Know the steps to set up your simulation and get useful results.
Centrifugal Fans: Basics and Importance
Centrifugal fans, also known as radial fans, are mechanical devices designed to increase the pressure and flow of air or gases by converting kinetic energy into potential energy. They operate by drawing air in axially and then discharging it radially, using the centrifugal force generated by a rotating impeller. This design allows for efficient handling of high-pressure applications and the ability to move large volumes of air against system resistance.
There are several types of centrifugal fans, each suited for different applications. The main types include (see the image on the right):
- Radial-bladed fans. They are robust and capable of handling particulate-laden air, commonly used in material handling and dust collection systems.
- Forward-curved fans. There are compact and quiet, ideal for HVAC systems in buildings.
- Backward-curved fans (or backward-inclined). are more efficient and self-limiting in power consumption, making them suitable for industrial processes and large-scale ventilation.
Centrifugal fans find applications across a wide range of industries. In HVAC systems, they provide air circulation and temperature control in buildings. Industrial processes use them for ventilation, fume extraction, and pneumatic conveying. They are crucial in power plants for draft induction and in the automotive industry for engine cooling.
The selection of a centrifugal fan depends on factors such as required airflow, pressure, efficiency, noise levels, and the nature of the medium being moved. Advancements in impeller design and motor efficiency continue to improve the performance and energy consumption of these versatile machines.
Components
These fans consist of an impeller and a housing. The impeller, typically made of metal or plastic, rotates at high speeds, drawing in air from the inlet and accelerating it towards the outlet. The housing of a centrifugal fan is designed to guide the airflow and increase its pressure. It is important to consider factors such as the shape and size of the housing, as well as the number and arrangement of impeller blades, to achieve the desired performance.
Simulation
The simulation of turbomachinery is possible using different CFD tools. In this case, we use Ansys Discovery to rapidly assess the hydraulic power of the fan. It is a simulation-driven design tool that integrates interactive modeling (including parametrization) with multiple simulation capabilities. In Explore Mode, the user obtains preliminary results quickly, even when changing the model or physics. This allows for the exploration of different designs prior to a more accurate simulation in Refine Mode, where the model is meshed and run using a more robust solver.
Steps. Simulating a centrifugal fan using Ansys Discovery involves the following steps:
- Geometry Creation. Start by creating a 3D model of the centrifugal fan geometry, including the impeller, housing, and any other components. In this Demo, we have a dummy geometry of a Backward-curved fan (Outlet Diameter: 546 mm, Plate thickness: 5 mm, Blade Thickness: 10 mm).
Create the Housing using proper design guidelines. Extract the volume of the impeller enclosure shown above. Disable the impeller for the simulation. You can use another CAD tool to create your own geometry, Ansys Discovery imports files from many CAD Software.
- Physics Setup. Go to the Simulation Tab and click on 'Fluid Flow', select the first option from the list: 'Flow'. Pick the inlet area shown below (left), 1) select 'Inlet' and 2) the option you prefer, 3) type a proper value and 4) the temperature of the air (optional). In this Demo, the inlet is defined as an isothermal flow with 1 m/s. Repeat the process following the steps from (5) to (8) for the outlet section, pick the area on the housing (right). Here we have a section with reference pressure of 0 Pa. Notice that the inlet and outlet values can be parametrized.
Go back to the Simulation Tab and click on 'Fluid Flow'. This time select 'Rotating Fluid Zone' from the list. Now you must select a geometry, then pick the 'Impeller Enclosure'. Ensure you see the (1) name of the zone on the screen, and (2) type the rotational speed in the Unit you prefer. For this Demo, the fan rotates at 360 rpm (usually this speed is higher) in the direction shown. Make sure that the fluid zones ('Impeller enclosure' and 'housing') have air as the material assigned. The Structural Steel is for the impeller, but it is disabled for this simulation.
- Run the model in Explore Mode. Ensure you are on the Explore Mode by selecting the option at the bottom of the screen. Next, select the Fidelity you want for the simulation keeping in mind that each fidelity has a cell size associated. If you want to know the cell size, go to the Simulation Tab and click on 'Size Preview'. Move your mouse pointer over the geometry and the cell size will be visible. For more details, watch the following video. To solve the model click on the green button on the bottom-right side of the screen. The results will be assessed in the next section.
- Run the model in Refine Mode. Move to the Refine mode by clicking on the white arrow at the right side of the Explore tab. Next, select the Fidelity in the same manner as in the Explore Mode. Recall the fact that in this mode the user can create a mesh. There are different mesh controls to set up, but in this demo we will work with the options below. For more details watch this video. To solve the model for each Fidelity value, click on the green button on the bottom-right side of the screen, as in the Explore Mode.
Postprocessing Results: Explore Mode
The following results are presented for different levels of Fidelity (Resolution). As the purpose of the fan is to rise the static pressure, check those contours first to have an overview of the flow within the domain. Select the Units you prefer and use the 'Direction Field' tool and select the meridional plane as shown below.
The number 1 on the bottom-left side in the figure above means that this is the first model solved using the lower Fidelity. As the Fidelity decreases, the element size does and the results are more accurate. Table 1 presents the results for different Fidelity values, where the inlet volumetric flow rate and the hydraulic power are calculated:
- The volumetric Flow rate is the product of the air density, the air speed and the inlet area.
- The Hydraulic power is the product of the volumetric flow rate and the pressure rise. Verify the units.
Postprocessing Results: Refine Mode
Now, the use the third environment of Ansys Discovery. As mentioned before, the Refine Mode allows the user to create and control the mesh for more accurate results. we can also make use of the Fidelity tab to refine the mesh. If you want to refine the mesh by typing the cell size (min/max), the growth rate and the number of inflation layers, please refer to this . Again, the picture below is the one for the first model having the smallest number of cells, but Table 2 presents the complete information about the results when the mesh is refined.
Go further!
Download the project file (2023R2) and check the velocity vectors, streamlines, particle flow and create the Fan curve for this geometry. You can do it by parametrizing the inlet velocity. If you want try using your own geometry with the same set up.