The effect of strain rate on heat release rate markers in lean premixed hydrogen flames

We investigate lean, premixed axisymmetric hydrogen flames stabilised on a bluff-body and compare the effect of strain rate and equivalence ratio on -chemiluminescence, OH-Planar Induced Fluorescence (OH-PLIF), and the dilatation rate as markers of the mean heat release rate (HRR). To compare with and complement the experimental measurements, laminar counterflow flame simulations were performed, which included an sub-mechanism. The simulations show that the dilatation rate is well correlated to the heat release rate (HRR) for increasing tangential strain rate. Moreover, the ratio of the dilatation rate to the HRR was found to be independent of strain rate over the ranges of lean equivalence ratios tested, confirming it as a robust marker of the HRR. However, the ratio of -chemiluminescence to HRR was found to vary non-monotonically with strain rate, with its magnitude depending on equivalence ratio. Examining key steps in the sub-mechanism suggests that this effect results from preferential diffusion of hydrogen, which modifies the reaction zone and the radical concentrations. We then examine this phenomena experimentally through analysis of the mean , OH-PLIF, and dilatation rate of bluff-body stabilised axisymmetric flames measured for several bulk velocities and lean equivalence ratios. Close to the bluff-body, the flame is flat and parallel to the flow and is subjected to decaying levels of tangential strain rate moving downstream. At low bulk velocities, strain-induced appear near the flame anchoring region, leading to a significant overestimation compared to the dilatation profiles. Overall, the measurements show that the response for different strain rates and equivalence ratios closely follow the non-monotonic behaviour observed in the counter flow flame simulations. These results highlight that can introduce significant errors when used to obtain the flame length, which is important for measuring the mean flame speed, or obtaining convective time delays, which are crucial for thermoacoustic analysis. Novelty and Significance Statement : Through simulations and experiments, we systematically assess the effect of strain rate and equivalence ratio on three markers of heat release rate (HRR): -chemiluminescence, OH-PLIF, and the dilatation rate in lean premixed hydrogen flames. Local discrepancies between these markers are shown to arise from the thermodiffusive properties of hydrogen flames in the presence of high tangential strain rate, which modifies the local flame structure. The dilatation field is found to provide a robust quantitative marker for HRR, becomes increasingly decorrelated with the HRR for increasing tangential strain, and OH-PLIF is challenging to interpret due to the appearance of OH-concentration in the post-flame region.