Effects of Prandtl number in quasi-two-dimensional turbulent Rayleigh-Bénard convection*

We report an experimental study of the Prandtl (\textit{Pr}) number effects on flow pattern and local temperature fluctuation in quasi-two-dimensional turbulent Rayleigh-Bénard convection. The experiments were conducted in four rectangular cells with same aspect ratio but different heights, the Rayleigh number \textit{Ra} range (1e9 -- 2e10) remains unchanged while \textit{Pr} is varied from 11.6 to 157.4. The flow patterns visualized by the shadowgraph show that thermal plumes become more slender as \textit{Pr} increases, and their organized-motion is more concentrated towards the sidewall. The mean flow strength, characterized by the Reynolds number \textit{Re}, becomes weaker with the increase of \textit{Pr}, i.e. \textit{Ra}$^{\mathrm{0.57}}$\textit{Pr}$^{\mathrm{-0.81}}$. It is further found that the temperature fluctuations in the center ($\sigma_{\mathrm{c}}$/$\Delta $T) and near sidewall ($\sigma_{\mathrm{s}}$/$\Delta $T) behave different, i.e. Pr$^{\mathrm{-0.19}}$Ra$^{\mathrm{-0.28}}$ and Pr$^{\mathrm{0.10}}$Ra$^{\mathrm{-0.20}}$, respectively. This result quantitatively demonstrates that, as \textit{Pr} increases, thermal plumes prefer to move along the sidewall rather than traveling through the center of the cell.