On the Effects of Water Injection on Highly Strained Premixed Hydrogen Flames

Abstract

The aerospace industry is under increasing pressure to reduce emissions and transition toward a more sustainable future. With recent technological advances and stronger emphasis on environmental protection, liquid hydrogen has gained attention as an alternative fuel for civil aviation due to its a vorable thermo chemical properties, including low minimum ignition energy, wide flammability limits, high energy content, and potential as a zero-carbon alternative to fossil fuels. However, operating hydrogen under such conditions presents challenges, including potentially higher engine-out NOx emissions due to higher flame temperatures, onboard storage difficulties, higher laminar flame speeds, and an adia batic stoichiometric flame temperature higher than that of natural gas. To mitigate NOx formation, water injection has emerged as a possible method due to water’shigh specific heat capacity and latent heat of evaporation absorb heat and lower combustion temperatures, with liquid water injection generally prov ing more effective than steam. Additionally, introducing flame strain has emerged as another potential strategy as highly strained flames have shown promising results for reducing NOx emissions. This thesis aims to qualitatively and quantitatively analyze the effect of water injection on highly strained premixed laminar hydrogen flames through the use of computational methods and in this way infer the possibility of combining these two methods to reduce NOx emissions in hydrogen combustion. In order to do that, Direct Numerical Simulations (DNS) of highly strained hydrogen flames with water injection with different setups were performed to conducted a parametric analysis of the effect of spray injection velocity, droplet diameter, and strain rate on the flame structure and emissions of NOx related species. The results revealed that for the baseline case with water injection a sharp reduction in the presence of key flame radicals and reductions in hydrogen reactivity and domain temperature, which lead to reductions of NNH, N2O, and NO emissions. These effects are enhanced with increasing water injection velocity likely due to the increasing momentum resulting in the evaporation of the droplet occurring closer to the flame front. Regarding the effect of increasing droplet diameter, it was verified that increasing diameters are also associated with larger reductions in NO emissions and radical presence, likely due to the higher droplet volume requiring more energy to evaporate and therefore evaporation occurring closer to the flame front. In a computational setup with a flame with higher strain rate than the baseline case, when injected with water at similar water loading, the reductions in radical compositions, hydrogen rate of production and NO emissions are more significant in the case with higher bulk flame strain rate. From these results, it can be concluded that increasing droplet diameter and water injection velocity has positive effects on reducing emissions of NOx related species. Furthermore, it can also be concluded that higher strain rates enhance the effects of water injection, presenting sharper reductions in key radicals and emissions of NOx related species.