Modifications in the existing power plants in reducing emissions of carbon monoxide (CO) and nitrogen oxides (NOx)is central to global reduction of pollutant emissions. Varying the swirl number (SN) of the combustor inflow can be used to modify the dynamics of the combustion and emissions. This paper presents numerical investigation into the effect of swirl number on the combustion and emission characteristics in a gas turbine model combustor. Flame with swirl numbers of 0.38, 0.67 and 1.15 were studied using hydrogen enriched flame. A modified two step global chemical reaction mechanism with 1-step global mechanism of hydrogen by Marinov was used to model the combustion of hydrogen enriched methane. The discrete ordinates (DO) radiation model with Weighted Sum of Gray Gases model (WSGGM) was used to account for the radiation heat loss between the combustion gases and combustion chamber wall. The developed model predicted the temperature profile and emissions within 10% of the experimental values. Results further show that flames with SN of 0.67 have the highest size and strength of the recirculation zone that enhance flame stabilization as compared to flames with other swirl numbers. The NOx emission was observed to decrease with increasing SN at all operating conditions. This was due to the decreased combustion maximum temperature as the SN increases. However, the dynamics of the CO emission depends on the fuel composition. For instance, pure methane (0% H2), flames with SN of 0.67 gave the minimum CO emission, while, the CO emission decreased with increasing SN when methane and hydrogen composition in the fuel mixture were of the same order. The decreased CO emission was due to improved mixing and higher flame temperature in the combustor that aid the complete combustion of CO to CO2.
Key words: NOx emissions; CO emissions; hydrogen-enriched methane; Swirl number; swirl stabilized combustor.
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