Heat transfer of flow boiling carbon dioxide in vertical upward direction

Heat transfer measurements of flow boiling Carbon Dioxide (CO$_2$) in vertical upward direction have been carried out with a dedicated test facility at the European Organization for Nuclear Research (CERN). The investigation covers saturation temperatures within a range of $-25 \degree \textrm{C} \leq T_{\mathrm{sat}} +5 \degree \textrm{C}$, mass velocities from 100kg m$^{-2}$ s$^{-1}$ $\leq G \leq $ 450 kg m$^{-2}$ s$^{-1}$ and heat fluxes of 5.3 kW m$^{-2}$ and 11.4 kW m$^{-2}$. The experiments have been conducted in a vertical upward evaporator of 8 mm inner diameter and 8 m length and a database of more than 1900 measurements has been compiled within the present study. The heat transfer mechanism is dominated by the nucleate boiling contribution and dryout is observed as a function of saturation temperature, mass velocity and heat flux. A correlation is proposed to predict the dryout inception within the experimental range where the onset of dryout has been observed. The results suggest that most commonly used heat transfer prediction models underpredict the heat transfer mechanisms of CO$_2$. Moreover, the heat transfer coefficients of CO$_2$ increase in vertical upward direction, compared to the data of horizontal studies. For that reason, a vertical multiplier is suggested to capture the trends of vertical upflow with two existing prediction models.