Effect of staged combustion on instabilities

Lean premixed prevaporized combustion is widely used to reduce NOx emissions from gas turbines. However, it is highly susceptible to acoustic oscillations, which can lead to large pressure fluctuations and structural damages. In the modern combustors, staged combustion technologies are usually adopted in lean premixed combustor. Generally, a staged combustion burner includes outer premixed and central combustion parts. In experiment it is observed that when the ratio of the fuel supplied to premixed and central part is changed, combustion instabilities occurring in the model combustor are influenced significantly. A one dimensional analytical model is introduced to analysis the problem, and the unsteady heat release model is supposed to be n-x model. A good agreement is found between the calculated and experimentally observed frequencies. It is also found that when the time delay in the n-r model changed, the unstable mode is moved from 3^rd to 2^nd In experiment it is also observed that when the fuel supplied ratio between premixed and central parts is changed, the unstable frequencies jump between two values, which are agreed with the calculated results. This supplies a possible explanation why the staged combustion technology has effects on instabilities: the time delay of the whole flame transfer function is changed with the variation of the fuel ratio between premixed and the central part of the burner.