Numerical Study of Flame Anchoring in Micromixer Tubes

Abstract This paper investigates the influence of thermo-physical conditions and mixture compositions on the development and anchoring of flame inside a set of micromixer tubes with characteristic dimensions of those used in industrial gas turbines for a partially premixed CH4-H2 blended fuel mixture, and thereby attempts to develop measures to avoid flame inside the tubes. An experimental test rig was built to study the thermo-physical factors that govern the onset of flame anchoring inside the tubes, over a wide range of operating conditions, geometric configurations and fuel compositions. An initial flame is established in the tubes by lighting up a flame torch over a very short span of time. The high-speed flow flushes the flame out of the micromixer tubes until a thermo-chemical state is reached that allows the flame to anchor inside the tubes. This study is primarily focused on using high resolution CFD model to predict the onset of flame anchoring for one geometric configuration. The air flow rate upstream of micromixer tubes is fixed and the fuel composition is varied by increasing the mole fraction of H2 from 70% until a flame is detected inside any of the micromixer tubes. The fuel flow rate is adjusted to maintain a fixed adiabatic flame temperature, derived from equilibrium, across all the conditions. The CFD results are validated with the experimental results. A high fidelity LES model with species transport combustion model, a 22-species chemical mechanism, stiff chemistry solver and adiabatic wall boundary conditions is used for the simulations. The role of turbulence fluctuations in mixture composition is also investigated in the development of flame and flow rate in each micromixer tube using CFD solutions. Experimental and numerical results are compared for characteristic unburnt air velocity, combustor inlet air temperature, and target adiabatic flame temperatures that can sustain GE Vernova F class combustor operation with margin. In the experiment, a stable flame is detected inside the tubes with high H2 percentage, which the numerical simulation predicted with around 8% variation in H2 proportion. The scope of the work includes improvement of CFD prediction by accounting for heat loss through the micromixer walls with realistic wall boundary models.