Axial Fans

Recommended Practices

Qualify Using Steady State

Before running a transient analysis, it is vital to first qualify the model by ensuring that the fan performance curve is predicted well using steady state simulations. Use the Realizable K-Epsilon or the K-Omega SST turbulence models for consistency with the DES model in the unsteady analysis.

To avoid pressure reflections, set the inlet or outlet boundaries to be freestream depending on which is the important side. If it is necessary to control the mass flow, then set the mass flow at a boundary farther away from the fan. Aim to place boundaries at least ten fan diameters away from the fan.

Use Compressible Flow

Rotors passing relative to stators create flow and pressure pulsations which are strictly a compressible phenomenon. The pulsations can result in accumulated mass (monopoles) within the system, which an incompressible calculation does not permit. Even at low Mach-numbers, compressibility is preferred.

Recommendations for Obtaining Steady Performance Curves

1. No special initialization procedures are required. To accelerate convergence, set the initial axial velocity to give the expected mass flow through the fan.
2. No preferences exist between mixing plane and frozen rotor.
3. The segregated solver is preferred to coupled due to the existence of a follow-on step to transient where it is definitely faster. Prediction accuracy between the two is marginal. Use the same solver for both steady state and transient calculations to minimize flow-wide disturbances.
4. Low Reynolds number resolution on the blade is preferred for fan performance prediction.
5. Start by running with the known boundary conditions (pressure-pressure or mass flow-pressure) obtained from the experimental conditions.
6. Sample the Mach number at the boundary and reset the steady boundary conditions to freestream with the sampled pressure, Mach number, and temperature balance.
7. Run with freestream boundaries, and the system reregulates itself to find the correct mass-flow / pressure balance.

Using the recommended settings, a performance curve of 10 points can be generated in approximately a day when running a 4–6 million-cell mesh on 48 processors.

Recommendations for the Unsteady Simulation

1. Start the acoustics simulation from the steady solution with non-reflective boundaries as described.
2. Change the turbulence model to Detached Eddy Simulation (DES) using the SST k-omega model.
3. Use the recommended Segregated Flow settings.
4. Use the Segregated Flow Model setting Hybrid-Bounded Central Differencing (BCD) or full BCD, if cell quality allows with blend = 0.3. This higher blending factor can be necessary due to non-orthogonality across the sliding mesh. Use second-order time.
5. Freeze the wall-distance calculation. The wall distances are needed mainly near to the surfaces to calculate wall damping characteristics. Since the near-wall distances are unlikely to change, it is inefficient to calculate this distance at every time-step.
6. Convert to transient and run.
7. Run for between 100–200 full rotations to get a good statistical sample for digital signal processing.

Using the settings approximately 15 full rotations a day are possible when running a 4–6 million-cell mesh on 48 processors.

Choosing Transient or Harmonic Balance

Axial fans, that are used for under-hood cooling or building HVAC, are placed close to casing, mounting struts or other geometries which are not rotating. The correct way to simulate this system is to account for the periodic interaction of the rotating and stationary parts, namely downstream wake, and upstream pressure gusts of turning vanes, blades, and supports.

There are two ways to model axial fans in Simcenter STAR-CCM+: using transient Rigid Body Motion (sliding mesh) or the Harmonic Balance method. Both are more time-consuming, but more accurate, than steady state approximations that use frozen-rotor or mixing-plane stator-rotor interfaces. The Harmonic Balance method runs faster than transient. However, the Harmonic Balance method resolves tones that are based on the modes solved (blade passing frequency, harmonics, and Strouhal shedding). It does not resolve broadband noise which is associated with resolved turbulence.

Recommended Time-Step

Recommended Solver Settings