Experimental investigation of bubble dynamics and flow patterns during flow boiling in high aspect ratio microchannels with the effect of flow orientation

Multiphase flow and boiling phase-change within microchannels are of paramount importance to thermal management and electronics cooling, among others. The present experimental study investigated the bubble dynamics and flow patterns during flow boiling in a high aspect ratio microchannel at different channel orientations. Hydrofluoroether (HFE−7000) was used as the working fluid within a transparent glass microchannel with a width-to-height aspect ratio of 10 and a hydraulic diameter of 909 μm. A tantalum layer coating was applied to the channel, allowing visual observation to be conducted whilst imposing uniform heating along the channel. Under horizontal and vertical upward flow, the mass fluxes were varied between 14 kg m−2 s−1 and 42 kg m−2 s−1 for Reynolds numbers between 28.4 and 85.2, while the heat fluxes were set from 2.8 kW m−2 to 18.7 kW m−2. The results show that single bubble growth can be divided into three stages, namely: bubble-free growth, partially confined bubble growth, and fully confined growth. The effects of mass flux and heat flux on bubble behaviour and transition points between the defined stages are discussed. Equally important, the channel orientation influences the bubble behaviour in terms of bubble evolution, bubble shape, and bubble nose velocity, with no significant differences observed for the flow patterns when comparing horizontal and vertical upward cases. The comparison of the present work with previous flow pattern map results from the literature shows that earlier flow pattern transition models cannot accurately predict the observed flow patterns in the present configuration. Two types of instabilities are observed, which are associated with the dominant flow pattern during flow boiling and hence a function of the mass flux to heat flux ratio. The first type of instability, high amplitude and long period oscillation, is found during slug flow which is characterised by the presence of bubble rapid expansion along the channel. In addition, a second instability ensues when churn and annular flow create short period oscillations due to the high number of nucleation events, bubble coalescence, and the breakage of liquid slug or liquid film.