Pool boiling heat transfer on micro-fins with wire mesh – Experiments and heat flux prediction

Abstract

The article presents experimental data for two kinds of enhanced surfaces: a micro-finned structure without a covering (MF) and micro-fins covered with a copper wire mesh (MFM). The experiments were carried out for water, ethanol and FC-72 at atmospheric pressure. Micro-fins of 0.5/1 mm in height were uniformly spaced on the base surface. The wire mesh with apertures of 0.32, 0.4 and 0.5 mm was sintered with the micro-fin tips. The widths of spaces between the micro-fins (tunnel widths) were 0.6, 1.0 and 1.5 mm. The highest heat transfer coefficients for ethanol used as the working fluid and 1 mm high micro-fins were obtained at the largest mesh aperture size and tunnel widths of 1 mm and plain micro-fins with 0.6 mm wide tunnel. For 0.5 high micro-fins, the highest heat transfer coefficient was obtained for the structure with the widest, 1.5 mm tunnels.The measurement data collected for the three working fluids helped generalize the results in the form of a graph of the increase in heat transfer coefficient as a function of the dimensionless numbers.Visualization studies aimed at identifying nucleation sites and determining the diameter and frequency of departing bubbles for boiling water. A different mechanism of pool boiling was observed for plain micro-fins and micro-fins covered with wire mesh.A simplified model was proposed for determining the total heat flux for micro-fins with the wire mesh. It was assumed that the structure formed a system of connected perpendicular horizontal tunnels limited with a porous top covering. Regarding the calculated bubble parameters (diameter, nucleation sites density, frequency), the heat fluxes were determined for the evaporation in the tunnels between the micro-fins and for the convection on the wire mesh surface. The predicted heat fluxes, when compared to the experimental results, showed satisfactory agreement for boiling water at medium and high heat fluxes (range from 70 to 355 kW/m2). Less accurate results were obtained for ethanol and FC-72.