Drag reduction in boiling Taylor-Couette turbulence

TL;DR

This study demonstrates that vapor bubbles in turbulent Taylor-Couette flow can cause up to 45% drag reduction, highlighting the importance of bubble deformability and providing insights into boiling turbulence physics.

Contribution

It reveals significant drag reduction due to vapor bubbles in turbulent flow and links global effects to local bubble deformability, a novel insight in boiling turbulence research.

Findings

Up to 45% global drag reduction observed.

Bubble deformability (Weber number > 1) is crucial for drag reduction.

Similarities found between vapor and gas bubble turbulence despite different physics.

Abstract

We create a highly controlled lab environment-accessible to both global and local monitoring-to analyse turbulent boiling flows and in particular their shear stress in a statistically stationary state. Namely, by precisely monitoring the drag of strongly turbulent Taylor-Couette flow (the flow in between two co-axially rotating cylinders, Reynolds number $\textrm{Re}\approx 10^6$) during its transition from non-boiling to boiling, we show that the intuitive expectation, namely that a few volume percent of vapor bubbles would correspondingly change the global drag by a few percent, is wrong. Rather, we find that for these conditions a dramatic global drag reduction of up to 45% occurs. We connect this global result to our local observations, showing that for major drag reduction the vapor bubble deformability is crucial, corresponding to Weber numbers larger than one. We compare our findings with those for turbulent flows with gas bubbles, which obey very different physics than vapor bubbles. Nonetheless, we find remarkable similarities and explain these.