A new experimental method for characterizing boiling regimes of water in vertical milli-channels

Abstract

Over the past few decades, high-performance heat-transfer devices have been studied for numerous applications. Boiling flow in such confined spaces induces high-frequency flow instabilities that directly influence the heat-transfer performances. A small-scale water vaporizer with 10 mm-sized channels is experimentally studied over a wide range of vapor quality (0.05–1). The peculiarity of the setup is a transparent window that allows for high speed imaging of the boiling flow up to 5 bars. This work focuses on a post-processing method based on spatio-temporal diagrams. Hydrodynamics of the boiling flow can be observed on the diagrams enabling to understand the boiling mechanisms and the flow regimes inside the channels. Four regimes are observed, namely bubbly, slug, churn and annular flow. Diagrams are also used to estimate the local mean void fraction over time in channels, the local vapor quality, the slip ratio and a flow-regime map. For qualities under 10 %, the bubbly flow is predominant in channel. For higher qualities, if the specific mass flow rate of water is below 37 kg/s/m², only the slug flow and the annular flow are observed, whereas the churn flow is also observed for larger flow rates.