In flow boiling, two different mechanisms, namely nucleate boiling and convective boiling, are considered to be dominant at different working conditions while superimposing in a transition regime. The transition between these two regimes has been considered in different ways in available correlations in the literature. However, few experimental studies have focused on the characteristics of such transition. In this work the nucleate flow boiling-convective flow boiling transition is studied experimentally in a horizontal heated pipe of 5mm ID using R134a as working fluid. The study consist on varying the local hat flux while maintaining the mass flux, pressure and local quality fixed. It is shown that the transition is quite sharp indicating that the superimposing of both mechanism is rather limited.

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