The paper focuses on theoretical and experimental studies of boiling heat transfer on double-extended surfaces with tunnel structures (TS), covered with perforated foil. The experiments were conducted for water at atmospheric pressure. The enhanced structures make use of three passive techniques of boiling heat transfer enhancement. The tunnel external covers were manufactured from perforated copper foil of 0.05 mm thickness (pores diameter 0.3, 0.4, 0.5 mm), sintered with the mini-fins, formed on the vertical side of rectangular fins and horizontal inter-fin surfaces. Basing on their previous studies and existing analytical boiling models, the authors proposed their own one for extended surfaces with connected horizontal and vertical tunnels (TS). Characteristic mechanisms of vapor bubbles generating, shifting and detaching in tunnels were accounted for, as well as the nonisothermality of the extended surface. Regarding the calculated bubbles parameters (diameter, nucleation sites density, generation frequency), the heat fluxes for evaporation within the tunnels and convection on the tunnel external surfaces were determined. Substantial enhancement of heat transfer coefficient was observed for the investigated structures. Calculations, based on the developed simplified model, indicated a predominating convection heat contribution into the heat transfer for tunnel structures (TS). The predicted heat fluxes, when compared with experimental results, showed satisfying congruence.
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