Effect Of Finite Mixing Timescales In Nonpremixed Rich Relaxation Lean (NRRL) Combustors

A novel Nonpremixed Rich Relaxation Lean (NRRL) combustor concept has been recently introduced as a method to enable fuel- and operational flexibility in gas turbines. It features a multistage process starting with a non-premixed zone where a portion of air and fuel burn stoichiometrically. The resulting products mix with the remaining fuel, relaxing to a rich equilibrium before the balance of air is added, creating a lean burning zone. Fuel flexibility is enabled by both the nonpremixed flame and the rich relaxation zone. The latter requires long residence times and high pressures for low NOx levels. Previous research focused on the chemical kinetics of NOx production, examining timescales and sensitivities to staging levels, residence times, and operating conditions. This paper investigates how finite-rate fluid mechanics impact NOx emissions, particularly through relaxation dynamics in the rich zone and pre-flame mixing in the lean zone. Entrainment parameters are introduced into a 1D Lagrangian chemical reactor network to model these effects. This work shows that in the rich zone, finite-rate mixing effects can promote faster relaxation towards rich equilibrium but also result in 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.