Effect of Finite Mixing Timescales in Nonpremixed Rich Relaxation Lean Combustors

Abstract A novel nonpremixed rich relaxation lean (NRRL) combustor concept was recently introduced as a method to enable fuel- and operational-flexibility in gas turbines. To review, the multistage combustor starts with a nonpremixed zone, modeled as a portion of the overall air and fuel initially burning stoichiometrically, and whose products then mix with the remaining fuel. This mixture ideally relaxes to a rich equilibrium and then the balance of air is added, resulting in a lean burning zone. The critical components in this staging strategy are the nonpremixed flame and the rich relaxation zone. The nonpremixed flame is inherently fuel flexible. The rich relaxation zone requires long residence times and high pressures to reach low rich equilibrium NOx levels. Previous work focused on the fundamental chemical kinetic behavior of the concept. Namely, the characteristic chemical timescales and kinetic mechanism responsible for NOx production, and the sensitivities of these emissions-controlling characteristics to staging levels, residence times, and operating conditions. The purpose of this paper is to show how finite-rate fluid mechanics affect exit NOx emissions, especially via the relaxation dynamics in the rich zone and preflame mixing in the lean zone. These finite-rate mixing effects are modeled by introducing entrainment parameters into the 1D Lagrangian chemical reactor network used previously. This work shows that in the rich zone, finite rate mixing effects can promote faster relaxation toward rich equilibrium despite a lower mass-averaged residence time for relaxation to occur. In the lean zone, finite rate effects can provide time to destroy NO forming precursors but can also extend residence times in states with high NO formation rates. The net result of these competing effects on exit NOx depends on the differing mixing timescales, and the different combinations of results are explored in this paper.