- Title
- Vaporisation of single and binary component droplets in heated flowing gas stream and on solid sphere
- Creator
- Nguyen, Thi Bang Tuyen
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2018
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Droplet vaporisation is significant to a number of multiphase process engineering applications which include but not limited to Fluid Catalytic Cracking (FCC) process for producing transport fuel; fluid coking for producing fuel gas, distillate and petroleum coke; spray coating of tablets in pharmaceutical industry; drying of seeds in spouted bed in food industry and spray drying of milk in dairy industry. The principal aim of this study was to improve the physical understanding the droplet vaporisation in a multiphase environment due to both convective (homogeneous vaporisation) and conductive heat transfer (heterogeneous vaporisation) with the aid of experimental measurement and numerical modelling. The principal aim was met first by numerically quantifying the feed droplet vaporisation time in a typical multiphase application (FCC riser) including both homogeneous and heterogeneous modes; and then separately investigating these two modes by experimentally quantifying the vaporisation behaviour of a suspended droplet in a hot convective flow and on a heated spherical particle, respectively. A comprehensive quantitative comparison of the existing models (both homogeneous and heterogeneous) was conducted to predict FCC feed droplet vaporisation time under typical industrial operating conditions. Noting a dearth of suitable physical model that accounts for the conductive heat transfer between feed droplets and catalyst particles, a new vaporisation model based on the particle-droplet collision mechanism was proposed which provided a reasonable agreement with the available heterogeneous models. It was noted that all homogeneous models predicted a larger droplet vaporisation time compared to the heterogeneous models which could be attributed to the large difference in the Nusselt number in these two modes of heat transfer. Evaporation behaviour of binary mixtures droplet in high Reynolds number (~ 714) environment was next studied experimentally and a numerical model was developed. Transient change in droplet size and temperature were measured for both pure component system (water) and a polar binary system (70 % water and 30 % glycerol) at free stream temperature ~ 353 K and superficial gas velocity ~ 4.3 m/s. Reasonable agreements with the model predictions were obtained for single component system however some deviation was noted for binary system specifically at the transition stage which was attributed to the liquid phase diffusional resistance due to high system viscosity. Transient droplet temperature measurements were performed which showed an unsteady heating stage followed by a thermal equilibrium stage. The unsteady heating stage was shown to be within the two limits of characteristic thermal convection and mass diffusion time scale. Heterogeneous vaporisation behaviour was examined by the experimental studies of binary mixture droplets evaporating on heated spherical particle. Effect of liquid composition for three different binary system droplets (water-glycerol, water-IPA and water-butanol) and solid surface temperature (range) on the droplet vaporisation rate were studied. It was observed that droplets exhibited pinned mode evaporation (i.e. evaporation with constant wetting area and reducing contact angle) for major duration of its lifetime, at ~95 % for pure water and a major time for binary systems. A model was given to determine time varying theoretical contact angle based on droplet evaporation rate incorporating the effect of Marangoni flows which provided good agreement with the experimental data. Furthermore, local temperature measurements of the droplet showed a short initial unsteady heating duration followed by a longer thermal equilibrium stage regardless droplet compositions and solid surface temperature; the actual heating duration was found to be less than 10 % of the droplet lifetime and fell within the range of the calculated thermal diffusion time-scales. Finally, a scaling analysis was carried out to quantify the internal motions within the droplet. It was shown that under the given operating conditions, surface tension driven flow component (thermal Marangoni flow) dominates over the convective flow component due to density difference (Rayleigh flows) which justifies inclusion of the additional Marangoni number based correction factor in the evaporation model to correctly predict the vaporisation rate. This study aimed to shed light on the two different modes of droplet vaporisation process in multiphase system and it is expected that some of the models developed in this study can be incorporated in CFD framework to aid design of the relevant process equipment.
- Subject
- droplet evaporation; heat transfer; mass transfer; contact angle; spreading diameter; internal motions; unsteady heating
- Identifier
- http://hdl.handle.net/1959.13/1392739
- Identifier
- uon:33450
- Rights
- Copyright 2018 Thi Bang Tuyen Nguyen
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 10 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 150 KB | Adobe Acrobat PDF | View Details Download |