The Study On The Mechanism Of Enhancement Of Solution Crystallization By Power Ultrasound

Ultrasound-assisted freezing is an excellent method for long preservation of someliquid and solid food products. With the enhancement by ultrasound during freezing,nucleation rate of crystal can be enhanced and crystal size can be reduced. Althoughultrasound has long been used to influence crystallisation of solution, the exactmechanism that explains this influence is still not clearly understood. In order to clarifyeffects of ultrasonic waves on supercoold water, the following studies were carried out:The mechanism of nucleation of ice induced by ultrasound was studied. Theultrasonic waves have been applied to supercooled pure water and degassed water,respectively. For each experiment, water sample is cooled at a constant cooling rate of0.15°C/min and the ultrasonic waves are applied from the water temperature of0°Cuntil the water in a sample vessel nucleates. This nucleation temperature is measured.The use of ultrasound increased the nucleation temperature of both degassed water andpure water. However, the undercooling temperature for pure water to nucleate is lessthan that of degassed water. It is concluded that cavitation and fluctuations of density,energy and temperature induced by ultrasound are factors that affect the nucleation ofwater. Cavitation is a major factor for sonocrystallisation of ice.The mechanism of secondary nucleation of ice induced by ultrasoundwasinvestigated. Experiments on ice crystals in degassed sucrose solutions and untreatedsolutions have been carried out using a novel ultrasonic cold stage device. For eachexperiment, the ultrasonic waves that are applied operated at a power output of120W.The results show that the ice dendrite crystals growing in a sucrose solution can bebroken up into smaller fragments when ultrasound was applied for2s, but thepre-existing ice dendrite crystals in a degassed sucrose solution are still unbroken whenultrasound was applied for6s. These findings confirm that cavitation induced byultrasound is a major factor for the fragmentation of ice crystals.The mechanism of ultrasound inhibition of dendritic ice growth was studied. Theeffect of ultrasonic waves on the growth of dendritic ice in degassed sucrose solutionsand untreated solutions was investigated using a novel ultrasonic cold stage device. Theresults show that the use of ultrasound could inhibit the growth of dendritic ice in bothuntreated and degassed solutions, but the growth velocity of dendritic ice in untreated sucrose solutions is much less than that in degassed solutions. It is concluded thatcavitation and heat effect induced by ultrasound are factors that affect the the growthprocess of ice crystal. But cavitation is a major factor for the inhibition of dendritic icegrowth.The mechanism of ultrasound reduction of crystal size was investigated. The effectsof ultrasonic waves on crystallization process of saturated ammonium chloride solutionand degassed solution are investigated by using a novel ultrasonic cold stage device.The results show that the use of ultrasound could accelerate the crystallization processof both ammonium chloride solution and degassed solution, but the cooling rate andcloudiness time of ammonium chloride solution are much less than those of degassedsolution. Furthermore, sonocrystallisation of ammonium chloride solution can result insmaller crystals than those induced by sonocrystallisation of degassed solution. It isconcluded that cavitation and fluctuations of density, energy and temperature inducedby ultrasound are factors that affect the crystallization process of saturated solutions.But cavitation is a major factor for the enhancement of saturated solution crystallization.Immersion freezing of potatoes with the aid of ultrasound was studied. The effect ofultrasonic waves on freezing rate of potatoes in degassed coolant and untreated coolantwas investigated. The results show that cavitation and vibrational motion of the coolantmolecules induced by ultrasound are factors that affect the heat transfer between thepotato samples being frozen and the refrigerating medium. But cavitation is a majorfactor for the enhancement of the heat transfer.The Mechanism of enhancement of solution crystallization by ultrasound putforward in this thesis could be valuable to optimization of sonocrystallisation.