Abstract: The paper focuses on theoretical and experimental studies of boiling heat transfer on double-extended surfaces with tunnel structures, covered with perforated foil. The experiments were conducted for water and R-123 at atmospheric pressure. The tunnel external covers were manufactured from perforated copper foil of 0.05 mm in thickness (pore diameters: 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. On the basis of their previous studies and existing analytical boiling models, the authors proposed their own model for extended surfaces with connected horizontal and vertical tunnels. Extended surface nonisothermality and the characteristic process of vapor bubbles formation and departure were accounted for. 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 the heat transfer coefficient was observed for the investigated structures. Calculations, based on the developed simplified model, indicated a predominating convection heat contribution to the heat transfer for tunnel structures. The predicted heat fluxes, when compared to the experimental results, showed satisfying agreement in low and medium heat flux ranges for water, and in all heat flux ranges for R-123.
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