RADIATION-INDUCED MELTING OF SEMITRANSPARENT MATERIALS (HEAT TRANSFER, NATURAL CONVECTION)

BRENT WOODRUFF WEBB, Purdue University

Abstract

Radiation-induced melting of semitransparent materials has been studied from both theoretical and experimental perspectives. Radiation-induced natural convection in a vertical rectangular cavity in the absence of phase change was considered. Experiments were performed in which a vertical layer of water was exposed to a radiation source simulating the solar spectrum. The temperature distribution and flow structure were compared to predictions of a theoretical model based on solution of the governing transport equations with internal radiative heating. Parametric calculations illustrate the effect of such problem variables as modified Rayleigh number, Prandtl number, fluid layer opacity, opaque wall reflectivity, and cavity aspect ratio. The radiative melting of an unconfined layer of a pure, polycrystalline paraffin (n-octadecane) was considered. Radiation sources approximating emission from blackbodies at 3200 K and 5800 K were used in the study. Crystallographic effects in the solid proved to be very important; multiple internal scattering was shown to be of first order importance in the radiative transfer. A model is outlined which involves solution of the radiative transfer equation with treatment of the internal scattering in the solid. Model predictions show good agreement with experimental data. Finally, radiative melting of a confined layer of semitransparent paraffin (n-octadecane) was investigated. Radiation-induced buoyancy-driven flow in the melt was found to influence significantly the shape and motion of the melting front. The effects of radiative flux incident on the layer, phase change material opacity, initial solid subcooling, and layer aspect ratio were studied. The results reveal that radiative transfer and natural convective flow in the melt are of equal importance in governing the motion of the melting front and overall melting rate. A numerical methodology is presented capable of predicting the absorption of radiation, buoyancy-driven flow in the melt, and advancement of the solid-liquid interface for radiative melting of a semitransparent material. Sample predictions underline the importance of both natural convection and radiation transfer in the melting process.

Degree

Ph.D.

Subject Area

Mechanical engineering

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