Large Eddy Simulation (LES) Of Nonpremixed Flames By Hydrogen-enriched Fuels With Detailed Chemical Mechanisms

Combustion of fossil fuels possesses the highest consumption rate in human activities to get energy. More and more attentions are paid on carbon-free energy and environmental pollution which requires reducing greenhouse gas (CO2) emissions, improving combustion efficiencies, and developing more clean fuels. Nonpremixed combustion of hydrogen or syngas is one of the most fundamental and potential subject for designing new combustor configurations, pre-combustion of carbon capture and controlling the emissions of NOx. Numerical simulation with high resolution approach is a productive tool to investigate turbulent combustion. LES has the ability to capture the large scale turbulence motion directly, and the small vortical structures are described by subgrid scale model, which made it widespread in performing studies on combustion science.Standalone linear eddy model (LEM) is based on a phenomenological description of turbulent mixing on a one-dimensional domain, which can be considered to be a line of sight through the turbulent flow. LEM treats the three-dimensional advective turbulent stirring by one-dimensional stochastic rearrangements, in which the diffusive molecular mixing and the chemical reactions are solved directly. It has the nature to represent turbulence-chemistry interactions in flame propagation when coupled to LES code. Hydrogen-enriched fuels have some drawbacks such as high diffusivity, wide flammability range, short auto-ignition time, and high flame temperature that not only make the use of hydrogen unsuitable for the traditional combustion systems, but also increase the potential of explosion hazards. Meantime the combustion kinetics of H2 or H2/CO are essential part of most hydrocarbon fuels. It’s highly demanded to employing detailed chemistry when modelling the flames.This paper discretizes the LES governing equations by finite volume method, and the LEM method is implemented into the original code to enclosure the subgrid unsolved terms. After a thorough review of H2 and H2/CO chemical kinetics in combustion, several new mechanisms of different research groups have been selected and coupled into the LES-LEM code, which leads to a new simulation tool for turbulent combustion. A test of its computational performance in many aspects, including an evaluation of computation complexity, a speedup test and a simulation of complex cold swirl flow, indicates that the new model has the ability to capture the complex vortical structure in turbulence flow. The following studies have been accomplished with the new coupled model.A 19-step H2/O2 combustion mechanism and a 31-step H2/CO mechanism have been coupled into the LES-LEM code. Extensive numerical tests were performed for two typical nonpremixed flame combustors from TNF database with hydrogen-enriched fuels. Comparisons with the experimental data, especially in the shear layer with high flame instabilities, show that the flame temperature profile and species mass fraction profile as well as their RMS fluctuation were well-predicted. The submodels of LEM and combustion mechanisms are capable to describe the flame-turbulence interactions in nonpremixed flames by hydrogen-enriched fuels.Based on the profiles of temperature and other variables as well as the production and consumption of radical H, the fuel variability analyses indicate that when the hydrogen content is increased in the fuel mixture, the flame becomes less vortical and wrinkled due to the more diffusive and less viscous flow field. Lower burning speeds and lower temperature values was well as more vortical structures are found for the hydrogen-leaner mixtures. The case with increasing pressure show that pressure has the positive effects in flame development and pollutant formulation. The addition of H2O in the coflow play a role in the auto-ignition process and NO formulation by more OH and HO2 depending on the dominant chemistry at the flame base.Five different chemical mechanisms for hydrogen combustion are selected in large eddy simulation of hydrogen jet flames to investigate the ability of these mechanisms to represent the turbulence-chemistry interaction and other combustion phenomena. The mechanisms studied include a three-step reduced mechanism, two detailed H2/O2 reaction mechanisms, as well as a detailed H2/CO mechanism and the GRI3.0 mechanism. Linear eddy model is incorporated to evaluate the effect of turbulence-chemistry interactions. Extensive simulations of a well-known experimental case (German Aerospace Centre DLR nonpremixed flame M2) have been performed for the purpose of validation. Comparisons against experimental data including scalar distribution profiles are presented where a reasonable agreement is observed for the detailed mechanisms. Flux analyses of the species conservation equations and ignition delay time tests showed that chemical kinetics plays a role in the development of flame structures in the jet flame. The reduced mechanism has a damping effect on turbulence due to the dissipation caused by an excess of heat-release and a poor prediction of some intermediate species as well as the flame/turbulence interactions. The O Conaire mechanism exhibit the best overall performance.The five mechanism are also studied with simple reactors for the calculation of ignition delay times and laminar flame speed. The aim of this study is to identify a reaction mechanism that accurately represents H2/O2 kinetics over a large range of conditions, particularly at elevated pressures as present in a gas turbine combustor. And same work is done for nonpremixed flames by hydrogen-enriched fuels in three-dimensional domain after a definition of ignition delay time. Great deviations are found for ignition delay times between simple reactors and real flames, which means that some original theories cannot be applied to real flame directly.This study highlights the importance of precise descriptions of the chemical kinetics in LES of nonpremixed hydrogen/syngas combustion. It is hoped to provide some practical reference information for designing hydrogen/syngas combustor.