Analysis of CH$_2$O ⋅ 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 (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. ... mehrAlthough 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.