Design and investigation of a central air jet flameless combustor

Abstract

Due to strict regulations on harmful emissions such as NOx and CO, many novel combustion techniques have been investigated in order to decrease emissions from gas turbines. One of these novel techniques is colorless distributed combustion (CDC) or flameless combustion (FC). CDC has been examined under reacting conditions using ANSYS Fluent at 27 different operating cases for the aim of developing design equations and a 3D flameless combustion map that includes the effect of air diameter, confinement size and excess air ratio on recirculation ratio to allocate flameless combustion regions. A simple lab scale combustor with central air jet arrangement is employed for the analysis. Eddy dissipation concept (EDC) either global or with detailed mechanism (DRM19) fails to predict the effect of excess air ratio in flameless combustion. A numerical approach for calculating the recirculation ratio is presented. As the air jet diameter decrease 40% there is about 54% increase in the recirculation ratio and about 90% decrease in the NOx emissions. It is found that above a recirculation ratio of 3 there is an insignificant effect on NOx emissions. Reducing the combustor diameter by a constant value for a given air diameter is found to result in a decrease in the recirculation ratio and the percentage of decrease is higher at high thermal intensities combustors. Moreover, it is observed that as the combustor diameter decrease to half of its value at constant air jet diameter and constant excess air ratio, there is an 80–90% increase in the NOx emissions inside the combustor. Furthermore, design equation is obtained to predict the appropriate air diameter that if combined with the available combustor diameter achieves a minimum recirculation ratio suitable to sustain flameless combustion.