上方冷却による2成分系水溶液の凝固プロセス(熱工学,内燃機関,動力など)

概要

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Freezing and melting phenomena are inportant in many different fields, including crystal growth, casting, metallurgy, geophysics and oceanography. Solidification from multi-component solution is the one often observed in nature. In order to investigate basic features of freezing processes of binary system, we conducted a series of laboratory experiments in a rectangular box cooled from above using aqueous NaNO_3 solution. During the freezing, the solid phase always grows into many needle-like crystals called mushy layer. We measured growth of the mushy layer thickness, the solid fraction, temperature and concentration distributions. The average solid fraction is found to increase in the mushy layer with time. This is probably due to a slow descent of the released solute in the mushy layer and an eventual fall into the liquid region below with gravitational force. We propose a one-dimensional model to explain the horizontally-averaged mushy layer growth. In the model, the estimate of a heat flux at the mushy-liquid interface due to natural convection is found essential for a correct prediction. The proposed theory predicts well the mushy-layer growth and the average solid fraction, once the convective heat flux is properly given.
2009-05-25