Muffler Design for a Refrigerator Compressor

During its operation, a refrigerator compressor produces pulsating noise, primarily driven by the suction and discharge processes. Sound attenuating mufflers need to be designed without any additional pressure drop on both suction and discharge side. An additional pressure drop at the suction and discharge side will lead to lower charging and discharging of the compressor and hence reduces cooling capacity. Since a one dimensional formulation with plane wave assumption to calculate transmission loss is not applicable for small mufflers (ratio of length to diameter is less than 1), a numerical methodology was established and validated using an impedance tube. Detailed three dimensional Computational Fluid Dynamics (CFD) simulations were used to further study the pressure drop across the mufflers for a given flow and compressor operating frequency. In the proposed new methodology, the pressure distribution is observed as a function of frequency and an optimal position of inlet and outlet pipes is decided to improve transmission loss. Mufflers designed with this approach showed better acoustic performance on the suction and discharge side of refrigerator compressor. The effect of various refrigerants on acoustic frequencies is also studied, which would additionally help in tuning the muffler to improve its effectiveness

Effect of shell compliance on the axial transmission loss of concentric tube resonator

A simple expansion chamber can be turned into a concentric tube resonator by connecting the inlet and outlet pipes through a perforated pipe. This arrangement will reduce the pressure drop and aerodynamic noise generated due to sudden area changes. It also improves the transmission loss through the inertance of the perforations. Thus, a concentric tube resonator offers a great advantage over a simple expansion chamber in flow, aerodynamics, and acoustics point of view. In general, while calculating the TL, the walls of the chamber are assumed to be acoustically rigid. However, in some applications, the walls are compliant to the sound waves incident on them. Thus, a part of the incident energy will break out of the chamber in the transverse direction, and the rest will propagate and gets attenuated in the axial direction. Therefore, the presence of compliant walls will influence the transmission loss of the chamber calculated in the axial direction. The current paper discusses an analytical methodology developed by using Green’s function to calculate the axial transmission loss of a concentric tube resonator having a compliant wall in transverse direction. In the analysis, end walls of the chamber are assumed to be acoustically rigid except at the inlet and outlet ports. The effect of the compliant wall, annular cavity, and perforated sheet together is transferred to the inner pipe as a reflection load coefficient. The Green’s function for this configuration is expressed as the summation of modal eigenfunctions of the inner pipe by using the division-of-region method. The modal amplitudes and the corresponding wavenumbers are calculated by substituting appropriate boundary conditions. The inlet and outlet ports are treated as hypothetical rigid pistons moving back and forth with uniform velocity. From the Kirchoff Helmholtz integral equation, the total velocity potential generated inside the chamber is calculated with the aid of the principle of superposition. By using the relation between the velocity potential and acoustic pressure, the total pressure acting on each piston is calculated. Thence, the transfer matrix relating the acoustic pressure and volume velocity at the inlet port to that of outlet port is evaluated to predict the transmission loss. A numerical model has been prepared by using the finite element method to validate the results obtained from the current methodology. The comparison shows that there is a good agreement between the results obtained from the proposed method and the numerical model

Green's function approach for the transmission loss of concentrically multi-layered circular dissipative chamber

An analytical method has been proposed by using Green's function analysis to find the transmission loss (TL) of a concentrically multi-layered circular dissipative chamber. Each layer of the chamber is filled with a porous acoustic absorptive material and is separated from the adjacent one by a thin perforated screen. A tailored Green's function for this configuration, in the absence of mean flow, is expressed as the summation of eigenfunctions of the central duct. In the analysis, the walls of the chamber are assumed to be acoustically rigid. The cumulative effect of the layers has been incorporated in terms of the reflection coefficient in the eigenfunctions of the central duct. By using the piston analogy approach at the inlet and outlet ports of the chamber, the total velocity potential generated inside the central duct is estimated. Thence, the transfer matrix is evaluated to predict the TL. The results obtained from the current method are in good agreement with the numerical models and available literature. A parametric study has been conducted to investigate the effect of the number of layers, and their arrangement with respect to thickness and flow resistivity, on the acoustic performance of the chamber

Application of NAH method for the prediction of sound radiation from a flexible box structure

Prediction of noise radiation from the thin flexible structures, which encloses sound sources is important for the structural design to develop a quieter product. There are different direct and indirect techniques are existing to know the sound radiation of the machine or structure. The direct method such as sound intensity method and indirect methods or inverse techniques includes Near-field Acoustic Holography (NAH) and beam forming. NAH is one of the good inverse techniques to predict the vibro-acoustic properties of the sound source. In this method, acoustic quantities can be reconstructed on the surface of a vibrating structure based on sound pressure measurements made at a set of points in the near-field of the source. These NAH methods are the ill-posed inverse problems due to the existence of strongly decaying, evanescent like waves. Regularization is used to overcome the ill-posed problem. The purpose of this investigation is to predict the sound radiation characteristics of a box structure with one flexible wall. The acoustic quantities are reconstructed with NAH technique

Prediction of Acoustic Natural Frequencies for two dimensional simplified aircraft cabin by Impedance Mobility and Compact Matrix(IMCM) approach

Prediction of modal characteristics of acoustic cavities with irregular shapes is an important topic in vibro-acoustic analysis of systems. In this paper, proposed method of impedance and mobility compact matrix (IMCM) provided the approach of Integro Modal Method (IMM) in the matrix form for prediction of modal characteristics of acoustical cavities with irregular shape. This method, consists of discretizing the whole cavity into a series of sub cavities either regular or irregular cavities. Acoustic pressure in regular sub cavities has been decomposed over a modal basis and for irregular sub cavities over that of bounding surface. Continuity of both pressure and velocity between adjacent sub cavities is ensured using a membrane with zero mass and stiffness. This method aims to develop a physical basis rather than a numerical approach. Mathematical formulation of the method has been explained in detail. Coupling equations being in a matrix form, so, it is easily solved using numerical methods in computer. The current approach is demonstrated by considering two dimensional simplified aircraft cabin as an irregular cavity. Predicted natural frequencies of the simplified aircraft cabin based on the proposed method have been compared with the available results in the literature and are in good agreement

Effect of a joint on breakout noise characteristics of rectangular duct

Breakout from a flexible rectangular duct depends on its structural properties as well as acoustic properties of the medium. Majority of duct breakout noise prediction models in the literature consider an ideal duct (without joint). However, ducts used in applications have joints. So, current research interest is to study the effect of duct joint in predicting the breakout noise and its influence on modal parameters. For this purpose, duct with a joint is considered in the experimental study and an ideal duct for numerical analysis. As a first step, an experimental setup is developed to measure breakout noise in terms of transverse transmission loss and radiation efficiency, and furthermore an experimental modal analysis is performed to measure modal parameters of the duct. Numerical analysis is performed on an ideal rectangular duct (without considering the joint condition) to calculate the breakout noise and modal parameters. Both, experimental and numerical results are compared, and it is observed that joint has a less significant influence on breakout noise as compared with modal parameters

Study on Effect of Joints on the Modal Parameters of Rectangular Duct.

A duct is an enclosed passage for transmission of a substance, especially liquid or gas with extensive usage in heating, ventilation, and air conditioning (HVAC) applications and gas industries. Practically, duct shapes have deviations from ideal geometric shapes, predominantly in thin-walled structures due to joint conditions. The aim of the present research is to understand the effect of duct joints on modal parameters. In this study, Experimental Modal Analysis (EMA) is carried out on three different rectangular ducts having different joints and materials. Numerical Modal Analysis (NMA) is then performed on these ducts by modelling the corresponding joints. The Modal Assurance Criteria (MAC), frequency comparison, and auto-MAC plots are used to check the correlation between the experimental and numerical results. Based on these comparisons, a good match in natural frequencies but a mismatch in mode shapes are observed