| Summary: | The heat transfer rate during nucleate flow boiling conditions inside a horizontal heated pipe is assumed to increase with the heat flux until the boiling crisis while being independent of the mass flux. Contrary to this, we present experimental observations of a heat transfer deteriorated regime prior to the occurrence of the boiling crisis. We show that in this regime, the heat transfer coefficient becomes independent of the heat flux, and experimentally, we identify the limits of this boiling regime that elucidate how the near-wall interaction of the bubbles can hinder the heat transfer process. Furthermore, we show that in order to avoid this heat transfer deteriorated regime, the mass flux needs to exceed a threshold value, which drastically reduces as the working pressure decreases. This fact offers useful insights on how to design thermal management systems, and it is more important that the role of the mass flux during nucleate flow boiling needs to be reconsidered.
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