Simulation Study On Heat And Mass Transfer Characteristic Of Droplet Vacuum Freezing Process

Since the industrial revolution,a variety of industries in society have achieved rapid development.However,some crises have hidden in the phenomenon of rapid development,namely the new crisis of human existence,such as environmental pollution,ozone depletion,global warming and other global issues.The CFCs is the most important destruction of the ozone layer,therefore the refrigeration with non-CFC attracts more and more attention of the people.Refrigeration has the important position in the industry,and the refrigerant plays a key role in the process of the refrigeration.If using water,which is green environmental protection material,as refrigerant combined with the corresponding refrigeration mode,we believe that it will possess a wide application prospect.This paper chooses the liquid droplet as the research object,meanwhile,makes a detailed and in-depth study of some attributes of the liquid droplet such as freezing temperature under the condition of vacuum,mass transfer rate,diameter and so on.Firstly,the paper made an introduction of seven different kinds of strengthen freezing mechanisms including vacuum spray freezing,high voltage conversion freezing,ultrasonic assisted freezing,radio frequency auxiliary freezing,active freezing and made the analysis and discuss the advantages and disadvantages respectively.And then,based on the diffusion control mechanism we established the corresponding stages mathematical model in the process of the droplet freezing under vacuum condition.Compared with the mathematical model in the references,there are some innovations of the new model:In the new model,the analysis of transient phase transformation and constant temperature stage during the freezing process is added,and the freezing process is expressed with three stages.By using MATLAB software to simulate the established mathematical model,the computing results of the new model in accord with the three stages of the temperature change in the actual freezing process.The influence of vacuum chamber pressure,droplet diameter and initial temperature on the characteristics of heat and mass transfer in freezing process is analyzed.What's more,the heat and mass transfer model of the vacuum freezing of droplet was established in COMSOL,by using dynamic mesh method to follow the tracks of droplet boundary.The numerical simulation reveals the change law of the relevant parameters in the freezing process of the droplet.The correctness and rationality of the model are verified by the comparison between the experimental data and the calculated results.Furthermore,finite cell division makes model developed in COMSOL not can express quality and temperature variation on the surface of droplet but also can reflect the droplet freezing process of the internal parts on the heat and mass transfer characteristics.