Research On The Mechanism Of Dynamic Ice Slurry Production By Vacuum Method

The use of ice slurry as a secondary refrigerant has drawn considerable interest because of the high energy storage density associated with the latent heat of phase change.Ice slurry also has a fast cooling rate because of the large surface area available for heat transfer created by its numerous particles.Presently,the situations of energy shortage and imbalance between electricity supply and demand are becoming increasingly grim.Therefore,the development of ice slurry as the carrier of cold medium cold storage technology is of great significance.Vacuum method,which uses an aqueous solution as a refrigerant and relies upon direct contact heat transfer,has received considerable attention in recent years.However,the mechanism of the formation of the ice crystals has not been accurately understood,which is the main obstacle preventing the development and application of vacuum method.Based on this background,this paper uses aqueous solutions of ethylene glycol and sodium chloride as ice-production solutions,and the mechanisms of ice slurry production by vacuum method are researched systematically.The research results have certain academic significance and reference value in engineering.The vapor-liquid-solid equilibrium of nonvolatile solute was analyzed theoretically and the vapor-liquid-solid equilibrium parameters(the balancing pressure p and temperature t)of the ethylene glycol aqueous solution and the sodium chloride aqueous solution were measured.It was found that the phase equilibrium parameter can be described by the vapor-ice equilibrium equation of ice,and the results were consistent with theoretical analysis.The effects of ethylene glycol additive,solution volume and stirring speed on the supercooling degree of ice slurry preparation were investigated experimentally.The results indicated that the higher the concentration of ethylene glycol or sodium ethylene additive,the smaller the average supercooling crystallization pressure difference.Furthermore,the increase of cooling rate could increase the average differential pressure.The stirring speed had little influence on supercooling crystallization pressure difference.In the aspect of dynamic research,batch dynamic method was used to research the drowning-out crystallization kinetics behavior of ice particle making.Based on the moment transformation method,the population balance equations were solved,and the nucleation and crystal growth rate equations were determined.The influences of particle suspension density,stirring speed and supercooling degree on ice crystal nucleation and growth process were studied.The total crystallization rate equations were developed based on both the Jeziorny and Mo equations.The relationship between IPF and time,the cooling rate was revealed from a macro perspective.Furthermore,it was determined that the higher the concentration of ethylene glycol or sodium ethylene additive,the smaller the IPF,which showed that the additives could inhibit the formation of ice crystals to a certain extent.To understand the average particle size and size distribution,images of ice particles in ice slurry were obtained using a microscopic observation system.Experimental particle size distributions were also compared to normal,log-normal,Gamma and Weibull distributions.The results indicated that size distributions of ice particles under different experimental conditions were in accordance with Gamma distribution.The results indicated that the higher the concentration of ethylene glycol and chloride sodium,the smaller the average particle size.Furthermore,the average particle size increased with the increase of ice packing fraction and decreased with the increase of stirring speed slightly.Silica nanofluids of different concentrations were prepared by dispersing silica nanoparticles in either aqueous ethylene glycol solution or aqueous sodium chloride solution as the base fluid.As a result,different samples of ice slurries were prepared.The thermodynamic and kinetic characteristics of the ice slurry generation process and the ice crystal morphology were researched.The results showed that nano-SiO2,by acting as a kind of typical nucleating agent,could effectively reduce the crystallization degree of supercooling,could promote the formation of ice crystals,and could increase the nucleation rate,growth rate and total crystallization rate.Additionally,nano-Si02 could also cause thinning of the ice particles.Based on the Material Studio software,the mechanism of inhibing ice particle formation of ethylene glycol and sodium chloride was studied.The hydrogen bonding characteristics and diffusion coefficient of the ethylene glycol solution system and the sodium chloride solution system were analyzed.Under a vacuum environment,ethylene glycol and sodium chloride could destroy the hydrogen bonding between water molecules,which inhibited the formation of ice crystals.The diffusion coefficient of water molecules decreased with increasing ethylene glycol and sodium chloride concentration.Moreover,the movement of the crystal nucleus was blocked,which to a certain extent,can inhibit the nucleation and the growth of crystals.