Negative heat release rate with non-premixed combustion model

[georgesnaffaa246]

I am running a 3D transient combustion simulation with Ansys Fluent (2025 R1). I am using reduced order models (SDF and FGM), and I am computing the heat release rate.

At the start of the simulation, I am running into an issue: the heat release rate computed using the volume integral on my domain (PDF -> Heat release rate) is negative for the first 0.1-0.2s of the simulation, it remains constant at negative values for a while before it increases again, even when a flame is present. I have tested both the mesh and the timestep, and it does not seem to be the issue. 

The setup is basically a cylinder filled with air at 25 °C, with a mass flow inlet (0.5 g/s) for fuel set at 900 °C, fuel composition is H2, CO2, CO, CH4 and C2H4. 

Pressure-based solver with coupled pressure-velocity coupling, second-order implicit transient formulation.

Has anyone encountered such an issue before? If so, what could cause that? And what are some potential solutions? 

Thank you

[Ren]

Could you share some images of the combustion model setup in the "Species Model" dialog box (all the tabs), and also contour plots of the following:

• velocity
• mean mixture fraction
• progress variable
• temperature
• heat release rate