Multi-scale vibration behavior of a graphite tube with an internal vapor–liquid–solid boiling flow

• Multi-scale phenomena were detected by wavelet analysis of acceleration signal.
• Vibration signal is decomposed into micro-, meso- and macro-scale components.
• Power spectra of vibration signal reflect axial particle or bubble distribution.
• Energy of tube vibration shows a slight increase with heating steam pressure.
• Higher steam pressure is acceptable operation conditions for graphite tube.

In this study, a fluidized bed evaporator mainly made of a brittle graphite heat transfer tube was developed and the vibration acceleration signals of the tube were measured by means of accelerometer sensors and datum acquisition systems at varied steam pressure, solid holdup, particle diameter and axial position to investigate the vibration characteristics of the tube induced by an internal vapor–liquid–solid boiling flow. Multi-scale characteristics of the vibration acceleration signals are identified by the power spectral density and wavelet transformation analyses. The high (3000–9000 Hz), intermediate (500–3000 Hz) and low (0–500 Hz) 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 axial distributions of solid particles and vapor bubbles in the opaque graphite tube can be well reflected by the power spectral density analysis. The tube vibration energy enhances with the increase of solid holdup. Especially, the energy of the micro-scale vibration sub-signal gradually rises and then dominates the system with solid holdup. However, the energy of the micro-scale sub-signal shows a slight increase with heating steam pressure due to the uniform axial distribution of solid particles. The influence of particle diameter is mainly reflected in the enhancement of micro-scale motion. In order to balance the possible destruction caused by tube vibration and heat transfer enhancement due to the addition and fluidization of solid particles, operating conditions of lower solid holdup and higher steam pressure are recommended for the industrial application of vapor–liquid–solid fluidized bed evaporators.

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