Drag reduction in boiling Taylor–Couette turbulence

We create a highly controlled laboratory environment - accessible to both global and local monitoring - to analyse turbulent boiling flows and in particular their shear stress in a statistically stationary state. By precisely monitoring the drag of strongly turbulent Taylor-Couette flow (the flow in between two coaxially rotating cylinders, Reynolds number Re approximate to 10(6)) during its transition from non-boiling to boiling, we show that the intuitive expectation, namely that a few volume per cent of vapour bubbles would correspondingly change the global drag by a few per cent, 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 vapour 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 from those of vapour bubbles. Nonetheless, we find remarkable similarities and explain these.