Dual fuel issues related to performance, emissions, and combustion instability in lean premixed gas turbine systems

Lean-premixed combustion has become one of the most promising means of meeting stringent environmental requirements for the reduction of NO x emissions in land-based gas turbine engines. These systems must also be capable of operating with either natural gas or liquid hydrocarbon fuels for increased versatility; however, high performance, low pollutant emissions, and the absence of detrimental combustion instabilities must be maintained for these dual fuel conditions. The major objective of the present study is to determine the effect of various fuels (natural gas, ethylene, n-heptane, and JP-8) on the combustion instability characteristics of a model lean-premixed gas turbine combustor under well-controlled conditions.; The results indicate strong dependence of combustion instability on inlet air temperature, overall equivalence ratio, and fuel type. CH* chemiluminescence imaging indicates that two distinct instability mechanisms exist for natural gas combustion: flame-vortex interaction with periodic extinction and reignition, and ignition near the bluff centerbody. Acetone planar laser-induced fluorescence studies indicate that large-scale fuel unmixedness at leaner equivalence ratios results in increased localized equivalence ratio fluctuations, whereas the reduced unmixedness at higher equivalence ratios provides a more uniform fuel distribution. A comparison between the ignition and chemical time scales reveals regions where the ignition time is less than the period of the instability, which provides an explanation for the instability temperature dependence. Analysis of the measured species temperatures and concentrations with a transient homogeneous CHEMKINRTM model indicates that ignition of the corner recirculation zone mixture is a possible mechanism for instability initiation at lean equivalence ratios.; The CHEMKINRTM model indicates much lower ignition temperatures and times for ethylene than for natural gas, which explains the dramatic difference in the observed inlet air temperature dependence for these two fuels. Computational analysis of the initial fuel droplet size and droplet lifetimes indicates that the heavier hydrocarbon fuel (JP-8) could convect to the flame front and burn in a more diffusion style flame than that of n-heptane. Comparison of the liquid and prevaporized n-heptane tests reveals little difference in the instability patterns, which indicates that good atomization is achieved for this fuel and supports this hypothesis.