Effect of oxide layers on spray water cooling heat transfer at high surface temperatures

The influence of surface oxidation phenomena on spray water cooling heat transfer is an important issue in steel industry. In practical applications, spray cooling is regularly used for the cooling of steel surfaces from high temperatures with an (initial) oxide (scale) layer. This paper investigates the changes in the heat transfer due to the oxide layer and its removal during cooling. After a theoretical treatment of the principal effects of thin and stable layers, the heat transfer coefficient was measured by an automated cooling test stand (instationary method) for oxidized steel samples. The heat transfer is described by the concept of an effective heat transfer coefficient. Compared to the clean surface state, scale layers show a different effective heat transfer coefficient in the transition boiling regime. Additionally, the local delamination of the oxide, the formation of vapor gaps at the oxide–metal interface and the movement of scale plates makes the heat transfer more erratic. The heat transfer is therefore inhomogeneous and has to be described statistically. For water temperatures around 291 K, surface temperatures between 473 and 1173 K, i.e. ΔT>180K and water impact densities between VS=3VS=3 and 30 kg/(m2 s) the heat transfer coefficient αα was measured. As sample material, different steel grades, oxidized in air for a specific time (0, … , 80 μm scale layer) were used. The results are compared with the clean surface state.