Abstract
The paper presents experimental investigations of boiling heat transfer for a system of connected narrow horizontal and vertical tunnels. These extended surfaces, named narrow tunnel structure (NTS), can be applied to electronic element cooling. The experiments were carried out with ethanol at atmospheric pressure. The tunnel external covers were manufactured out of 0.1 mm thick perforated copper foil (hole diameters 0.5 mm), sintered with the mini-fins, formed on the vertical side of the 10 mm high rectangular fins and horizontal inter-fin surface. Visualization studies were conducted with a transparent structured model of joined narrow tunnels limited with the perforated foil. The visualization investigations aimed to formulate assumptions for the boiling model through distinguishing boiling types and defining all phases of bubble growth.
Highlights
INTRODUCTIONHeat transfer to boiling liquid on enhanced surfaces is widely used for cooling purposes in technical equipment that generates high heat fluxes and where it is critical that none of the elements or integrated circuits of the device reaches the temperature higher than acceptable for its working life, reliability and correct performance
The surface for determining the boiling curves was composed of a system of 13 mini-fins arranged with 2 mm pitch (1 mm inter-fin areas) and grouped in a form of three main fins, while the segment selected for internal visualization consisted of only 3 mini-fins attached at the distance of 1 mm from the transparent plates which acted as the vessel walls
The visualization experiments allow drawing the following conclusions: x Internal visualization allowed reconstruction of the basic boiling features relating to the confined regions of the interconnected subsurface tunnels. x Observations of the horizontal tunnel showed that the classic mode of suctionevaporation may occur in both tunnels only at large superheats, i.e. above 13 K. x Pores in the perforated foil of the horizontal tunnel are inactive at small superheats. x The tunnels do not “work” independently
Summary
Heat transfer to boiling liquid on enhanced surfaces is widely used for cooling purposes in technical equipment that generates high heat fluxes and where it is critical that none of the elements or integrated circuits of the device reaches the temperature higher than acceptable for its working life, reliability and correct performance. Nakayama et al [6] were among the first to conduct boiling visualization in horizontal structural surfaces (subsurface tunnels limited by foil with triangular holes), using a section of the structure, limited by transparent walls Their visualization study of boiling R-11, water and nitrogen suggested an important role of evaporation inside the tunnel in heat transfer enhancement. Arshad and Thome [7] built a similar set-up for the observation of the boiling process on the plane, which was perpendicular to the axis of the tunnels with triangular, rectangular and circular cross-sections They confirmed, in part, the findings of Nakayama et al [6], pointing to the fact that evaporation of a thin film is the basic condition that has to be satisfied to make vapor generation inside the subsurface structure possible. The objectives of the present study include the elucidation of the pool boiling mechanism in a system of connected horizontal and vertical narrow tunnels, and an attempt to define particular phases in bubble generation and growth
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