Influence of surface morphology, water flow rate, and sample thermal history on the boiling-water heat transfer during direct-chill casting of commercial aluminum alloys

An experimental investigation has been conducted on as-cast samples from th ree commercially significant aluminum alloys (AA1050, AA3004, and AA5182) t o quantify the influence of surface morphology, water flow rate, and sample thermal history on the boiling-water heat transfer under conditions simila r to those experienced in the direct-chill (DC) casting process. The study involved characterization of the as-cast surface morphology using a laser p rofilometer and quantification of the sample surface temperature and heat e xtraction to the cooling water using a DC casting simulator in combination with an inverse heat-conduction (IHC) analysis. The results from the study indicate that alloy's thermal conductivity, surface morphology, and sample initial temperature all dramatically influence the calculated "boiling curv e." The intensity of the heat extraction was found to be enhanced at high h eat fluxes in the nucleate boiling regime as the thermal conductivity was i ncreased and was also found to increase as the surface of the sample became rougher, presumably through promotion of nucleation, growth, and/or detach ment of bubbles. The heat transfer was also found to increase with increasi ng sample starting temperature, resulting in a series of boiling curves dep endent on initial sample temperature. Finally, the effect of the water flow rate on heat transfer was found to be comparatively moderate and was limit ed to the sample with the smooth (machined) surface.