Abstract
Complex experimental investigations of boiling heat transfer on structured surfaces covered with perforated foil were performed. Experimental data were discussed for three kinds of enhanced surfaces: tunnel structures (TS), narrow tunnel structures (NTS) and mini-fins with the copper wire net (NTS-L). The experiments were carried out with water, ethanol, R-123 and FC-72 at atmospheric pressure. The TS and NTS surfaces were manufactured out of perforated copper foil (hole diameters: 0.3, 0.4, 0.5 mm) sintered with the mini-fins, formed on the vertical side of the 5 and 10 mm high rectangular main fins and horizontal inter-fin surface. The NTS-L surfaces were formed by mini-fins of 0.5 and 1 mm height uniformly spaced on the base surface. The wire mesh with an aperture of 0.32, 0.4 and 0.5 mm sintered with the fin tips formed a system of connected perpendicular horizontal tunnels. The tunnel width was 0.6 - 1.0 - 1.5 mm and the depth was 0.5 or 1.0 mm. The effects of the Bond number and dimensionless parameters for three kinds of enhanced structures on heat transfer ratio at nucleate pool boiling were examined.
Highlights
Boiling heat transfer enhancement is effective provided that specific surfaces are used
The narrow tunnel structures (NTS)-L surfaces were formed by mini-fins of 0.5 and 1 mm height uniformly spaced on the base surface
The studies conducted since the 1960s at numerous research centers have shown that the most efficient method of increasing the density of the heat flux and the value of the heat transfer coefficient is the use of coverings that are either porous or porous and capillary, the employment of enhanced surfaces or forming the channels, tunnels and hollows/cavities linked with the surface by a narrow „outlet”
Summary
Boiling heat transfer enhancement is effective provided that specific surfaces are used (enhanced boiling surfaces). The studies conducted since the 1960s at numerous research centers have shown that the most efficient method of increasing the density of the heat flux and the value of the heat transfer coefficient is the use of coverings that are either porous or porous and capillary, the employment of enhanced surfaces or forming the channels, tunnels and hollows/cavities linked with the surface by a narrow „outlet”. The following factors facilitate boiling heat transfer intensification: x artificially produced nucleation sites in the form of pores contribute to the increase in the nucleation site densities, x increasing the wetted surface by using the tunnels/channels and extending the surface through the use of the fins/mini-fins/microfins, x film vaporization or vaporization from the menisci in the subsurface tunnels/hollows/cavities, x internal convection associated with the liquid flow in the tunnels/channels, forced by the capillary forces and the pumping action of the growing bubbles, x external convection associated with the growth, departure and movement of the vapor bubbles. The measurement data for the four types of working fluid used in the studies were taken from [1,2,3,4]
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