Analysis of CH2O ⋅ OH as marker for the heat release rate from air to pure oxy-fuel flames at elevated preheat temperature, pressure and strain

This work presents a numerical investigation of the applicability of the product of formaldehyde (CH2O) and hy
droxyl radicals (OH), denoted as CH2O ⋅ OH, as a marker for the local heat release rate (HRR) in CH4 flames over
a broad range of boundary conditions. Both laminar freely propagating one-dimensional and premixed counter-
flow CH4 flames were simulated using several detailed reaction mechanisms. The analysis systematically varied
the oxidizer composition (from air to pure oxy-fuel), equivalence ratio (Φ), inlet temperature, pressure, and strain
rate. The correlation between the CH2O ⋅ OH profiles and the HRR was quantified using the Pearson correlation
coefficient. Overall, the study confirms CH2O ⋅ OH as a practical and reliable HRR marker in CH4 flames, particu
larly under high-temperature and high-pressure oxy-fuel conditions. The results show that in slightly rich CH4-air
flames (about Φ = 1.5), CH2O ⋅ OH provides an accurate estimate of the HRR. In oxygen-enriched and pure oxy-
fuel flames, the strongest correlation shifts to ultra-rich conditions (about Φ ≈ 3.0). Increasing pressure enhances
the correlation at lower equivalence ratios, whereas preheating weakens the correlation locally but expands the
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overall range of validity. Although increasing strain rate generally degrades the correlation, CH2O ⋅ OH remains a
robust marker under technically relevant turbulent conditions (strain rates below 10,000 s−1), maintaining Pearson
correlation coefficients of approximately 𝑅 ≈ 0.9 even close to extinction. This demonstrates the strong feasibility
of using CH2O ⋅ OH for HRR estimation in highly turbulent flames. The demonstrated robustness of CH2O ⋅ OH
highlights its suitability as an HRR marker, providing valuable insights for experimental diagnostics.