Turbulent heat transport near critical points: Non-Boussinesq effects

Measurements of the Nusselt number N as a function of the Rayleigh number R using ethane gas near its critical point show that N is enhanced by non-Oberbeck-Boussinesq (NOB) effects when the top temperature approaches the coexistence curve. The empirical criterion that a small ∆ρ/ρ (ρ is the density) implies Boussinesq behavior is found not to be valid. An examination of data by Chavanne et al. suggests that the observed transition near R = 10 to a powerlaw N ∼ Reff with γeff ≃ 0.39 may be due to NOB effects rather than to a cross-over to the “ultimate” Kraichnan regime. The data of Niemela et al. (which did not show the above-mentioned transition) are further away from the coexistence curve and are expected to conform to the Boussinesq approximation.