Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

The present study presents new flow boiling heat transfer results of R-134a flowing inside a 13.84 mm internal diameter, smooth horizontal copper tube. The heat transfer measurements were made over a wide range of test conditions: saturation temperatures of 5, 15 and 20 C, (corresponding to reduced pressures of 0.08, 0.12 and 0.14), vapor qualities ranged from 0.01 to 0.99, mass velocities of 300 and 500 kg/m(2) s, and heat fluxes of 7.5 and 17.5 kW/m(2). The experimental results clearly show that a local minimum heat transfer coefficient systematically occurs within slug flow pattern or near the slug-to-intermittent flow pattern transition. The vapor quality x(min) at which the local minimum occurs seems to be primarily sensitive to mass velocity and heat flux. Thus, it is influenced by the competition between nucleate and convective boiling mechanisms that control macroscale flow boiling. The experimental results were compared to four types of predictive methods: (a) strictly convective, (b) superposition, (c) strictly empirical and (d) flow pattern based. Generally, all the methods tend to underpredict the experimental data and the higher errors occur in two particular regions: low and high vapor qualities. These vapor qualities correspond to slug and annular patterns, respectively. For slug flow, methods that require the identification of nucleate boiling related regions tend to predict the heat transfer coefficient accurately. This emphasizes that for slug flows, heat transfer is not a simple juxtaposition of nucleate and convective boiling contributions, but that the integration of these two heat transfer mechanisms is also a function of flow parameters. The comparisons between experimental and predicted data show that the best overall results are obtained with superposition and flow pattern based methods. (C) 2008 Elsevier Ltd. All rights reserved.

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