Experimental Investigation of Spray Flame Dynamics with Variable Degree of Premixing
Supervisor | Subject |
|---|---|
Editor | Cooperation/Funding |
| Prof. Dr.-Ing. Thomas Sattelmayer | Combustion stability, flame transfer function, optical equivalence ration measurements, spray combustion, thermoacoustics |
| Jan Kaufmann, M.Sc., Manuel Vogel, M.Sc. | This project is funded by GE Power and BMWi, whose support is gratefully acknowledged. |
Motivation
Fuel flexibility is of great importance in modern gas turbines in order to reliably supply electrical energy, even in cases of limited primary fuel availability. In this context, the capability for liquid fuel combustion is also desirable for natural gas systems.
Liquid fuel combustion is prone to thermoacoustic instabilities, which arise from a coupling between combustion chamber acoustics and unsteady heat release. Due to the potential detrimental consequences, these instabilities must be avoided during operation.
The combustion stability characteristics of liquid fuels are fundamentally different to those of natural gas. Fuel atomization, droplet-air mixing and fuel evaporation have a major influence on flame dynamics. Understanding these fundamental mechanisms is a prerequisite for thermoacoustically stable design. In the field of gaseous fuel combustion extensive investigations have already been carried out. However, the mechanisms leading to thermoacoustic instabilities in liquid fuel operation are less understood.
Methods and Objectives
The goal of the current project, which is carried out in cooperation with GE power, is the identification and decomposition of thermoacoustic instability mechanisms in spray combustion. The thermoacoustic stability is quantified in terms of Flame Transfer Functions (FTF). The FTF provides information on the amplitude- and phase-response of the flame in a complex-valued, frequency dependent function as illustrated in \mbox{Fig. \ref{fig:FTF}}. Physically, the FTF relates heat release rate fluctuations to acoustic perturbations. By comparing the FTFs for different degrees of fuel-air premixing and a further comparison to the corresponding FTFs of perfectly premixed natural gas flames, the impact of the individual sub-processes can be isolated.
For liquid fuel combustion, local equivalence ratio fluctuations are expected to have a significant impact on thermoacoustic stability. Observing the flame chemiluminescence proved to be a useful approach to determine the local equivalence ratio of a flame. Therefore, temporally highly resolved flame images of OH* and CH* chemiluminescence are recorded in order to determine the locally resolved equivalence ratio in the flame.
Experimental Setup
For the experimental analysis, a gas turbine combustion test rig equipped with a swirl stabilized dual fuel burner is used as depicted in Fig. 2. An image of the resulting cyclic flame structure is shown in Fig. 1.
Acoustic perturbations are introduced to the combustion chamber via sirens. The resulting acoustic field and the flame response are recorded by pressure probes and band-pass-filtered high-speed imaging.
Results
The acoustic arrangement has been set up and FTFs for the operation range under perfectly premixed natural gas operation have been measured. A representative FTF plot is provided in Fig. 3. The FTF exhibits a minimum of the absolute value associated with a phase jump in the low frequency regime and a linear phase decline for higher frequencies.
Furthermore, the optical setup for determining the local equivalence ratio in the flame has been established and successfully demonstrated under perfectly premixed conditions for natural gas combustion. The derived calibration chart for mapping the OH*/CH*-ratio to the equivalence ratio shows a monotonic trend and can be approximated by a power law function as indicated in Fig. 4.
References
[1] Stadlmair, N. V., Mohammadzadeh K., P., Zahn, M., and Sattelmayer, T. (2017).Impact of Water Injection on Thermoacoustic Modes in a Lean Premixed Combustor under Atmospheric Conditions. In Proceedings of ASME Turbo Expo 2017, Charlotte, NC, USA.