Comparison of oscillatory flow conditions in Newtonian and non-Newtonian fluids using PIV and high-speed image analysis

Oscillatory flow in pipelines is common in many industrial applications, including oil well drilling, An experimental investigation of oscillatory flow inside a vertical U-shaped circular pipe is presented in this paper to mimic the practical scenario takes place within a vertical oil well. Flow visualization was deployed to compare the velocity distributions in Newtonian (deionized water) arid non-Newtonian fluid (a mixture of three water based polymeric liquids). The experiments were performed in a 1.2m high, 50 mm diameter transparent test section, at room temperature (21 degrees C) and atmospheric pressure. Particle image velocimetry (My) technique was used to obtain non-invasive instantaneous flow velocity profiles. Based on local velocities, the streamwise (axial) velocity component within the pipe across its diameter was determined and the cross-sectional average velocity together with the normalized axial velocity were also calculated. In addition, high-speed motion pictures were used to determine the displacement of the air-liquid interface at the top of the U-tube limb. This enabled comparison of the flow field with the overall volumetric oscillating flow. A piston was driven at harmonic motion via a gas buffer, to provide the driving force for the test fluids at four different low frequencies ranging from 0.1 to 0.75 Hz. Oscillatory Reynolds number (Res) based on Stokes layer thickness was used as the criteria for determining the specific flow regime. According to the literature, the critical value for the oscillating Reynolds number was considered to be 500, and with this as reference all the experimental cases were within the laminar regime. Eight different experimental cases were tested within the ranges of (4 < Res < 116) and Womersley number (3 < Wo < 55). At higher frequencies, the viscous effects for the Newtonian fluid are confined to the Stokes layer and the central core of the velocity profile is plug-type. At the same time, higher frequencies resulted with increased velocity amplitudes. The viscous resistance of the non-Newtonian fluid and the presence of shear layer contribute to an uneven velocity profile across the pipe cross-section. Cross-sectional average velocity provides a more complete picture of the kinematic structure of oscillating flow and its dynamic distribution across the cross-section. Non-Newtonian fluid tends to achieve higher normalized axial velocities compared to that for Newtonian fluid, which is more or less equals to unity. The study was partly motivated by challenges associated with operational procedures during drilling and maintenance of petroleum wells. Furthermore, this study is also part of a comprehensive study aimed at investigating the influence of low frequency oscillations on particle settling in non-Newtonian drilling fluids.