Bubble nucleators to enhance external pool boiling from the bottom row of a tube bundle

•Passive nucleators underneath tube bundles enhance bundle heat transfer.•Bundle heat transfer enhancement is most pronounced at lowest heat fluxes.•Enhancement is due primarily to the early activation of nucleation sites.•Enhancement is weakly dependent on nucleator geometry and vapour generation rate. The influence of vapour bubbles impacting on boiling heat transfer surfaces is examined. Passive nucleators based on boiling heat transfer were used to create gas bubbles with varying properties that impinged upon two horizontal tubes above. Two nucleator devices were compared: a microscale nichrome wire nucleator, and a hollow tube nucleator situated below a column of cylinders with uniform heat flux. Nucleators significantly enhanced the heat transfer coefficient of the lower tube in the column, which was most pronounced at low heat fluxes due to the early activation of potential nucleation sites by the bubbly flow carried from the nucleator by buoyancy. Nucleator augmentation of heat transfer coefficient on the tube above was insensitive to nucleator heat flux once nucleate boiling was established and, for the nucleator geometries tested, only weakly dependent on nucleator geometry and vapour generation rate.