Simultaneous PIV and shadowgraphy measurements in slug flow in Newtonian and non-Newtonian liquids

Abstract

A recent technique for performing simultaneous Particle Image Velocimetry and Shadography applied for the first time to slug flow, is presented in this work. The unsteadiness of slug flow creates the need of recording the shadow of the Taylor bubbles at the precise instant and position of the PIV measurements. Therefore, this experimental technique is used in order to characterise simultaneously the flow in the liquid and the shape of a single gas slug (Taylor bubble) rising through a vertical column of stagnant liquid using only one CCD camera. The experimental facility and technique are described and the details of the synchronisation between the two techniques are also explained. This new method is based on the fact that the PIV particles and the shadow of the bubbles can illuminate the CCD camera sensor at different gray levels. The PIV images are obtained by seeding the flow with fluorescent seeding and placing an optical filter in front of the camera so that the intense laser reflections in the interface are avoided and only the PIV particles reach the CCD sensor. The shadowgraph images are obtained by backward illumination with a board of LEDs emitting light in a wavelength that passes through the optical filter. The processing of the images is explained. The reason for keeping recording images with 8 bits is discussed. During the post-processing it is possible to combine the shadowgraph results with the velocity field acquired with PIV, therefore, solving some of the basic processing errors, which appear at the interfaces. Some limitations of the technique itself, due to the highly 3D shape of the rear of the bubbles are also discussed. The flow around a gas slug rising through a vertical column filled with a non-Newtonian fluid is also presented. These are the first quantitative measurements in slug flow for non-Newtonian flows, as long as the authors are aware