Wave and drop periodicity in transient annular flow

Drop concentration frequency has been determined for the first time from time varying drop concentration measurement using light scattering technique. The study has been carried out using laser diffraction technique on a 19 mm internal diameter, 7 m length of vertical pipe using air and water as fluids. The gas superficial velocity was 13–43 m/s at liquid superficial velocities of 0.05 and 0.15 m/s. Additional tests were carried out with the gas velocity at 14 m/s for liquid superficial velocities of 0.03–0.18 m/s.Fluctuations of drop concentration and film hold-up with time have been analyzed. Both drop concentration and film hold-up shows evidence of periodicity with film thickness more periodic than drop concentration. Power spectrum density of auto-correlation function was generated from time series of the wave that produces the droplets and the droplet concentration to identify peak frequencies in both instances. Comparison of the peak structure frequencies shows that wave frequency is generally higher than frequency of the drops. This observation has been linked to drop coalescence rate and turbulent diffusion in the dispersed phase. Comparison of drop collision frequency with mechanistic model was carried out. Good agreement was achieved and a new, improved model proposed.The trend in drop frequency has been observed to be similar and directly proportional to the trend observed in drop size distribution (MMD). Traditional Strouhal number – Lockhart–Martinelli parameter provides a good correlation for the wave frequency. However, correlating drop frequency using this approach proves inadequate.

► Interrelationship between drop and wave frequencies has been elucidated based on dynamic, time varying information logged from drop-laden core flow and liquid film where drops are sourced.
► Waves are found to be more periodic than drops created by them.
► Correlating wave and drop frequency using Strouhal number and Lockhart–Martinelli parameter gives a good correlation for wave while for drop it is not adequate.
► Drop frequency and drop size distribution display similar signature and trend.