Flow Boiling in a Microtube: Flow Pattern and Heat Transfer

This paper presents the results of the flow boiling patterns of FC-72 in a microtube. The internal diameter of the tube is 0.48 mm, with a heated length of 73 mm. The mass flow rate varies from 50 to 3000 kg/m²-s. The microtube is made of Pyrex in order to obtain the visualisation of the flow pattern along the heated channel. Different types of flow pattern have been observed: bubbly flow, deformed bubbly flow, bubbly/slug flow, slug flow, slug/annular flow, and annular flow. The flow pattern map is compared with those obtained for larger tubes (2.0, 4.0, and 6.0 mm). Flow patterns in the microtube, present less chaotic behaviour and regular vapour-liquid interfaces. Besides, as the tube diameter decreases, the intermittent flow regime shifts from the saturated boiling region towards the subcooled boiling region. The experiments, in the microtube, show the presence of flow instabilities in a large portion of the tests at low mass flow rates and low subcooling. Flow patterns in presence of flow instabilities are mainly characterized by bubbly/slug flow and slug/annular flow. Heat transfer rates have been studied in all flow pattern conditions. The two groups of results, with flow instabilities and without flow instabilities, show similar heat transfer behaviour. The experimental results of flow pattern are compared with the flow pattern maps of McQuillan and Whalley (1985), Mishima and Ishii (1984), and Ong and Thome (2010).