Experimental investigation of collision behavior of fluidized solid particles on the tube wall of a graphite evaporator by vibration signal analysis

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It is necessary to investigate the collision behavior of fluidized solid particles on the wall of a brittle graphite heat exchange tube of a fluidized bed evaporator for a better fundamental investigation on the vibration risk assessment and heat transfer process enhancement. Vibration acceleration signals of the single graphite tube with an inner vapor-liquid-solid boiling flow at varied steam gauge pressure, solid holdup and particle size were measured with developed signal acquisition and processing system. Main results are as follows. Circulating rate of the vapor-liquid-solid flow, kurtosis and standard deviation of vibration signals were considered as measures of collision force, collision frequency and vibration intensity, respectively. The largest standard deviation occurs in axial middle position of the graphite tube, and kurtosis of vibration signals decreases from the top to the bottom. As the steam gauge pressure increases, the collision frequency of smaller solid particles (1.3, 2.4mm) increases first and then gradually reaches to a stable value, while a fluctuating or slight decreasing tendency of kurtosis appears for the larger solid particles (3.5mm). The collision force and the comprehensive vibration intensity factor of all three sizes of solid particles enhance with the increase of steam gauge pressure. As the solid holdup increases, the collision frequency increases obviously, while the collision force decreases. Vibration intensity is higher both with the addition of solid particles and the increase of solid holdup. The boiling coefficient shows a clear enhancement and can be explained according to the conclusions on the collision behavior of solid particles. The results effectively support the reliability of collision behavior research between fluidized solid particles and tube walls by vibration signals analysis, which lays the foundation of reasonable design and further application of the fluidized bed evaporator.