Subcooled Flow Boiling of Carbon Dioxide Near the Critical Point Inside a Microchannel

Subcooled flow boiling heat transfer of carbon dioxide (${\mathrm{CO}}_{2}$) at the microscale near the critical pressure is experimentally studied. The onset of nucleate boiling and the local heat-transfer coefficient are obtained under 26 experimental conditions, consisting of 17 sample points each, yielding 442 data points. Excess temperatures of about 1 K at the onset of nucleate boiling and heat-transfer coefficients of up to $200\phantom{\rule{0.25em}{0ex}}\mathrm{kW}/{\mathrm{m}}^{2}\phantom{\rule{0.25em}{0ex}}\mathrm{K}$ are recorded. The heat-transfer coefficient and the onset of nucleate boiling models and correlations developed for low reduced pressures are compared to current experimental results. Modifications to these models are introduced to account for conditions pertinent to reduced pressures around unity. Overall, the correlations do not fully capture current measurements, suggesting that the physics of flow boiling heat transfer near the critical conditions exhibits some unique characteristics. Furthermore, it is observed that the experimentally obtained heat-transfer coefficient is better predicted by a model developed for flow boiling of ${\mathrm{CO}}_{2}$. However, with increasing pressure, deviations between experiments and predictions increase.