New correlations for bubble departure and liftoff diameters in vertical subcooled boiling flow under low-pressure conditions

The bubble departure and liftoff diameters are crucial parameters affecting wall boiling heat transfer in sub -cooled boiling flow. This study aimed to develop new correlations for these diameters of a growing bubble on the heated wall in vertical subcooled boiling flow. By analyzing a force balance model, the major non-dimensional parameters were derived to express the bubble diameters. Especially, it revealed that, in addition to the Reynolds number, the Capillary number should be considered to better account for the effect of mass flux on the bubble departure diameter. Finally, the correlations of the two bubble diameters were developed by utilizing the available experimental data. The mean absolute percentage errors of the new correlations were 18.57% and 22.16%, respectively for the bubble departure and liftoff diameters when compared with experimental data. The applicable ranges of correlation of bubble departure diameter cover the superheated Jacob number of 0.0056 -0.035, subcooled Jacob number of 0.0081-0.086, liquid Prandtl number of 1.006 -1.76, product of Capillary and Reynolds numbers of 181.6 -575.23, and density ratio of 0.0006 -0.05. For the bubble liftoff diameter, the applicable ranges are 0.0014 - -0.045 for the superheated Jacob number, 0.0024-0.111 for the subcooled Jacob number, 1.29-1.75 for the liquid Prandtl number, 2883.4-99,716.21 for the Reynolds number, and 0.000615-0.00175 for the density ratio.