Study on sound pressure level and acoustic damping ratio of one dimensional sound field partitioned by perforated plate with geometric similarity

Abstract

In heat exchangers like a boiler, as a tube bank is set in a duct the alternative vortices occur behind a tube bank and the vortex shedding frequency increases with the flow velocity when the boiler is operated. High level sound is suddenly generated when the vortex frequency comes close to the acoustic natural frequency of the duct. The high level sound keeps the frequency and the amplitude constant even if the flow velocity increases in the case of small acoustic damping due to the self-excited mechanism. This is generally called the self-sustained tone and the factory is forced to stop the operation due to the complaints of the neighborhood. The effective countermeasure is required in the design stage. In general, the baffle plate is inserted in the tube bank but it is difficult to treat it, especially to the duct with plural tube banks. Then the other effective countermeasures are keenly anticipated. The author has many studies concerned to the clarification of the generation mechanism and the countermeasures and their effects. In recently, it is clarified that the perforated plate suppresses the high level sound. The author has a question what is the relation between the suppression effect of the perforated plate and the dimension ratio. Then in this paper, the acoustic experiments will be conducted by using the three different sizes of the ducts and perforated plates. The acoustic damping obtained by these experiments and analyses is compared. As a result, it was clarified that the acoustic damping ratio becomes smaller with larger of dimensional ratio.