Emission Characteristics and Visualization of an Axial-Fuel-Staged MILD Combustor

Abstract

ABSTRACT Axial-fuel-staged combustion is a promising approach to control NOx emission at high combustor outlet temperatures by shortening the residence time of burnt gas in high-temperature region. In this paper, an axial-fuel-staged MILD combustor was studied experimentally. The combustor consists of a first MILD burner producing hot burnt gases and a secondary transverse jet nozzle injecting premixed fuel and air mixtures. By varying the secondary stage equivalence ratio (Φ2) at several constant overall equivalence ratios (Φt), NOx reduction (up to 35%) has been achieved at higher Φt but failed at lower Φt. And by increasing Φ2 at constant first stage equivalence ratios (Φ1), a continuous NOx emission increase was observed, which is insensitive to the variation of Φ1. Furthermore, the liftoff behavior of the reaction region was also studied and it was found to play important roles in the dilution of the jet mixtures, as well as OH* intensities and NOx emissions. With the decrease of Φ1, the OH* chemiluminescence images present a decreased OH* intensity when Φ2 is lean but an increased OH* intensity when Φ2 is rich. Finally, an opposed-flow flame calculation was performed and the experimental results were well-supported by this numerical study.