Multi-Scale Vibration Behavior Of A Graphite Heat Transfer Tube With Vapor-Liquid-Solid Flow Boiling

Understanding of the vibration behavior of a heat transfer tube made of graphite material affected by an inner vapor-liquid-solid boiling flow is of great significance for the safe operation of the fluidized bed evaporator.The vibration behavior was investigated by means of vibration accelerometers,thermocouples,pressure transmitters,electromagnetic flow meters and a system of measurement and analysis dynamic signals using signal analysis tools of the standard deviation,kurtosis,power spectra and wavelet decomposition.The rules of the liquid-phase circulating flow,vapor bubble behavior and solid particle behavior in the vapor-liquid-solid boiling flow were investigated,respectively.The mechanism and rule of the vibration behavior of the graphite tube at different experimental conditions were further studied.The main results of the present investigations are as follows.The circulating velocity of vapor-liquid-solid boiling flow increases with the growth of steam pressures,and the circulating velocity decreases with the increase of solid holdup.The downward spikes of the differential pressure signals represent the generation or break up of large vapor bubbles,and the dominant frequency of power spectra corresponds to the fluid pulsation frequency,and the standard deviation is positive relation to the length of vapor-liquid-solid boiling section.The standard deviation of differential pressure signals increases with the enhancement of steam pressures and keeps near constant with the enhancement of solid holdup.The impact force of solid particles with tube walls is positive relation to the circulating rate of solid particles and the impact frequency is negative relation to the kurtosis of vibration acceleration signals.With the increasing steam pressures,the kurtosis of vibration acceleration signals of solid particles with sizes of 1.2 mm and 2.4 mm shows a decreasing trend and the kurtosis of 3.5 mm particles shows a stable tendency.With the increasing solid holdup,the kurtosis of all kinds of solid particles shows a decreasing trend.All of the vibration signals,including acceleration,velocity and displacement signals,show a multi-scale characteristic.The high,intermediate and low frequency components of the signals are motivated by the micro-scale motion of solid particles,meso-scale motion of vapor bubbles and macro-scale motion of the circulating liquid flow,respectively.The vibration intensity of the axial middle position of the graphite tube is larger than the upper and lower ends.With the increase of steam pressures and diameter of solid particles,the standard deviation of three-scale sub-signals of vibration acceleration,velocity and displacement signals enhances,and the enhancement amplitude gradually decreases.With the addition of solid particles into vapor-liquid boiling flow,vibration acceleration signal enhanced significantly,and the vibration velocity signal shown a slight increase,and the vibration displacement signal decreased.With the enhancement of solid holdup,the standard deviation of three-scale sub-signals of vibration acceleration signal enhances;the macro-scale sub-signal of vibration velocity signal decreases,and the meso-and micro-scale sub-signals of vibration velocity signal increase;the macro-and meso-scale sub-signals of vibration displacement signal decrease and the micro-scale sub-signal of vibration displacement signal increases.The investigation of vibration behavior of graphite tube with vapor-liquid-solid boiling flow can provide some practical guidelines for the industrial application and effective design and safe operation of fluidized bed evaporator.Meanwhile the vibration detection in this work,as a kind of non-intrusive measurement method,can serve as a reference for the experimental research of opaque fluidized bed evaporator.