Reduce Exhaust Noise Research Articles

Experimental analysis of biomimetic silencer to reduce exhaust noise in pneumatic devices

Pneumatic devices such as pneumatic artificial muscles (PAMs) generate excessive noise during operation, thus inducing safety hazards and becoming a nuisance to nearby communities. In this study, biomimetic silencers consisting of polyurethane foam and slit structures that mimic shark gill slits to reduce the exhaust noise caused by pneumatic devices were developed and experimentally analyzed. Two separate experimental approaches were employed to evaluate and analyze the performance of the developed biomimetic silencer. During these experiments involving the compressor and PAM exhaust noise measurements, the sound pressure signals, noise, and flow rate were obtained for the cases of with and without silencer. The results demonstrated that the slit structure of the developed silencer decreased the relative velocity with the surrounding air, thus resulting in approximately 6 dB improvement in noise reduction performance of the developed silencer compared with that of a general pneumatic silencer. Although the use of porous materials in the silencer causes an exhaust time delay, the delay is insignificant compared with that of a general pneumatic silencer in terms of the exhaust area. Thus, the developed silencer is an effective solution for reducing the exhaust noise in pneumatic devices, particularly in situations where noise reduction is critical.

Case study: Noise attenuating performance of perforated corrugated lined straight-through perforated pipe-resistant muffler

Noise of internal combustion engine is the main source of noise pollution, and mufflers are the main means to reduce exhaust noise of internal combustion engine. In the process of improving the acoustic performance of the exhaust muffler based on the three-dimensional numerical method, the influence of the high speed and high temperature airflow discharged by the engine in the actual working process on the acoustic performance of the exhaust muffler is often ignored, which leads to the obvious deviation between the predicted results and the actual situation. In this paper, based on the structural optimization design of the straight-through perforated pipe resistance muffler, the noise characteristics of the straight-through perforated pipe resistance muffler with perforated corrugated lining was analyzed considering the airflow velocity and temperature inside the muffler. The influence of temperature and velocity fields on the acoustic performance of muffler was studied by using numerical simulation method, which takes the solution results inside muffler as boundary conditions of sound field analysis. The influence of changing the internal structure of muffler on the aerodynamic performance of muffler was discussed, and the pressure loss is analyzed. The research has shown that adding perforated corrugated lining inside muffler could effectively improve the transmission loss of muffler and the noise reduction performance of prototype straight-through perforated pipe-resistant muffler.

Numerical Modeling and Geometry Enhancement of a Reactive Silencer

Internal combustion engines and blowers frequently utilize silencers to reduce exhaust noise. In the current paper, the transmission loss of reactive silencers is predicted using the plane wave decomposition method and a three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach. A mass-flow-inlet boundary condition is first used to perform a steady flow computation, which serves as an initial condition for the two subsequent unsteady flow computations. At the model's inlet, an impulse (acoustic excitation) is placed over the constant mass flow to perform the first unstable flow computation. Once the impulse has fully propagated into the silencer, the non-reflecting boundary condition (NRBC) is then added. For the scenario without acoustic excitation at the inlet, a second unsteady flow computation is performed. During the two transient computations, the time histories of the pressure and velocity at the upstream measuring points as well as the history of the pressures at the downstream measuring point are recorded. The related acoustic quantities show variations between the two unsteady flow computational findings. As a result, the transmitted sound pressure signal is just the sound pressure downstream, while the incident sound pressure signal is obtained by utilizing plane wave decomposition upstream. The transmission loss (TL) of the silencer is then calculated after the Fast Fourier Transform (FFT) converts the two sound pressure signals from the time domain to the frequency domain. The numerical calculations and the reported data are in good agreement for the published results, in addition to geometry enhancement by increasing number of holes in the cross section for muffler.

Acoustic Characteristics of Rectangular Hypersonic Mixer Ejector

JAXA is currently studying the pre-cooled turbojet engine (PCTJ) to be installed in a hypersonic transport. As a device to reduce exhaust noise at takeoff, a mixer-ejector that is applicable to a rectangular hypersonic nozzle having a complicated geometry with an external ramp has been proposed and tested. The ejector introduces the low-speed secondary flow into the high-speed main jet and reduces the exhaust velocity by mixing. As a result, the mean jet velocity decreases and the jet noise decreases. A 1%-scale mixer-ejector was fabricated, and the mixing process between the main jet and the secondary flow was visualized using Mie scattering of water droplets. The direct relationship between the degree of mixing and the amount of noise reduction was investigated.

Development of combined resistance muffler for reduced noise of farm tractor

A farm tractor is the most important component of farm mechanization. Farm tractor noise, on the other hand, has a negative impact on human health. The exhaust system produces the most noise of all the noises produced by a tractor. A muffler is an acoustic filter that is used to reduce exhaust noise. Perforated ducts, baffles/perforated baffles, expansion chambers, and longer inlet/outlets are used to make mufflers. A person working in a workplace with a noise level of 90 dBA is limited to 8 h of work each day under the Occupational Safety and Health Act (OSHA, 1983). As a result, utilizing the finite element approach and acoustic theory, an existing muffler (Reactive type muffler) was optimized to a combined resistance muffler after installing in a 47.6 PS, 2200 rated RPM three-cylinder diesel engine. Except for the frequency range of 138–455 Hz, 515–851 Hz, and 1524 Hz, the optimized combined resistance muffler had a considerable increase in transmission loss in all other frequency ranges. The improved muffler's mean transmission loss was 3.77 dB higher than the original muffler's in the 0–2000 Hz range, which indicates notable improvement in the noise canceling effect. The noise reduction of this combined resistance muffler was tested at operator ear level (OEL). Near rated engine speed, the combined resistance muffler suggests a noise reduction of 1.2 dBA–1.9 dBA over the existing muffler. The upgraded muffler also served as a benchmark for similar muffler structural improvements.

Analysis of Composite Scrubber with Built-In Silencer for Marine Engines

The International Maritime Organization (IMO) is strengthening regulations on reducing sulfur oxide emissions, and the demand for reducing exhaust noise affecting the environment of ships is also increasing. Various technologies have been developed to satisfy these needs. In this paper, a composite scrubber for ships that can simultaneously reduce sulfur oxide and noise was proposed, and the flow characteristics and noise characteristics were analyzed. For the silencer, vane type and resonate type were applied. In the case of the vane type, the effects of the direction, size, and location of the vane were analyzed, and in the case of the resonate type, the effects of the hole location and the number of holes were analyzed. The result shows that the length increase of the vane increased the average transmission loss and had a great effect, especially in the low frequency region. The transmission loss increased when the vane was installed outside, and the noise reduction effect was excellent when the vane was in the reverse direction. In the resonate type, increasing the number of holes is advantageous for noise reduction. The condition for maximally reducing noise in the range not exceeding 840 Pa, which is 70% of the allowable back pressure, is a vane length of 225 mm in the outer vane reverse type. The pressure drop under this condition was 777 Pa, and the average transmission losses in the low frequency region and the entire frequency region were 43.5 and 54.5 dB, respectively.

Local Structural Optimization Method Based on Orthogonal Analysis for a Resistant Muffler

A resistant muffler is the most direct and effective means of reducing exhaust noise from earthmoving equipment. Different types of construction machinery require different mufflers. Current muffler design is mostly experience-based, and there are blind spots in the design process. The optimization scheme must constantly be revised to achieve satisfactory noise reduction. The use of a large number of optimization schemes are wasteful in terms of both cost and time. Therefore, an effective and time-saving design method for muffler optimization is needed. An optimization approach, known as the “local structural optimization method for a resistant muffler”, based on orthogonal analysis is proposed in this study to optimize the design of a resistant muffler with a complex structure. The results of an orthogonal analysis for a simulation of the muffler structure show that the muffler acoustic and aerodynamic performances are affected by the inlet pipe, outlet pipe, insertion pipe, cross-flow perforated pipe and other local structures, and the results are employed to determine the influence law for the transmission loss (TL) and the exhaust back pressure, considering the local structural parameters of the muffler. The method is applied to optimize the design of a muffler suitable for an excavator and verified by performing an installation test. The experimental results show that the improved designs reduces the exhaust noise by 5 dB and the exhaust back pressure by 0.6 kPa. The improved designs exhibit higher aerodynamic and acoustic performances than a muffler prototype, showing that the proposed method is accurate, effective and reliable. The proposed method considerably simplifies the design process and saves time and cost. This method provides theoretical guidance and an optimization process for the design of a muffler for construction machinery.

Noise sources in a commercial supersonic jet

Stringent noise regulations currently limit commercial aviation. These regulations make supersonic commercial flight impractical. The development of an engine that can meet these strict rules is paramount to making supersonic commercial flight a reality. One method of noise reduction is to add additional streams to an engine. As such, the three-stream jet has potential to help reduce exhaust noise. Understanding the noise sources in the jet plume can help to design nozzles that are quieter. To accomplish this, high-fidelity, high-speed data are required. Data for an axisymmetric and offset three-stream nozzle were generated using the LES code JENRE developed by the Naval Research Laboratory. The simulation data has been shown to match well with experimental data. Advanced analyses methods that are based on Proper Orthogonal Decomposition (POD), wavelet decomposition, and Stochastic estimation have been applied to extract noise sources in the jet plume.

Design and evaluation of an Integrated SCR and exhaust Muffler from marine diesels

Integrated, compact, diesel after treatment system that combines selective catalytic reduction (SCR) for NOX control with a reactive muffler to reduce exhaust noise was designed and evaluated under various conditions. Considering the installation size, noise reduction, performance of SCR, exhaust back pressure and its effects on engine performance, it provides the basis for the design of the Integrated SCR–Muffler. Based on the two-load method and engine model established by AVL/Boost, the source impedance of engine has been found out. The transfer matrix of exhaust system was calculated from three-dimension finite element method which used equivalent fluid to describe the problem of wave propagation in the catalyst. On the basis of computational results, comprehensive analysis of noise reduction was made, the computed insertion loss of original muffler, SCR and Integrated SCR–Muffler was 32.1, 23.5 and 32.8 dB(A), respectively. The CFD simulation results prove that the Integrated SCR–Muffler is capable of increasing homogeneity of NH3 and improving NOX reduction efficiency due to the effect of silencing elements including perforated pipe and perforated plate, even though the pressure loss increased considerably when compared with SCR. Furthermore, the numerical results from simulations are compared with test. For most investigated operating conditions of D2 driving cycle, simulated emission results show a good consistency with experimental data. The Integrated SCR–Muffler shows a better performance than original muffler and SCR as far as IL, NOX reduction efficiency and pressure loss are concerned.

Gas turbine engine exhaust nozzle having a noise attenuation device driven by shape memory material actuators

A gas turbine engine exhaust nozzle arrangement for the flow of exhaust gases therethrough between an upstream end and a downstream end thereof comprising a nozzle and a plurality of tabs which extend in a generally axial direction from a downstream portion of the nozzle wherein the nozzle further comprises an actuation mechanism capable of moving the tabs between a first deployed position, where the tabs interact with a gas stream to reduce exhaust noise thereof, and a second non-deployed position, where the tabs are substantially aerodynamically unobtrusive.

Development of the braided wire damper for exhaust radiation noise reduction

Single pipes used in a conventional vehicle exhaust system are not enough to reduce exhaust noise to a satisfactory level, especially at high frequencies. Double pipes are effective for reducing noise, but still not enough, and they are heavy. The new method, `Braided Wire Damper’, is an exhaust pipe partly wrapped with braided wire net. Proposed here is the use of `Braided Wire Damper’ and it is found to be very effective for reducing not only the noise but also the weight of the exhaust system.

Control of the Multi-Mode Muffler for Low Noise and Low Backpressure

To reduce backpressure for improving engine power in exhaust system, a large difference of pressure is required, but this is in conflict with the requirement for reducing exhaust noise that needs a small pressure difference. In this paper, the controllable muffler designed by simplifying the structure of the exhaust system has a low backpressure and a proper sound specification to the rotation of engine. The exhaust system in conventional studies has been designed to have maximum noise reduction over the whole driving domain, but due to its complex structure this led to increased backpressure. If the muffler is designed according to the driving frequency, which is a dominant noise component in stationary driving speed, the backpressure is reduced due to the simplified structure of the muffler. Furthermore, a multi-mode muffler able to change structure with varied driving speed was designed.

Development of an active muffler system for reducing exhaust noise and flow restriction in a heavy vehicle

This paper presents an application of an active muffler system for reducing exhaust noise in heavy vehicles. The signal-to-noise ratio associated with the process of estimating the transfer function of a secondary path was improved using conventional inverse modeling combined with an adaptive line enhancer as a control strategy. The simplified passive muffler using 2-duct perforated elements was combined with the active muffler system instead of a conventional passive exhaust system to reduce exhaust noise and flow restriction simultaneously. In experiments to estimate the transfer function of a secondary path, estimates using the inverse modeling method alone show many errors at the dominant frequencies of primary noise. Results obtained using the proposed algorithm correspond well to the actual transfer function as estimated without the primary noise. Control experiments with the proposed active muffler system have been implemented for stationary and driving states, respectively. Compared to the conventional passive muffler, the experimental results showed that the proposed active muffler could reduce the backpressure up to 60% as well as the overall sound pressure level below about 2200 rpm.

Active noise control with the active muffler in automotive exhaust system

This study experimentally demonstrates the use of active muffler attached to the automotive exhaust system to reduce exhaust noise. For improving the signal to noise ratio in the process of estimation of secondary path transfer functions, the on-line algorithm that conventional inverse modeling is combined with adaptive line enhancer is used as the control algorithm. Active muffler is designed that the primary noise and the control sound are propagated as a plane wave in the outlet. Therefore, the error microphone could be placed out of the tail pipe center of a high temperature and the radiation noise to the outside could be reduced in the whole area around the outlet. The control experiment for reducing exhaust noise with active muffler is implemented during run-up at no load. From the experimental results presented, compared with the conventional off-line method, the proposed on-line method is capable to acquire a reduction of exhaust noise above 5 dB in overall sound power level.

Practical means of implementing vehicle noise control. II

Engine noise is usually the main source of noise on a commercial vehicle. Palliative treatment by engine/gearbox enclosure and by attention to the exhaust system can give some 7–8 dBA reduction of noise. A weight penalty of 10 lb/dBA and a cost penalty of £3–4/dBA results from these treatments. Reduction of engine noise at source is a long term solution. It is usually not difficult, if expensive, to reduce exhaust noise to an acceptable level by careful design of the exhaust system. Engine enclosure can be conveniently effected by absorbent-lined steel panels although practical difficulties can occur with existing vehicles. The choice of absorbent material is restricted by lubricating and fuel oil contamination and the resultant fire hazard.