Boiling Rayleigh-Benard flow*

We report on heat transport due to boiling of Novec7000 (1-methoxyheptafluo-ropropane) at the bottom plate of a turbulent Rayleigh-B\'enard sample filled with liquid (except for small vapor bubbles when boiling took place). The top surfaces of the bottom plates were silicon wafers etched with a triangular lattice of 30 $\mu$m diameter and 100 $\mu$m deep cavities. The lattice spacing was different for each wafer (100 $\mu$m and 1 mm). The plate diameter and sample height both were 10 cm. Only a central bottom-plate area of 2.5 cm diameter was heated. When the cavities were activated (deactivated) by assuring that they were filled by vapor (liquid), then they nucleated (did not nucleate) bubble formation for bottom-plate temperatures $T_b$ larger than the boiling point. Results of the heat transport as a function of $T_b$ with a fixed applied temperature difference $\Delta T = T_b - T_t = 20$K and $\Delta T = 15$K ($T_t$ is the top plate temperature) will be reported. The effective conductivity of the 2-phase flow was enhanced relative to the supersaturated single-phase system by up to 40\%. Direct visualization of the boiling surface showed that larger (smaller) spacing lead to weak (strong) interaction between neighboring sites, which was determining for bubble departure size.