SURFACE CHARACTERIZATION OF NITROGEN ION IMPLANTED TITANIUM AND ITS ELECTROCHEMICALLY FORMED PASSIVE FILM IN SIMULATED BODY FLUID CONDITION

Melting of aluminium spheres within the up-welling, gas-liquid, two phase plume of gas stirred aluminium bath has been analysed theoretically. Heat transfer coefficients between the submerged solid and the molten metal in such system were deduced from a simple thermal energy balance viz., q " Ait m=ma delta H, incorporating directly the experimentally measured complete melting times of aluminium spheres. Such estimates of heat transfer coefficients were subsequently compared with the corresponding predictions derived from the correlation viz., Nu D-2=(0.4Re D 0.5+ 0.06Re D 0.66) Pr0.4(mo blk/mo)0.25. It was found that experimental heat transfer coefficients within the two phase plume region of the gas stirred system can be described adequently via the above correlation up to about 40(degree)C of melt super heat. For larger superheats ( e.g.,65°C), significant differences between predicted and experimental heat transfer coefficients were, however, observed. The phenomena was investigated embodying a time averaged, instead of the initial sphere diameter. It was found that regardless of such considerations, experimental heat transfer coefficients at larger melt superheats tended to be smaller than their corresponding theoretical estimates. To rationalise the discrepancy between theory and experiments at relatively large melt superheat, an order of magnitude analysis of the phenomena involved during melting of solids in metallic baths has been carried out. This suggests that heat transfer coefficients between solid and fluid in metallic melts, in much contrast to their aqueous counterpart, is dependent on the melt super heat, increasing super heat leading to smaller heat transfer coefficients and vice-versa. The present work appears to indicate that usual heat transfer correlations are not applicable to molten metal systems, particularly at high melt super heat.