Boiling of Liquid Subcooled to Saturation Temperature in Channels as a Method for Removal of Ultimate Heat Fluxes

Abstract

The results of a comprehensive experimental study of the boiling of water subcooled to the saturation temperature in a channel are presented. This technology is used to remove extreme heat fluxes in modern equipment. An emphasis is placed on the study of the characteristics of vapor bubbles, their changes under the effect of various regime factors, and characteristics of the heating surface. For the realization of the goals, a high-speed video of the process was used. Experiments were carried out with distilled deaerated water at atmospheric pressure, heat flux densities of up to q = 8 MW/m², subcooling of the liquid to the saturation temperature of Δt_s = 30–80°C, and the flow velocity of the liquid of up to w = 0.7 m/s. Smooth and structured surfaces with coatings, most of which were produced by microarc oxidation, were used in the experiments. Significant subcooling of the liquid to the saturation temperature is shown to cause a deep deactivation of the active nucleation cites after the collapse of the vapor bubble and spatial and temporal randomness of the distribution of the nucleation cites over the heating surface. The bubble size distribution, the density of nucleation cites that is approximately proportional to the heat flux density, the bubble lifetime, and the duration of individual stages of its life cycle are determined. A picture of the bubble collapse is clarified. The subcooling of the coolant to the saturation temperature is shown to be the strongest among the parameters that determine the boiling of the subcooled liquid. The phenomenological Snyder–Bergles model of the boiling process was established to agree best with the measurement results. Such engineering aspects of the problem as the choice of limiting design parameters of the cooling system and the use of coatings for boiling enhancement of a subcooled coolant are considered.