*3.2.1 Natural frequency and peak frequency of self-sustained tone*

**Figure 15** represents the natural frequency of the duct and the peak frequency of the self-sustained tone. The vertical axis shows the frequency while the horizontal axis shows the pattern of the baffle plate positions as shown in this figure. The natural frequency of the duct can be obtained by the speaker test that was performed using the setup of the experiment shown in **Figure 13**. The peak frequency of the self-sustained tone was obtained by the ventilation experiment. The symbol ∆ shows the peak frequency in the case "with baffle plate". In this case, the self-sustained tone was not generated when the baffle plate positions are −1, 0, and + 1. Therefore, we cannot see the symbol ∆ in these positions. The natural frequency of the duct corresponds to the peak frequency of the self-sustained tone as represented in **Figure 15**. For a countermeasure of a self-sustained tone, a method involving the insertion of a baffle plate in the duct is generally adopted to suppress the self-sustained tone. This method is based on the idea that the baffle plate can prevent the resonance within the range of the usage flow velocity by introducing a new partition, thus increasing the natural frequency

**65**

**Figure 16.**

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

of the duct [14, 15]. However, **Figure 15** represents that the natural frequency of the duct decreases by the insertion of the baffle plate regardless of the position in **Figure 15**, because the baffle plate cannot divide into two parts of the acoustic field of the duct due to a small length. If the baffle plate length is the same with the length of the duct, then the natural frequency becomes higher and doubles.

**Figure 16** represents the onset gap velocity of the self-sustained tone. The vertical axis shows the gap velocity of the tube bank when the self-sustained tone is generated and the horizontal axis shows the pattern of the baffle plate positions as shown in this figure. In patterns (−1, 0, and + 1) where the baffle plate is inserted in the entire tube bank, the self-sustained tone was not generated within the range of the flow velocity that the setup of experiment can produce. The self-sustained tone is not generated because vortices are assumed to become very small in patterns (−1, 0, +1) as described later. Furthermore, the onset gap velocity of the selfsustained tone shows a significantly different tendency between the upstream and

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

*3.2.2 Onset gap velocity of self-sustained tone*

*Natural frequency and peak frequency ([12]).*

**Figure 15.**

the downstream positions of the baffle plate.

*Gap velocity and pattern of baffle plate positions ([12]).*

**Figure 14.** *Pattern of baffle plate positions ([12]).*

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

**Figure 15.** *Natural frequency and peak frequency ([12]).*

*Noise and Vibration Control - From Theory to Practice*

as shown in **Figure 13**. The position of this baffle plate is varied in the flow direction in increments of a row pitch (12 mm) in the tube array. The patterns of the baffle plate position are shown in **Figure 14**. An effective position of the baffle plate for

**Figure 15** represents the natural frequency of the duct and the peak frequency

of the self-sustained tone. The vertical axis shows the frequency while the horizontal axis shows the pattern of the baffle plate positions as shown in this figure. The natural frequency of the duct can be obtained by the speaker test that was performed using the setup of the experiment shown in **Figure 13**. The peak frequency of the self-sustained tone was obtained by the ventilation experiment. The symbol ∆ shows the peak frequency in the case "with baffle plate". In this case, the self-sustained tone was not generated when the baffle plate positions are −1, 0, and + 1. Therefore, we cannot see the symbol ∆ in these positions. The natural frequency of the duct corresponds to the peak frequency of the self-sustained tone as represented in **Figure 15**. For a countermeasure of a self-sustained tone, a method involving the insertion of a baffle plate in the duct is generally adopted to suppress the self-sustained tone. This method is based on the idea that the baffle plate can prevent the resonance within the range of the usage flow velocity by introducing a new partition, thus increasing the natural frequency

suppressing the self-sustained tone is derived from this experiment.

*3.2.1 Natural frequency and peak frequency of self-sustained tone*

**Figure 13.**

*Setup of experiment ([12]).*

**3.2 Results of experiments**

**64**

**Figure 14.**

*Pattern of baffle plate positions ([12]).*

of the duct [14, 15]. However, **Figure 15** represents that the natural frequency of the duct decreases by the insertion of the baffle plate regardless of the position in **Figure 15**, because the baffle plate cannot divide into two parts of the acoustic field of the duct due to a small length. If the baffle plate length is the same with the length of the duct, then the natural frequency becomes higher and doubles.
