Expansion Chamber Research Articles

Hybrid silencers with micro-perforated panels and internal partitions

A sub-structuring approach, along with a unit cell treatment, is proposed to model expansion chamber silencers with internal partitions and micro-perforated panels (MPPs) in the absence of internal flow. The side-branch of the silencer is treated as a combination of unit cells connected in series. It is shown that, by connecting multiple unit cells with varying parameters, the noise attenuation bandwidth can be enlarged. With MPPs, the hybrid noise attenuation mechanism of the silencer is revealed. Depending on the size of the perforation hole, noise attenuation can be dominated by dissipative, reactive, or combined effects together. For a broadband sound absorption, the hole size, together with the perforation ratio and other parameters, can be optimized to strike a balance between the dissipative and reactive effect, for ultimately achieving the desired noise attenuation performance within a prescribed frequency region. The modular nature of the proposed formulation allows doing this in a flexible, accurate, and cost effective manner. The accuracy of the proposed approach is validated through comparisons with finite element method and experiments.

Topology optimization for three-phase materials distribution in a dissipative expansion chamber by unified multiphase modeling approach

The sound attenuation performance of a dissipative expansion chamber is a combinational result of reflective and dissipative effects. Although it is well known that the performance can be substantially improved by altering the distribution of constituent materials, the process of finding an optimal distribution of the materials still remains a challenge. This work proposes a new design method for interior space of a dissipative expansion chamber by using topology optimization method. Different from the existing topology optimizations for expansion chamber designs based on simplified material modeling, the present design deals with fully-modeled multiphase constituent materials, such as acoustic, poroelastic and elastic one. Difficulties in the optimization formulation for the multiphase material distribution arise from extremely-different acoustic behavior of the materials and the use of various governing equations for different phase materials. To systematically vary the attributes of the chamber interior space, a unified multiphase modeling approach that allows continuous variations between the three-phase materials within the same implementation is employed with an elaborately-derived penalty parameters of material interpolation functions. Various design examples are successfully solved for wide frequency bands and the optimal configurations clearly demonstrate the importance of using specific configurations tuned to different target frequencies.

Integrating laboratory and field data to quantify the immersion freezing ice nucleation activity of mineral dust particles

Abstract. Data from both laboratory studies and atmospheric measurements are used to develop an empirical parameterization for the immersion freezing activity of natural mineral dust particles. Measurements made with the Colorado State University (CSU) continuous flow diffusion chamber (CFDC) when processing mineral dust aerosols at a nominal 105% relative humidity with respect to water (RHw) are taken as a measure of the immersion freezing nucleation activity of particles. Ice active frozen fractions vs. temperature for dusts representative of Saharan and Asian desert sources were consistent with similar measurements in atmospheric dust plumes for a limited set of comparisons available. The parameterization developed follows the form of one suggested previously for atmospheric particles of non-specific composition in quantifying ice nucleating particle concentrations as functions of temperature and the total number concentration of particles larger than 0.5 μm diameter. Such an approach does not explicitly account for surface area and time dependencies for ice nucleation, but sufficiently encapsulates the activation properties for potential use in regional and global modeling simulations, and possible application in developing remote sensing retrievals for ice nucleating particles. A calibration factor is introduced to account for the apparent underestimate (by approximately 3, on average) of the immersion freezing fraction of mineral dust particles for CSU CFDC data processed at an RHw of 105% vs. maximum fractions active at higher RHw. Instrumental factors that affect activation behavior vs. RHw in CFDC instruments remain to be fully explored in future studies. Nevertheless, the use of this calibration factor is supported by comparison to ice activation data obtained for the same aerosols from Aerosol Interactions and Dynamics of the Atmosphere (AIDA) expansion chamber cloud parcel experiments. Further comparison of the new parameterization, including calibration correction, to predictions of the immersion freezing surface active site density parameterization for mineral dust particles, developed separately from AIDA experimental data alone, shows excellent agreement for data collected in a descent through a Saharan aerosol layer. These studies support the utility of laboratory measurements to obtain atmospherically relevant data on the ice nucleation properties of dust and other particle types, and suggest the suitability of considering all mineral dust as a single type of ice nucleating particle as a useful first-order approximation in numerical modeling investigations.

Comparison of two methods of mathematical modeling in hydrodynamic sealing gap

The aim of work is to compare two possible methods of mathematical modeling of hydrodynamic instabilities. This comparison is performed by monitoring the formation and evolution of Taylor vortices in hydrodynamic sealing gap. Sealing gaps are a part of the hydraulic machines with the impeller, such as turbines and pumps, and they have an effect on the volumetric efficiency of these devices. This work presents two examples of sealing gaps. These examples are closed sealing gap and modified sealing gap with expansion chamber. On these two examples are applied procedures of solution contained in CFD software (ANSYS Fluent 14.5). In ANSYS Fluent is two possible basic approaches of solution this task: the moving wall method and the sliding mesh method. The result of work is monitoring the impact of the expansion chamber on the formation of hydrodynamic instabilities in the sealing gap. Another result is comparison of two used methods of mathematical modeling, which shows that both methods can be used for similar tasks.

Two-step design process for optimal suction muffler in reciprocating compressor

A systematic design process for an optimal suction muffler is proposed to reduce the noise of a reciprocating compressor. Because the outer shape of a suction muffler is complicated, the well-known internal configuration for simple expansion chamber mufflers is not easily applicable to the suction muffler design problem. To achieve an optimal design of a suction muffler, two sequential optimization problems are formulated to maximize the transmission loss value at a target frequency: acoustical topology and shape optimization problems. The key idea in the suggested method is to use an optimal topology obtained by solving the topology optimization problem as an initial shape for the shape optimization problem. The formulated optimization problems are solved at several target frequencies, and the acoustical characteristics of the optimal shapes are closely investigated. The experimental results for two optimal suction mufflers support the validity of our suggested two-step design process for optimal suction mufflers.

Thermophysical flow simulations of rapid expansion of supercritical solutions (RESS)

Thermophysical flows of rapid expansion of supercritical solutions (RESS) were numerically simulated from the nozzle inlet to the outlet of the expansion chamber. In these studies, supercritical carbon dioxide (SCO2) was used as the solvent, and naphthalene was used as the solute. Our approach seamlessly simulated SCO2 entering the nozzle, SCO2 crossing the critical pressure in the nozzle, CO2 gas expanding into the expansion chamber, and CO2 condensation in the chamber. The simulation is based on a preconditioning method developed by Yamamoto, together with mathematical models for thermophysical properties of substances in a program package for thermophysical properties of fluids (PROPATH). A primary issue was the extent to which changing the nozzle inlet and outlet pressures impacted the solubility of the solute. We also studied the effects of changing the nozzle throat diameter and throat length. In addition, we used a simple particle-formation model to predict nucleation, condensation, and coagulation of naphthalene particles. Our results indicate that the location at which the pressure crosses the critical pressure is sensitive to the pressure at the nozzle inlet, and nucleation of naphthalene starts near that location.

Numerical study of a particle separation method based on the Segré‐Silberberg effect

AbstractParticles that are placed in a laminar pipe flow rotate and migrate transversally to a radial equilibrium position. This so called Segré‐Silberberg effect is used in a new method for size separation of particles. The particles to be separated are placed in a pipe flow and subsequently enter an expansion chamber, where the flow is split and the particles are divided into two fractions. This paper reports the results of two‐dimensional Euler‐Lagrange simulations of the motion of neutrally‐buoyant particles inside the expansion chamber. The simulation results agree well with experimental data on the separation and show that the Saffman force has a significant impact onto the particle trajectories. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

내부 파티션을 갖는 단순확장관의 소음저감 및 배압특성의 전산해석

Mufflers have been widely used in the exhaust system to reduce the noise. However, installing muffler may deteriorate the efficiency due to the increase of back pressure. Mufflers usually consist of partition plates and perforated holes in a expansion chamber. In this paper, the influences of the location of the partition and hole on the acoustic TL and back pressure were examined. The acoustic TL was predicted using virtual lab commercial software, while the back pressure were predicted using CFX commercial software. The results were used to set up a database for finding their trends. The optimal muffler model in user-interested frequency range could be selected by analyzing this database.

Duct noise attenuation using reactive silencer with various internal configurations

The broadband sound attenuation characteristics of expansion chamber silencers can be altered by their internal configuration. Three-dimensional modeling of such systems, especially in the presence of complex internal partitions, remains a challenging task. In order to tackle the system complexity, this paper presents a systematic approach based on the sub-structuring modeling principle, to investigate the effects of several typical silencer configurations and provide guidelines for possible system optimization. Through numerical examples, the effects of various internal arrangements, including the side-branch partitions, multi-chamber partitions, non-symmetric inlet/outlet, and their combined effects are investigated. Numerical predictions show good agreements with both finite element method (FEM) and experiments. Investigations suggest some critical issues and possible solutions for better silencer design.

Near-unity mass accommodation coefficient of organic molecules of varying structure.

Atmospheric aerosol particles have a significant effect on global climate, air quality, and consequently human health. Condensation of organic vapors is a key process in the growth of nanometer-sized particles to climate relevant sizes. This growth is very sensitive to the mass accommodation coefficient α, a quantity describing the vapor uptake ability of the particles, but knowledge on α of atmospheric organics is lacking. In this work, we have determined α for four organic molecules with diverse structural properties: adipic acid, succinic acid, naphthalene, and nonane. The coefficients are studied using molecular dynamics simulations, complemented with expansion chamber measurements. Our results are consistent with α = 1 (indicating nearly perfect accommodation), regardless of the molecular structural properties, the phase state of the bulk condensed phase, or surface curvature. The results highlight the need for experimental techniques capable of resolving the internal structure of nanoparticles to better constrain the accommodation of atmospheric organics.

Experimental Investigation of a Double Expansion Chamber Reactive Muffler for Stationary Diesel Engine

The increasing need to control noise levels in our environment produced by internal combustion engines, demands the accurate designing of mufflers to meet specific noise emission norms. This paper describes a theoretical and experimental study of a circular double expansion chamber reactive muffler designed specifically for a stationary generator engine. For the theoretical approach, the equations are solved by four pole transfer matrix method using MATLAB software to obtain optimized dimensions of three mufflers at desired maximum transmission loss and at frequencies at which highest sound level is observed. MATLAB software is also used to obtain the graphs for transmission loss versus frequency. In the experimental method a white noise 1D plane wave is generated in the impedance tube attached on both sides of the muffler to obtain the transmission loss. The theoretical results obtained are then compared with experimental results for validation. The theoretical results are well correlated with the experimental results.

Formation of a Double Layer in Electronegative <inline-formula> <tex-math notation="TeX">${\rm O}_{2}$ </tex-math></inline-formula> Plasma

A double layer (DL) was observed at the boundary between the source region and expansion region of an inductively coupled, radio frequency, plasma reactor when an oxygen discharge was ignited at low pressure. A DL is a narrow localized region with relatively large potential difference and electric field, which can be formed in electropositive as well as electronegative plasmas. In an inductively coupled plasma reactor of this type, it seems to be formed at the interface between the smaller source region and the larger expansion chamber and acts as an internal boundary separating two plasma regions of significantly different compositions, density, and plasma potentials.

Simplification of the Delany–Bazley approach for modelling the acoustic properties of a poroelastic foam

The acoustic performance of mufflers is often enhanced by the inclusion of dissipative materials. A simplified implementation of the Delany–Bazley method is presented which facilitates the acoustic characterisation of poroelastic foam in computational models in the absence of material airflow resistivity data. The acoustic characteristics of a polyurethane foam material were experimentally obtained using a two-cavity impedance tube method. A finite element model of an expansion chamber design incorporating foam is presented. Numerical results of the transmission loss for the foam-filled muffler are compared with results obtained experimentally using a two-microphone acoustic pulse method.

A computational fluid dynamics study on the heat transfer characteristics of the working cycle of a β-type Stirling engine

A compressible CFD code has been developed to study the heat transfer characteristics of a β-type Stirling engine with a very simple design and geometry. The results include temperature contours, velocity vectors, and distributions of local heat flux along solid boundaries at several important time steps as well as variations of average temperatures, integrated rates of heat input, heat output, and engine power. It is found that impingement is the major heat transfer mechanism in the expansion and compression chamber, and the temperature distribution is highly non-uniform across the engine at any given moment. The results, especially the rates of heat transfer, are quite different from those obtained by a second-order model. The variations of heat transfer rates are much more complicated than the simple variations returned by the second-order model. This study sheds light into the complex heat transfer mechanism inside the Stirling engine and is very helpful to the understanding of the fundamental process of the engine cycle.

Mathematic Analysis and Performance Simulation for Exhaust Muffler Based on Transmission Loss

Transmission loss is an important noise reduction parameter of exhaust muffler. Based on the finite element method, transmission loss is calculated, and the influential effect of the fluid is considered in this paper. Through calculation of transmission loss of the single expendable muffler and composite muffler, the results show that expansion ratio affects amplitude of transmission loss, and expansion chamber length affects the attenuation frequency bandwidth. The analysis method and conclusions are instructive to the design parameters of exhaust muffler.

Plasma expansion across a transverse magnetic field in a negative hydrogen ion source for fusion

High power negative hydrogen ion sources operating at 0.3 Pa are a key component of the neutral beam injection systems for the international fusion experiment ITER. To achieve the required large ion current at a tolerable number of co-extracted electrons the source is equipped with a magnetic filter field (up to 10 mT). The IPP prototype source (1/8 of the area of the ITER source) has been equipped with a flexible magnetic filter frame to perform filter field studies (position, polarity, strength). Axial profiles of the plasma parameters are measured with two Langmuir probes, positioned in the upper and the lower half of the expansion chamber. In addition to the expected decrease in electron temperature and density a vertical drift develops the direction depending on the polarity of the field. Without field no drift is observed. The drift is less pronounced in caesium seeded discharges and almost vanishes in deuterium, indicating an influence of the ion mass on the drift. A comparison with results from a half-size ITER source reveals that the plasma is much more uniform in the large source.

Determination of transmission and insertion loss for multi-inlet mufflers using impedance matrix and superposition approaches with comparisons

This paper details how transmission and insertion loss can be defined for the multi-inlet case. Two different procedures are used for calculations and are compared against one another. The first is an impedance matrix method that has been proposed in the past and is especially suitable for deterministic approaches like the boundary element method. The other is a superposition method that does not require assembling or inverting a matrix. Though each method can be extended to the n-inlet situation, the superposition approach is mathematically simpler but requires the source impedance to be known a priori. Transmission and insertion loss can be determined using either method, and results using both are shown to be equivalent. The methods are demonstrated and compared for a two-inlet one-outlet expansion chamber. After which, the superposition method is used to determine the insertion loss for a two-inlet one-outlet generator set muffler experimentally.

A Study on Exhaust Muffler Using Counter-Phase Counteract

The exhaust noise, which falls into low-frequency noise, is the dominant noise source of a diesel engines and tractors. The traditional exhaust silencers, which are normally constructed by combination of expansion chamber, and perforated pipe or perforated board, are with high exhaust resistance, but poor noise reduction especially for the low-frequency band noise. For this reason, a new theory of exhaust muffler of diesel engine based on counter-phase counteracts has been proposed. The mathematical model and the corresponding experimental validation for the new exhaust muffler based on this theory were performed.

Improved tuning of the extended concentric tube resonator for wide-band transmission loss

Transmission loss (TL) of a simple expansion chamber (SEC) consists of periodic domes with sharp troughs. This limits practical application of the SEC in the variable-speed automobile exhaust systems. Three-fourths of the troughs of the SEC can be lifted by appropriate tuning of the extended inlet/outlet lengths. However, such mufflers suffer from high back pressure and generation of aerodynamic noise due to free shear layers at the area discontinuities. Therefore, a perforate bridge is made between the extended inlet and outlet. It is shown that the TL curve of a concentric tube resonator (CTR) can also be lifted in a similar way by proper tuning of the extended unperforated lengths. Differential lengths have to be used to correct the inlet/outlet lengths in order to account for the perforate inertance. The resonance peak frequencies calculated by means of the 1-D analysis are compared with those of the 3-D FEM, and appropriate differential lengths are calculated. It is shown how different geometric characteristics of the muffler and mean flow affect the differential lengths. A general correlation is obtained for the differential lengths by considering seven relevant geometric and environmental parameters in a comprehensive parametric study. The resulting expressions would help in design of extended-tube CTR for wide-band TL. (C) 2014 Institute of Noise Control Engineering.

Study on plate silencer with general boundary conditions

A plate silencer consists of an expansion chamber with two side-branch rigid cavities covered by plates. Previous studies showed that, in a duct, the introduction of simply supported or clamped plates into an air conveying system could achieve broadband quieting from low to medium frequencies. In this study, analytical formulation is extended to the plate silencer with general boundary conditions. A set of static beam functions, which are a combination of sine series and third-order polynomial, is employed as the trial functions of the plate vibration velocity. Green׳s function and Kirchhoff–Helmholtz integral are used to solve the sound radiation in the duct and the cavity, and then the vibration velocity of the plate is obtained. Having obtained the vibration velocity, the pressure perturbations induced by the plate oscillation and the transmission loss are found. Optimization is carried out in order to obtain the widest stopband. The transmission loss calculated by the analytical method agrees closely with the result of the finite element method simulation. Further studies with regard to the plate under several different classical boundary conditions based on the validated model show that a clamped-free plate silencer has the worst stopband. Attempts to release the boundary restriction of the plate are also made to study its effect on sound reflection. Results show that a softer end for a clamped–clamped plate silencer helps increase the optimal bandwidth, while the same treatment for simply supported plate silencer will result in performance degradation.