Elusive transition to the ultimate regime of turbulent Rayleigh-Benard convection

By using cryogenic He-4 gas as the working fluid in a cylindrical cell 0.3 m in both height and diameter, we study the influence of non-Oberbeck-Boussinesq (NOB) effects on the heat transfer in turbulent Rayleigh-Benard convection (RBC). We show that the NOB effects increase the heat transfer efficiency when the top plate temperature closely approaches the saturation vapor curve even far away from the critical point. Viewed in this light, our analysis points to the likelihood that the claim of having observed the transition to Kraichnan's ultimate regime, under nominally similar conditions in the experiments with SF6 [Phys. Rev. Lett. 108, 024502 (2012)], is probably an NOB effect and the important issue of the transition to the ultimate state of RBC remains open.