Bernhard Wegner

Analysis of NOX Formation in an Axially Staged Combustion System at Elevated Pressure Conditions

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bars. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30 deg. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2 , CO, CO2 , and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX ) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bars and 5 mm at 6.5 and 8 bars. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model. This model was also able to predict some key features of the flow such as the change of penetration depth with the pressure.

CFD Prediction of Partload CO Emissions Using a Two-Timescale Combustion Model

Todays and future electric power generation is characterized by a large diversification using all kind of sources, including renewables resulting in noncoherent fluctuations of power supply and power usage. In this context, gas turbines offer a high operational flexibility and a good turn down ratio. In order to guide the design and down select promising solutions for improving partload emissions, a new combustion model based on the assumption of two separate timescales for the fast premixed flame stabilization and the slow post flame burnout zone is developed within the commercial computational fluid dynamics (CFD) code ANSYS CFX. This model enables the prediction of CO emissions generally limiting the turn down ratio of gas turbines equipped with modern low NO x combustion systems. The model is explained and validated at lab scale conditions. Finally, the application of the model for a full scale analysis of a gas turbine combustion system is demonstrated.

Investigation of a Flame Anchored in Crossflow Stream of Vitiated Air at Elevated Pressures

The objective was to study the effect of equivalence ratio of secondary stage combustible mixture injected into the cross flow stream of vitiated air in a two staged combustion system on the characteristics of the secondary stage combustion zone. The primary cylindrical combustor equipped with low swirl air blast nozzle operating with kerosene generates vitiated air. A methane injector was flush mounted to the inner surface of the secondary combustor. It was used to inject the premixed methane-air mixtures perpendicular into the crossflow of vitiated air. An optical, double shell, secondary combustor with three optical windows on its outer shell was used to image the secondary stage flames. The inner shell was a quadratic fused quartz tube which acts as a thermal barrier and the outer thick quartz windows mounted in the quadratic stainless steel chamber withholds the pressure. Chemiluminescence imaging technique equipped with ICCD camera was used to image the OH* emissions of the secondary stage flame.The vitiated air was generated at 2 bar and 1700 K. The velocity of the vitiated air in the secondary combustor was 57 m/s. A premixed methane air mixture was injected into the cross flow stream of vitiated air. The momentum flux ratio between the jet and the vitiated air was maintained at 1.4. The equivalence ratio of the premixed methane-air mixture was varied from 0.5 to 1.0. As the equivalence ratio of the secondary stage combustible mixture moves towards stoichiometric condition, the secondary stage combustion zone becomes compact and also the distance between the burner and the combustion zone decreases.The turbulent flame stabilized in the secondary combustor exhibited large scale structures and other unsteady phenomena that require time-resolved computational methods. Large eddy simulations (LES) are well suited to the calculation of such complex flows. The flame was embedded in a strong turbulent flow where auto-ignition and quenching are important, which poses a significant challenge for the reaction modeling. The presumed JPDF turbulent reaction model, which has been proven to be a reliable model for these challenging conditions, was successfully coupled with the LES simulation. The qualitative agreement between the results of simulations and measurements was quite satisfactory.Copyright

Experimental Investigations of Flame Stabilization of a Gas Turbine Combustor

While today’s gas turbine (GT) combustion systems are designed for specific fuels there is an urgent demand for fuel-flexible stationary GT combustors capable of burning natural gas as well as hydrogen-rich fuels in future. For the development of a fuel flexible, low-emission, and reliable combustion system a better understanding of the flow field – flame interaction and the flame stabilization mechanism is necessary. For this purpose, a down-scaled staged can combustion system provided with an optical combustion chamber was investigated in a high pressure test rig. Different optical diagnostic methods were used to analyze the combustion behavior with a focus on flame stabilization and to generate a comprehensive set of data for validation of numerical simulation methods (CFD) employed in the industrial design process. For different operating conditions the size and position of the flame zone were visualized by OH* chemiluminescence measurements. Additionally, the exhaust gas emissions (NOx and CO) and the acoustic flame oscillations were monitored. Besides many different operating conditions with natural gas different fuel mixtures of natural gas and hydrogen were investigated in order to characterize the flashback behavior monitored with OH* chemiluminescence. For selected operating conditions detailed laser diagnostic experiments were performed. The main flow field with the inner recirculation zone was measured with two-dimensional particle image velocimetry (PIV) in different measuring planes. One-dimensional laser Raman spectroscopy was successfully applied for the measurement of the major species concentration and the temperature. These results show the variation of the local mixture fraction allowing conclusions to be drawn about the good premix quality. Furthermore, mixing effects of unburnt fuel/air and fully reacted combustion products are studied giving insights into the process of the turbulence-chemistry interaction and reaction progress.Copyright

Analysis of NO

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bar. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30°. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2 , CO, CO2 and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX ) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bar and 5 mm at 6.5 and 8 bar. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model; this model was also able to predict some key features of the flow as the change of penetration depth with the pressure.Copyright