A set of air and highly viscous oil experiments was conducted to analyse the inner flow field of slug flow for low mixture Reynolds number conditions. The unsteady behavior of the wall shear stress profile was measured, modelled, and compared to an existing model. The Constant Temperature Anemometer (CTA) was utilized to measure the wall shear stress around the pipe circumference. Particle Image Velocimetry (PIV) was applied to investigate the inner liquid velocity field, simultaneously. A total of 9 flow conditions has been studied in a horizontal 50.8-mm (2-in.) inner diameter (ID) pipe and highly viscous fluid properties (nominal viscosity value of 0.68 Pa·s). From the slug front to the slug tail, a continuous increase of the velocity was observed by PIV, resulting in the increase of the wall shear stress in the liquid slug body at all the circumferential locations of the pipe as reflected by CTA. The wall shear stress decreased again from the tail of the slug into the liquid film region. This unsteady behavior of the wall shear stress profile was modelled by unsteady viscous flow theory. The proposed point model showed a fair agreement not only with the measured wall shear stress profile at the pipe bottom, but also along the pipe perimeter. On the other hand, the constant values provided by the existing correlation gave a deviation from the measured profiles.

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