UBC Theses and Dissertations
Heat transfer model of the hot rolling runout table-cooling and coil cooling of steel Hernandez-Avila, Victor Hugo
The controlled cooling of steel strips is required to attain the high quality standards of flat-rolled steels employed in important industries such as the automobile, petro-chemical, house-appliances and construction. Strict control of the temperature of the strip during cooling in the hot-strip runout table is necessary, but little success has been reached in the optimization of the heat removal since no real understanding of the physical mechanisms involved has been attained. Given that the experimental measurements of the local heat-transfer coefficients may involve very complex procedures, the modeling of the boiling mechanisms is presented as the best way to obtain the local thermal response of steel strips during their processing, and mathematical models for the runout table and subsequent coil cooling are presented as powerful tools to predict the thermal and the microstructural response of the steel. The runout table model is unique in the sense that it is mechanistic in nature and predicts the local heat-transfer coefficients during cooling. The model adopts the extrapolation of the "macrolayer evaporation mechanism" into the forced-flow transition boiling regime. The analysis in terms of the nucleation process, fluid flow, liquid-solid contact area, and the liquid-vapor interface instability allow succesful prediction of pilotplant and full-scale operations and of the most fundamental microscopic parameters measured elsewhere. The liquid-solid contact found in the transition boiling regime is responsible for most of the heat released, and explains why previous assumptions with regard to film boiling failed to account for the effect of variables such as water temperature or strip velocity on the cooling process. This study shows that bottom jet cooling is much lower than top cooling not only because of the smaller contact but also because of the inherently lower stability of the liquid-vapor interface of the latter.
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