| Popis: |
Flow boiling heat transfer enhancement using porous coatings in microchannels has been experimentally investigated with HFE-7200. Results of the coated microchannel heat sink were compared to baseline results in a plain, micro-milled copper microchannel heat sink at similar operating conditions, namely inlet pressure of 1 bar, mass flux of 200 – 400 kg/m2 s and inlet subcooling of 10 K at wall heat fluxes between 24.5 kW/m2 to 206.6 kW/m2. Flow visualisation results and SEM surface analysis are presented. The coated surface was densely populated with well-defined cavities between 0.6 µm to 3.3 µm wide. The plain channels had fewer cavities for a given area and these were larger, i.e. up to 6 µm. Bubble generation frequency in the coated channels is significantly higher than in the plain channels due to the presence of more favourable nucleation sites on the coated surface. Flow pattern development occurred similarly in both heat sinks, namely bubbly to slug, churn and annular flow with increasing heat flux, with earlier flow pattern transitions in the coated heat sink. Enhancement in microchannel flow boiling heat transfer was shown to be influenced by mass velocity and may reach up to 44% at low heat fluxes, where the nucleate boiling mechanism is dominant. It diminishes with increasing heat flux, corresponding to nucleate boiling suppression following flow regime transition. Pressure drop increase posed by the coated heat sink is relatively small in terms of overall system power consumption but appears to be influenced by mass flux, pressure oscillations as well as channel rewetting behaviour. |
