To conform with ever-stringent emission regulations, accurate prediction of pollutant formation is essential in developing modern propulsion engines. With the aim of single-digit ppm nitrogen oxides (NOx) emission for gas turbines, significant research thrust has been put forward in developing detailed gas phase chemical kinetic models. The development of such models significantly depends on the homogeneous experimental targets (ignition delay data, speciation, and temperature etc.) from shock tubes, jet-stirred reactors, and flow reactors, and with lesser attentions directed towards transport-dependent validation targets (e.g., laminar flame speed, speciation) from laminar premixed and opposed diffusion flames. In this study, several recent detailed chemical kinetic models have been computationally investigated to assess the predictive competency of nitrogen oxides for gaseous low carbon number fuels, namely- methane (CH4), ethylene (C2H4), and ethane (C2H6). The primary objective is to understand how the recent models perform in predicting nitric oxide (NO) emissions at atmospheric and high-pressure flame conditions including rich flames. Three different detailed kinetic models are selected for this assessment - Ranzi et al. (2012), Glarborg et al. (2018), and Ahmed et al. (2019), and each of these models includes its own hydrocarbon and NOx reaction pathways. To perform the kinetic analysis, we employed one-dimensional laminar premixed flame calculations using chemical kinetics package Cantera (v2.3) against two different experimental targets: (a) laminar flame speed measurements, and (b) speciation data of nitric oxide (NO). The study is conducted at atmospheric and higher pressure (12 atm) for a wide variety of equivalence ratios, Φ=0.5-1.55. The kinetic simulation shows that for these high temperature flames, each of the kinetic models satisfactorily reproduce the experimental measurements of laminar flame speed at atmospheric and high pressure within the reported experimental uncertainty. For the predictions of NO species at atmospheric pressure, the simulation suggests acceptable emulation of experimental observations for lean to slightly rich flames (Φ=0.5-1.05), and the greater discrepancies emerge for all three models at the rich flame conditions (Φ=1.1-1.55). At higher pressure (12 atm), the NO prediction is found to be consistent with the above atmospheric pressure observation, with significant underprediction at the rich flame conditions. These contradictory observations for flame speed vs. NO speciation clearly signals that a global marker such as laminar flame speed alone is not adequate for model validation. We then studied the role of Zel’dovich (thermal-NO) vs. Fennimore (prompt-NO) reactions pathways by systematically switching off the thermal-NO pathway reactions and confirmed that the prediction discrepancies at the rich mixtures are primarily linked to the prompt-NO pathways. To further understand the key reactions involved in rich flames related to NO evolution, we performed sensitivity and rate of production (ROP) analysis at the Φ = 1.3 condition for C2H6 flames. It is found that the leading NO production reaction for Ahmed et al. model is HNCN+O = NO+HCN, whereas for the other two models primary production reaction is NCN+O=CN+NO. On the NO consumption side, the primary reaction is HCCO+NO=HCNO+CO for Ranzi et al. and Ahmed et al. models as opposed to CH+NO=HCN+O for Glarborg et al. model. Furthermore, the complete ROP analysis for all three models manifests that a consensus for the main production and consumption reactions for NO at rich flames is yet to be established for these recent models. Finally, this study highlights the key reactions responsible for the poor prediction performances of NO species at rich flame conditions and contribute in bringing those for kinetic modelers attention for future model development and refinement.

Kinetic Study of Nitric Oxide Formation at Atmospheric and High-Pressure Conditions for Different Primary Gaseous Fuels

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.