CHF IN BUBBLE BOILING IN CHANNELS

摘要

This paper presents a physical model of the heat transfer crisis in bubble boiling of fluids in channels in the field of positive as well as negative values of the relative enthalpy x_(cr) at the values of void fraction alpha less than 0.65. The proposed model is based upon the well-known fact that the onset of the crisis is linked to the full evaporation of a thin liquid film under a steam conglomerate that is formed as a result of coalescence of single bubbles, when such conglomerate exists near the wall. We propose new methods for finding the film thickness and the time of the conglomerate existence near the wall, as well as the equation for determining CHF. We apply proposed equation to generalize the results of the experimental and the look-up table data for the critical heat flux for water, nitrogen, helium, R134a, R115 and R12 in pipes, annular and rectangular channels. We further show that the presented model describes the experimental data not only quantitatively but also qualitatively, and explains the influence of the mass velocities on CHF in the wide range of its variation.