The Prandtl number has an influence on Rayleigh-Benard instabilities that is sometimes not really explicit in Fluent. The theoretical results given in literature are difficult to obtain, especially for low Prandtl number, in other words for little viscous fluid with a high power of heat transfer.
However this study is just a small part of what is needed to know Rayleigh-Benard. It should be good to verify what is obtained here. But the five fluids are 'numerical fluids' so everything but physical.
Concerning the study, it was interesting to use a CFD code (Fluent) even if the results were quiet easy to obtain for the simple geometry used. Consequently it was funny to use more complex grids.