EXPERIMENTAL STUDY ON THE MUTUAL EFFECTS BETWEEN FLUID-INDUCED VIBRATION AND INTERNAL FLOW

Transcription

1 The 21 st International Congress on Sound and Vibration July, 2014, Beijing/China EXPERIMENTAL STUDY ON THE MUTUAL EFFECTS BETWEEN FLUID-INDUCED VIBRATION AND INTERNAL FLOW Kong Weitao, Liu Gongmin, Li Shuaijun, Zhou Guocheng College of Power and Engry Engineering, Harbin Engineering University, Harbin, China Most researchers studying fluid-induced vibration(fiv) conducted researches without inner flow, neglecting the mutual effects between internal flow and FIV. The inner flow sound generated by fluid-induced vibration of tubes is an important part of condenser noise. Trying to realize the rules of FIV influence on internal flow pressure fluctuation, which is known as the flow noise, and how inner flow affects FIV, an experimental study was carried out. For the investigation of such a propose, a wind tunnel was designed and built. Since the mass of the sensor may affect results greatly, two strain gauges with negligible weight compared with the instrumented tube were carefully sticked to the fully flexible PVC tube. It s quite difficult to measure the pressure fluctuation of internal flow directly, while it is easy to measure the inlet and the outlet pressure fluctuation. To install the pressure fluctuation sensor, a pair of clamps was designed. Experiments were conducted with and without the internal flow. It is found that the pressure fluctuation of the internal flow is strongly affected by FIV and inner flow velocity. And the inner flow conditions also affect FIV. Besides, the velocity of internal flow contributes to the overall sound level of pipeline. 1.Introduction Heat exchanger is an important part in many industries. It is susceptible to serious damage duo to flow induced vibration. There are cases showing that fluid-induced vibration cause huge loses 1. Many researchers have dedicated themselves to explore the nature of FIV. It is well known that the mechanisms of fluid induced vibration are vortex shedding, turbulent buffeting, fluid-elastic instability and acoustic resonance 1,2. Blevins 3 gave a comprehensive introduction to vortex shedding. Paidoussis 1 summarized generic idealized response with increasing flow velocity of a structure in either axial or cross-flow and the mechanism under each period. By measuring the tube surface pressure distribution and tube vibration, Mahon 4 found there was time delay between the tube motion and the resulting fluid forces. The tube vibration could be locked-in or synchronized with the vortex shedding, resulting in large deflection and stress. Even though there are many researchers have done their work on FIV, only a few work was conducted with internal flow. Chen 5 firstly explored how internal flow affects vortex-induced vibration(viv) responses in his paper. Bokaian 6 developed a mathematical model for the prediction of the cross-flow displacement of a deep water marine riser due to vortex-shedding in a vertically sheared flow. Guo 7 studied further on this topic experimentally. However, the marine riser system includes the effects of Coriolis force and the corresponding results probably not able to work on heat exchanger tubes. Considering the effects of pipe wall thickness, fluid-structure and velocity, Li 8 gives a 14-equation model for the vibration analysis of pipe conveying fluid. Currently, the mutual effects between FIV and internal flow are still not clear. This paper reveals some 1

2 phenomenons and rules about the effect of internal flow on FIV and how FIV affects the internal flow fluctuation. 2.Experimental setup Fig.1 Schematic of the test Tests were conducted on a wind tunnel with a diameter of 425 mm, which can provide air at a velocity from 4 m/s to 25 m/s and the turbulence intensity is relatively low. This velocity range corresponds to Reynolds numbers from to In this range of the Reynolds number, a fully turbulent vortex street is formed in the wake 3. The flow velocity is measured by a Pitot tube flowmeter. Figure 1 gives a schematic of this experiment. The instrumented tube is made up of PVC with an effective length of 530 mm. It has an outer diameter of 20 mm and a thickness of 2 mm. Its inner and outer surfaces are smooth. It is fixed horizontally by special designed clamps, which is shown in Fig.2. The tapped hole on top is used to install the pressure sensor, which is a piezo-electric one and can detect the pressure fluctuation of the water flow. Two strain gauges are installed at the position, whose distance to the fixed end is 20 mm. They are 180 apart(lift direction), and wired in half bridge mode. The sketch of data acquisition system is shown in Fig. 3. The inlet and the outlet pipe is of pliable plastic, which can reduce the water flow turbulence level. Noting that there are two signals of pressure fluctuation, this paper adopts the arithmetic mean of them for data process. Fig.2 Schematic of clamp Fig.3 Schematic of data acquisition 2

3 Water Flow Velocity Lift Direction Order Table 1. Impact test results Without Water 1m/s 2m/s 3m/s 4m/s 5m/s Hz 80.5Hz 79.69Hz 79.69Hz 78.91Hz 78.91Hz Hz 213.3Hz 217Hz 217Hz 215Hz 218.5Hz Hz 417.2Hz 418.8Hz 414.1Hz 415.6Hz 417.2Hz The following test is conducted by holding the internal flow condition constant while changing the air velocity. And then changes the internal flow conditions, the procedure comes again. f stands for the vortex shedding frequency, and fn is the tube fundamental frequency. Double Y figure of the lift direction RMS strain and f/fn verse air velocity at without internal flow and inner flow velocity Vw=3 m/s is given in Fig.4. In Fig.4a, once the air velocity reached Va=13 m/s, the ratio of vortex induced vibration frequency verse fundamental frequency remains unchanged. From 13m/s to 25m/s is the lock-in range 6,9. While for water velocity reached Vw=3 m/s, the lock-in range is from 9 m/s to 20 m/s. Even though the f/fn of Fig.4a is about 0.88, the result is still acceptable because of the elasticity of the tube. The interaction between the tube displacement and vortex shedding reduces the ratio. For Vw=3 m/s, the water flow greatly reduces the tube displacement that makes the interaction much weaker, making the ratio approach 1 closely. a. Without water b. Vw=3m/s Fig.4 Double Y display of the variation of lift direction rms response and f/fn with air velocity To visualize the effects of FIV on pressure fluctuation, the lift direction response is transformed into decibels. Besides, the vibration level and pressure fluctuation level is displayed on one double Y figure. Confined by the length of this paper, here only displays some of the results. The first peak of pressure fluctuation of the internal flow should be around 80 Hz. However, as is shown in Fig.5, the background noise level of low frequency band is too strong that makes the first 3

4 a. Water velocity Vw=1m/s b. Water velocity Vw=3m/s c.water velocity Vw=5m/s Fig.5 Double Y display of lift direction response and pressure fluctuation peak nearly disappeared. In Fig.5a, the similarities of two curves makes it safe to draw the conclusion that the internal flow pressure fluctuation is strongly depended on FIV. The strain second peak frequency decreased from 211 Hz to 207 Hz, while the pressure one increased from 139 Hz to 206 Hz. This phenomenon suggests that there is a time delay between FIV and pressure fluctuation. And the delay becomes shorter as the air velocity increase. Even though the deference between second strain peak frequency and pressure fluctuation one, as is shown in Fig.5c, keeps narrowing as the air velocity increase, it s not the same phenomenon as Fig.5a. In Fig.5c, the pressure second peak frequency is higher than its vibration counterpart. Besides, the air flow velocity barely has any effect on the pressure fluctuation level. There are at least two major aspects determining the pressure fluctuation. One is the FIV, another is the inner flow velocity. At low inner flow velocity, the FIV dominates. However, at high enough velocity, the turbulence takes control and the effect of FIV is not that distinct as low water velocity. Certainly, velocity between 4

5 them is the transition period. In Fig.5b, the second peak do not be excited by FIV until Va=20 m/s. This suggests that Vw=3 m/s falls into the transition period. The pipeline overall sound level can reach 180 db, which can cause serious problems to the whole system. Figure 6 Shows that the overall sound level is greatly affected by the air flow velocity at Vw=1 m/s. And it has a very sharp increase from 8 m/s to 12 m/s, while the vibration amplitude is decreased. The reason of this phenomenon is deemed that FIV has a mechanism shift from vortex shedding to turbulent buffeting 2,9. Compared with vortex shedding, turbulent buffeting has a better effect on the pressure fluctuation. For the rest circumstances, the background pressure fluctuation level is too high that the phenomenon at Vw=1 m/s do not occur. Fig.6 Pressure fluctuation overall level 3.Conclusions The mutual effects between FIV and internal flow we investigated experimentally. By processing available experimental data, the following conclusions can be verified: (i) The inner flow velocity adds the effective mass of the instrumented tube and decreases the fundamental frequency. (ii) The internal flow can reduce the FIV greatly and weakens the interaction between FIV and tube displacement. (iii) There is time delay between FIV and pressure fluctuation and the delay decreases as the air velocity increase. (iv) There are at least two major aspects determining the pressure fluctuation and three divisions. The first division is FIV dominated, the third is turbulence lead, and the second in between is the transition period. (v) The internal flow velocity contributes to the overall sound level of pipeline. (vi) Compared with vortex shedding, turbulent buffeting has a better effect on the pressure fluctuation of internal water flow. 5

6 Reference M.P. Paidoussis, Real-life experiences with flow-induced vibration, Journal of Fluids and Structures 22, , (2006) S.J.Price, A review of theoretical models for fluidelastic instability of cylinders in cross-flow, Journal of Fluids and Structures, , (1995) Blevins, R.D., Flow Induced Vibration. Van Nostrand Reinhold Co., New York.(1990) John Mahon, Interaction between fluidelastic instability and acoustic resonance, PH.D thesis, University of Dublin, (2008) Chen, B.C.M., A marine riser with internal flow-induced vibration., Proceedings of Offshore Technology Conference, (1992) A. Bokaian, Lock-in Prediction Of Marine Risers And Tethers, Proceedings of Offshore Technology Conference, Houston, USA, (1994) H. Y. Guo, M. Lou., Effect of internal flow on the vortex-indunced vibration of risers. Journal of Fluids and Structure, 24: P, (2008) Li Shuaijun, Liu Gongmin, Kong Weitao, Vibration analysis of pipes conveying fluids by transfer matrix method. Nuclear Engineering and Design, 266:78-88P, (2014) R.D. Gabbai, H. Benaroya. An overview of modeling and experiments of vortex-induced vibration of circular cylinders. Journal of Sound and Vibration, 282: P, (2005) 6