Time-resolved measurements of the local wall heat flux in turbulent Rayleigh–Bénard convection

Abstract

We report direct and time-resolved measurements of the local wall heat flux in turbulent Rayleigh–Bénard convection in air. The measurements have been performed in a cylindrical test section with a diameter of 7.1 m and a diameter-to-height ratio of 8. They cover Rayleigh numbers 8.5×105<Ra<2.6×109, while the Prandtl number was fixed at Pr=0.7. In order to measure the local wall heat flux, we use heat flux plates that have been flush mounted into the hot/cold surfaces of the convection experiment. The results of our measurements show that the local wall heat flux in turbulent Rayleigh–Bénard convection strongly fluctuates. With increasing Rayleigh number, the variance of the fluctuations goes down with Ra−0.5 and the probability of large excursions from the mean decreases. The probability density of the fluctuations can be well-described by a distribution according to the Generalized Extreme Value Theory. We also analysed the typical time scales, and we found that the power of the fluctuations becomes more weighted towards higher frequencies, if the Rayleigh number increases.