Characterisation of Interaction between Combustion Dynamics and Equivalence Ratio Oscillations in a Pressurised Combustor

Abstract

In regular operation, all gas turbine combustors have a significant spontaneous noise level induced by the turbulent high power flame. This noise is characteristic for the operation as it is the result of the interaction between turbulence and combustion. Pressure fluctuations may also be generated by thermoacoustic instabilities induced by amplification by the flame of the acoustic field in the combustor. This paper focuses on the characterisation of the latter process, the combustion dynamics, in a pressurized premixed natural gas combustor. In order to predict the thermo-acoustically unstable operating ranges of modern gas-turbines with the use of an acoustic network model, it is essential to determine accurately the flame transfer function. This transfer function gives the relationship between a perturbation upstream of the flame and its combustion response, leading to acoustic forcing. In this paper, the flame transfer function is obtained by experimental means in a combustor test rig. This test rig was built in the framework of the European DESIRE project, and has the ability to perform thermo-acoustic measurements up to an absolute pressure of 5 bars. The maximum power of the setup is 500 kW. The paper presents a method to determine the flame transfer function by factorizing it in six subfunctions. Systematically these subfunctions are determined. With the method presented, acoustic measurements on the steady, unperturbed flame and on the unsteady, actively perturbed flame are performed. The effect of pressure is investigated. The steady measurements are used to provide an acousto-combustion finger print of the combustor. In the unsteady measurements, the flame transfer function is reconstructed from the measured acoustic pressures. These flame transfer functions are compared to transfer functions obtained from a numerical experiment in CFD. Good agreement is obtained.

Experimental Investigation of Thermoacoustics and High Frequency Combustion Dynamics with Band Stop Characteristics in a Pressurized Combustor

“…The FTF was obtained by Pozarlik [5] for various combustion chamber geometries. Significant activity was present between 420 and 450 Hz and maybe even beyond, according to previous investigations conducted in the same test unit [3][4][5]. This higher frequency activity is also present for a blend of H 2 , which was observed by Pater [6], although this was in non-premixed combustion.…”

“…where, 𝑄 and 𝑄 represent the mean rate of heat release and its fluctuations, 𝑚 and 𝑚 represent the mean and perturbed mass flow, and 𝜔 is the frequency. The wellknown relation between the equivalence ratio fluctuations and mass flow fluctuations is discussed in references [3,4].…”

“…However, most work has been completed for atmospheric combustion. Van Kampen and Kok [3,4] presented a method to measure the FTF at an elevated pressure with the use of pressure transducers and CFDbased FTF calculations. The FTF was obtained by Pozarlik [5] for various combustion chamber geometries.…”

Experimental investigations of ion current in liquid-fuelled gas turbine combustors

“…Most investigations of dynamic instabilities of swirled combustion cover premixed conditions and use gaseous fuels, such as methane or natural gas, to reduce the complexity of the process. Lieuwen et al 2 and Sattelmayer 3 focused on the direct relationship between oscillations and the equivalence ratio under atmospheric conditions, while Van Kampen et al 4 performed experiments relating to this interaction in a pressurized rig. Other authors like Durox et al 5 varied the flow field with the swirl number, while Moeck et al 6 studied the influence of the precessing vortex core (PVC), which is a rotating coherent structure induced by the swirl.…”

“…2 and Sattelmayer 3 focused on the direct relationship between oscillations and the equivalence ratio under atmospheric conditions, while Van Kampen et al. 4 performed experiments relating to this interaction in a pressurized rig. Other authors like Durox et al.…”