Experimental Study of the Influence of Structured Capillary-Porous Coatings on the Dynamics of Development of Transient Processes and the Crisis Phenomena at Stepwise Heat Release

Abstract

The effect of structured plasma-sprayed capillary-porous coatings on transient processes and the development of crisis phenomena at boiling under pulsed heat release was studied. The working fluid was liquid nitrogen on the saturation line at atmospheric pressure. It is shown that under unsteady heat release, there is a degeneration of the development of the boiling crisis on heaters with structured capillary-porous coatings at q < q_CHF (critical heat flux at steady heat release). Under unsteady pulsed heat release, no rapid transition to the film boiling regime (without passing through the nucleate boiling stage) is observed on heaters with such coatings until the thermal load is more than two times higher than the critical heat flux for steady heat release. This significantly increases the times of transition to post-critical heat transfer. Analysis of synchronized measurements of surface temperature of heaters and high-speed video recording of transient processes shows that the degeneration of the heat transfer crisis at q < q_CHF on samples with coatings occurs due to significantly lower liquid boiling temperature differences and specific features of the dynamics of propagation of self-sustaining evaporation fronts in comparison with a smooth heater.