SUMMARY

Dielectric fluids, including refrigerants, are electrically inert and are a good candidate as working fluid in two-phase microsystem cooling applications. In this study, R245fa is investigated for flow boiling in microgaps with height of 200 μm, spacing in the ranges of 200-225 μm (dense device) and 400-450 μm (sparse device) are studied. For heat fluxes up to 498 W/cm2, mass flux values ranging from 193 kg/m2s to 7896 kg/m2s and inlet temperatures ranging from 10 oC to 18 oC, average two-phase heat transfer coefficient up to 60 kW/m2K the devices are tested. High speed flow visualizations at frame rate of 2229 fps elucidate the flow boiling patterns inside the microgaps. The visualizations have shown different two-phase flow regimes such as bubbly, foggy, and slug flows that are generated in the pin finned area. The surface temperature values are calculated based on the obtained heaters temperature data and are plotted against the dissipated power for a wide range of heat flux for each experiment. An uncertainty analysis is also performed for the reported data. Single-phase and two-phase heat transfer coefficients and a comprehensive flow regime mapping is represented as part of this documentation. Also, pressure drop and heat transfer coefficients characterizations are done by developing correlations for single-phase and two-phase pressure drops, single-phase and two-phase heat transfer coefficients that perfectly match on all experimental data from devices.