Spray Drying Technology.pdf - National University of Singapore

Depending on the case, such high loading can significantly affect the flow field
near the atomizer which subsequently affects the flow field in the other
downstream regions [4] . Recently, in a full transient simulation, the effect of
particles on the flow field was further illustrated by Jin and Chen [26] .
In the wake of recent developments in transient CFD simulations of
spray dryers, usage of the steady or transient particle tracking has also
emerged as an important consideration. Figure 8 shows the difference in
numerical procedures when implementing full coupled steady or transient
particle tracking. In the steady approach, particles are injected and tracked
until it reaches the boundary of the simulation domain [11][23][28] . On the other
hand, in the transient approach, particles are typically tracked and penetrate
the flow field following the simulation time step and duration [21][22] . This has
three major implications: (1) relatively large simulation time is required for all
the injected parcels to initially penetrate the simulation domain, (2) once the
particles have penetrated the flow field, further transient duration is required
for any statistical analysis of the particle simulations, (3) particles have to be
injected at each time step drastically increasing memory requirements
(increasing total number of particle in simulation). It is also clear that the
coupling iterations are required for each time step in the transient tracking
making it computationally expensive. Therefore, full transient air-particle
simulation is a very computationally expensive undertaking. In terms of
coupling with the continuous phase as alluded to earlier, for steady state
particle tracking, it is important to incorporate these source terms in
developing the flow field by repeatedly injecting the particles and reiterating
flow field until insignificant change occurs in the computed flow field.
Many different atomizers can be found in spray drying. For example: eg.
pressure nozzle, ultrasonic nozzle, two fluid nozzle and rotary disk. In a CFD
simulation, these different atomizers are characterized by the particle size
distribution, mass flow rate and the initial particles velocity components. As
noted by Kieviet et al. [10] , getting these initial parameters can be a big
challenge by itself. While the mass flow rate can be easily known as an
overall simulated operation capacity, the latter two parameters are less
apparent. Most commercial nozzles and rotary disks come with specifications
of the droplet size distribution; the former includes the cone angle.
Woo, Huang, Mujumdar, Daud – CFD modeling of spray dryers 11