One of the preferred ways to reduce NOX formation in an aero-engine is to operate lean throughout the whole operational range; however the lean combustion suffers from poor stability. To avoid the problem associated with stability, often a rich pilot flame is used along with a main flame to act as a source of heat and radicals to the main flame. The focus of the paper is to discuss the influence of the liquid fuel spray characteristics and effect of flow parameters on the lean blow out (LBO) limits of a piloted burner. In order to understand the observed remarkable LBO limits of the pilot flame with Jet A-1 (LBO = 145 kg-air to kg-fuel at 0.1 MPa of combustor pressure), velocity field measurements by laser Doppler Anemometry (LDA) technique have been performed. Furthermore, the flame structure has been analyzed with OH* chemiluminescence imaging. Experimental results show that the LBO limits of the burner running with liquid fuel further improves with an increase in combustor pressure. Such improvement in LBO limits is attributed to the change in the liquid fuel distribution caused by the change in the combustor pressure. For gaseous fuel measurements, results indicate that the equivalence ratio and the momentum ratio of the pilot jet to the co-annular flow are the dominating parameters that control the mixing process in the combustor and the ensuing effect on the flame structure and location of flame stabilization is substantial. The flame stabilizes either along the centreline or along the shear layer between two jets. Such information is useful in designing a lean partially premixed combustion system where a pilot flame is required to stabilize a main lean flame.
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.
In order to extend the operation regime of existing gas turbine combustion systems to lower the minimum loads, the applicability of matrix burners (arrays of jet flames) as an alternative to conventional swirl stabilized burners has been considered. In comparison to well-studied single jet flame systems, the effects of geometry and thermodynamic parameters on characteristics of matrix burner systems have not been studied in detail. Information, which is essential for design processes e.g. scaling of matrix burners, is not yet available in public domain. This work involves a systematic investigation of a matrix burner system operating at highly turbulent flow conditions (Reynolds Number ≈ 20000) prevailing in gas turbine combustion systems. In order to understand the effects of geometrical scaling, three variants of jet diameter have been investigated. A detailed test campaign including lean blow out limits detection, velocity field measurement and hydroxyl radical (OH*) chemiluminescence recording has been conducted. Influence of variation in stoichiometry and exit velocity of fuel-air mixture has been captured. The results show that it is possible to generalize the scaling of the matrix burner using the well-known Peclet criterion.
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