| Among various drying technologies,spray drying is widely used in chemical,pharmaceutical,food and other industrial fields by virtue of its fast drying rate and good quality control.It seems that the process of spray drying is quite simple,but the flow inside the drying tower is very complicated.The flow is multiphase turbulent flow,and there is mass and heat transfer between the drying medium and the droplet.It takes a lot of time and financial resources to measure the micro-physical quantity in the tower in the experimental research,and it is difficult to accurately observe the change of particle trajectory during the drying process that people are concerned about.CFD technology is introduced into the study of spray drying process,so that people can have a more intuitive analysis of the multiphysics of the flow and movement and drying process of the droplet.Therefore,the aim of this paper is to deeply study the flow characteristics in spray drying tower and further explore the influencing factors of flow with ZLG-5 spray dryer,by combining the theoretical analysis and experimental research with numerical simulation analysis.In this paper,the characteristics of the flow in the spray drying tower were investigated by experiments and numerical simulation.The potassium sorbate solution was selected as the experimental material for spray drying.In the experiment,after selecting the appropriate experimental operating conditions,the temperature of the characteristic points in the tower was measured,and then the temperature distribution trend in the tower was analyzed.It can be obtained that the temperature gradient on the central axis of the drying tower is large.The cooler region appears in the area around the atomization disk where the material liquid initially enteres the flow field.The temperature in most areas of the tower is close to the exhaust temperature.The moisture content of the product obtained by spray drying is 0.85%,which achieves a better drying effect and meets the storage requirements.In the numerical simulation,according to the drying experiment conditions,ICEM CFD was applied to construct the physical model of spray drying tower and divide the grid.Based on the basic understanding of the characteristics of the flow field in the drying tower,the models of droplet movement,heat transfer and mass transfer in the drying process were established.The discrete phase model(DPM)was used to describe the droplet/particle stress and movement.The continuous phase is described by RNG k-? turbulence model.The numerical simulation of three-dimensional rotary spray drying process was realized by CFD.The temperature of the predicted characteristic points was in good agreement with the experimental results.The maximum absolute error is 2.76%,which verifies the accuracy and reliability of the simulation results.Further analysis of the water vapor content in the drying tower shows that the distribution of water vapor presents an opposite trend to the temperature.The hot air enters the drying tower from the annular air inlet,and then swirls downward in the vertical direction of the axis.At the same time,it can also be observed that there are large whirlpools in the connection between the cylinder and the cone,and there are also some small whirlpools in other areas.The droplets move randomly in the tower.Analyzing the droplet drying process,we can get the independence of each droplet drying process.After absorbing the heat provided by the hot air,the droplet evaporates the internal water,and then the droplet diameter is reduced to a fixed value.The diameter of atomized droplets is not uniform.Larger droplets require longer drying time.Finally,the influencing factors of spray drying process were discussed.The simulation predicted the influence of different inlet air temperature,inlet air angle and atomizing disk speed on the spray drying process,which is beneficial to better understand the spray drying process. |
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