Harald Funke

Numerical and Experimental Characterization of Low NOx Micromix Combustion Principle for Industrial Hydrogen Gas Turbine Applications

The international effort to reduce the environmental impact of electricity generation, especially CO2-emissions requires considerations about alternative energy supply systems. An effective step towards low pollution power generation is the application of hydrogen as a possible alternative gas turbine fuel, if the hydrogen is produced by renewable energy sources, such as wind energy or biomass. The use of hydrogen and hydrogen rich gases as a fuel for industrial applications and power generation combined with the control of polluted emissions, especially NOx, is a major key driver in the design of future gas turbine combustors.The micromix combustion principle allows a secure and low NOx combustion of hydrogen and air and achieves a significant reduction of NOx-emissions. The combustion principle is based on cross-flow mixing of air and gaseous pure hydrogen and burns in multiple miniaturized diffusion-type flames. For the characterization of the jet in cross-flow mixing process, the momentum flux ratio is used.The paper presents an experimental analysis of the momentum flux ratio’s impact on flame anchoring and on the resultant formation of the NOx-emissions. Therefore several prototype test burner with different momentum flux ratios are tested under preheated atmospheric conditions. The investigation shows that the resultant positioning and anchoring of the micro flames highly influences the NOx-formation.Besides the experimental investigations, numerical simulations have been performed by the application of a commercial CFD code. The cold flow simulation results show the mixing of the air and hydrogen after the injection, in particular in the Counter Rotating Vortices (CRV). Furthermore, the hydrogen jet interacts also with another vortex system resulting from a wake flow area behind the combustor geometry. Furthermore, reacting flow simulations have been performed by the application of a Hybrid Eddy Break-Up (EBU) combustion model. The combustion pressure has been varied from atmospheric conditions up to a pressure of 16 bar.The experimental and numerical results highlight further potential of the micromix combustion principle for low NOx-combustion of hydrogen in industrial gas turbine applications.Copyright

Experimental and Numerical Characterization of the Dry Low NOx Micromix Hydrogen Combustion Principle at Increased Energy Density for Industrial Hydrogen Gas Turbine Applications

In the future low pollution power generation can be achieved by application of hydrogen as a possible alternative gas turbine fuel if the hydrogen is produced by renewable energy sources such as wind energy or biomass. The utilization of existing IGCC power plant technology with the combination of low cost coal as a bridge to renewable energy sources such as biomass can support the international effort to reduce the environmental impact of electricity generation. Against this background the dry low NOx Micromix combustion principle for hydrogen is developed for years to significantly reduce NOx emissions. This combustion principle is based on cross-flow mixing of air and gaseous hydrogen and burns in multiple miniaturized diffusion-type flames. The two advantages of this principle are the inherent safety against flash-back and the low NOx concentrations due to a very short residence time of reactants in the flame region of the micro-flames.The paper presents experimental results showing the significant reduction of NOx emissions at high equivalence ratios and at simultaneously increased energy density under preheated atmospheric conditions. Furthermore the paper presents the feasibility of enlarged Micromix hydrogen injectors reducing the number of required injectors of a full-scale Micromix combustion chamber while maintaining the thermal energy output with significantly low NOx formation.The experimental investigations are accompanied by 3D numerical reacting flow simulations based on a simplified hydrogen combustion model. Comparison with experimental results shows good agreement with respect to flame structure, shape and anchoring position. Thus, the experimental and numerical results highlight further potential of the Micromix combustion principle for low NOx combustion of hydrogen in industrial gas turbine applications.Copyright

NUMERICAL AND EXPERIMENTAL INVESTIGATIONS ON THE FLOW IN A 4-STAGE TURBINE WITH SPECIAL FOCUS ON THE DEVELOPMENT OF A RADIAL TEMPERATURE STREAK

In the development of modern gas turbines the increase of the turbine inlet temperature is restricted by the need to cool the first stages of the turbine. In addition the flow leaving the combustor is thermally inhomogeneous. Since the blade cooling has to be designed for the actual local hot gas temperatures, it is important to know how these temperature inhomogeneities develop and attenuate inside the multistage flow passage.In this investigation the flow inside a 4-stage turbine, which is set up in a test rig at the Institute of Steam and Gas Turbines, Aachen University of Technology, is calculated with a state-of-the-art fully three-dimensional Navier-Stokes solver based on an accurate finite volume scheme. The stator and rotor rows are coupled via mixing planes. The turbine is a scaled down original turbine with realistic axial gaps.The homogeneous reference case is qualified by comparison to recent experimental data gathered at the test rig. Therefore, the flow is extensively measured at several locations. In a second step a radial temperature streak is set at the inlet for the same point of operation. The results show the development of the temperature streak through the four stages. With this information the underlying mixing processes are described and analysed. It is found that the hot streak segregation effect is present in all four stages.Copyright

COMPARISON OF NUMERICAL COMBUSTION MODELS FOR HYDROGEN AND HYDROGEN-RICH SYNGAS APPLIED FOR DRY-LOW-NOX-MICROMIX-COMBUSTION

The Dry-Low-NOx (DLN) Micromix combustion technology has been developed as low emission combustion principle for industrial gas turbines fueled with hydrogen or syngas. The combustion process is based on the phenomenon of jet-incrossflow-mixing. Fuel is injected perpendicular into the aircross-flow and burned in a multitude of miniaturized, diffusionlike flames. The miniaturization of the flames leads to a significant reduction of NOx emissions due to the very short residence time of reactants in the flame. In the Micromix research approach, CFD analyses are validated towards experimental results. The combination of numerical and experimental methods allows an efficient design and optimization of DLN Micromix combustors concerning combustion stability and low NOx emissions. The model is evaluated and compared to a detailed hydrogen combustion mechanism derived by Li et al. including 9 species and 19 reversible elementary reactions. Based on this mechanism, reduction of the computational effort is achieved by applying the Flamelet Generated Manifolds (FGM) method while the accuracy of the detailed reaction scheme is maintained. For hydrogen-rich syngas combustion (H2-CO) numerical analyses based on a skeletal H2/CO reaction mechanism derived by Hawkes et al. and a detailed reaction mechanism provided by Ranzi et al. are performed. The comparison between combustion models and the validation of numerical results is based on exhaust gas compositions available from experimental investigation on DLN Micromix combustors. The conducted evaluation confirms that the applied detailed combustion mechanisms are able to predict the general physics of the DLN-Micromix combustion process accurately.

Numerical and Experimental Investigations on Endwall Contouring in a Four-Stage Turbine

Secondary flows and leakage flows create complex vortex structures in the 3-D flow field of a turbine stage. Aerodynamic losses are the consequence. Reducing the aerodynamic losses by endwall contouring is subject of an actual investigation of the flow field in a 4-stage test turbine with repeating stages.Numerical 4-stage simulations are performed for a reference case of a turbine without endwall modifications and two different geometric configurations with endwall contouring. The numerical results for the reference case are compared to corresponding experimental investigations. Both, the experiment and the CFD focus on the stage exit flow field of the second, the third and the fourth stage of the actual four stage turbine. The 3-D flow field is calculated by application of a steady 3-D Navier-Stokes code.The numerical results of an arc-like endwall contouring at the casing are presented a) with a maximum deviation from the reference contour in the axial gap within the stages (“arc contour”) and b) with a maximum deviation in the axial gap between the stages (“off-set arc contour”). The results show a significant influence of the bumps on the blade’s profile pressure distribution near the radial gap, the leakage flow and the radial pressure field. A detailed secondary flow analysis shows the influence of the different endwall contours on the leakage vortex development.Finally, the aerodynamic efficiencies of the geometric configurations are compared. It is predicted that the off-set arc contour has a remarkable positive influence on the machine’s performance.Copyright

Numerical and Experimental Investigations of the Influence of Different Swirl Ratios on the Temperature Streak Development in a 4-Stage Turbine

In the development of modern gas turbines the increase in the turbine inlet temperature is restricted by the need for cooling the first stages of the turbine. In addition, the flow leaving the combustor is thermally inhomogeneous. Since the blade cooling has to be designed for the actual local hot gas temperatures, it is important to know how these temperature inhomogeneities develop and attenuate inside the multistage flow passage.In this investigation the development of a circumferential and a radial temperature inhomogeneity inside a 4-stage turbine is analyzed at three different swirl ratios. Since the experimental setup allows a circumferential temperature streak, a radial temperature streak has also been applied at different swirl ratios to the same geometrical configuration for a numerical investigation.The first stage has a significant impact on the attenuation and change in form of a circumferential temperature streak depending on the swirl. For the radial streak the hot streak segregation effect can be eliminated by increasing the swirl. Consequently, the temperature equalization process is weakened.Copyright