No abstract

The regulatory restrictions, currently acting, impose a significant reduction of the Greenhouse Gas (GHG) emissions. After the coal-to-gas transition of the last decades, the fossil fuel-to-renewables switching is the current perspective. However, the variability of energy production related to Renewable Energy Sources requires the fundamental contribution of thermal power plants in order to guaranty the grid stability. Moving toward a low-carbon society, the industry is looking at a reduction of high carbon content fuels, pointing to Natural Gas (NG) and more recently to hydrogen-NG mixtures. In this scenario, a preliminary study of the BERL swirled stabilized burner is carried out in order to understand the impact of blending natural gas with hydrogen on the flame morphology and CO emissions. Preliminary 3D CFD simulations have been run with the purpose to assess the best combination of combustion model (Non Premixed and Partially Premixed Falmelets), turbulence model (Realizable k ɛ and the Reynolds Stress equation model) and chemical kinetic mechanism (GriMech3.0, GriMech 1.2 and Frassoldati). The numerical results of the BERL burner fueled with natural gas have been compared with experimental data in terms of flow patterns, radial temperature profiles, O2, CO and CO2 concentrations. Finally, a 30% hydrogen in natural gas mixture has been considered, keeping fixed the thermal power output of the burner and the global equivalence ratio.

Nowadays hydrogen is gaining more and more attention by Industry, Academia and Politics. Being a carbon free fuel, it is supposed to have a key role in the future energy scenario, especially if produced by renewable sources. The use of mixtures of hydrogen and conventional hydrocarbons in gas turbines is one of the most promising technical solutions for obtaining a sustainable combustion during the transition toward a full decarbonization. For this reason, it is fundamental to investigate the behaviour of fuels enriched with hydrogen in combustion processes. In this work, a lab-scale swirled premixed burner has been investigated by means of a fully 3D URANS approach. Firstly, a numerical simulation with cold flow has been performed to validate the model against experimental data. Then, reactive flow simulations have been performed. Initially, a combustion with 100% methane was considered. Then, a 30% by volume hydrogen blending has been investigated. The partially premixed combustion model has been implemented to take into account the inhomogeneities of the mixture at the chamber inlet. The variation of the flame structure due to the hydrogen enrichment will be described in terms of the temperature and species concentration distributions.

Hydrogen can play a key role in the gradual transition towards a full decarbonization of the combustion sector, e.g., in power generation. Despite the advantages related to the use of this carbon-free fuel, there are still several challenging technical issues that must be addressed such as the thermoacoustic instability triggered by hydrogen. Given that burners are usually designed to work with methane or other fossil fuels, it is important to investigate their thermoacoustic behavior when fueled by hydrogen. In this framework, the present work aims to propose a methodology which combines Computational Fluid Dynamics CFD (3D Reynolds-Averaged Navier-Stokes (RANS)) and Finite Element Method (FEM) approaches in order to investigate the fluid dynamic and the thermoacoustic behavior introduced by hydrogen in a burner (a lab-scale bluff body stabilized burner) designed to work with methane. The case of CH4-air mixture was used for the validation against experimental results and benchmark CFD data available in the literature. Numerical results obtained from CFD simulations, namely thermofluidodynamic properties and flame characteristics (i.e., time delay and heat release rate) are used to evaluate the effects of the fuel change on the Flame Response Function to the acoustic perturbation by means of a FEM approach. As results, in the H2-air mixture case, the time delay decreases and heat release rate increases with respect to the CH4-air mixture. A study on the Rayleigh index was carried out in order to analyze the influence of H2-air mixture on thermoacoustic instability of the burner. Finally, an analysis of both frequency and growth rate (GR) on the first four modes was carried out by comparing the two mixtures. In the H2-air case the modes are prone to become more unstable with respect to the same modes of the case fueled by CH4-air, due to the change in flame topology and variation of the heat release rate and time delay fields.

This work deals with numerical simulations of an annular burner with cross-section variations by using the FEM method. The main novelty is the introduction of the axial component of the mean flow in 3D simulations, which is usually neglected in the literature. The geometry investigated consists of an annular plenum chamber and an annular combustion chamber connected by 12 swirl burners. New insights are provided for a wide range of mean flow values, thus of Mach numbers. Firstly, a numerical assessment has been performed. Indeed, a simple annular geometry, which accurately approximates a real LP combustor, has been simulated and the results compared with the ones obtained with the software OSCILOS. The results achieved agree with the ones presented by Dowling and Stow [1], which demonstrated by applying a loworder one-dimensional model that the higher the mean flow velocity, the lower the frequencies and the growth rates gained.