Experimental studies of heat transfer regimes with boiling crisis in a vertical channel at high heat fluxes

The results of experimental studies of heat transfer during water boiling in a vertical heated tube with a diameter of 0.013 m and a length of 2.6 m are presented. The studies were conducted in the following ranges of variation: pressure from 4 MPa to 16 MPa, mass velocity from 120 kg/m 2 s to 2000 kg/m 2 s, and heat flux from 0.13 MW/m 2 to 2.59 MW/m 2 and temperatures up to 800°C. Correlations have been obtained for calculating the boundaries of the onset of boiling, the transition to the equilibrium boiling regime, and the heat transfer coefficient in the non-equilibrium boiling region. The influence of flow parameters on the heat transfer coefficient is shown. In the equilibrium boiling region, the mass velocity does not significantly affect the heat transfer coefficient. The heat transfer coefficient increases with increasing pressure and gradually decreases with increasing vapor quality (steam content). In some modes, prior to the heat transfer crisis (critical heat flux, CHF), an intensification of heat transfer and an increase in the heat transfer coefficient up to 162 kW/(m 2 ·K) are observed. Such regimes are characteristic of the second-kind heat transfer crisis (CHF-II), which occurs as a result of the drying out of the near-wall liquid film in a dispersed-annular flow at high values of vapor quality.