The vibration response of the disk to the cavity excitation is used to analyze the interaction between the solid parts of the medium D, that is, the vibration response of the disk to the sound field excitation in the cavity. A 3 mm thick steel disc was placed at one end of the cylinder as described in 2.1. The vibration was excited by a small vibration generator and the response was measured at 65 points. These data are used as excitations for the sound field in the cavity. In the intracavity disc, this time the cavity is coupled to the second disc placed at the other end. In order to obtain a clear surface acceleration measurement grid (left), the experimental and calculated results of the simple geometric cavity sound pressure of the disk grid (middle) and the cylindrical cavity grid (right) compare the coupling of the medium to the solid part. That is to say, the vibration of the disk can be obtained without considering the coupling effect on the sound field. Therefore, it is possible to analyze the sound field using only the vibration distribution of the disk as a boundary condition.
The housing unit is used in the FE representation to represent a thin disc placed at the opposite end of the excitation disc (3 mm). The intracavity fluid unit is coupled to the disc structural unit. The response of the thin disk is also obtained by the modal overlap method. A comparison between the calculated results and the experimental results shows that the frequency is better than 1 kHz.
Since the cavity does not resonate in the low frequency region, it is expected that the results in this region will vary greatly. To confirm this, the acoustic material is filled in the cavity to re-measure the response of the thin disk. It was found that the response of the high frequency band was significantly reduced. It indicates that the disc is mainly excited by the sound field, and the vibration transmitted through the cylinder wall can be ignored. For a hermetic compressor, this procedure is sufficient to describe the propagation of acoustic energy through the cavity to the casing. Solid and motor surface vibrations are used as boundary conditions for velocity in the cavity model to determine the response of the enclosure.
However, detailed geometric and related vibration information is still required for compressor operation. The effects of geometric factors are discussed in the next section.
The sound field in the actual compressor chamber firstly performs a simple geometric simplification of the body and the motor to analyze the sound field in the actual compressor casing. This is to have a deep understanding of the accuracy of the geometrical features affecting the calculation results. In these analyses, the cavity was modeled by BEM and solved using the Pade expansion method. Firstly, the sound field model of the empty casing is established, and the vibration of the casing is decoupled. The cavity model consists of 7,500 TERA3 units with an analysis frequency of up to 10 kHz. The cell value of the particle velocity is defined in the cell area at the top of the chamber, and the response of the sound pressure is calculated at several points. This model was verified by experiments.
The particle velocity is measured with a microfluidic transmitter that measures the pressure response at a specified point. Indicates the sound pressure frequency relationship at a node relative to the specified unit particle velocity value. When the cavity is filled with air and the frequency is greater than 10 kHz, it shows good consistency. It is a comparison of 1/3 octave, the average error is about 3dB. But in some 0.75mm steel discs, the measured and calculated vibration response of the acoustic field excited by the cavity is compared with the particle velocity unit value boundary condition experimental device frequency band, the error can be Up to 9dB.
In the second experiment, the motor stator was represented by a similar simple geometry made of nylon, supported on four solid steel cylinders representing the spring, as shown. This figure also shows the comparison between the calculation result of 1/3 octave and the experimental result. The average error is also 3dB, and the maximum error is 9.3dB.
In the third experiment, the effect of the copper wire coil was replaced with a nylon sheet placed above and below the stator in a simple geometry, as shown. Comparing the calculated results with the experimental results, the average error is 3 dB and the maximum error is 10 dB. It is worth noting that large errors occur continuously in the 4 kHz and 5 kHz bands due to the uncertainty of the experimental process.
In the final experiment, the body of the compressor included the actual body and motor, at which time the chamber was excited and the sound pressure was measured at several points. Boundary conditions are used to represent all geometric details. The numerical model takes into account the real geometry and simplifies it to reduce the number of boundary elements. The boundary elements must be able to fully represent all geometric features. The comparison between the calculated results and the experimental results shows a similar trend to the previous analysis, with an average error of 3 dB and a maximum error of about 10 dB.
Conclusion The numerical model of a simple geometric cavity has shown that for 3 mm discs, the dynamics of the medium-solid interaction are negligible. Therefore, from the perspective of the cavity, it can be regarded as a rigid boundary. The excitation of the sound field can be represented by relatively few surface velocity measurement points.
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