MODELING AND NUMERICAL ANALYSIS OF SINGLE DROPLET DRYING DALMAZ, Nesip M.Sc., Department of Chemical Engineering Supervisor: Prof. Dr. H. &Ouml / nder &Ouml / ZBELGE Co-Supervisor: Asst. Prof. Dr. Yusuf ULUDAg August 2005, 120 pages A new single droplet drying model is developed that can be used as a part of computational modeling of a typical spray drier. It is aimed to describe the drying behavior of a single droplet both in constant and falling rate periods using receding evaporation front approach coupled with the utilization of heat and mass transfer equations. A special attention is addressed to develop two different numerical solution methods, namely the Variable Grid Network (VGN) algorithm for constant rate period and the Variable Time Step (VTS) algorithm for falling rate period, with the requirement of moving boundary analysis. For the assessment of the validity of the model, experimental weight and temperature histories of colloidal silica (SiO2), skimmed milk and sodium sulfate decahydrate (Na2SO4&amp / #8901 / 10H2O) droplets are compared with the model predictions. Further, proper choices of the numerical parameters are sought in order to have successful iteration loops. The model successfully estimated the weight and temperature histories of colloidal silica, dried at air temperatures of 101oC and 178oC, and skimmed milk, dried at air temperatures of 50oC and 90oC, droplets. However, the model failed to predict both the weight and the temperature histories of Na2SO4&amp / #8901 / 10H2O droplets dried at air temperatures of 90oC and 110oC. Using the vapor pressure expression of pure water, which neglects the non-idealities introduced by solid-liquid interactions, in model calculations is addressed to be the main reason of the model resulting poor estimations. However, the developed model gives the flexibility to use a proper vapor pressure expression without much effort for estimation of the drying history of droplets having highly soluble solids with strong solid-liquid interactions. Initial droplet diameters, which were calculated based on the estimations of the critical droplet weights, were predicted in the range of 1.5-2.0 mm, which are in good agreement with the experimental measurements. It is concluded that the study has resulted a new reliable drying model that can be used to predict the drying histories of different materials.

<p>Explosive boiling is a phenomenon encountered in severe nuclear reactor accidents during quench cooling, core relocation or through fuel-coolant interactions. The mitigation of accident conditions is important from a safety standpoint since explosive boiling is potentially capable of destructive forces. Explosive boiling occurs when coolant water encounters a hot solid surface and absorbs a high degree of superheat. The resultant boiling mode is violent and features the rapid decomposition of liquid on a microsecond timescale with liquid atomization and ejection. In this study, the explosive boiling force of a single water droplet impacting hot solid surfaces was estimated with secondary droplet analyses using high-speed imaging.</p> <p>A water droplet at 25°C with a Weber number of 432 impacted perpendicular to solid surfaces at temperatures from 30-700°C. Solid surfaces of copper, brass and stainless steel varied in thermal diffusivity from 3.48 x10^-6to 1.17 x10^-4m²/s. Curved and flat impact surfaces were also tested. Explosive boiling was most prominent when the instantaneous interface temperature attained the superheat limit temperature (300°C ±17°C). Maximum boiling force was encountered at the superheat limit with reduced force at surface temperatures in the nucleate boiling regime and near zero force in the film boiling regime. Thermal disintegration dominates over inertial break up of the droplet near the superheat limit region. Thermal diffusivity effects were only distinguishable in the 250-450°C region where increasing thermal diffusivity translated to larger boiling forces. Secondary droplet counts, size, trajectories were dependent on the boiling mode present at the interface with very strong variances caused by thermal break up of the initial droplet. Explosive boiling caused greater fragmentation creating more secondary droplets with smaller sizes and larger ejection trajectories. A curved surface showed slightly higher explosive boiling force in the superheat limit region but with negligible effects on secondary droplet properties.</p> / Master of Applied Science (MASc)

Le séchage par atomisation est un procédé relativement bien maîtrisé, certains aspects de la transition goutte-particule n’étant pas encore totalement compris. Ainsi, comprendre précisément comment la particule est formée et comment ce phénomène peut être contrôlé reste aujourd’hui un défi majeur. Ce projet visait à découpler la complexité du phénomène de séchage via une approche multi-échelles. La formation de particules à partir de protéines laitières (protéines de lactosérum et micelles de caséines) a été étudiée avec différents systèmes expérimentaux (gouttes suspendue, confinée, mono-dispersées et enfin pulvérisées) dans des environnements de séchage contrôlés (température de séchage: 20°C à 190°C et l'humidité relative: 40% à 2%).Les résultats obtenus montrent que le séchage d'une goutte de protéine comprend trois étapes distinctes, mises en évidence par l’apparition d'événements morphologiques spécifiques (rétrécissement à vitesse établie, flambage, formation de vacuole). Selon le type de protéines, ces étapes diffèrent en termes de cinétique de séchage et de dynamique structurale, conduisant à des formes de grains caractéristiques. Ces différents comportements peuvent être rattachés aux conditions particulières de la formation d’une peau en surface et aux modes de dissipation des contraintes internes par les matériaux protéiques. De manière générale, l’approche multi-échelles de ce travail a permis de mettre en évidence la signature particulière de protéines laitières dans un état concentré et l'impact de la matière dans le processus de séchage. / Spray drying is a well-established process but certain aspects of droplet-particle transition are not yet fully understood, resulting in variability in terms of powder quality and performance. Therefore, understanding precisely how the particle is formed and how it can be controlled still remain a major challenge. This PhD project aims to break down the complexity of the drying phenomenon using an exploratory multi-scale approach. Particle formation of milk proteins (whey proteins and casein micelles) was investigated using different experimental systems (single pendant droplet, confined droplet, mono-dispersed droplets and spraying cone droplets) in controlled drying environments (drying temperature: 20°C to 190°C and relative humidity: 40% to 2%).The results showed that the drying of a single protein droplet included three distinct stages highlighted with the occurrence of specific morphological events (constant rate shrinkage, buckling instability, vacuole nucleation). According to the type of proteins, these drying stages differed in drying kinetics and droplet dynamics, leading to characteristic and reproducible particle shapes whatever the droplet configuration and the drying conditions. These different kinds of drying behaviour were related to specific skin formation conditions and different responses of the protein material to internal stress. Finally, by means of this multi-scale approach, this work highlighted the particular signature of milk proteins in a concentrated state and in general the impact of the matter in the droplet drying process.

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