Effect of channel height and mass flux on highly subcooled horizontal flow boiling

- Identified heat transfer regions: forced convection, transition & nucleate boiling.
- Smaller channel height at constant mass flux enhances heat transfer.
- Thermal boundary layer thickness and nucleation site density dictate heat transfer.
- Aging of copper boiling surface reduces heat transfer rates by 10% at most.

Experiments of highly subcooled flow boiling of water in horizontal macrochannels with orthogonal cross-section are performed. Explored parameters are channel height (3 and 10 mm) and mass flux (330-830 kg/m2 s). The range of applied heat fluxes is 200-1000 kW/m2. Aging of the copper boiling surface is examined during a 48 h continuous operation and is found to gradually reduce heat transfer rates compared to a polished surface. Yet, aging reaches a steady condition already at 24 h of operation with about 10% lower heat transfer rates than for the polished surface. Using the steady aged surface in the main experiments, three heat transfer regions are identified: (a) forced convection region, before the onset of boiling, depending highly on mass flux but for the most part being channel height independent, (b) nucleate boiling region depending highly on channel height but for the most part being mass flux independent, and (c) a transition region in-between depending on both mass flux and channel height. The 3 mm channel promotes initiation of boiling at lower wall superheats leading to better heat transfer efficiency compared to the 10 mm channel, except for the highest mass fluxes where their performance is comparable. The largest enhancement in heat transfer coefficient provided by the 3 mm channel compared to the 10 mm channel is ∼15-20% and is found at the lowest mass flux 330 kg/m2 s. Analysis of the present data supports the notion that heat transfer is dictated by the thickness of the thermal boundary layer and the density of active nucleation sites.

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