Nucleate pool boiling performance of finned and high flux tube bundles in R-114/oil mixtures
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The heat transfer characteristics of pure R-114 and R-114/oil mixtures during nucleate pool boiling from a small bundle of finned and High Flux tubes were measured. The bundles had 5 instrumented and 10 additional heated tubes of 15.8 mm outside diameter which were arranged in an equilateral triangular pitch of 19.1 mm giving a pitch-to-diameter ratio of 1.2. Pure refrigerant with York-C lubrication oil mass con-centrations of 1, 2, 3, 6 and 10% was used. All experiments were performed at 2.2C corresponding to a pressure slightly below atmospheric. Data sets were taken using decreasing heat flux only in order to avoid the boiling hysterisis phenomenon. An enhancement in heat-transfer performance of the finned tube bundle due to oil applications was observed. The enhancement increased with up to 3% oil concentration over all heat flux ranges. Further oil additions showed better performances than pure R-114 at high heat flux levels but poorer performance was obtained at lower heat fluxes. High flux tube experiments indicated that the heat transfer performance of the bundle was approximately 2.5 times better than the finned tube bundle performance for pure R-114. No positive performance enhancement was observed from the High Flux tube bundle due to oil addition. The performance immediately degraded with 1% oil and stayed almost constant with 2% and 3% oil additions. A decrease of performance became significant at high heat flux levels with oil concentrations of 6% and 10%. The performance of these two tube bundles was compared to smooth tube operation.
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