Heat transport properties of plates with smooth and rough surfaces in turbulent thermal convection

We present an experimental study of turbulent thermal convection with smooth and rough surface plates in various combinations. A total of five cells were used in the experiments. Both the global Nu and the Nu for each plate (or the associated boundary layer) are measured. The results reveal that the smooth plates are insensitive to the surface (rough or smooth) and boundary conditions (i.e. nominally constant temperature or constant flux) of the other plate of the same cell. The heat transport properties of the rough plates, on the other hand, depend not only on the nature of the plate at the opposite side of the cell, but also on the boundary condition of that plate. It thus appears that, at the present level of experimental resolution, the smooth plate can influence the rough plate, but cannot be influenced by either the rough or the smooth plates. It is further found that the scaling of N u with R a for all of the smooth plates is consistent with the classical 1/3 exponent. But the scaling exponent for the global N u for the cell with both plates being smooth is definitely less than 1/3 (this result itself is consistent with all previous studies at comparable parameter range). The discrepancy between the N u behaviour at the whole-cell and individual-plate levels is not understood and deserves further investigation.