Universidade Tecnica de Lisboa, Instituto Superior Tecnico Departamento de Engenharia Mecanica, Av. Rovisco Pais 1049-001 Lisboa, Portugal
Antonio L. N. Moreira
IN+ Center for Innovation, Technology and Policy Research, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon 1049-001, Portugal
Studies of boiling instabilities induced by the nucleation of bubbles within micro-channels are of particular interest, especially when accompanied by in-situ viewing of the phenomena, to improve the performance of micro-cooling systems. In this paper, experiments are conducted to study the effects of liquid flow rate and heat input on two-phase flow instabilities in a complete wetting liquid-surface system formed by a dielectric liquid inside a glass microchannel. This is the typical configuration of silicon microprocessor cooling systems which make use of low surface energy coolants. New measurements are reported of the instantaneous pressures at inlet and , taken simultaneously with temperature profiles and fast video recording, which are related with flow patterns to provide a comprehensive understanding of the destabilization mechanisms.
Flow boiling patterns are described in terms of the Boiling Number alone Bo = q"s/(G.hfg) which allows to identify two types of flow, non-reversal and reversal, respectively, and four boiling regimes. In the low Boiling Number region before the Onset of Flow Reversal (OFR) the flow pattern is associated with a steady incipient boiling flow and then with elongated bubbles which form, grow and later disrupt by a mechanism of shear at the front interface. In this regime of Elongated Flashing Bubbles the bubbles are lubricated by a liquid film, and their motion is not reversed; the frequency and amplitude of pressure oscillations continuously increase but the pressure drop sharply decreases and the heat transfer coefficient reaches the maximum value. The frequency of pressure oscillations is suggested to be associated with the frequencies of bubble nucleation and interfacial disruption. The second mechanism, which is determined by the imbalance between shear strain and surface tension, is anticipated to play the major role. The onset of flow reversal in caused by an increased rate of nucleation within the liquid film surrounding bubbles which further increases the amplitude and frequency of pressure oscillations until it turns into cahotic at the same time that the heat transfer deteriorates.