Enhanced tunneled surfaces for water pool boiling heat transfer under low pressure

•Sub-atmospheric pool boiling of water from TS and NTS is studied.•The range of analyzed saturation pressure range is 0.75–4kPa.•Both TSes and NTSes enhanced with perforated foil improved the HTC at low pressures.•Small differences between the design of the surface reduce the wall superheat•The TS-2 surface yields the highest heat transfer coefficients and the lowest superheat. In this paper the low pressure boiling of water from two different types of enhanced boiling surfaces: Narrow Tunnel Structures NTS and Tunnel Structures TS is analyzed. Heat transfer coefficients that can be achieved under low-pressure boiling conditions (0.75–4kPa) are estimated and compared with the one for boiling on a plain surface. Small differences between the design of the surface reduce the wall superheat, thus allowing to mitigate the influence of sub-atmospheric conditions. These differences were described in detail and it was determined that among six analyzed samples the TS-2 surface yields the highest heat transfer coefficients and the lowest superheat. Heat transfer measurements are supplemented with visual observations based on high speed camera recordings. The boiling curves, the heat flux, the wall superheat, and the heat transfer coefficients were determined and compared.