Crystallization Mechanism And Process Control Of Supercooled Water Under Ultrasonic Wave

Ice slurry cold storage is an important part of low-carbon district heating and cooling system with tenacity.Now,the subcooling process of ice slurry production has two major problems:"ice blockage"and"insufficient utilization of subcooling",which need to be solved.To solve the above problems,theoretical analysis and numerical simulation are used to study the mechanism and process control of subcooled water crystallization under the action of ultrasonic waves in the paper,and optimal matching relationships between the main design parameters of super-cooler,crystal promotion tank and ice slurry transport pipeline are analyzed.Firstly,the classical nucleation theory is used to elucidate the crystal suppression mechanism of the subcooled water,and a model of the super-cooler is established according to the solidification melting theory,and the influence of the Reynolds number,evaporation temperature and subcooled water subcooling degree in the heat exchanger tube on the liquid phase volume fraction of the super-cooler were analyzed.The results show that when the evaporation temperature and Reynolds number are constant,the larger the subcooling degree of subcooled water,the smaller the volume fraction of liquid phase.When the subcooling degree and Reynolds number are constant,the lower the evaporation temperature,the smaller the volume fraction of liquid phase.When subcooled water is under the condition of laminar and transitional flow state,the larger the Reynolds number would result in smaller liquid phase volume fraction.When it is in vigorous turbulent flow state,the liquid phase volume fraction firstly rises,and then drops as the Reynolds number increases.Besides,there is a correspondence between the location of severe fluctuation of turbulent kinetic energy and the location of icing.Thus,there are optimal matching relationships between the average life of the super-cooler and the evaporation temperature,subcooling degree and Reynolds number threshold.For safely operating of the super-cooler,the subcooling degree should be within the range of 2°C-3°C,and the evaporation temperature should be within range of-4°C-6°C.When the subcooling degree at 2°C and evaporation temperature at-6°C and the Reynolds number within range of 9000-12000,the super-cooler can operate safely about 14380s.Secondly,the molecular kinetic energy nucleation theory was used to elucidate the crystallization mechanism of subcooled water under the condition of ultrasound.According to the theories of ultrasound,nucleation rate and particle dynamics,the multiphase flow mixture model of the crystal promotion tank was established,and the effects of ultrasound parameters,subcooled water flow rate,height-diameter ratio of the tank on the ice content of ice slurry were analyzed.The results show that the critical ice core size firstly increases,then stays constant,and finally decreases with the increase of ultrasonic parameters.The ice content firstly increases,and then decreases with the increase of subcooled water flow rate,height-diameter ratio of the crystal promotion tank.The maximum ice content matches the ultrasonic parameters,subcooled water flow rate,height-diameter ratio of the crystal promotion tank.Based on the ability of the crystal promoter to produce ice slurry with high ice content,the ultrasonic intensity should be within range of 40W-50W,the ultrasonic frequency should be within range of 30k Hz-35k Hz,the height-to-diameter ratio of the crystal promotion tank should be within range of 3-4.5.Under the condition of subcooling degree at 3℃,the subcooled water inlet flow at 3.5m~3/h,and the height-to-diameter ratio of the crystal promotion tank at 4,the maximum ice content rate of the ice slurry in the crystal promotion tank can be close to 3.8%.Finally,the change of flow state during ice slurry transport is elucidated by using the particle dynamics theory,and the model of ice slurry transport is established according to effects of grain aggregation and fragmentation.The results show that the maximum local ice content firstly decreases,then remains constant,and finally increases with the increase of flow velocity.The maximum local ice content become large with the increase of both ice content and initial ice particle size.Besides,there is a correspondence between the location with a larger percentage of large particles and the location with a larger ice content.The ice particles of the suspended flow type are distributed uniformly,and the percentage of particles is more stable,which contributes to transporting ice slurry.It was found that the suspended flow pattern is formed when the flow velocity is in the range of 1.0m/s to 2.0m/s.In order to easily control the flow pattern of the slurry by using flow velocity,the ice particle size should be shorter than 0.2mm and the initial ice content rate should not be higher than 8%.The development of design guidelines for the dynamic ice storage systems by using subcooling method contributes to solving the problem of dual-side uncertainty of the district heating and cooling system during cooling period.