Peculiarities of convective flows and heat transport in cryogenic helium

We propose dedicated experimental studies of peculiarities observed over years in flowing behaviour and heat transfer efficiency in thermally driven flows of all forms of cryogenic helium. As for classical thermally driven flows, we shall focus on puzzling behaviour of heat transfer efficiency in Rayleigh-Bénard convection (RBC) in the very high Rayleigh number regime (Ra > 1E12), in particular on claimed transitions to the ultimate or "Grenoble" RBC regimes, paying attention to experimental details and non Oberbeck-Boussinesq effects. In view of striking similarities between RBC in classical fluids and thermal counterflow of superfluid 4He, we propose to use spatially distributed sensitive thermometry and powerful method of second sound attenuation and study various aspects of counterflowing superfluid helium, especially the generation and propagation of this quantum flow in a long macroscopic channel. We argue that, in order to resolve existing peculiarities, these complementary classical and quantum flows ought to be investigated simultaneously.