Simplified reaction mechanisms for the oxidation of hydrocarbon fuels in flames have been systematically examined using a numerical laminar flame model. These mechanisms include both one- and two-step global reactions, as well as quasi-global mechanisms. The aim of this study was to investigate the effects of various reaction rate parameters on the predicted flame properties, specifically flame speeds, by comparing them to experimentally observed values in selected fuel-air mixtures.
Several fuels were studied, including n-decane, methyl-substituted n-paraffins, acetylene, and a variety of olefin, alcohol, and aromatic hydrocarbons. By adjusting reaction rate parameters, the study sought to achieve the best agreement between the computed and experimental flame speeds. This led to the identification of the influence of these parameters on key laminar flame properties such as flame speed, flame temperature, and the composition of the burned gases.
In the study, it was found that the often-used choice of simultaneous first-order dependence for both fuel and oxidizer in global rate expressions was insufficient to accurately predict the rich flammability limits. Specifically, this approach did not capture the correct behavior at higher fuel concentrations, leading to inaccuracies in predicting the conditions under which a flame would propagate.
However, the best choice of reaction rate parameters was able to reproduce the rich and lean flammability limits. Additionally, this choice adequately reflected the dependence of flame speed on pressure and equivalence ratio across all fuels examined. This approach provided a more accurate representation of flame propagation and burning conditions compared to the first-order global rate expressions.
Furthermore, the two-step and quasi-global mechanisms were able to offer insights into the flame temperature and the composition of the burned gases, which are crucial for understanding the combustion process in more detail. Despite these improvements, it was clear that none of the simplified mechanisms studied were able to accurately describe the chemical structure of the flame itself. This limitation highlights the complexity of flame chemistry, where simplified mechanisms may fail to capture the finer details of molecular interactions and reaction pathways.
Overall, the results underscore the importance of selecting the appropriate reaction rate parameters when modeling combustion processes. Although simplified mechanisms can provide valuable insights into flame behavior and combustion characteristics, they still cannot fully capture the intricate chemical dynamics of the flame. Thus, further refinement of these mechanisms or the use of more detailed models may be necessary for more accurate predictions of combustion processes, particularly in more complex systems.
