Direct numerical simulation of supercritical carbon dioxide oxy-methane combustion

Supercritical CO2 (sCO2) oxy-methane combustion is a key component of zero-carbon technologies in direct-fired sCO2 power cycles, i.e., the Allam cycle which offers promising solutions for clean and sustainable energy production. The use of sCO2 as both working fluid and diluent to moderate the combustor exit temperature at high pressure and high preheat temperature in the Allam cycle poses a unique combustion behavior. The effects of high sCO2 dilution on sCO2 oxy-methane combustion behavior, flame propagation, and flame stability are not fully resolved due to experimental challenges at such extreme conditions. This study addresses this major challenge by providing an understanding of the effect of sCO2 dilution on supercritical mixing and the combustion behavior in sCO2 oxy-methane combustion. A direct numerical simulation (DNS) integrated with the real-fluid equation of state is developed to provide the first DNS dataset for the realistic operating conditions of sCO2 oxy-methane combustors designed by Southwest Research Institute. The combustion behavior shows that sCO2 dilution has a major impact on mixing, heat release rate, temperature, and flame thickness. A peak in the heat release rate is identified for a given air–fuel ratio and the lowest CO production for 75%–80% CO2 dilution which results in a maximum temperature of 2000 K. By comparing the results obtained from ideal- and real-fluid equation of state, this study shows that real-fluid effects can significantly affect density gradient distribution and heat release rate, impacting supercritical mixing and flame dynamics under high sCO2 dilution. The results provide crucial insight for designing future sCO2 oxy-combustors.