the model section calculated in this paper, the finite element method is used in this paper. By establishing a three-dimensional geometric model of the heat exchange silencer, the finite element calculation software is used to carry out the acoustic finite element simulation calculation of the silencer. During calculation, the internal fluid domain is divided into air domain and sound absorbing material domain. The finite element mesh is divided. And the solution domains are assigned different temperatures along the axis. We consider the finned tubes in the heat exchanger as rigid walls. In this way, not only the outer shell of the heat exchanger plays the role of expansion and noise reduction, but the heat exchange tube can also play a blocking role in high-frequency noise.

We first simulate the acoustic performance of the heat exchanger shown in Figure 4, and the calculation results are shown in the open circles in Figure 5. The results show that the heat exchanger exhibits attenuation lobes only at low frequencies. As the frequency increases, due to the appearance of high-order waves in the chamber, the noise reduction effect is greatly reduced.

Figure 4 Schematic diagram of heat exchanger Resistive sound-absorbing materials are generally used to achieve broadband noise reduction, and obtaining accurate material properties is the key to predicting their acoustic performance. The sound-absorbing material is generally a porous medium. Due to the friction, viscous force and heat conduction between the gas molecules in the exhaust gas and the gap of the sound-absorbing material, the energy of the sound wave is converted into other forms of energy to achieve the effect of noise reduction. Acoustic models of sound-absorbing materials include local models and volume models. The local model only considers the surface acoustic impedance and does not consider the standing wave effect of sound waves in the sound absorbing material. It is suitable for sound absorbing materials with high flow resistivity or relatively small thickness. Marine diesel exhaust silencers are generally larger in size, so the sound-absorbing material laid inside is also thicker. Obviously, the volume model considering the propagation of sound waves in the sound-absorbing material is more suitable for its performance prediction. In this acoustic model, the sound-absorbing material can generally be regarded as an equivalent fluid with a specific complex sound speed and complex density. How to describe the complex sound speed and the complex density becomes the key to the acoustic performance prediction of the resistive silencer. Although there are empirical models such as Delany-Bazley, Attenborough, Zwikker-Kosten and Wilson that can describe the above complex properties[5], the acquisition of complex properties still needs to know the basic parameters of the material such as flow resistivity, porosity, and structure factor.

Therefore, in order to accurately predict the acoustic performance of the silencer, it is necessary to measure the properties of the sound absorbing material before the acoustic calculation of the silencer. Before the test, it is assumed that the complex properties of the sound-absorbing material can be expressed as follows [5]:

B D c z z A f iC f = + − (2)

1 1 0 1 1 1

2 2 0 2 2 / 1 B D c k k A f iC f = + − (3) where 0 ρ and 0 c are the density and speed of sound of air, respectively. The characteristic impedance of air is 0 0 0 z c ρ = , and the air wave number 0 k .

The sound-absorbing material is aluminosilicate rock wool with a bulk density of 95g/L. The sample is 100mm in diameter and 25mm in thickness. The test was performed in a B&K-4206T impedance tube measurement system. In the test, the forward and reverse waves are decomposed by dual sensors, and then the quadrupole parameters of the sample are obtained by decomposing the two-load method, and then the complex properties of the material are obtained by fitting according to the calculation results.

Three samples were measured three times, the results were averaged, and the experimental data were fitted to obtain the complex impedance and complex wavenumber parameters of the batch of sound-absorbing materials:

34 0.42 70 0.64

A B C D

= = − = = −

= = − = = − (4)

1 1 1 1

30 0.28 28.37 0.40

A B C D

2 2 2 2

Using the above parameters, the transmission loss of the heat exchange silencer shown in Figure 1 is simulated, and the results are shown in Figure 5. The results show that the silencer greatly improves the high frequency silencer performance of the heat exchanger. When the frequency is higher than 200Hz, the transmission loss is above 20dB. However, resistive silencers drop sharply in the main low frequency noise band of diesel engines (below 250Hz). In order to improve the low frequency performance of the heat exchange silencer, we replaced the silencer with a hybrid silencer composed of an expansion chamber, a 1/4 wavelength resonator, etc., as shown in Figure 6.

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heat exchanger heat exchanger & dissipative silencer

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Figure 5 Acoustic calculation results of heat exchange silencer

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Figure 6 Structure diagram of heat exchange silencer (hybrid type)

Due to the different lengths of the three expansion chambers, the arched attenuation domains of the transmission loss curves of each chamber are also different. This can play a complementary role in acoustics. After adding the complex hybrid silencer, the silencer exhibits peak transmission loss values at the main noise frequency bands of the diesel engine at 63, 125 and 250 Hz. At other frequency bands, the transmission loss of the silencer is close to the scheme in Figure 1. This shows that the acoustic performance of the heat exchange silencer is greatly improved by our optimization with the fixed outer dimensions.

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Figure 7 Comparison of acoustic calculation results of heat exchange silencers Figure 8 shows the calculation results of the transfer loss of the heat exchange silencer under different heat exchange rates. The thin solid line represents the transfer loss of the entire device when the heat exchange is sufficient (exhaust outlet temperature is 110°C).

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Figure 8 shows that as the heat transfer intensity increases, the overall transfer loss curve shifts to low frequencies. Considering that the overall dimensions of the silencer have not changed, our design is equivalent to improving the compactness of the silencer.

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Figure 8 Acoustic calculation results of heat exchange silencer at different temperatures 4. EXPERIMENTAL VERIFICATION

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To validate our design, we carried out acoustic tests on the heat exchange silencer acoustic performance test bench shown in Figure 9. The outlet radiated noise of the pipeline was measured with and without the heat exchange silencer installed.

Figure 9 Experimental bench for acoustic performance of heat exchange silencers

The hot gas source (1) is the power source of this experiment. The high-temperature exhaust gas is discharged from the gas source (1), passes through the bellows (2), the flow meter (3), the thermometer (4), the mechanical sound source (5), the reducing pipe (7), and finally enters the silencer (10). ) or replace the straight pipe (10a). A microphone is arranged at the exhaust outlet of the silencer or replacement pipe to measure the sound pressure level of the outlet. At the same time, a static pressure measuring point is arranged upstream and downstream of the silencer, and the resistance loss of the silencer is determined by the static pressure difference between the two measuring points.

The silencer insertion loss at different cooling amounts is shown in Figure 10. Figure 10 shows that the greater the cooling, the lower the outlet temperature. As the cooling capacity increases, the overall noise reduction capacity of the heat exchange silencer will be improved. At the same time, the overall curve will shift to low frequencies, which is consistent with our predictions.

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experimental results-100 ° C experimental results-250 ° C experimental results-350 ° C

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Figure 10 Measured noise reduction of heat exchange silencer at different temperatures

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Figure 11 Comparison of measured noise reduction and simulation calculation of heat exchange silencer Figure 11 shows the comparison between the measured noise reduction and the simulation of the heat exchange silencer. First of all, in terms of trend, the actually measured 1/1 octave insertion loss and transmission loss line spectrum curves are in good agreement, and the curves show a trend of increasing gradually from low frequency to high frequency. When the frequency is greater than 2000Hz, the transmission loss begins to decrease gradually, which is caused by the high frequency failure of the sound absorbing material.

f /Hz

Secondly, in the frequency between 63-200Hz, the theoretical calculation results and the experimental results have a large deviation. This aspect stems from the imperfection of the measured sound-absorbing material model. On the other hand, the noise source in the laboratory is not strong enough, so that the signal-to-noise ratio of the experiment cannot meet the requirements in these frequency bands. In addition, the theoretical calculation uses the transmission loss to evaluate, while the experiment measures the insertion loss.

5. CONCLUSION

(1) The heat exchanger itself has a certain silencing ability, but mainly concentrates on low frequencies. When the frequency of the incident noise is higher than the cut-off frequency of the pipe, a silencer must be used to obtain a satisfactory amount of noise reduction.

(2) A hybrid silencer in series downstream of the heat exchanger is more suitable for marine diesel engine exhaust noise control.

(3) After cooling by the heat exchanger, the noise reduction curve of the heat exchange silencer shifts to the low frequency. Considering that the lower the frequency, the more difficult the noise control is, so the acoustic performance of the silencer is improved after heat exchange.

6 REFERENCES

[1]Mats, Abom, Antti, et al. Acoustic Source Characterization for Prediction of Medium Speed Diesel Engine Exhaust Noise[J]. Journal of Vibration & Acoustics Transactions of the Asme , 2014.

[2]Pingjian M, Meng Y, Wenping Z, et al. Numerical simulation of heat exchange and resistance characteristics of heat exchanger in muffler of diesel engines[J]. Journal of Harbin Engineering University , 2019(40)

[3] Munjal M L . Acoustic of Ducts and Mufflers. 1987. [4] Weirman C .Correlations ease the selection of finned tubes[J]. Oil and Gas Journal ， 1976, 74(36):94-100.

[5]. Jean F. Allard ， Noureddine Atalla ， Propagation of Sound in Porous Media: Modelling Sound Absorbing Materials, Second Edition[M]. John Wiley & Sons, Ltd, 2009.