Study on acoustic natural frequency and its mode of one dimensional sound field partitioned by perforated plate

Abstract

Karman vortex shedding occurs when the gas passes through the duct with tube bank of the heat exchangers, such as gas heaters and boilers. Very high level sound in called "self-sustained tones" occurs due to the interference of the vortex and the sound field in the duct. In general, baffle plates are used to suppress the self-sustained tone. However, it is difficult to use them effectively, because insertion conditions have not been established. Then, perforated plates are installed in both sides of the duct to suppress the self-sustained tones. As a result, it was clarified that the perforated plate could suppress the self-sustained tone as expected. The perforated plates are generally used with a cavity to obtain the suppression effect effectively. However the perforated plates change the acoustic natural frequency of the sound field of the duct. In previous paper, this was not referred. This paper aims to clarify the role of the perforated plates when it changes the acoustic natural frequency of the duct. In this study, one dimensional duct will be used for simplicity and the relationship between the aperture ratio and the acoustic natural frequency will be obtained analytically and experimentally. As a result, it was clarified that the analytical results are in good agreement with the experimental ones and the plates with holes had a role of acoustic damping in calculation of the acoustic natural frequency of the one dimensional sound field like a duct with plates with holes. 熱交換器のダクト内には管群(熱交換を行うため，温水や蒸気などを通す管の集合部)が存在する．この管群と

Countermeasure for High Level Sound Generated from Boiler Tube Bank Duct

“…Table 2 represents the peak frequency in each case of the aperture ratio. It has been also confirmed that the resonance frequency increases as the aperture ratio increases in the previous study [31] in which an acoustic resonance frequency has been experimentally and analytically verified by assuming a one-dimensional sound field in a duct partitioned by a perforated plate, and similar results are obtained in this study. Figure 33 shows the acoustic mode at each peak frequency in the cases of ϕ = 0, 1, 2, and 4%.…”

Relation between peak value and acoustic damping of sound pressure level of one dimensional sound field partitioned with perforated plates

“…The mode orthogonality as shown in the equation (11) is even true for the case of one dimensional sound field partitioned with the perforated plate. Figure5 (a) and (b) are 1 st mode and 2 nd mode shapes of the pressure obtained by the reference [6] respectively. The horizontal axis shows the position from the origin and the vertical axis shows the normalized sound pressure.…”

“…A perforated plate is widely used as an acoustic absorption material for compressors [1] and acoustic barriers for roads and railways [2]. It has been confirmed that the perforated plate suppressed the self-sustained tone generated from heat exchangers like a boiler by using it on a duct wall [3]～ [6]. The effects of the perforated plate on the acoustic natural frequency of an one dimensional sound field partitioned with the perforated plate has been studied in our previous investigations [7], [8].…”