Influence of coolant and material properties on cooling high-temperature steel spheres in subcooled ethanol-water mixtures

Abstract

The paper presents the new experimental results on cooling high-temperature steel spheres in subcooled ethanol-water mixtures at mass fractions of ethanol in the range of 10–80% under atmospheric pressure. Three test samples of 39 mm in diameter each have been prepared for the experiments: a carbon steel sphere with the oxidized surface and two stainless steel spheres. In the latter samples, one sphere had a polished surface, while the other one had a carbon coating. Usage of binary liquid mixtures as cooling liquids allows changing their thermophysical properties in a wide range. In general, when decreasing ethanol mass fraction boiling heat transfer intensifies, and it leads to an earlier (at higher surface temperature) transition to the intensive heat transfer regime. The experimental results are qualitatively consistent with a model that was proposed by authors earlier. This model describes the conditions of transition from a stable film boiling to an intensive heat transfer regime (so-called “microbubble boiling”). The main idea of the model is that this regime can arise due to the local contacts between the crests of the wavy liquid flow at the liquid/vapor interface and the protrusions of the surface roughness. The model considers the properties of a coolant (latent heat of evaporation, surface tension and kinematic viscosity) and thermophysical properties of a cooled surface (thermal effusivity). The experimentally obtained values of heat transfer coefficients in stable film boiling regimes are in agreement with the calculation results of the semi-empirical model of heat transfer in stable film boiling of subcooled liquids developed by authors earlier.