Takeaway from Airflow Cooling Project with Flotherm

Pre-Simulation Check: Hand Calculations

Before running the simulation, I perform simple hand calculations to establish a baseline expectation.
These checks generally include:

1. Estimating airflow rates using fan P–Q curves

1. Approximating pressure drop
2. Estimating convection coefficients (h)
3. Predicting approximate temperature rise (ΔT) based on total thermal resistance
4. Ensuring the expected results fall within a physically reasonable range

This helps validate the setup and prevents unrealistic CFD outcomes.

Boundary Condition Setup

Closed surfaces (Top and Bottom)

Since the enclosure in my project is a closed chassis with both a top cover and a bottom plate,
the top and bottom boundaries are defined as solid walls.
This allows the simulation to accurately capture:

1. Internal recirculation
2. Heat transfer through the enclosure surfaces
3. Proper pressure development inside the closed volume

Open sides (Left and Right)

The left and right sides of the domain are relatively open with no nearby obstructions.
Therefore, I set these boundaries as openings (e.g., Opening, Pressure Outlet, or Entrainment),
allowing air to freely enter or exit depending on the flow conditions.

This better represents a semi-open environment and avoids artificial confinement of the airflow.

Meshing Setup:

1. Regions that require mesh refinement

Mesh refinement is needed in areas where the physics are more complex, such as:

1. Key components like fans, heat sinks, or geometrically complex features 
2. Regions with strong flow gradients, recirculation, or turbulence
3. Hot spots, where temperature gradients or local heat transfer rates are high

Finer mesh in these regions helps capture local flow and thermal behavior more accurately.

2. Methods of applying refinement: Localized refinement vs. Volume regions

1. Localized refinement is applied directly on surfaces, edges, or critical geometric features.
2. It is useful for sharp corners, narrow channels, and small details.
3. Volume refinement regions are applied to a 3D volume surrounding a component. The refinement region should be larger than the target component because flow development, mixing, and convection occur around the component, not only on its surface. A larger refinement volume also avoids abrupt mesh transitions that may introduce numerical errors.

3. Minimum number of elements in each direction (X, Y, Z)

Any geometric feature or physical thickness should have at least three elements across each direction.
This ensures:

1. Proper resolution of flow or temperature gradients
2. Avoiding numerical smearing
3. Allowing higher-order solvers to operate correctly

4. Handling convergence: refine first, then coarsen

If the simulation does not converge well or the results appear unstable:

1. Increase mesh density, especially in critical regions.

2. Continue refining until the solution becomes mesh-independent, meaning further refinement no longer causes significant changes in key outputs (e.g., temperature, pressure drop).

3. Only after reaching mesh independence should you coarsen less important regions to reduce computational cost.

Post-Simulation Checks: Result Validation

After completing the simulation, I verify the results using planes, slices, and surface plots.
Key validations include:

(1) Temperature field

• Whether the temperature distribution is physically reasonable

• Whether any unexpected hot spots appear

• Whether similar components with similar power show consistent temperature levels

(2) Pressure field

• Ensuring inlet pressure > outlet pressure, especially when fans are involved

• Checking for any unrealistic high-pressure regions that may indicate boundary or mesh issues

(3) Velocity field and flow direction

• Confirming that airflow is directed toward the components that require cooling

• Identifying recirculation zones, dead zones, or reverse flow that may cause local overheating

• Examining velocity vectors for flow uniformity across components

(4) Component-level throughflow

• Verifying that each component receives reasonable and consistent airflow

• Checking whether some components are "shaded" or starved of flow

• Ensuring the actual throughflow matches expectations from the fan curve and system pressure loss

(5) Identifying anomalies

• Large temperature discrepancies among identical components

• Unusually low or high velocity regions

• Flow not following expected thermal-fluid behavior