SUMMARY

Turbulent thermal convection is a crucial part of heat transport in several important natural and engineering flows. Large scale natural systems such as the Earth's atmosphere---its oceans as well as the interior---and the interior of stars such as the Sun, are all affected to various degrees by thermal convection. The simplified physical model used to understand this ubiquitous heat transport mechanism is the Rayleigh-Bénard convection (RBC), which is a fluid flow driven by a temperature difference between the top and bottom plates of an experimental cell with adiabatic sidewalls. Despite the long history of the subject and the recent progress in theoretical, numerical and experimental domains, many questions remain unresolved. A fundamental question concerns the heat transfer scaling in highly turbulent convective flows, and particularly the existence of a possible transition to an asymptotic regime of enhanced heat transfer. Multiple theories predict a variety of possible asymptotic regimes, some of which literature disagreeingly claims to observe. The current study explores RBC at extreme parameters to help elucidate the true nature of convection in this regime. A facility was developed for conducting natural convection experiments at Rayleigh numbers exceeding 10^17. The study employs cryogenic nitrogen as the working fluid, taking advantage of its thermophysical property variations following the saturation curve from atmospheric pressure up to its critical point. This inaugural experimental campaign reveals a consistent heat transfer scaling relationship of Nu∝Ra^0.306 for the aspect ratio 1 test cell configuration over the entire parameter range (10^9 < Ra < 3×10^14) with negligible Prandtl number dependence (0.7 < Pr < 22), indicating that no transition to an enhanced heat transfer regime has yet been achieved. Attend in person or virtually at: https://gatech.zoom.us/j/95916039479.