A TWO-ZONE THERMAL MODEL FOR PREDICTING THE EFFECTS OF HEAT TRANSFER ON THE VARIATION OF ICE CRYSTAL SIZE DISTRIBUTION IN ICE SLURRIES

摘要

A two-zone thermal model has been developed to predict the effects of heat transfer on the size distribution of ice crystals in ice slurries under stagnant conditions. The model extends the homogeneous model recently developed by taking into account the stratification of ice crystals due to gravity. Under adiabatic conditions, the crystal sizes slowly grow due to Oswald ripening, but when heat is externally applied and the ice crystals melt and diminish in size, smaller crystals tend to float to the top of the cell and separated zones of clear liquid and an ice slurry mixture emerge. The temperatures in the two zones diverge as the temperature of the liquid zone rises while that of the ice slurry zone remains constant due to phase change. Measurements of ice crystal size distributions were obtained using direct video observation of the ice slurry crystal behaviour in a stagnant flow cell. For the heating cases, the time to melt all the crystals was sufficiently short (＜300 s) such that the effects of Ostwald ripening could be neglected. The model predictions and experimental results for 5-10 %-wt ethanol solutions with initial ice fractions ranging from 2 to 6 % agreed reasonable well suggesting that the stratification effect was adequately considered in the new model. The two-zone model can be applied to the prediction of ice crystal size distributions in storage tanks where stratification, heat gain from the environment and Ostwald ripening can simultaneously occur.