Thermally induced stresses and NO_x formation by hydrogen and natural gas combustion: Applying fluid-structure interaction on forced-draft burner components

The combined application of CFD and FEM within a fluid-structure interaction framework reveals the effects of thermally induced stresses on burner components resulting from the transition from pure natural gas to pure hydrogen operation. The analysis focuses on a commercially available forced-draft gas burner operating at 358 kW and an air excess ratio of 1.2, with particular attention to an operating point located in the lower turndown ratio range. The temperature fields were obtained from detailed reactive flow simulations using the steady diffusion flamelet model in combination with the nitrogen augmented San Diego reaction mechanism. These data were used and applied as thermal loads in the structural analysis. Results from experimental and numerical investigations indicate that low load operation with pure hydrogen causes elevated temperatures in the near burner region. As a consequence of the thermophysical and combustion related properties of hydrogen, the flame root shifts closer to the stabilization region, resulting in locally intensified thermal loads. The present study demonstrates that, despite significant temperature increases during hydrogen operation, the resulting stresses in the most heavily loaded regions of the baffle plate remain comparable in both shape pattern and local maxima. A global increase in stress levels across the entire baffle plate is observed, with particular focus on the localized rises at manufacturing related features such as the bends of the swirl guiding vanes. Furthermore, the study demonstrates that variations in the baffle plate geometry influence NO_x emissions, thermal loads and the shape of the stabilizing flame.