Investigating Taylor Bubble Dynamics in Counter-Current Air-Water Flows: A Combined Numerical and Experimental Analysis

In pressurised water reactor (PWR), understanding two-phase flows is critical during events like boiling heat transfer and Loss of Coolant Accident (LOCA). During these scenarios, the slug flow regime, characterized by elongated Taylor bubbles, occurs in steam generators, challenging system stability. This study investigates Taylor bubbles in counter-current air-water flows using high-fidelity numerical simulations and advanced experiments. We examined two flow regimes: transitional (Re = 1400) and fully turbulent (Re = 5600), focusing on bubbles under stagnant conditions where buoyancy is balanced with the downward flow's inertial drag. Experimentally, we analyzed bubble disintegration, interface dynamics, and liquid phase velocity using Particle Image Velocimetry. High-speed video and image analysis revealed asymmetries in Taylor bubbles, deviating from the expected axisymmetric profiles. Disturbance waves on the bubble interface with amplitudes ranging from 10 to 100 micrometers were tracked. Numerically, we simulated bubble behavior and disintegration mechanisms, employing Volume-Of-Fluid (VOF) approach with geometric interface reconstruction and high-order Runge-Kutta time schemes. We focused on the transitional flow regime and notably observed the formation of a secondary vortex in the turbulent wake at finer mesh resolutions. To counter bubble breakup in turbulent regime we propose new grid-scale surface tension model.