NO formation in high pressure premixed flames: Experimental results and validation of a new revised reaction mechanism

Abstract

This paper presents an experimental and modelling study of NO formation in high pressure premixed flames.Experiments were performed in a high-pressure counterflow burner in which laminar premixed CH4/air flames were stabilised at equivalence ratios of E.R=0.7, 1 and 1.2 and for pressures varying from 0.1 to 0.7 MPa. We report quantitative NO mole fraction profiles measured by Laser Induced Fluorescence. The effects of pressure and equivalence ratio on NO formation are discussed. These results are compared to the simulations using two reaction mechanisms: NOmecha2.0 associated to a detailed mechanism for methane oxidation: GDFkin ® 3.0 and the mechanism from Klippenstein et al., which is the most recent high-pressure NOx formation mechanism available in the literature. In general, both mechanisms are able to predict NO correctly in lean and stoichiometric high pressure flames; however, in rich flames, GDFkin ® 3.0_NOmecha2.0 gives the best predictions. The performances of these mechanisms are also tested on NO measurements in high-pressure flames from the literature.A kinetic analysis is then presented to identify the main pathways that lead to the formation and consumption of NO and highlight the differences between the two mechanisms, as well as a sensitivity analysis to identify important reactions that influence the formation/consumption of NO in our high pressure flames.

Effect of hydrogen addition on NOx formation in high-pressure counter-flow premixed CH4/air flames

“…Very recently, Lamoureux et al [33] proposed a final version of their new detailed NOx chemistry sub-mechanism, named NOmecha2.0, validated at high temperature on a large experimental database obtained in laminar premixed flames, jet-stirred and plug-flow reactors under sub-atmospheric and atmospheric pressure conditions. This mechanism (GDFkin ® 3.0_NOmecha2.0: NOmecha2.0 [33] associated to GDFkin ® 3.0 [34]) was very recently validated in our high pressure counterflow CH4/air flames [35] and compared to the mechanism from Klippenstein et al [36], which is the most recent high pressure NOx formation mechanism available in the literature. In the present work, GDFkin ® 3.0_NOmecha2.0 [35] and Klippenstein [36] mechanisms will be used for validation in high pressure CH4/H2/air flames.…”

“…φC=0.7. This lean methane flame was previously studied in [35,40]. Hydrogen addition to CH4/air flames allows to extend the flammability limits towards very lean conditions.…”

“…Below those values of equivalence ratios, the flames became unstable. As for our previous work [35,40], the pressure was varied from 0.1 to 0.7 MPa. Figure 1 shows pictures of the CH4/H2/air flames stabilised in this work.…”

“…Figure 1 shows pictures of the CH4/H2/air flames stabilised in this work. As mentioned elsewhere [35,40], a stabilisation criterion was defined, using a telescopic sight with high magnification (allowing to distinguish a wire of 50 μm), placed on one of the visualisation window of the chamber in order to scrutinise the flamefronts. For each pressure condition, flames were considered as stable if their central shape appear flat in a diameter greater than the nozzle diameter (7 mm), and their position above the burner do not fluctuate by more than +/-50 μm around their average position.…”

“…Flame conditions are summarised in Table 1, together with adiabatic flame temperatures Tf and laminar flame velocities SL computed for a free flame configuration using LOGEsoft software (LOGEresearch v1.10.0 [37]) with GDFkin ® 3.0_NOmecha2.0 [35]. Note that the flame with equivalence ratio φC=0.61 could not be stabilised at 0.7 MPa and the flame with φC=0.57 was stabilised only at 0.3 MPa.…”