Computational Fluid Dynamic Models of Spray Dryers
Components of a CFD Model of Spray Dryers
This section of the book is aimed at providing the reader with an introduction to some key considerations in using the CFD technique to model the spray drying process. Specific numerical details and strategies in setting up a CFD model of a spray dryer, which is beyond the scope of this section, can be found in Woo (2016). Detailed discussion on some of the key ideas presented in this section can also be found in the book cited.
At the core of a CFD model (any CFD model) of a spray dryer is the simulation of the turbulent airflow within the chamber. There are reports in the literature providing experimental validation to selected spray dryer geometries and operations (more details on the cases can be found in Woo (2016)). It is noteworthy that different turbulence models are reported for different dryer configurations and in many cases. As such, there are many reports in which the selection of a suitable turbulence model was mainly based on past reported “similarity” in spray drying chamber and operation (discussed in the introduction of Jubaer etal, 2019c). Such an approach in turbulence model selection will inevitably introduce a certain degree of uncertainty in the airflow prediction, which forms the core of the simulation framework. In the authors’ opinion, while detailed validation is possible for academic- or research-oriented simulation work, such luxury is not available for industrial applications or in the routine use of the CFD technique for spray dryer design. Therefore, it is important to keep in mind this aspect of uncertainty in the interpretation of a CFD prediction of a spray dryer.
Building upon the turbulent airflow prediction, particle parcels are then tracked within the flow field in the Lagrangian mode with two-way coupling between the air phase and the particles. Different forces interacting with the particles in addition to the drag force may be incorporated into Lagrangian tracking, and this certainly depends on the level of detail required from the simulation. Stochastic turbulent dispersion of the particles is typically accounted for in most reported CFD simulations. Incorporated into the Lagrangian particle tracking framework are all the other important sub-models, which “convert” the entire CFD simulation framework into a spray dryer CFD simulation. Some of these sub-models include:
- 1. Droplet drying model
- 2. Particle-wall interaction model
- 3. Agglomeration model
While the droplet drying model has already been discussed in Chapter 2 of this book, the particle-wall interaction model and the agglomeration model will be discussed later on in this chapter. Regardless of the type of sub-model, particle tracking model and turbulent airflow prediction used, underpinning the entire simulation framework is another aspect that has to be carefully considered: to undertake the simulation in the steady state or transient state? This will be discussed in greater detail in the next section.