**1. Introduction**

Heat exchangers and boilers are widely used in various plants such as power plants and chemical plants. In heat exchangers such as boilers and gas heaters, a high level sound is sometimes generated and it results in a serious problem such as a plant shutdown or non-operation. A high level sound is generated in tube banks installed in a duct. In tube banks, water flows inside of tubes, warm gas flows outside of tubes, and Karman vortex shedding occurs. The vortex shedding frequency depends on the flow velocity. In contrast, a resonance frequency called an acoustic natural frequency, which is independent of the flow velocity, is determined by the duct size and the sound speed. When the two frequencies coincide, a resonance phenomenon occurs at a certain velocity [1–9]. Ziada and Oengören [10] have shown that vortex excitation results from the formation of periodic vortices in the space between tubes by visualization experiments in the waterway. Hamakawa et al. [11] focused on effect of arrangement of tube banks, and investigated the characteristics of vortex shedding and acoustic resonance from in-line and staggered tube banks.

When a resonance phenomenon occurs at a certain velocity, if the acoustic damping is small, a high level sound continues as flow velocity increases. This phenomenon is called the self-sustained tone [12, 13]. The self-sustained tone might cause the surrounding noise problem, and also cause plant shutdown and, hence, production losses, etc.

For a countermeasure of the self-sustained tone, a method of inserting a partition plate called a baffle plate inside the duct is generally used. In this method, the baffle plate inserted inside the duct is assumed to increase the natural frequency of the duct, detune the frequency of the vortex shedding from the tube bank and the acoustic natural frequency of the duct, and suppress the resonance phenomenon [14–16]. However, Ishihara et al. [12] demonstrated that the natural frequency of the duct decreases by inserting the baffle plate, and a decision of an appropriate insertion position of the baffle plate is not easy. Hamakawa et al. [16] have investigated the effect of the baffle plate on the acoustic resonance generation from in-line tube banks with small cavity, and they clarified that although sound pressure level of an acoustic mode perpendicular to the flow (lift mode) is suppressed by a baffle plate, that of an acoustic mode parallel to the flow (drag mode) increases. Ishihara and Takahashi proposed that flexible walls such as rubber boards are set on the duct walls for suppressing the self-sustained tone [17]. They expected that the vibration of the flexible walls damp the lift resonance mode when self-sustained tone is generated. They demonstrated that the suppression effect of the rubber sheet appeared when the tension of it is small and it is located at just the tube bank and downstream of the tube bank. On the other hand, to suppress the self-sustained tones, a method using perforated plates and cavities has been proposed by Ishihara and Nakaoka [13]. A perforated plate has long been used in various noise-control applications, such as vehicle exhaust systems, ducts, hearing protection devices, and acoustic panels, because it is well known that perforated plates have an acoustic damping effect [18–20]. Ishihara and Nakaoka [13] thought that a resonance mode perpendicular to the flow (lift mode) might be suppressed by a damping effect of perforated plates, when the self-sustained tone occurred.

In this chapter, we review the generation mechanism of the self-sustained tone clarified experimentally and numerically, and the methods for suppressing a selfsustained tone using baffle plates and perforated plates.

## **2. Generation mechanism of self-sustained tone**

#### **2.1 Self-sustained tone**

The Karman vortex shedding frequency *f*v is generally proportional to the flow velocity and the natural frequency of the duct *f*a is constant value determined by the duct size and the sound speed. When the flow velocity increases, the *f*v approaches *f*a. Before the *f*v reaches the *f*a, the vortex shedding frequency suddenly locks on to the natural frequency of the duct. The resultant high level sound occurs at or nearly at the natural frequency of the duct, and this phenomenon is called a lock-in or lock-on. There are many studies on the excitation mechanism of a lock-in or lock-on phenomenon in the tube bank [1–11, 21, 22].

The relation between frequency and flow velocity in a lock-in phenomenon is represented in **Figure 1**. A high level sound called a self-sustained tone occurs due to a lock-in phenomenon [12, 13]. In a lock-in phenomenon, as shown in **Figure 1**, the frequency slightly rises as the flow velocity increases. Furthermore, the lock-in occurs at a certain flow velocity, and does not occur in accordance with the large acoustic damping of the duct if the flow velocity increases. However, with the small

**57**

**Figure 2.**

*Acoustic resonance and lock-in phenomenon.*

tube bundles [23].

**Figure 1.**

**2.2 Setup of experiment**

*Countermeasure for High Level Sound Generated from Boiler Tube Bank Duct*

acoustic damping of the duct, the lock-in continues as the flow velocity increases, and the sound pressure level remains high [13]. **Figure 2** shows that when the shedding frequency of the strong vortices generated in the tube bank almost coincides with the acoustic resonance frequency of the duct, the strong sound field in the duct is excited. As a result, the vortices and the sound field in the duct cause the

Also, focused on the self-excited acoustic resonance of two side-by-side cylinders in a duct, the mechanism of the self-excited acoustic resonance is investigated by experiments and numerical solutions [21, 22]. It was found that dynamic lift fluctuation on the cylinders and strong in-phase vortex shedding synchronization are generated by the acoustic resonance. Shahab Khushnood et al. reviewed and summarized the flow-induced vibrations and acoustic resonance in heat exchanger

Ishihara et al. [12, 13] performed the experiments to investigate the selfsustained tone. **Figure 3(a)** and **(b)** represents the setup of the experiment and the tube bank. The duct is made of acrylic plates that have a thickness of 1 cm. The tube bank consists of an array of bronze tubes whose diameter is *D* = 6 mm. The array geometry is represented in **Figure 3(b)**, where the spacings *T*/*D* and *L*/*D* are

strong interaction. This phenomenon is a self-excited mechanism.

*Relation between frequency and velocity in the case of lock-in phenomenon.*

*DOI: http://dx.doi.org/10.5772/intechopen.86039*

*Countermeasure for High Level Sound Generated from Boiler Tube Bank Duct DOI: http://dx.doi.org/10.5772/intechopen.86039*

**Figure 1.** *Relation between frequency and velocity in the case of lock-in phenomenon.*

acoustic damping of the duct, the lock-in continues as the flow velocity increases, and the sound pressure level remains high [13]. **Figure 2** shows that when the shedding frequency of the strong vortices generated in the tube bank almost coincides with the acoustic resonance frequency of the duct, the strong sound field in the duct is excited. As a result, the vortices and the sound field in the duct cause the strong interaction. This phenomenon is a self-excited mechanism.

Also, focused on the self-excited acoustic resonance of two side-by-side cylinders in a duct, the mechanism of the self-excited acoustic resonance is investigated by experiments and numerical solutions [21, 22]. It was found that dynamic lift fluctuation on the cylinders and strong in-phase vortex shedding synchronization are generated by the acoustic resonance. Shahab Khushnood et al. reviewed and summarized the flow-induced vibrations and acoustic resonance in heat exchanger tube bundles [23].
