Influence of iron oxide layer on intensification of heat dissipation from the surface of a horizontal plate cooled with a water nozzle

Abstract

The paper presents the changes in the heat transfer coefficient (HTC) and heat flux (HF) during the water spray cooling of the metal plate on the surface of which the thickness of the oxidized layer changed. The experimental investigations consisted in measuring the temperature changes inside the plate during spray cooling with water from 700 °C to the ambient temperature. The plate was made of the material (Armco iron), the surface of which had been oxidized during heating. Three measurements were made for three different thickness of the oxidized layer. The size of the area related with the directed impact of the spray water on the surface (high impingement zone) and the size of the secondary water flow zone (horizontal flow zone) that was placed beyond the high impingement zone were assessed. The distribution of the water flux density over the cooled surface was determined as well as the increase in the thickness of the oxidized layer with the heating time. The experimental investigations allowed identify HTC and HF by using the inverse method. The uncertainty tests that allowed determine the accuracy of the inverse solution were performed. The obtained results shows that the development of the boiling process on the surface covered with the oxidized layer is local. In the high impingement zone, the increase in the thickness of the oxidized layer from 0.018 to 0.035 mm significantly influences the changes of HF and HTC during each boiling phase. The increase in the thickness of the oxidized layer caused a shift of the maximum HF and the beginning of the transition boiling towards a higher surface temperature. Outside the high impingement zone the influence of the increase in the thickness of the oxidized layer is small during film boiling phase. The obtained results were compared with those obtained for the surface characterized by a low oxidation state, obtaining 35% lower maximum values of HF in relation to the oxidized surface.