Laminar burning velocities of rich NH3+N2+O2 flames: Comparing the effects of elevated temperatures and oxygen ratios on model validation

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Ammonia (NH3) is a promising carbon-free alternative fuel. One of its fundamental combustion properties, the laminar burning velocity (SL), is widely used to validate the combustion kinetic models. This study investigates the SL across varying unburned mixture temperature Tu and oxygen ratio xO2, particularly regarding their impact on the model validation. The focus is on the fuel-rich conditions that exhibit noticeable discrepancies among different kinetic model predictions. To provide accurate experimental data for the analyses, atmospheric measurements were carried out using the heat flux method, covering equivalence ratios ranging from 1.2 to 1.9, Tu from 298 K to 448 K, and xO2 from 0.30 to 0.38. Notably, many of these conditions have not been thoroughly explored in existing literature. Seven kinetic models were used to compare with the experimental data and assess the simulation discrepancies, where the increment parameters were used to compare the effect of SL vs. Tu and SL vs. xO2 dependences on the model validations. Through these analyses, the α coefficient, describing the SL vs. Tu dependence, is found too difficult to distinguish the performances of the various models and serve as validation targets. Sensitivity analyses reveal that the reactions governing the dependences of SL on both Tu and xO2 are fundamentally the same. Similar flame conditions with nearly identical sensitivity values were identified, suggesting that a model accurately reproducing the SL vs. Tu trend is promised to replicate the SL vs. xO2 trend, and vice versa. Consequently, the SL measurement with elevated Tu, which is difficult in experiments and in distinguishing the performance of different models, is suggested to be substituted by the much easier measurement with elevated xO2. This conclusion, alongside the methodology, holds potential for application in other flame types and help reduce the experimental efforts for model updating.