Impact of the oxide scale on spray cooling intensity

Abstract

Paper presented to the 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Florida, 14-16 July 2014.

Heat treatment of steel is attended by oxide scales growth with various physical properties. The most common and most dominant impact of the oxide scale layers is on the surface quality and mechanical properties of steel. This paper is focused on study of influence of the oxide scale on cooling intensity. Spray cooling is a typical technique used in heat treatment and other metallurgical processes where controlled temperature regimes are required. Cooling intensity is primarily affected by spray parameters as pressure and coolant impingement density. It is not frequently reported but even thin layers of oxides can significantly modify the cooling intensity. This effect is dominant in the cooling of steel surfaces at high surface temperatures. Study of the influence of the oxide scale layers on cooling intensity was carried out by experimental measurements and numerical analysis. Experimental measurements compare the cooling of scale-free surfaces and oxidized surfaces. Experimental investigations show a difference in the cooling intensity. Numerical analyses were prepared to simulate cooling of the samples with different oxide scale layers and different thermal conductivity of scales. Even a scale layer of several microns can significantly modify the cooling intensity. A low thermal conductivity of the oxides can make the cooling more intensive. The paper provides experimental evidence of this fact and numerical study of the oxide scale layer thickness and thermal conductivity on the influence on the spray cooling with boiling. The Leidenfrost phenomenon and change in surface temperature provides key to the explanation why the hot surface covered by the oxides is sometimes cooled more intensively than the clean surface.