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Boiling water heat transfer study during dc casting of aluminum alloys Li, Dianfeng

Abstract

During Direct Chill (DC) casting of aluminium alloys, the majority of the heat (-80%) is extracted in the secondary cooling zone where water contacts the periphery of the solidifying ingot as it is drawn from the mould. The high heat extraction rates during secondary cooling induce thermal gradients and mechanical contraction within the thin newly solidified shell and result in deformation known as "butt curl" and a contraction (pull-in) of the rolling face. Since aluminium has a relatively low melting point, heat extraction to the cooling water is strongly dependent on the temperature of the aluminium sheet ingot surface, and will vary dramatically during cooling as different heat transfer regimes are encountered (i.e. film boiling, transition /nucleate boiling and convection cooling). For the aluminium DC casting process, the heat transfer to the cooling water is particularly complicated as ingot surface temperatures at the initial point of water contact cause transition/film boiling to occur followed by nucleate boiling and convection cooling as the surface temperature of the ingot is cooled. To model the DC casting process accurately, it is necessary to develop accurate boiling curve data (i.e. heat flux Vs surface temperature) for as-cast alurninium as it is being cooled. This study investigated the influence of the ingot surface topography, sample starting temperature and water flow rate on the boiling curve for three commercially significant aluminum alloys namely: AA1050, AA3004 and AA5182. The project involved both experimental measurements (using industrial as-cast aluminium samples and an experimental set-up designed and built at UBC), a 2-D LHCP (inverse heat conduction problem) model to calculate the heat flux on the sample surface as it is was being cooled and measurements at NRC (National Research Council) to quantify the surface roughness for each sample using a laser profilemeter. The experimental test facility was designed and built at UBC and included: a vertical furnace to heat the samples to the desired temperature, a pneumatically operated lowering platen to move the sample out of the furnace and position it in front of the water box and a water box, which was built out of Plexiglas and duplicated a typical section of an aluminium mould used for industrial DC casting. For each test, a sample, instrumented with a number of thermocouples, was put into the furnace and heated to the desired temperature. The sample was then lowered in front of the water box (2~3 mm away from the water box), the water was turned on to the desired flow rate and the sample was cooled. During cooling of the sample the data acquisition system recorded the sample temperature as a function of time at a frequency of 20 Hz. The inverse heat transfer model was developed to calculate the boiling curves for direct water chill cooling using the thermal response of the ingot and the application of a 2D finite element based heat conduction model to iteratively calculate the heat flux at the surface of the sample. This technique was verified using both analytical solutions and hypothetical data obtained using a known heat flux profile in the commercial FEM code ABAQUS. The results from the study indicate that a variation in alloy surface morphology (machined versus as-cast), water flow rate and sample initial temperature all dramatically influence the calculated boiling curve. The intensity of the heat extraction was found to be enhanced as the surface of the sample became rougher because nucleation and growth of bubbles became easier thereby enhancing the heat transfer. Sample starting temperature also had a significant influence on the calculated boiling curve and it was found that a unique boiling curve for a given surface temperature did not exist for each of the alloys studied. A few tests were also run whereby the sample was moved slowly down into the water spray. Comparison of the experimentally calculated heat fluxes using the test rig at UBC to those calculated in industry by freezing thermocouples into solidifying ingots were similar in shape and it was found that the calculated boiling curves from the industrial data fell in between the boiling curve calculated using a stationary sample and the boiling curve calculated using a moving sample. Indicating that the boiling curves calculated using the UBC rig reflect the heat transfer phenomena occurring in industry during water spray cooling.

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