Convection at very high Rayleigh number: signature of transition from a micro-thermometer inside the flow

In 2001, a change in the statistics of temperature fluctuations in Rayleigh-Bénard convection was reported at very high Rayleigh number (Ra ~ 1012) [1]. This change was concomitant with an enhancement of the heat transfer which had been interpreted [2] as the triggering of Kraichnan Convection Regime [3]. But a systematic study of finite probe size effect showed that the 200 µm probe used in the 2001 study was about three times too large to be free from finite size correction [4], calling for a confirmation of these results. We report new measurements of temperature fluctuations performed with a probe ten times smaller than the one used in 2001. In this proceeding, we discuss experimental aspects of this experiment and complementary measurements made in the “Barrel of Ilmenau”. The first experiment was conducted with cryogenic helium and the second with air. Once combined, those two experiments lead to evidence of a signature of a transition in the local temperature fluctuations, supporting the conclusion of the 2001 study.