NUCLEATE POOL BOILING OF R-114 AND R-114-OIL MIXTURES FROM SMOOTH ANDENHANCED SURFACES .2. TUBE BUNDLES

Measurements of pool boiling heat-transfer coefficients in pure R-114 and R-114-oil mixtures are reported for a bundle of smooth tubes and t hree enhanced tube bundles (finned; structured and porous). Each bundl e contained 15 electrically heated tubes (five of which were instrumen ted) in a staggered triangular-pitch layout. Tests were carried out at atmospheric pressure while decreasing the heat flux. For pure refrige rant, the structured and porous bundles provide the highest average en hancements of between 4 and 6. For all but the porous bundle, the bund le effect is only significant at low heat fluxes: the porous tube bund le exhibits no bundle effect. With addition of oil, the performance of the smooth and finned tube bundles at first increases before dropping off slightly. For the structured and porous bundles, oil addition lea ds to a steady decrease in performance, especially for the porous bund le at high heat fluxes.