Flow regimes and convective heat transfer of refrigerant flow boiling in ultra-small clearance microgaps

• Published in: International journal of heat and mass transfer Vol. 108; pp. 1702 - 1713
• Main Authors: Nasr, Mohamed H., Green, Craig E., Kottke, Peter A., Zhang, Xuchen, Sarvey, Thomas E., Joshi, Yogendra K., Bakir, Muhannad S., Fedorov, Andrei G.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.05.2017
• Subjects: Boiling; Flow regime; Microgap; Pin fins; Pressure drop; Two-phase heat transfer coefficient; Microgap; Flow regime; Two-phase heat transfer coefficient; Pressure drop; Boiling; Pin fins
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2016.12.056

•Convective boiling of R134a in ultra-small clearance microgaps was investigated.•Two microgap configurations were investigated: bare and pin fin populated microgap.•Microgap ability to dissipate ultra-high heat fluxes was experimentally assessed.•Flow regimes were correlated with trends in two-phase heat transfer coefficient.•Thin film convective boiling was the dominant heat transfer mechanism observed.•Pin fin microgap showed 4x larger heat transfer coefficient than bare microgap.•Two-phase heat transfer coefficient correlations found in literature were assessed. Understanding two-phase convective heat transfer under extreme conditions of high heat and mass fluxes and confined geometry is of fundamental interest and practical significance. In particular, next generation electronics are becoming thermally limited in performance, as integration levels increase due to the emergence of ‘hotspots’ featuring up to ten-fold increase in local heat fluxes, resulting from non-uniform power distribution. An ultra-small clearance, 10μm microgap, was investigated to gain insight into physics of high mass flux refrigerant R134a flow boiling, and to assess its utility as a practical solution for hotspot thermal management. Two configurations – a bare microgap, and inline micro-pin fin populated microgap – were tested in terms of their ability to dissipate heat fluxes approaching 1.5kW/cm2. Extreme flow conditions were investigated, including mass fluxes up to 3000kg/m2s at inlet pressures up to 1.5MPa and exit vapor qualities approaching unity. Dominant flow regimes were identified and correlated to two phase heat transfer coefficients which were obtained using model-based data reduction for both device configurations. The results obtained were compared to predictions using correlations from literature, with the maximum heat transfer coefficient reaching 1.5MW/m2K in the vapor plume regime in the case of the finned microgap.