Laminar burning velocity and Markstein length of ammonia/air flames up to the initial mixture pressure of 2.0 MPa

The utilization of ammonia as a fuel is a promising method to achieve carbon neutrality by 2050. Ammonia utilization in the power generation sector is one of potential applications with numerous studies on ammonia combustion having been carried out for its application in gas turbines. The pressure ratio of the latest large-scale gas turbines is as high as over 20. Therefore, fundamental combustion characteristics at high pressure need to be clarified to employ ammonia for the fuel of large-scale gas turbines. In this study, laminar burning velocity and Markstein length were experimentally evaluated up to 2.0 MPa for the first time using a newly designed constant volume combustion chamber withstand a maximum pressure of 12 MPa. Spherically propagating ammonia/air premixed flames were observed using high-speed schlieren photography with a continuous light source and highspeed camera. Since laminar burning velocity of ammonia is slow, significant influence of buoyancy on laminar flame propagation characteristics should be taken into account. In this study, ignition influenced period and buoyancy influenced period were carefully determined. Using a non-linear relationship between flame propagation speed and flame stretch rate, the laminar burning velocity and Markstein length were determined. Laminar burning velocity was also evaluated using numerical simulations with detailed reaction mechanisms. As results, it was clarified that the reaction mechanisms developed by Gotama et al. and Han et al. agreed well with experimental results. In addition, an increase in flame propagation speed was observed especially at high pressure conditions due to the hydrodynamic instability even for ammonia flames which has a greater flame thickness. The results of this study are valuable for understanding and validating ammonia combustion chemistry at high pressure conditions.