Although heat release rate is one of the important combustion characteristics, its direct measurement is impossible. Recently, Najm et al. have reported that the heat release rate is well correlated with the product of OH and HCHO fluorescence signals in PLIF measurement. So far, it is well confirmed that this approach can be applied to premixed combustion. In this study, we tried to evaluate the heat release rate in diffusion flame in the same manner. Before that, we numerically investigated

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... e correlation between heat release rate and the product of OH and HCHO concentrations in counter-flow diffusion flame. It is found that the profile of large heat release rate is well correlated with the product of OH and HCHO concentrations, because the heat release rate largely depends on HCO production rate, described by OH and HCHO concentrations through HCHO + OH → HCO + H2O. However, in the region where the heat release rate is relatively small, the same correlation is not observed. In PLIF measurement, since the profiles of OH and HCHO fluorescence signals match those concentrations predicted by simulation, these radial concentrations are well determined by experiments. The product of OH and HCHO fluorescence signals shows the similar profile to that of the simulated heat release rate. However, different from the premixed flame, the disagreement was found especially in the region of small heat release rate. Thus, it is concluded that the region of large heat release rate in diffusion flame is well correlated with OH and HCHO PLIF measurement.