Learning Forum

[didi06071120]

Hello everyone,

I am currently performing a CFD simulation using ANSYS Fluent to study CO₂ distribution in a classroom with multiple occupants. The model includes supply air (SA), return air (RA), and CO₂ sources from human breathing.

I initially attempted to simulate HVAC recirculation by defining the CO₂ concentration at the supply inlet using an expression:

C_SA = 0.3 × C_OA + 0.7 × Average(CO₂ at RA)

where:

• C_OA is the outdoor air CO₂ concentration (500 ppm),
• C_RA is the area-weighted average CO₂ at the return outlet.

However, I encountered several issues:

1. The indoor CO₂ concentration becomes unrealistically high (around 1800–2000 ppm).
2. The spatial distribution becomes overly uniform, losing expected airflow-driven gradients.
3. I am not sure whether Fluent updates the "Average()" expression at every iteration, or if it remains fixed during the solution process.
4. I suspect this setup creates a feedback loop where the boundary condition depends on the solution itself.

My questions are:

• Is it appropriate to directly use expressions referencing surface averages (e.g., Average(CO₂ at RA)) as boundary conditions in Fluent?
• Does Fluent update such expressions every iteration, or only at initialization?
• Would it be more correct to implement this type of recirculation using an iterative (manual) coupling approach instead?
• Are there recommended practices for modeling HVAC recirculation (mixing of return and outdoor air) in Fluent without using UDF?

Any insights or references would be greatly appreciated.

Thank you!

`

[Rob]

Using 70% return air is fine, as it the use of an area average in that way. But... depending on convergence how confident that you're getting a sensible value on the report? What is your air change rate on the domain? 

[didi06071120]

Hello Rob,

Thanks for your reply.

My model has ~8.7 ACH (≈6550 m³/h total airflow), with ~2.5 ACH outdoor air (30% OA). However, the indoor CO₂ still reaches ~1500–1800 ppm.

Is this level physically reasonable, or could there be an issue with my model setup?

Thank you!

[didi06071120]

Based on your experience, does this CO₂ distribution look physically reasonable for a classroom with ~8.7 ACH and 30% outdoor air?

Thank you!

[Rob]

Pass, you need to do a Google (or other search) to see what the standards are.  Do check if you're monitoring CO2 mass or volume/mole weighted as CO2 has a much higher molecular weight relative to air.