- \n
- mass-flow-inlet<\/li>\n
- mass flow = 1.43e-4 kg\/s<\/li>\n
- Total Temperature = 2000 K<\/li>\n
- Mean Mixture Fraction Z = 1<\/li>\n
- normalized Progress Variable C = 1<\/li>\n
- both variances = 0<\/li>\n<\/ul>\n
Z=1 represents the premixed CH4\/air reactant stream at equivalence ratio 0.8; Z=0 is ambient air.<\/p>\n
Original numerical symptom<\/h3>\n
We initially investigated persistent
\"Maximum PDF table enthalpy exceeded\"<\/code> warnings and a very large high-temperature population. At a trusted saved checkpoint:<\/p>\n- \n
- T > 2200 K: 607,239 cells<\/strong><\/li>\n
- T > 2400 K: 454,109 cells<\/strong><\/li>\n
- T > 2600 K: 282,747 cells<\/strong><\/li>\n
- T > 2800 K: ~103,329 cells<\/strong><\/li>\n
- Tmax = 2849.998 K<\/strong>, exactly the upper temperature limit of the current PDF table.<\/li>\n<\/ul>\n
Of the cells above 2400 K, 82.3%<\/strong> have cell enthalpy above their local PDF-table maximum enthalpy. Above 2600 K this fraction is 87.8%<\/strong>.<\/p>\n
Numerical\/model sensitivities already tested<\/h3>\n
We have already performed a substantial set of controlled sensitivity tests, so we would particularly appreciate guidance beyond generic URF\/time-step recommendations:<\/p>\n
- \n
- Time step:<\/strong> reducing dt from 2e-5 to 1e-5 s did not materially change the solution at matched physical times.<\/li>\n
- Energy URF:<\/strong> reducing Energy URF from 1.0 to 0.8 produced essentially the same high-temperature population; in a clean sibling comparison it was slightly worse rather than better.<\/li>\n
- PDF enthalpy resolution:<\/strong> increasing the enthalpy coordinate from 20 to 80 points (H80)<\/strong> produced only a modest reduction (~5% in T>2400 and ~3% in T>2600 populations at comparable times); the underlying behaviour remained.<\/li>\n
- Adaptive Grid Refinement of the PDF table:<\/strong> AGR = 0.10 changed the high-temperature counts by less than approximately 0.5%.<\/li>\n
- Extrapolation:<\/strong> turning solution extrapolation OFF produced essentially the same result. At the same physical time, T>2200 \/ T>2400 \/ T>2600 counts were 676,287 \/ 485,091 \/ 288,294 versus 671,044 \/ 482,312 \/ 288,988 in the comparison branch.<\/li>\n
- Poor-mesh numerics:<\/strong> Poor Mesh Numerics correction was tested and had essentially zero effect on the high-temperature population.<\/li>\n
- Operating density:<\/strong> changing the specified operating density to a representative value produced essentially identical early behaviour.<\/li>\n
- Pilot temperature\/flow adjustments:<\/strong> increasing the pilot temperature toward the experimental ~2230 K worsened the PDF-enthalpy warnings; reducing the pilot flow did not eliminate the problem and in one test increased the warning count.<\/li>\n
- Heat-gain range:<\/strong> increasing the heat-gain range moved the PDF temperature ceiling from ~2425 K to ~2850 K, but the solution then populated temperatures up to the new ceiling rather than removing the underlying anomaly.<\/li>\n<\/ul>\n
Global transient mass balance closes to approximately 1% of the net boundary flux. All walls are adiabatic and radiation is off. Therefore we have found no indication that a gross mass imbalance, wall heat input, Energy URF, time step, table resolution, extrapolation, or isolated poor cells are the primary origin.<\/p>\n
Key boundary-enthalpy diagnostic<\/h3>\n
We then measured the pilot boundary state directly.<\/p>\n
On all pilot inlet faces:<\/p>\n
- \n
- F_T = 2000.000 K<\/li>\n
- F_Z = 1<\/li>\n
- F_PREMIXC = 0.1221857, corresponding to the burnt maximum of the transported un-normalized progress variable<\/li>\n
- F_H = +2.015300e6 J\/kg<\/strong><\/li>\n<\/ul>\n
Fluent’s own native heat-transfer report independently gives:<\/p>\n
pilot heat-transfer \/ pilot mass-flow = +2.0153e6 J\/kg<\/code><\/p>\nso this value is not a UDF post-processing artefact.<\/p>\n
However, cells at essentially the same burnt thermochemical state (Z≈1, normalized C≈1, T≈2000–2050 K, enthalpy inside the PDF bounds) have:<\/p>\n
H ≈ -2.064e5 J\/kg<\/strong><\/p>\n
and
Pdf_Adiabatic_Enthalpy(Z=1)<\/code> also gives approximately -2.0644e5 J\/kg<\/strong>.<\/p>\nTherefore the pilot boundary carries approximately:<\/p>\n
+2.219 MJ\/kg more total enthalpy than the corresponding burnt state.<\/strong><\/p>\n
The anomalous enthalpy is already present on the pilot boundary and in the first downstream cell layers, whereas the main heat-release-rate peak occurs farther downstream. Thus the excess enthalpy is not first generated inside the main reaction zone.<\/p>\n
Furthermore, the pilot boundary value +2.015300e6 J\/kg<\/strong> agrees almost exactly with an independently measured PDF upper enthalpy value on the unburnt (C=0) side<\/strong>, +2.0152998e6 J\/kg. We regard this only as an observation; we have not determined whether it represents clipping\/limiting, a particular T→H inversion path, or another mechanism.<\/p>\n
The main cold velocity inlet provides a useful sanity check: T=300 K, Z=1, C=0 gives F_H≈-2.0644e5 J\/kg. We recognize that this does not isolate boundary type because the main inlet is a velocity inlet whereas the pilot is a mass-flow inlet.<\/p>\n
Mesh\/modeling questions<\/h3>\n
The flame-region mesh size is approximately 0.30 mm<\/strong>. Our estimated laminar flame thickness is approximately 0.534 mm<\/strong>. Poor-mesh-cell treatment had essentially no effect, but we would like your assessment of whether the spatial resolution itself is inadequate for this Partially Premixed FGM\/C-equation LES formulation and could materially contribute to the observed temperature field.<\/p>\n
More importantly, our PDF table uses adiabatic premixed flamelets with the Non-Adiabatic Energy Treatment \/ heat-loss-gain enthalpy extension<\/strong>. Because a ~2000 K fully burnt pilot stream enters immediately adjacent to a ~300 K premixed reactant stream, is this application within the intended validity of that standard non-adiabatic FGM treatment?<\/p>\n
Could the large local enthalpy difference between the hot burnt pilot and cold main stream require true non-adiabatic flamelet generation at multiple reference temperatures\/enthalpies (or burner-stabilized flamelets)<\/strong> rather than an adiabatic flamelet manifold with enthalpy extension?<\/p>\n
Requested guidance<\/h3>\n
We would like a specific recommendation for Fluent 2022 R2:<\/p>\n
- \n
- How should a physically required ~2000 K fully burnt pilot be specified in Partially Premixed FGM?<\/li>\n
- For a mass-flow inlet with T0=2000 K, Z=1 and normalized C=1, why does Fluent assign approximately +2.0153 MJ\/kg total enthalpy instead of the ~-0.206 MJ\/kg adiabatic burnt-state value?<\/li>\n
- Is this expected behaviour, a known 2022 R2 limitation\/defect, or an incorrect boundary\/model specification on our side?<\/li>\n
- Is a different inlet treatment required for the burnt pilot?<\/li>\n
- Is our ~0.30 mm flame-region mesh insufficient for this model?<\/li>\n
- For a hot burnt pilot immediately adjacent to a cold premixed burner stream, should we generate genuinely non-adiabatic flamelets instead of using adiabatic flamelets with the heat-loss\/gain extension?<\/li>\n
- What is the recommended correction that preserves the physical pilot and avoids the observed PDF-table enthalpy exceedance\/high-temperature population?<\/li>\n<\/ol>\n
We can provide the full case\/data checkpoint, PDF table, native Fluent flux reports, UDF diagnostic and detailed sensitivity results.<\/p>\n
![\"\"](\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2026\/08\/image-129.png\")
<\/p>\n","protected":false},"template":"","class_list":["post-463072","topic","type-topic","status-publish","hentry","topic-tag-cfd-combustion","topic-tag-fgm","topic-tag-methane-air"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t
- Energy URF:<\/strong> reducing Energy URF from 1.0 to 0.8 produced essentially the same high-temperature population; in a clean sibling comparison it was slightly worse rather than better.<\/li>\n
- T > 2400 K: 454,109 cells<\/strong><\/li>\n
- T > 2200 K: 607,239 cells<\/strong><\/li>\n