Vapour bubble drag reduction in boiling Taylor-Couette turbulence*

1. The presence of air bubbles in high-Reynolds number (O(10⁶)) Taylor-Couette (TC) flow has proven to be successful in reducing the drag of the flow. Approximately, 40% drag reduction (DR) is achieved when the volume fraction α in the system is 4%. In this study, we explore the drag reduction mechanism in TC flow using vapor bubbles instead. To do so, a low-boiling point liquid (C3F7OCH3) is used as the fluid phase, which boils at 34 °C at atmospheric pressure. Our experiments are done in the Boiling Twente Taylor-Couette (BTTC) facility. We fix the angular velocity of the inner cylinder (IC) (the outer cylinder is at rest) and increase the temperature continuously from 20 °C to 50 °C. Eventually, the boiling point is reached and the nucleation of vapor bubbles is observed. We quantify the amount of DR due to the presence of vapor bubbles and compare it to the DR obtained with air bubbles. In order to study the influence of bubble deformability, we use high-speed imaging to measure the bubble size during our experiments and estimate the Weber number.