Identification of universal heat transfer characteristics along the boiling curve for vertical subcooled flow boiling of refrigerant Novec 649

Local measurements along the boiling curve of heat flux, surface temperature and void data from fiber-optic microprobes are presented for vertical subcooled flow boiling of refrigerant Novec 649. Experiments were conducted in a square flow channel with a cross section of 40mm by 40mm. Mass flux varied between 1000kgm^{− 2}s^{− 1} and 2000kgm^{− 2}s^{− 1} and subcooling ranged from 9K to 24K. Two heaters of technically relevant thickness made of two different materials but with identical surface characteristics were investigated: a wall-mounted copper heater and a single-rod copper-coated stainless steel heater. Boiling curves for both heater configurations are provided with data on void fraction, bubble detection frequency and bubble length obtained from fiber-optic microprobes placed at a distance of 100μm relative to the heater surface. A comparison of the void morphology along the boiling curve between the two heater configurations is presented. The data is discussed with respect to recent advancements in identifying the governing parameters for boiling heat transfer in pool boiling. It was found that the entire boiling curve correlates well with the triple-phase contact line density on the heater surface. A common stability limit for the boiling process for the two heater configurations was identified based on the fiber-optic microprobe data. It was found that the boiling process becomes unstable when the thermal diffusivity time of the heater substrate becomes much longer than the void interaction time at the heater surface.