Effect of combustor pressure on emissions in flameless combustion mode: An experimental and computational investigation

Several experimental and numerical studies show complex trends of NO_x formation at elevated pressures. The present study aims to address this aspect through both experimental and computational investigations and reports the combustion characteristics of flameless combustion with gaseous fuel at elevated pressure conditions. The work focuses on understanding the effects of pressure, residence time, and air jet diameter on NO_x and CO emissions from a laboratory-scale flameless burner operating with LPG fuel up to 2 bar absolute pressure. A conical combustor with tangential air injection and central fuel injection is employed to generate strong recirculation of hot combustion products within the combustor volume. Comprehensive CFD analysis reveals the influence of air injection velocity and operating pressure on flow recirculation patterns, temperature uniformity, and residence time distribution within the combustor. The numerical simulations show the formation of significant recirculation zones and thermally uniform regions, measured using the reactant dilution ratio ($R_{dil}$) and temperature uniformity index ($R_{tu}$​). All cases exhibit a maximum $R_{dil}$ ​ greater than 2.5, which ensures the presence of a sufficiently heated and diluted environment conducive for sustaining flameless combustion under high-pressure conditions. A temperature uniformity index, $R_{tu}$, exceeding 0.9 indicates the formation of a uniform thermal field with minimal variation in the combustor volume. This study helps establish a detailed relationship between elevated pressure and its impact on CO and NO_x emissions. Increased combustor pressure is shown to reduce the $R_{dil}$, leading to higher NO_x emissions due to reduced flow rate, lower injection velocities, and increased residence time within the combustor volume. This results in a simultaneous reduction of CO emissions OH* chemiluminescence studies revealed an increased homogeneity in OH* species distribution during the flameless combustion mode with a reduction in the maximum combustion intensity.