The effect of secondary air inlet conditions on natural gas combustor is investigated numerically. Secondary air inlet conditions include its amount, position, total number of inlet ports and its arrangement along the combustor. The secondary air is introduced normally through inlet ports at different levels along the combustor. Each level includes a number of ports distributed around the combustor periphery. The number of ports levels varied from four up to sixteen and the number of ports in each level varied from four up to sixteen ports. Thus, the total number of ports varied from 16 up to 256. The combustor used has an air swirler at its upstream. Primary air, secondary air and fuel lines are also included. The sheer-stress transport (SST) k-omega model was used to simulate the turbulent isothermal flow and the non-premixed combustion model was used to simulate the turbulent reacting flow. For validating the model, a comparison between the measured and the calculated axial temperature distribution is made which show a reasonable agreement. Primary air swirl number of 0.87 and air to fuel ratio of 30 are used in this study. Secondary air leads to a decrease in flame size. For secondary to primary air ratio (SPAR) greater than 0.3, the flame became narrower in diameter and shorter in length. For certain secondary air configuration, NO, CO, CO2 are decreased with secondary air and are further decreased when increasing the value of SPAR.

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The present work numerically investigates the effect of secondary air conditions on the combustion of natural gas/air mixture in a gas turbine combustor. Secondary air is introduced normally at the combustor first half. Secondary air test conditions include its flow rate, entry position, and its arrangement around the combustor periphery and the total number of inlet ports. Secondary air inlet ports are located at different levels along the combustor length. Each level includes a number of inlet ports uniformly distributed around the combustor periphery. The number of ports levels varied from four to sixteen and the number of ports in each level varied from four to sixteen ports. Thus, the total number of ports varied from 16 up to 256. The primary air swirl number is kept constant during tests taking the value of 0.87. A three dimensional (SST k-omega) model is used to simulate the turbulent isothermal flow and the non-premixed combustion model was used to simulate the turbulent reacting flow using a CFD package Fluent 12. For validation of the models used, a comparison between the calculated axial temperature distributions with the measured results of other investigators was made and showed a satisfactory agreement. Secondary air showed a remarkable effect on temperature distribution inside the combustor. For secondary to primary air mass ratio (SPAR) above 0.3, the flame becomes wider in diameter and longer in length when SPAR is increased. The NO increases by about 58% and 12 % when the SPAR increases from 0 to 90 % for the ports arrangement of 4x4. KEY WORDSNatural gas combustion, Pollutant emissions, primary air, secondary air, swirl number and CFD.---