EXPERIMENTAL AND NUMERICAL INVESTIGATIONS OF CHARACTERISTICS OF AN AXIAL JET IN THE VICINITY OF A CONFINED CONCENTRIC SWIRL FLOW IN A MODEL COMBUSTOR

Gas turbine combustion engineers strive to obtain high flame stability at low power conditions (idle); whereas good emissions characteristics are desired at high power conditions. Although lean combustors provide good emissions characteristics, they suffer from the issue of poor flame stability. Often a pilot flame along with a lean main flame is used in order to improve flame stability issue. Positioning of an axial jet concentric to a swirl jet is one of commonly used configurations for a pilot flame. The interaction between both co-annular jets leads to distinct flow patterns in the combustor and subsequently the flame structure is impacted. If the axial jet is able to penetrate the inner recirculation zone (IRZ) generated by the swirled flow of the main jet, then a jet type of flame is obtained. The jet flame in vicinity of co-annular swirled flow is also denoted as type 1 flame, and is known to have good flame stability. On the other hand, if the pilot jet is not able to penetrate the IRZ, then the recirculating type of flame is obtained. A recirculating flame type has good emissions characteristics but suffers from poor flame stability. In this work the numerical predictions have been performed to gain more insight into occurrences of these two different flame structures which have been experimentally recorded. Using ANSYS Fluent CFD software, non-reactive steady state turbulent flow simulations have been performed to understand the flow and mixing field in a 3D combustor. Laser Doppler Anemometry (LDA) and hydroxyl radical (OH*) chemiluminescence have been used to measure the flow and characterize the flame structure, respectively.