On the efficiency of tubes with integral fins in pool boiling: Effect of fin height

The phase-down of high global warming potential (GWP) refrigerants motivates low-charge flooded evaporators, for which enhanced external pool boiling on tube surfaces is essential. Pool boiling of the low-GWP azeotropic blend R515B is investigated on a smooth copper tube and on three integrally finned copper tubes (16 mm OD) with fixed fin pitch (≈0.81mm) and fin heights of 0.09, 0.14, and 0.25 mm, corresponding to height-to-pitch ratios (h/p) of 0.11, 0.16, and 0.31, at saturation temperatures of 10–40 °C over a wide heat-flux range. Relative to the smooth surface, the integrally finned tubes deliver a substantial enhancement and show higher heat transfer coefficients at higher saturation temperatures. For all h/p values, the heat transfer coefficient (HTC) increases quasi-linearly with heat flux up to a distinct transition, after which the growth rate of heat transfer coefficient with heat flux decreases. The transition heat flux varies systematically with h/p and saturation pressure, revealing a trade-off whereby larger h/p improves pre-transition performance but promotes earlier transition. High-speed visualization links the transition to bubble growth beyond the inter-groove spacing and coalescence across adjacent grooves, which reduces independent nucleation sites and limits liquid replenishment. Based on this mechanism, a dimensionless criterion is formulated to combine geometric confinement and capillary effects with heat-flux-driven vapor generation, suggesting a path to generalize the transition condition across geometries through h/p (and related spacing scales).