Study of Buffeting Excitation Acting on Coil Tube Bundles in Cross Flow

Abstract

Although the amplitudes induced by random excitation forces on the tubes are relatively small, the vibrations do cause continuous friction between the tube and supports, which results in gradual failure of the tubes due to fretting wear. Therefore, it is very important to determine the guild lines of the random excitation force spectrum for the coil tube. To the authors' knowledge, there are no published studies on the normalized force spectrum of coil tube. A simplified three-layer experimental model was established. The robustness of the numerical method was demonstrated by comparing the experimental and simulated results, including the vibration response and the fluid excitation force spectrum. And then a semi-empirical equation for predicting the dominant frequency of turbulent buffeting was constructed by employing the threshold envelope method. Through the observation of time history and RMS data, it was found that pitch diameter ratio between adjacent tube layers had the greatest influence on the force coefficients. Pitch diameter ratio in the same layer and helix angle had little effect on the force coefficients. With the increase of a, the flow instability in the shell-side flow enhanced and the fluctuation of force coefficients became larger. Finally, the mechanism of the tube position, Reynolds number and bundle structure on the normalized force spectrum was studied. The normalized envelope force spectrum for coil tube was proposed as the guidelines to predict and evaluate the random excitation force acting on the tubes.