Enhanced boiling heat transfer in horizontal tube bundles

Two-phase flow boiling from bundles of horizontal tubes with smooth and enhanced surfaces was investigated experimentally in two staggered tube bundles consisting of fifteen rows and five columns laid out in equilateral triangular arrays with pitch-to-diameter ratios of 1.17 and 1.5. The enhanced surfaces tested included a knurled surface (Wolverine's Turbo-B) and a porous surface (Linde's High Flux). Experiments were conducted in pure refrigerant R-113, pure R-11 and mixtures of approximately 25, 50, and 75% of R-113 by mass at pressures of 2 and 6 bar, heat fluxes from 5 to 80 kW/m;Values of the heat transfer coefficients for the enhanced surfaces were significantly larger than for the smooth tubes and were comparable to the values obtained in pool boiling. The heat transfer coefficients for the enhanced surfaces did not vary significantly with mass flux, quality, or pitch-to-diameter ratio. Similar results were found for the smooth surface at all but the lowest heat fluxes. Visual observations aided in determining that the onset of nucleation was delayed for the High Flux surface especially in the first tube row. Fully developed nucleate boiling was dominated the heat transfer process throughout the majority of the tube bundle. The enhanced tubes were unaffected by the convective flow in the free stream because the enhanced surface caused the boiling process to occur within the subsurface structure. Heat transfer coefficients decreased significantly for all surfaces when refrigerant mixtures were used with the largest degradation occurring at a mixture of 25% R-11/75% R-113. However, the enhanced surfaces consistently performed better than the smooth tubes.;The performance of the enhanced tubes could be predicted using the pool boiling results, and visual observations aided in developing phenomenological interpretations of the behavior of the smooth and enhanced tubes. The degradation in the smooth tube heat transfer coefficients obtained in fluid mixtures was found to depend on the difference between the molar concentration in the liquid and vapor.