Turbulent Diffusion of Heat at High Rayleigh Numbers

Abstract

Thermal convection is observed in controlled laboratory experiments at very high Rayleigh numbers using a relatively large apparatus filled with low temperature helium gas. The low temperature environment offers two advantages toward the study of turbulent convection; namely the favorable properties of the working fluid in achieving very high Rayleigh numbers and the low thermal mass of the heated metallic surfaces at cryogenic temperatures. The latter property is exploited in order to provide a means of measuring an effective thermal diffusion coefficient of the buoyancy-driven turbulence by propagating thermal waves into the bulk and observing the damping of their amplitude with distance. The diffusivity measured directly in this way compares well with values inferred from the time-independent measurements of the global turbulent heat transfer at Rayleigh numbers of order $10^{9}$ but are significantly different at Rayleigh numbers of order $10^{13}$ which can be interpreted as a consequence of the formation of well developed bulk turbulence decoupled from the thermal boundary layers at the heated horizontal surfaces.