Measurement and Analysis of Flame Transfer Functions in a Lean-Premixed, Swirl-Stabilized Combustor with Water Injection

Abstract

Water injection offers a high potential for raising the electrical power output of stationary gas turbines without drawbacks in emissions. To achieve that, liquid water is injected into the primary combustion zone of the flame while simultaneously reducing the equivalence ratio. Using this method, both the thermal power output of the combustor and the mass flow through the turbine increase, which leads to a considerable augmentation of the electrical power at a constant flame temperature. Beyond that, it is well-known that present-day lean premixed combustors tend to drive thermoacoustic instabilities if operated under unfavorable conditions. In order to establish power augmentation by injection of liquid water in premixed combustors, more insight into the underlying mechanisms must be provided to avoid potential drawbacks regarding flame stability. In this work, we present an experimental study which is focused on the analysis of flame dynamics by means of Flame Transfer Functions (FTF) and phase-resolved OH∗-chemiluminescence. Experiments were carried out on a single burner test-rig with a swirl-stabilized, lean premixed flame. Water is injected on the burner central axis, whereby water-to-fuel ratios up to 1 are investigated. Additionally, spectra of dynamic pressure activity are discussed for different levels of stability and water-to-fuel ratios. Analyses presented here distinguish between water injection at a constant equivalence ratio (φ = 0.63 . . . 0.86) and a constant adiabatic flame temperature (Tad = 1773 . . . 2070K). For a constant equivalence ratio, both amplitude and phase of the FTF have found to be strongly influenced by the dilution effect of water; whereas, for water injection at a constant adiabatic flame temperature mainly the phase response was affected. When operating the combustor beyond the thermoacoustic stability limit, we achieved stable conditions by injecting water, even when the thermal power is increased at constant flame temperature.