Fig. 1 Experimental Apparatus
Fig. 2 Condenser Microphone and Microphone Tube
There is agreement in the literature that the acoustical tones generated by flow over tubes and cylinders in ducts can be functions of the following non-dimensional parameters: (1) tube-to-tube pitch-to-diameter ratio; (2) number of tube rows; (3) Reynolds numbers. The magnitude of sound in-creases with flow energy; sounds emitted from tubes are generally described as a soft tone, while the sound emitted from heat exchanger tube bundles can reach painful intensities.
It was the aim of this paper, first to present some recent experimental results which clearly show the distinction between vortex shedding and acoustical resonances and then show how an alternative approach may resolve some of the inconsistencies in the commonly accepted design procedures. The test, results as well as additional data obtained from earlier tests, in addition to available data from the literature are used to evaluate the existing design criteria. Finally, an effort is made to correlate the data points such that the regions of resonance and no-resonance be better distinguished.
2. TEST FACILITY AND ANALYSIS PROCEDURES
2.1 Wind Tunnel and Test Section
The experiments were conducted in an open-circuit wind tunnel that, was driven by a centrifugal fan. The working section of the tunnel had a 200 mm x 200 mm cross-section. The streamwise turbulence intensity upstream of the test section is less than 1.0 %.
The test section had the staggered tube banks shown in Fig. 1. The tubes are manufactured of stainless steel tube, which are 25.4 mm in outside diameter and 200 mm in length. They are fixed at both ends to the test section side walls. The test section, which is fabricated of steel plates 5 mm in thickness, has a fixed width of 200 mm, a fixed height of 200 mm and three types of longitudinal tube pitch (L) of 80 mm, 65 mm and 50 mm. Each tube bank consists of 10 rows. The geometry of L = 50 mm equals to the tube banks of the real exhaust gas economizer employed in the ship. The origin of co-ordinates was taken at the center of the tube midway between the first row and the second row. The x-axis was measured in the direction of uniform flow (U); the y-axis was perpendicular to the flow and the z-axis coincided with the tube axis (Fig. 1). To measure the sound pressure level (SPL) in the test section, a, row of pressure holes of 7 mm diameter were drilled on the top wall in the z-direction and the side wall in the y-direction at the same x position, respectively. For the test series, SPL measurements were taken at, three locations in each array for a range of velocities. The measurement locations within the test section are shown in Fig. 1.
2.2 Method of Measurements
SPL in the test section was measured with a Bruel & Kjaer Sound Pressure Meter and 1/4" Condenser Microphone (Fig. 2). To avoid the influence of the air flow upon the frequency response of the microphone, a probe mounted on the microphone was used for the measurements of the SPL on the inner wall (designated SPL) of the test section. The probe may then influence the frequency response of the microphone, but this influence was corrected. A pistonphone type Bruel & Kjaer was used to calibrate the microphone.
