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

•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.