Abstract-Experimentalresults and an analytical model on the boiling mechanisms in a closed two-phase thermosyphon are presented in order to define the frontiers between the main boiling regimes and, in particular, the frontier between intermittent boiling and fully-developed boiling. The boiling regimes are classified on the basis of the frequency of bubble nucleation and the ratio of bubble diameter to device diameter. A criterion for the intermittent/developed-boiling frontier is based on the ratio of the bubbienucleation waiting time and the bubble growth time. From this criterion a correlation between power throughput, working-fluid pressure and nucleation critical superheat are obtained. Experimental data on operating conditions, temperatures, and nucleation frequencies show the same functional dependence as the proposed correlation.

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An experimental facility was designed to measure pressure drop and heat transfer coefficient during flow boiling of azeotropic refrigerants in horizontal tubes. The apparatus is made of the refrigerant circuit, including the test section; the water circuit, to provide the power for evaporation; the glycol-water circuit, to fix the refrigerant pressure. The test section is a counterflow tube-in-tube heat exchanger (refrigerant inside, water outside). Plant assessment involved the measurement of pressure drop and heat transfer during R134a flow boiling in a smooth tube (outer diameter 9.56mm, inner diameter 8.92mm). Such a case study is a benchmark for the availability of a wide experimental database in the literature, which has been also summarized in many correlations. Then further tests involved a microfin tube. The experimental conditions were: evaporation temperature, 5°C; mass flux, 111÷333kg/m2s; average quality, 0.15÷0.93; heat flux, 8.8kW/m2 and 17.6kW/m2. The uncertainty affecting the pressure drop and the heat transfer coefficient resulted lower than 1% and 5% respectively. The comparison with the literature shows satisfactory agreement with the major findings and enables using the data for the assessment of the existing models to predict both the pressure drop and the heat transfer coefficient.

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