Evaluation of heat flux extracted from steel surface with oxide scale utilizing evaporation rate of water droplet

The present work has proposed a new method to evaluate heat flux extracted from steel surface with iron oxide scale using the mass change of a water droplet and the standard enthalpy of evaporation for water at 100°C. The apparatus was composed of a furnace, an electronic balance, a water droplet supplying device, a combination of a thermocouple and a digital multi-meter, in addition to a video camera. Ultra-low carbon steel plate samples were oxidised in air at 850°C in the furnace so as to have iron oxide scale (mainly FeO) with desired thicknesses, and then moved onto the electronic balance. About 3 g of water was supplied onto the sample, and the mass change of water and temperature inside the sample were measured by the balance and the thermocouple, respectively. In addition, boiling behaviour of water was also recorded by the video camera. The mass and temperature changes well corresponded to the video images which recorded film boiling and nucleate boiling of water. The quench temperatures obtained in experiments using the samples with scales 40, 58, 77 and 103 μm thick were 131, 167, 121 and 182°C, respectively, and the respective heat fluxes just before quench were estimated to be 31, 35, 33 and 43 kWm^-2. Assuming that the present system is at a quasi-steady state, Fourier's law has been applied to estimate the thickness of water vapour film during film boiling; resultantly, the thickness decreases from ca 100 μm to ca 20 μm with cooling of steel.