Heat-flux enhancement by vapour-bubble nucleation in Rayleigh-Bénard turbulence

We report on the enhancement of turbulent convective heat transport due to vapour-bubble nucleation at the bottom plate of a cylindrical Rayleigh-Bénard sample (aspect ratio 1.00, diameter 8.8 cm) filled with liquid. Microcavities acted as nucleation sites, allowing for well-controlled bubble nucleation. Only the central part of the bottom plate with a triangular array of microcavities (etched over an area with diameter of 2.5 cm) was heated. We studied the influence of the cavity density and of the superheat T_b - T_on (T_b is the bottom-plate temperature and Ton is the value of T_b below which no nucleation occurred). The effective thermal conductivity, as expressed by the Nusselt number Nu, was measured as a function of the superheat by varying T_b and keeping a fixed difference T_b - T_t ∼ 16 K (T_t is the top-plate temperature). Initially T_b was much larger than T_on (large superheat), and the cavities vigorously nucleated vapour bubbles, resulting in two-phase flow. Reducing T_b in steps until it was below T_on resulted in cavity deactivation, i.e. in one-phase flow. Once all cavities were inactive, T_b was increased again, but they did not reactivate. This led to one-phase flow for positive superheat. The heat transport of both one- and two-phase flow under nominally the same thermal forcing and degree of superheat was measured. The Nusselt number of the two-phase flow was enhanced relative to the one-phase system by an amount that increased with increasing T_b. Varying the cavity density (69, 32, 3.2, 1.2 and 0:3 mm^-2) had only a small effect on the global Nu enhancement; it was found that Nu per active site decreased as the cavity density increased. The heat-flux enhancement of an isolated nucleating site was found to be limited by the rate at which the cavity could generate bubbles. Local bulk temperatures of one- and two-phase flows were measured at two positions along the vertical centreline. Bubbles increased the liquid temperature (compared to one-phase flow) as they rose. The increase was correlated with the heat-flux enhancement. The temperature fluctuations, as well as local thermal gradients, were reduced (relative to one-phase flow) by the vapour bubbles. Blocking the large-scale circulation around the nucleating area, as well as increasing the effective buoyancy of the two-phase flow by thermally isolating the liquid column above the heated area, increased the heat-flux enhancement.