작동환경에 따른 열교환기 미세관의 진동특성에 관한 연구

Abstract

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 single U-tube, U-tube segment and Flat U-tube segment were applied to compare the vibration characteristics according to the geometry of bent tube under the operating conditions.

The pressurized tubes were applied the vibration test to find the vibration characteristics. The excitation test was carried out under initial pressure condition. The acceleration level was considered in order to measure the stress under elastic limit. So, the vibration test was carried out in a condition that the acceleration was 0.3 G and the pressure load was 0 bar, 30 bar, 55 bar and 80 bar, respectively. From the analysis and test results, we know that the natural frequency of U-tube is not affected by change of pressure load.

The heat exchanger and sub-models were applied for operating temperature and vibration environment. The operating temperature was 1,000 K. Tube shape of the heat exchanger was applied the favorable shape of tube through the vibration test.

The parameters such as bend radius and bent geometry were considered to estimate the vibration characteristics. Two models such as U-tube and Flat U-tube were employed to compare the vibration characteristics according to the geometry of bent tube. Bent tubes have a different shape and height. But, tubes of each model were designed to be the same length and same width in a particular position.

From the test results, we could see that the flat U-tube was superior to the U-tube in view of vibration. But, the maximum vibration displacement of 1st and 2nd tube in flat U-tube was equal to or greater than 1.5mm that it is the diameter of the tube. This displacement is likely to lead to wear on the tube.

And high cycle fatigue (HCF) test at high temperature was proceed from the previous test. The purpose of this study is to establish the test procedure and method, and to verify the test facility through preliminary HCF test. And two segment models were performed the excitation under elevated temperature. The sweep vibration test which has frequency sweep rate of 0.1 octave/min, temperature of 1,000 K, sweep endurance time of 7 hours and frequency range of 100 to 800 Hz was used. The fretting wear was observed in contact surface of tube side of two segment model. A large amount of fretting wear was observed in U-tube. On the other hand, a relatively small amount of fretting wear was observed in flat U-tube. From the test results, we could see that the flat U-tube was superior to the U-tube in view of fretting wear caused by vibration.