Effect of the Dilution Ratio and Inlet Structure on High CO₂-Diluted Methane Combustion for a Supercritical CO₂ Brayton Cycle

Adding a combustion heater to the traditional supercritical CO₂ (sCO₂) Brayton cycle can significantly improve the response speed of the system. This application scenario (combustion conditions) is proposed for the first time, and the influence of different operational and design parameters is numerically studied. The results show that the CO₂ dilution ratio can adjust the O₂/CO₂ ratio of 'the combustion stream', thus changing the flame location and combustion efficiency. For low inlet speed, the flame location will move upstream and become unstable, which is harmful to the injector. In addition, variations in pressure have a limited impact on the flame, except that the inflow velocities and densities are modified accordingly. For the inlet structure, different slope angles can provide different tangential and axial velocities of the inner CO₂ dilution, thus changing the flame characteristic. However, the combustion efficiency can be further improved. By providing a well-stirred mixture in the fuel and oxidizer stream, a recessed fuel injector has evidenced a high improvement in the combustion efficiency. However, adjusting the CO₂ dilution ratio is also needed to optimize the flame location. Finally, the optimal flame location can be obtained with a high efficiency of 95.2%. The current study can provide a basis for the combustion heater's application in the sCO₂ Brayton cycle.