Experimental study of non-Boussinesq Rayleigh–Bénard convection at high Rayleigh and Prandtl numbers

A set of experiments is performed, in which a layer of fluid is heated from below and cooled from above, in order to study convection at high Rayleigh numbers ~Ra! and Prandtl numbers ~Pr!. The working fluid, corn syrup, has a viscosity that depends strongly on temperature. Viscosity within the fluid layer varies by a factor of 6 to 1.8310 in the various experiments. A total of 28 experiments are performed for 10,Ra,10 and Pr sufficiently large, 10,Pr,10, that the Reynolds number ~Re! is less than 1; here, values of Ra and Pr are based on material properties at the average of the temperatures at the top and bottom of the fluid layer. As Ra increases above O~10!, flow changes from steady to time-dependent. As Ra increases further, large scale flow is gradually replaced by isolated rising and sinking plumes. At Ra.O~10!, there is no evidence for any large scale circulation, and flow consists only of plumes. Plumes have mushroom-shaped ‘‘heads’’ and continuous ‘‘tails’’ attached to their respective thermal boundary layers. The characteristic frequency for the formation of these plumes is consistent with a Ra scaling. In the experiments at the largest Ra, the Nusselt number ~Nu! is lower than expected, based on an extrapolation of the Nu–Ra relationship determined at lower Ra; at the highest Ra, Re ̃1, and the lower-than-expected Nu is attributed to inertial effects that reduce plume head speeds. © 1999 American Institute of Physics. @S1070-6631~99!00710-2#