The prediction of bubble departure and lift-off radii in vertical U-shaped channel under subcooled flow boiling based on forces balance analysis

Abstract

Bubble departure and lift-off phenomena as well as relevant pivotal parameters, including departure and lift-off radii, growth constants, and acting forces in a vertical U-shaped channel were studied by means of high-speed photography. To this goal, 68 experiments were conducted for subcooled flow boiling of distilled water at atmospheric pressure over a Nichrome heating surface installed on the outer wall of the vertical U-shaped channel, with heat flux ranging from 26.1 to 61.6 kW.m−2, mass flux ranging from 114 to 255 kg.m−2.s−1 , and inlet flow subcooling ranging from 1 to 8 °C. In order to develop new models for departure and lift-off radii the appropriate expressions for Radial pressure gradient force and Centrifugal force must be considered in forces balance analysis. The new obtained Force balance models were validated against the present experimental data. Moreover, these models were adapted to be capable of predicting the data reported in the literature for straight channels where the effect of radial pressure gradient is not present. Furthermore, the influence of flow conditions, involving wall heat flux, mass flux, and inlet flow temperature on departure and lift-off radii and growth constants were examined. The results demonstrated that increasing wall heat flux and inlet flow temperature leads to higher departure and lift-off radii, as well as the rise in bubble growth constants. Also, increasing mass flux contributes to the reduction in these radii and the rise in bubble growth constants. At the end, two correlations were developed to predict bubble departure and lift-off growth constants which unveiled a good agreement with associated experimental data.