Investigating the hydrodynamics of airlift pumps by wavelet packet transform and the recurrence plot

The hydrodynamics of airlift pumps at various gas superficial velocities were investigated using the wavelet packet transform and recurrence plot due to their capabilities for extracting the micro structures and determining the whole complexity in airlift pumps. For this purpose, the pressure fluctuations of an airlift pump were measured at various gas superficial velocities. The wavelet packet transform was first used to extract the integer approximation and detailed information from the pressure signals. Then, the energy distributions in each frequency band were calculated and the corresponding dynamical characteristics were obtained using a recurrence plot. It was found that the highest pumping efficiency under the slug-churn flow condition is caused by the highest mean pressure. In addition, the energy of the slug-churn flow regime has a dramatic increase in the frequency band of 31.25-62.5 Hz, due to the burst of big Taylor bubbles. In the low-frequency band (0-31.25 Hz), the pressure shows a chaotic behavior due to the turbulence of liquid flow. In the frequency band of 31.25-62.5 Hz, the pressure executes a transition from chaotic fluctuation in slug flow to periodic fluctuation in slug-churn due to the linear motion of small bubbles.