Experimental study of viscous effects on flow pattern and bubble behavior in small diameter bubble column

Abstract

An experimental study was carried out to explore viscous effects on the flow pattern and bubble behavior in a small diameter bubble column using silicone oil fluids of viscosities 5, 100, 1000, and 5000 mPa s. The flow pattern was obtained from the probability density function using the time-averaged void fraction traces of the real-time qualitative and quantitative measurements from Electrical Capacitance Tomography (ECT). This was confirmed from the high-speed camera and ECT images which also gave a vivid description of the bubble behavior. Further confirmation of the observed flow pattern was obtained using the diameter ratio, λ, where for slug flow λ > 0.6. The flow pattern was observed to vary from spherical cap bubbles to developing slug and slug flow within the gas superficial velocities considered (0.02 ≤ Ugs ≤ 0.361 m/s). As viscosity increases, the appearance of spherical cap bubbles decreases, while slug flow tendency increases. The observed flow patterns were compared with previous work. The developing slugs in liquid viscosities of 5 and 100 mPa s were observed to be deformed. The Taylor bubble obtained from 1000 mPa s is called prolate spheroid, while that obtained from 5000 mPa s is called oblate spheroid. In addition, as the superficial gas velocity increases, the length of Taylor bubbles increases, while that of slug flow decreases. The bubble behavior was further characterised using the inverse dimensionless viscosity and Eotovos number. The process of leading and trailing bubble merging to form Taylor bubbles via coalescence was also captured and explained using the high-speed camera video.