The objective of this paper is to develop the methods of vibration test in order to predict the vibration characteristics for Inconel 625 fine tubes used in tube bundle type heat exchanger. The two Flat U-tube segments with anti-vibration baffle and without anti-vibration baffle were applied to compare the vibration characteristics according to the baffle of bent tube under the operating conditions.
The excitation test was carried out under operating condition. The two segment models were performed the excitation under room temperature. Bent tubes have the same shape and height, but a segment model with anti-vibration baffle has an additional baffle.
First, we performed the modal analysis to predict the resonance frequency of bent tube. In segment model without anti-vibration baffle, the natural frequencies of 1st , 2nd and 3rd tube were 221 Hz, 264 Hz, 323 Hz, respectively. In segment model with anti-vibration baffle, the natural frequencies of 1st, 2nd and 3rd tube were 311 Hz, 384 Hz and 483 Hz, respectively. We peformed the sweep vibration test to observe the vibration characteristics of tubes in the frequency range of 50 to 2,000Hz and acceleration of 30 G. We measured the resonance frequency and maximum displacement of tubes using a high speed camera and camcorder. In segment model without anti-vibration baffle, the natural frequencies of 1st, 2nd and 3rd tube were 213 Hz, 250 Hz and 303 Hz, respectively. The maximum displacement of 1st and 2nd tube was 12 mm and 10 mm, respectively. In segment model with anti-vibration baffle, the natural frequencies of 1st, 2nd and 3rd tube was 285 Hz, 333 Hz and 455 Hz, respectively. The maximum displacement of 1st and 2nd tube was 5 mm and 3 mm, respectively.
The natural frequencies obtained from the test results are compared with those obtained from the analytical results. We could see that the difference between analysis data and test data was 3 ~ 15%. The cause of the difference is that the exactly boundary condition on the contact behavior at the gap between tube and baffle was not applied in dynamic analysis.
The maximum displacements at resonance frequency of segment model without anti-vibration baffle are compared with those at resonance frequency of segment model with anti-vibration baffle. The maximum displacements at segment model without anti-vibration baffle were approximately 150 - 250 % larger than those at segment model with anti-vibration baffle. The cause of the large difference is the anti-vibration baffle which is for preventing the vibration in the upper part of tubes.
Harmonic analysis was carried out to predict the stress and behavior of segment model. With the results of modal analysis of segment model, harmonic analysis was performed in the operating environment under room temperature. The amplitude at segment model without anti-vibration baffle was 55.8 % larger than those at segment model with anti-vibration baffle. Also, the stress was reduced upto 63.6 % at segment model with anti-vibration baffle. The maximum von-Mises stress of the tube part of 2 kinds of segment model were less than the fatigue strength at 108 cycle at 300 K.
Inconel segment model was required to verify dynamic characteristics under high temperature. The amplitude at segment model without anti-vibration baffle was 54.4 % larger than those at segment model with anti-vibration baffle. Also, the stress was reduced upto 62.3 % at segment model with anti-vibration baffle. The maximum von-Mises stress of the tube part of 2 kinds of segment model were less than the fatigue strength at 108 cycle at 1,000 K.
The aim of this study is to observe the vibration characteristics of full model. We carried out the modal analysis to predict a resonance frequency before vibration test for full model. First, a modal analysis was performed to predict natural frequency of case part of full model.
The resonance frequency in baffle was observed at approximately 749 Hz. This dynamic behavior of baffle was observed using a high speed camera and camcorder.