Numerical Simulation Of The Mono-disperse Droplet Spray Dryer

Mono-disperse droplet spray drying(MDSD)has great advantages in the production of uniform size droplets.In a mono-disperse spray dryer(MDSD),droplets must pass through the pre-dispersion chamber before entering the main dryer where rigorous drying takes place.On the other hand,tuning the operational settings can also greatly affect the final particles' moisture content.Experimental limitations in observing the dispersion dynamics has recently led to the use of computational fluid dynamics(CFD)simulations.In most recent studies,effects of droplet injections arrangement on drying dynamics were considered.However,coupling of the pre-dispersion and the main drying chambers was omitted.In this work,a systematic method is introduced to tackle this problem.This method allows simulation of different sized chambers separately to avoid the meshing difficulty.A unique coupling approach offers smooth transition of droplets from one chamber to the other with rigorous conservation of their mass,energy and momentum.The resulting.model has been verified against experimental final moisture contents from running a mono-disperse spray dryer.The coupled method is promising,and can be used to further optimize and design spray drying process.In order to improve the mass transfer and heat transfer efficiency and optimize the operation in the drying process,by resorting to the discrete phase model(DPM)and the reaction engineering approach(REA)based drying model,a 3D CFD model is developed in this work to describe a complete MDSD consisting of a small droplet pre-dispersion chamber and a big drying chamber.This model allows us to investigate the influence of introducing swirling flows in the pre-dispersion chamber and the drying chamber on particle trajectory and drying dynamics.It is shown that the introduction of swirling flow in the pre-dispersion chamber can lead to a wider dispersion of the droplets,and the dispersion states depends on the inlet angle of the dispersion gas.Introducing the swirling flow in the main drying tower and decreasing the hot air inlet angle can lead to enhanced mass and heat transfer.Furthermore,the co-current scheme is superior to the counter-current scheme when both pre-dispersion chamber and drying chamber have swirling flows.Based on this model,this thesis further explored the effect of different atomizer positions on droplet dispersion,and discussed the possible better way to arrange the atomizer position.