Learning Forum

[abtharpe42]

I'm trying to simulate an ammonia-fueled GT combustor in Fluent using an ammonia combustion mechanism made by Alessandro Stagni in 2025 and the Flamelet Generated Manifold (FGM) model. I'm using these steady-state simulations to help me iteratively size the dilution holes so that the primary flame zone is at an equivalence ratio of 1.1. The dilution air then brings the global equivalence ratio down to about 0.3. Here are a images of the combustor are shown below with the periodic walls removed.

The air enters at 1073 K and 0.921 kg/s and is 19.89% o2, 3.28% h2o, and 76.83% n2 by mass. The fuel is 60% nh3, 7.1% h2, and 32.9% n2 by mass and enters at 800 K and 0.031135 kg/s. The outlet of the combustor is set to 1500 kPa. I've used the FGM many times in the past and with a plethora of geometries and have not had any issues. But lately with combustors like this one, I constantly spammed with warnings during runtime saying that the min/max PDF table enthalpy has been exceeded with unignited and ignited flows, and I don't know why. I set the minimum table temperature to 700 K, and I know for a fact from Species Transport Simulations that nowhere in the domain gets below that temperature. Here are some screenshots showing when things go bad in my latest run.

 

Normally the temperature contour has an aggressive patchwork appearance. I've tried different flamelet options, pressure-velocity couplings, etc. but nothing works. If I take the flamelets from these simulations and use them in simulations with unrelated geometries, I don't have anywhere near the same amount of trouble. Any help would be greatly appreciated.