Three-dimensional investigation of liquid slug Taylor flow inside a micro-capillary using holographic velocimetry

Digital holography is an optical technique which is capable of providing instantaneous three-components of fluid flow velocity in three-dimensions (3D-3C) using a single camera. Digital holographic microscopy has been implemented in the present study to analyze liquid slug Taylor flow in a micro-channel of cross-sectional dimensions of 1,000 × 1,000 µm². The working fluids are water (liquid) and air (gas), with superficial velocities of liquid, U _L = 0.6 mm/s and gas, U _G = 1.2 mm/s, respectively. The corresponding Capillary number, Ca = 0.035 × 10⁻³ and Bond number, Bo = 0.144. The holographic velocimetry technique has been implemented and appropriately validated by comparing the velocity profile from present experiment with that from analytical velocity profile for single-phase flow. Complete flow field results, i.e., u-, v- and w-components of velocity inside the liquid slug volume, i.e., in both streamwise (x–y) and cross-stream (y–z) planes are presented. The present experiments on liquid slug Taylor flow show strong cross-stream velocity near the advancing and receding meniscus due to higher capillary pressure. The stream traces show converging and diverging radial flow in the cross-stream plane near the receding and advancing meniscus, respectively. Two three-dimensional recirculation bubbles are observed inside the liquid slug. Overall, this paper reports the complex three-dimensional flow field inside a liquid slug Taylor flow from the 3D-3C flow field measurements.