Flow pattern maps for convective boiling of CO2 and R410A in a horizontal smooth tube: Experiments and new correlations analyzing the effect of the reduced pressure

In this work flow visualizations and measurements are made and analyzed to identify flow regime transitions, slug-to-intermittent, intermittent-to-annular and the dry-out inception, during the flow boiling of CO2 in a horizontal smooth tube of 6.00 mm of internal diameter, varying the reduced pressure between 0.57 and 0.64, the mass velocity between 150 and 500 kg/m2 s and the heat flux between 5 and 20 kW/m2. Additional experiments for R410A show the effect of the reduced pressure over a wider range, from 0.19 to 0.52, varying the other operating parameters in the same ranges. All together, the new experimental dataset of 1420 observations and heat transfer measurements were utilized to determine the location of the flow pattern transitions, which showed a strong dependency of the vapor quality on the mass velocity for each transition line, for fixed reduced pressure. The dry-out inception line was influenced by the heat flux, as expected. The influence of the reduced pressure was also identified as an important parameter with a remarkable impact. This flow pattern database was then statistically compared with well established methods (Wojtan et al. (2005) [1] for R410A, Cheng et al. (2008) [2] for CO2) and recent methods [3] and [4], showing poor agreement in the determination of the intermittent-to-annular flow regime transition for all the methods and in the prediction of the dry-out inception for all the methods, except for Wojtan et al. (2005) [1]. Finally, new easy-to-use correlations are proposed to provide better agreement with the experimental dataset and to explicitly illustrate the effect of the reduced pressure in an effort to generalize this diabatic flow pattern map for broader application.