Coating Effect Of Droplet Impacting On Low Temperature Spherical Particles

With the continuous progress of society,the quantity demanded of people for the drug powder and extract of high protein is gradually increased.In order to meet the growing trend and obtain high quality powder products,spray freezing technology as a new drying technology has been applied in practical engineering.Spray freeze drying(SFD)provides a technological process for processing dispersed powders.It is of great significance to the preparation of powder materials such as high added value drugs,food and functional materials.However,there are some problems such as poor operability,low heat and mass transfer efficiency,equipment deposition and poor product quality during the process of spray freeze drying.Considering the above issues,this article proposes a new method of spray freeze drying to coat the atomized droplets on the surface of frozen fluidized carrier particles and form the powder of final product after the process of coating,freezing,drying,and peeling.The feed liquid of food and extract of medicine can be further dried into powder finished products without secondary operation.Moreover,the drying technique has the advantages of simple structure,convenient operation and integration of equipment.The dynamic behavior of a single droplet impinging on a cold spherical surface is investigated in the freezing process of the drying method described above.In order to study the freezing behavior of a single droplet impinging on a cold spherical surface,the visualization of the dynamic behavior of a physicochemical droplet impinging on a cryogenic spherical surface has been carried out.The dynamic behavior of atomized droplets with different diameters impinging on spheres with different diameters at different temperatures was photographed.The dynamic characteristics of droplets impacting on the surface of metal balls were obtained.Firstly,visualization experiment was carried out to study the atomized droplet impact metal ball surface with different diameters and different temperatures.Moreover,the effects of different impact speed and different undercooling on the droplet impact process were investigated.We studied the droplet impinging effects on the surface of metal ball with the diameter for 1.5mm,3mm,5mm in the freezing process.Finally,the influence of different droplet diameters on the dynamic behavior of the impinging process was studied.When the ambient temperature is-20 ? and-30 ?,droplets spread after the rapid retraction into the tower shape and then slowly spread until stable status while droplets impact the surface of low-temperature metal ball.Compared with the normal temperature,the droplet morphology did not change significantly without obvious oscillation process.Through theoretical analysis on the droplet surface temperature in tension and viscosity influence law,this paper deduces that a layer of ice film was formed at the junction of the liquid film in the case of the maximum spreading diameter in the droplets,and then the freezing interface gradually moved upwards to the liquid drop freezing.Therefore,there is no migration at the bottom of the liquid droplet during the liquid film retraction stage.The results show that the larger the impact velocity,the larger the initial kinetic energy of the droplet and the larger the maximum spreading diameter are.With the increase in the diameter of liquid carrier particles,the maximum spreading length increases.The film thickness is decreased with the increase of carrier particle diameter.Furthermore,the spreading length and the thickness of the liquid film increase with the increase of the droplet diameter.