This work presents a numerical investigation of the applicability of the product of formaldehyde (CH 2 O) and hydroxyl radicals (OH), denoted as CH 2 O ⋅ OH, as a marker for the local heat release rate (HRR) in CH 4 flames over a broad range of boundary conditions. Both laminar freely propagating one-dimensional and premixed counter-flow CH 4 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 CH 2 O ⋅ OH profiles and the HRR was quantified using the Pearson correlation coefficient. Overall, the study confirms CH 2 O ⋅ OH as a practical and reliable HRR marker in CH 4 flames, particularly under high-temperature and high-pressure oxy-fuel conditions. The results show that in slightly rich CH 4 -air flames (about Φ = 1.5 ), CH 2 O ⋅ 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 overall range of validity. Although increasing strain rate generally degrades the correlation, CH 2 O ⋅ 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 CH 2 O ⋅ OH for HRR estimation in highly turbulent flames. The demonstrated robustness of CH 2 O ⋅ OH highlights its suitability as an HRR marker, providing valuable insights for experimental diagnostics. • CH 2 O ⋅ OH confirms as robust HRR marker across a wide range of conditions including variations in oxidizer content, equivalence ratio, preheat temperature, pressure, and strain. • For CH 4 -air flames, the highest correlation between CH 2 O ⋅ OH and HRR occurs at slightly rich mixtures ( Φ ≈ 1.5), while in CH 4 /O 2 flames, the strongest correlation shifts to ultra-rich conditions ( Φ ≈ 3.0). • Elevated pressure generally improves the correlation at leaner mixtures but narrows the range of high correlation, whereas preheating shifts the optimal equivalence ratio to richer conditions and broadens the applicability range. • Even under high strain rates (up to 10,000 s − 1 , relevant for turbulent flames), CH 2 O ⋅ OH maintains a strong correlation (R ≈ 0.9) with HRR, highlighting its suitability for turbulent combustion diagnostics. • The results of this study are intended to support and guide experimental diagnostics by identifying reliable marker regimes.
Stelzner et al. (Wed,) studied this question.