Characterizing the Mechanism of Lean Blowout for a Recirculation-Stabilized Premixed Hydrogen Flame

Abstract

The stability of hydrogen combustion under lean premixed conditions in a back-mixed jet-stirred reactor (JSR), is experimentally and numerically investigated. The goal is to understand the mechanism of flame extinction in this recirculation-stabilized flame environment. Extinction is achieved by holding the air flow rate constant and gradually decreasing the flow rate of the hydrogen fuel until a blowout event occurs. In order to gain insight on the mechanism controlling blowout, two dimensional computational fluid dynamic (CFD) simulations are carried out for the lean premixed combustion (LPM) of hydrogen as the fuel flow rate is reduced. The CFD model illustrates the evolution of the flow-field, temperature profiles, and flame structure within the JSR as blowout is approached. A single element chemical reactor network (CRN) consisting of a plug flow reactor (PFR) with recirculation is constructed based on the results of the CFD simulations, and its prediction of blowout is in good agreement with the experimental results. The chemical mechanism of Li et al. is used in both the CFD and CRN models, and GRI is used in the CRN for comparison. The modeling suggests that lean blowout does not occur with the flame in a spatially homogeneous condition, but rather under a zonal structure. Specifically, the flame is stabilized by the entrainment of combustion products from the re-circulation zone into the base of the reactant jet. The mixture of hot products and incoming premixed reactants proceeds through an ignition induction period followed by an ignition event. As the fuel flow decreases, the induction period increases and the ignition event is pushed further around the recirculation zone. Eventually, the induction period becomes so long that the ignition is incomplete at the point where the recirculating gas is entrained into the jet. This threshold leads to overall flame extinction.

Design of a Small-Scale Supercritical Water Oxidation Reactor. Part I: Experimental Performance and Characterization

“…This is analogous to operating combustion systems, which become more uniform at conditions when the flames velocities are reduced or reactor loading increases. 33,34 CFD simulations confirm that increasing fuel concentration shifts the primary reacting zone…”

Design of a Small-Scale Supercritical Water Oxidation Reactor. Part I: Experimental Performance and Characterization

“…This is analogous to operating combustion systems, which become more uniform at conditions when the flames velocities are reduced or reactor loading increases. 33,34 CFD simulations confirm that increasing fuel concentration shifts the primary reacting zone Corresponding Author *ivn@uw.edu; +1 206 543-5248; ORCID: 0000-0002-6347-7450…”

Design of a Small-Scale Supercritical Water Oxidation Reactor. Part II: Numerical Modeling and Validation

“…Experimentally in combustion scenarios, it is challenging to achieve the PSR limit even in the high-intensity reactors for hydrocarbon fuels [80][81][82][83] and hydrogen. 71,84 As the combustion approaches the PSR limit, the reaction reaches the blowout. To evaluate the stability of the SCWO process, we examine local Da and turbulent Re of the location where maximum oxidation rates are observed.…”