Simulation of nitrogen emissions in a premixed hydrogen flame stabilized on a low swirl burner

Abstract

There is considerable interest in developing fuel-flexible, low emissions turbines for power generation. One approach is based on burning a variety of lean premixed fuels with relatively low flame temperatures. Such flames can be stabilized in a low swirl burner configuration, for example, using a variety of fuels such as pure hydrogen and hydrogen-seeded hydrocarbon mixtures. However, many hydrogen-rich fuels are thermodiffusively unstable and burn in cellular flame structures, which can have a significant impact on the local nitrogen chemistry. These cellular burning patterns are characterized by a local enhancement of fuel concentration and a corresponding increase in local flame temperature just downstream. In turn, these regions become sites for enhanced thermal NOx production. The structure of these cells, and their impact on the net emissions of a flame is influenced by the global flame stabilization mechanisms and by local turbulence properties. Here we investigate the role of thermodiffusive instabilities on NOx emissions in the context of a laboratory-scale low swirl burner fueled with a lean hydrogen-air mixture at atmospheric pressure. The simulations show how the cellular burning structures characteristic of lean premixed hydrogen combustion lead to local and global enhancements in the NOx emissions. We quantify the chemical pathways that lead to the formation of NO and N2O, and how they are enhanced within local regions of intense burning.

DNS assessment of relation between mean reaction and scalar dissipation rates in the flamelet regime of premixed turbulent combustion

“…Nevertheless, the range of Re t ≈ 100 reached in the DNS by Nishiki et al [17,18] is typical even for recent simulations. For instance, among eight contributions that aimed at the DNS of premixed turbulent flames and were published in the proceedings of the recent 34th Combustion Symposium [12,[29][30][31][32][33][34][35], Re t was substantially larger than 100 in only two papers [29,34], with the combustion chemistry being reduced to a single reaction in five papers [12,30,32,33,35].…”

A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames

“…Accordingly, the range of Re t ≈ 100 reached in the DNS by Nishiki et al 28,36 is typical even for recent simulations. For instance, among eight papers that aimed at DNS of premixed turbulent flames and were published in the proceedings of the latest 34th Combustion Symposium, 45,[47][48][49][50][51][52][53] Re t was substantially larger than 100 only in two papers, 45,52 with the combustion chemistry being reduced to a single reaction in five papers. 47,[49][50][51]53 We may also note that the DNS data by Nishiki et al 28,36 were analyzed in several recent papers.…”

“…Nevertheless, as far as the goals of the present study are concerned, this limitation should not be overestimated, because (i) the governing phenomena, i.e., the baroclinic torque and dilatation, are localized to flame fronts, which are much thinner than the width yz , and (ii) small-scale effects are commonly less sensitive to the size of the computational domain. On the other hand, a low ratio of yz /L allowed Nishiki et al 28,36 to reach sufficiently large ratios of L/δ L , whereas the vast majority of recent DNSs 44,45,[47][48][49][50][51][52][53] …”

“…While the former case was selected in vast majority of recent DNS studies, 44,45,[47][48][49][50][51][52][53] the latter case addressed, in particular, by Nishiki et al 28,36 is also worth investigating. To conclude this section, let us compare the present simulations with previous DNS research [31][32][33][34] into vorticity transformation in premixed turbulent flames.…”

Fuel flexibility, stability and emissions in premixed hydrogen-rich gas turbine combustion: Technology, fundamentals, and numerical simulations

“…As a result, NO x emissions can rise. Bell et al [79,[131][132][133] recently investigated this phenomena and shed light on this unwanted effect if hydrogen is intended to be used to reduce NO x emissions.…”