Evaluation of Combustion Models and Reaction Mechanisms to Predict NOx and CO Emissions From Densely Distributed Lean-Premixed Multi-Nozzle CH4/H2/Air Flames

Abstract With global initiatives to reduce CO2 emissions, several new designs of gas turbine combustors that can operate with higher hydrogen content are being explored. Computational Fluid Dynamics simulation (CFD) has been an integral part of the design process of Gas Turbines. However, with an increased focus on design for fuel blends with high hydrogen content and in the absence of sufficient experimental or historical design data availability, the importance of CFD simulations is becoming increasingly critical. Predictive CFD simulations can provide significant insights into combustion behavior. The accuracy of simulations depends upon factors such as combustion models, reaction mechanisms, mesh resolution, etc. Most of the simulation methodologies developed in the past were focused on hydrocarbon fuels. Researchers have started focusing on simulation workflows for hydrogen fuels, considering lab-scale burners. Scale-up of these studies for complex industrial cases is required to develop methodologies that can be deployed for the industrial Gas Turbine design process. This work explores the importance of reaction mechanisms and combustion models on the flame length and emission characteristic prediction by CFD simulations of complex multinozzle combustor configuration, operating under CH4/H2 blend variations. For the study, both RANS and LES turbulence models are explored. Test data used for the analysis is taken from work published by KAIST University, U. Jin and K.T. Kim 1,2 on the investigation of combustion dynamics and NOx/CO emissions from lean-premixed multi-nozzle CH4/H2 blended flames. The combustion domain consists of densely distributed small-scale multi-tube injectors called Micromix nozzles. This setup provides insights into the collective behavior of small-scale multi-nozzle flames and resultant emission rates. Test data for different inlet compositions, keeping a thermal power condition of 78 kW, are considered for evaluation. Results from simulations for OH* chemiluminescence, OH concentrations, NOx, and CO emissions are compared against the test data. Reduce Model Fuel Library (MFL) mechanism with relevant NOx pathways along with Flamelet Generated Manifold (FGM) model found to predict the trend of flame length and emissions concentration with change in fuel composition reasonably well, compared to detailed chemistry combustion model as well as test data. However, for capturing impact of local non-unity Lewis number effects, detailed chemistry model found to be better for the low turbulent flow conditions, as considered in the referred experimental data.