Heat transfer and convection onset in a compressible fluid:: 3He near the critical point -: art. no. 056310

Heat transport in He-3 above its critical temperature T-c was studied along the critical isochore in a flat Rayleigh-Benard cell (height h = 1 mm, diameter D = 57 mm). The range of the reduced temperature epsilon was 5 x 10(-4)less than or equal to epsilon less than or equal to2 x 10(-1). The temperature difference DeltaT(t) across the fluid layer as a function of the time t was measured for different values of the heat current q until steady state was reached. The crossover was observed from the regime dominated by the Rayleigh criterion for the convection onset to that controlled by the adiabatic temperature gradient (ATG), or "Schwarzschild criterion," in good quantitative agreement with predictions. The slope of the convective heat current versus the reduced Rayleigh number was found to be independent of compressibility and the same as for still less compressible fluids. Plots of Nu versus Ra, both corrected for the ATG effect, are presented for early-stage convective turbulence (1x10(5)<Ra<5x10(8)), with unexpected results for the highest values of Pr approximate to 590. The evolution of the transients DeltaT(t) upon turning q on and off are described. In the nonconvective regime, the observed transient relaxation curve agrees quantitatively with predictions. In the convective regime, the shape of DeltaT(t) changes qualitatively with increasing q and with epsilon. In the Appendix, new data for the thermal conductivity are presented, the impact of the "piston effect'' on the temperature profiles inside the fluid is described, and the derivation for DeltaT(t) in the diffusive regime is outlined.