58926 - Experimental and Numerical Investigation on the Effect of Pressure on Micromix Hydrogen Combustion 

Utilization of hydrogen as gas turbine fuel is a promising way to promote the transition to carbon free fuels for electrical energy and heat generation. The micromix (MMX) combustion principle is a technical approach for hydrogen combustion facing the high reactivity of hydrogen by multiple miniaturized non-premixed type flames. Based on a Jet in Crossflow (JICF) fuel injection configuration, the concept is inherently safe towards flashback. Although comparable hot temperatures arise in the flame, NOx generation is suppressed by short flame length and thus short residence time.

In gas turbines combustors, elevated pressure conditions are crucial for the combustion process. The pressure is a significant influencing variable on the reaction progress and thus the generation of NOx emissions. In order to guarantee a low NOx configuration over the entire operation range, three key phenomena have to be fulfilled at all times: (1) Stable vortex pair for recirculation and adequate flame stability, (2) proper momentum flux ratio of the JICF for flame anchoring, (3) merging of adjoining flames needs to be prevented for low NOx emissions.

The objective of this paper is to investigate the influence of pressure and combustor load variation on the micromix flame with focus on the MMX key phenomena and the NOx emissions. This paper shows, whether a numerical model can predict flame shape and NOx emissions at elevated pressure conditions. Furthermore, simulations of the MMX combustion configuration are done for the part load operation conditions. For validation purposes, the numerical results are validated with experimental results.

A computational fluid dynamic (CFD) model is presented for the design conditions of a hydrogen-fueled industrial gas turbine of 2 MW_el class. The numerical method bases on a steady RANS approach. The fuel oxidizing is modelled with the complex chemistry model. The reaction kinetics are based on a reduced GRI 3.0 mechanism, where carbon containing reaction and the nitrogen oxidation mechanism are excluded. Instead, the NOx emissions are simulated based on the Zeldovich mechanism. The computational domain comprises a complete single flame and the boundary conditions ensure its position inside an industrial combustor with MMX flames arranged annularly.

We find that the micromix flame can be simulated well with the underlying model. The MMX key phenomena, mentioned above, are well predicted. Although the absolute NOx emissions are exceeding the experimental results marginally, the trends regarding to a variation of pressure and load are well described. Interestingly, the flame shifts its anchoring point during the pressure increase from the segment step upstream to the injection point. However, the numerical model predicts a stable flame within all simulated operation conditions.