The target of net-zero emissions set by the 2015 Paris Agreement has strongly commissioned the energy production sector to promote decarbonization, renewable sources exploitation, and systems efficiency. In this framework, the utilization of hydrogen as a long-term energy carrier has great potential. This paper is concerned with the combustion characterization in a non-premixed gas turbine burner, originally designed for natural gas, when it is fed with NG-H2 blends featuring hydrogen content from 0 to 50% in volume. The final aim is to retrofit a 40 MW gas turbine. Starting from the operational data of the engine, a CFD model of the steady-state combustion process has been developed, with reference to the base load NG conditions, by reducing the fuel mass-flow rate by up to 17% to target the baseline turbine inlet temperature. When the fuel is blended with hydrogen, for a given temperature at turbine inlet, an increase in the peak temperature up to 800 K is obtained, if no countermeasures are taken. Furthermore, the flame results are more intense and closer to the injector in the case of hydrogen blending. The results of this work hint at the necessity of carefully analyzing the possible NOx compensation strategies, as well as the increased thermal stresses on the injector.

Gas turbines are expected to play a key role in the energy production scenario in the future, and the introduction of carbon-free fuels is fundamental for the development of a sustainable energy mix. The development of a reliable numerical model is thus fundamental in order to support the design changes required for the burners. This paper presents the results of a numerical investigation on a turbulent, diffusive, combustion test case, with the purpose of identifying the best compromise between accuracy and computational cost, in the perspective of the model application in real, more complex, geometries. Referring to a test case has two main advantages. First, a rather simple geometry can be considered, still retaining a few peculiar flow features, such as recirculation vortices and shear layers, which are typical of real applications. Second, the experimental setup is much more detailed than in the case of real turbines, allowing a thorough model validation to be performed. In this paper, the Standard 2-equations k-ε model and the Speziale-Sarkar-Gatski Reynolds Stress Model are considered. Moreover, both the FGM combustion model and the detailed chemistry model are used, coupled with two chemical reaction mechanisms, and their results are compared. Finally, a standard and an enhanced near-wall approach are employed to solve the transport equations close to the walls. The results show a good agreement in the temperature distribution at the axial positions corresponding to the experimental measurements. Overall, the standard wall function approach for describing the near-wall flow proved to be more effective at increasingly higher distances from the jet centre. Such differences are related to the formulations employed by the two near-wall approaches, which led to changes in the predicted flow field around the fuel jet. Finally, the adoption of a reaction mechanism describing in detail the species concentration is mandatory whenever the reliable prediction of the NOx formation is of primary importance. The conclusion reached in this paper can be helpful for the development of reliable and cost-effective CFD models of turbine combustors.

The present paper is concerned with part of the work performed by Renault, IFPEN and Politecnico di Torino within a research project founded by the European Commission. The project has been focused on the development of a dedicated CNG engine featuring a 25% decrease in fuel consumption with respect to an equivalent Diesel engine with the same performance targets. To that end, different technologies were implemented and optimized in the engine, namely, direct injection, variable valve timing, LP EGR with advanced turbocharging, and diluted combustion. With specific reference to diluted combustion, it is rather well established for gasoline engines whereas it still poses several critical issues for CNG ones, mainly due to the lower exhaust temperatures. Moreover, dilution is accompanied by a decrease in the laminar burning speed of the unburned mixture and this generally leads to a detriment in combustion efficiency and stability. The optimization of in-cylinder turbulence plays a fundamental role in compensating this trend.The present paper is specifically focused on the characterization of the diluted combustion in the direct injection engine. The results of an experimental activity have been presented, aimed at characterizing the in-cylinder combustion process and the exhaust temperatures at 2000 rpm and variable load, both without dilution and with 20% of external EGR. At the same time, a 3-D numerical model for the in-cylinder turbulence and combustion simulation has been developed in Converge. The model embeds a user-specified laminar-flame speed submodel, which was derived from a 1-D combustion simulation model with detailed chemistry. The model has been calibrated against experimental data and then used to characterize the heat release dependence on the dilution. The experimental activity has evidenced the potential of EGR to increase the engine efficiency, by allowing to increase the boost level at full load and by reducing pumping losses at partial load. As far as the maximum allowed EGR rate is concerned, the CFD activity showed that the limit can be detected on the basis of a threshold value of the MFB0-50 interval. At 2000 rpm and medium load the maximum EGR rate ranged around 35% and showed an increasing trend versus load. It also demonstrated a decreasing trend against the engine speed.