Spray Dryer CFD Simulation
The spray dryer is used to dry liquids and convert them to dry solid powders. These spray dryers are widely used in the food, pharmaceutical, ceramic, and other industries, and these dryers can deliver the final products to the desired moisture content and size. The resulting powder is removed from the chamber by separating systems such as cyclone and bag filter from hot air.
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Spray Dryer Mechanism
The feed solution (slurry, paste, etc.) is first conveyed to the atomizer via a tank by means of a pump; the atomizer disperses the liquid solution as a spray or liquid droplets into the dryer main compartment. On the other hand, hot air, which rises by a burner or steam heater, blows into the chamber by a fan. At this point, the liquid phase of the spray contact with hot air, so that the moisture within the droplets evaporates within a short time, leaving the powdered dry particles with the hot air out of the chamber.
Problem Description for Spray Dryer CFD Simulation
For this simulation, we use two types of continuous and discrete phases. In fact, the hot airflow, which is responsible for evaporating the moisture contained in the solution, enters the spray dryer chamber as a continuous phase and flows into the chamber (Eulerian). While a solution containing water and solid particles (Lagrangian) injects into the chamber as a discrete phase. In fact, the main purpose of this simulation is to investigate the behavior of particles that flow as a discrete phase in a continuous flow.
The assumption for Spray Dryer CFD Simulation
There are several assumptions to solve this problem:
The problem is STEADY.
We take the effect of the earth’s gravity into account for the spray dryer modeling, so that its value is 9.81 m.s-2.
We assume the incompressible flow, so the solution is pressure-based.
Geometry of Spray Dryer
The Design Modeler software design the 3-D geometry of the Spray Dryer. The drying chamber consists of two cylindrical and conical sections. The device injects Inlet hot air and feeds solution from the upper part of the chamber and the powder from the conical bottom.
In the present modeling, we use unstructured mesh. The total element number is 84047. In the inlet and outlet zones, we use a more accurate grid.
Here is a summary of the steps to define and solve the problem in the table:
|Standard wall function|
|O2 , N2 , H2O (vapor)|
|Intraction with continouos phase|
|Velocity inlet / mass flow inlet|
|hot air inlet||6 m.s-1|
|vapor mass fraction||0.009|
|O2 mass fraction||0|
|feed inlet||mass flow||0.0116 m.s-1|
|vapor mass fraction||1|
|O2 mass fraction||0|
|outlet air||pressure||-150 Pa|
|vapor mass fraction||0|
|O2 mass fraction||0|
|convection||wall thickness||0.002 m|
|convection heat transfer||3.5 W.m-2.K-1|
|bulk temperature||298.15 K|
|vapor diffusion flux||zero|
|O2 diffusion flux||zero|
|turbulent Kinetic||First-order upwind|
In the present case, the K-Epsilon-Standard model is used. Also, due to the use of this turbulence model, the Standard Wall Function is also defined to investigate fluid behavior in the areas near the walls. However, after the completion of the solution process, the Y+ should be considered. The most appropriate range being between 30 and 300.
Species Transport Equation
In this model, there are several species of gases, including water vapor, oxygen, and nitrogen. It is assumed that the inlet hot and dry airflow has a very small amount of (about 0.009%) water vapor. Also, some moisture air comes at the inlet air with the feed solution, so that inlet air with the solution just contain water vapor. Therefore, in addition to receiving moisture of the feed solution, the inlet hot air flow also has the task of drying the humid air with the solution.
Discrete Phase Model (DPM)
The DPM is used when the purpose is to investigate the behavior of the particles from a Lagrangian perspective. In the present model, there is a solution consisting of water and solid particles whose purpose is to separate the water droplets or the moisture from the particles. By choosing the Interaction With Continous Phase, the behavior of the selective discrete phase flow (water droplets) interacts with the continuous flow (hot air).
The injection process is defined for the solution flow in the inlet boundary, wherein the inlet solution, water droplet is injected into the chamber and evaporated into the vapor (defined in the Species Transport section). Water droplets evaporate and the solid particles in the solution become dry. The material is injected into the domain as the SURFACE condition. This conversion of water from liquid to vapor occurs at a rate of 49 m.s-1 and a mass flow of 0.0116 kg.s-1 at 300.15 K. Also, the minimum and maximum diameter are used to define the size of the liquid droplet diameter.
Convection Heat Transfer
Since in the present model, the inlet hot air flow rate is high, the effect of the convection heat transfer term is reinforced. So, convection heat transfer should occur between stainless steel walls and bulk flow.
All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, Training File presents how to solve the problem and extract all desired results.