CFD Investigation of the Effects of Different Diluents on the Emissions in a Swirl Stabilized Premixed Combustion System

Abstract

Recently, new cycles for power generation, such as wet cycles and cycles for CO2 capture, have gained increasing interest. These new cycles use some sort of dilution in the air/fuel mixture, e.g. steam or CO2. Gas turbine cycles using LCV gases can also be said to fit this description. Almost all modern gas turbines use a lean premixed combustion system, since it combines low NOx emissions with high combustion efficiency. The main objective of this paper is to study the influence of different diluents on the NOx and CO emissions at different inlet temperature, equivalence ratio, pressure and mass flow. The studied combustor was a premixed swirl stabilized combustor with optical access and emission sampling equipment. The combustor uses Danish natural gas as its main fuel. Computational fluid dynamics (CFD) has been employed to perform the investigations. It is common knowledge that turbulence models based on the Buissinesq assumption are not generally capable of handling a highly swirling flow in a correct way. Therefore, a differential Reynolds stress model (DRSM) has been employed for modeling of the turbulence. The turbulent combustion has been modeled with the level-set flamelet library approach (FLA). In this approach a laminar flamelet is linked to turbulent flow field via a non-reacting scalar G and its variance. The laminar flamelet is modeled with separate code. This code solves the combustion development with a detailed reaction mechanism for a laminar, non-stretched and premixed one-dimensional flame. This is of great importance when emissions are to be predicted. All fluid dynamics computations were performed with the commercial CFD code Star-CD, version 3.20, where the FLA combustion model was implemented through Fortran based user subroutines. The computed flow field was validated against experimental data during non-reaction flow conditions. The computations showed good agreement with the experimental data. The computed CO and NOx emissions showed the same trends as the experimental data for the reacting case with an undiluted flame, when the equivalence ratio was altered. The computed emissions were used to build up an emission map for different dilutions during different operation conditions.