In recent years, the institute has put great effort into the modeling and investigation of high-frequency thermoacoustic phenomena. In accordance with the historical order of occurrence, HF acoustics were first considered in the field of rocket engines, where transverse acoustic motion is most destructive for the engine operation. Due to the enormous damping effect on axially oriented acoustic modes through convective transport in the thrust nozzle, longitudinal modes are usually irrelevant. Similarly, since lean combustion technologies have been implemented, gas turbines are also regularly subjected to HF instabilities. Physically, these instabilities evolve from constructive, self-sustaining feedback couplings between the combustor's acoustic oscillations and the flame's heat release rate fluctuations that manifest as pressure pulsations through the chamber. The pulsations need to be avoided, as they can be detrimental to the engines’ hardware, and also hamper operational flexibility and low-emissivity.
The year 2016 has been particularly successful as considerable advances on the understanding of physical mechanisms leading to the thermoacoustic generation of sound at high-frequencies both in rocket engines and gas turbine combustors was achieved. In the framework of the DFG funded SFB TRR 40 Project, a complete linear stability assessment tool based on linearized and frequency space transformed Euler equations was developed and finally successfully validated on the basis of DLR’s cryogenic BKD test chamber (Figure 1 left, see also Ref. [1]). In the GT research, the group takes on the challenge of unraveling high-frequency combustion instabilities together with TUM IAS Rudolf Diesel Fellowship, Dr. Bruno Schuermans. For this purpose, an extensive network ranging from academic (TUM-IAS), industrial (GE) and government (BMWi/AG Turbo) research frameworks has been established. Within this framework, three doctoral candidates are collaboratively conducting high-frequency thermoacoustic research at the institute with experimental, numerical and theoretical emphases. A lab-scale premixed combustor exhibiting high-frequency thermoacoustic instabilities served as subject of investigation (Figure 1 right).
A crucial ingredient for the understanding of HF acoustic motion is the flame response. For its description Flame Transfer Functions have been determined and compared to experimental measurements. In the rocket engine, single flame CFD simulations are used for that purpose exploiting the radial compactness. It is found that the flame’s pressure sensitivity dominates strongly over its modulability by transverse acoustic velocity. The computational results reveal that due to pressure modulation two different coupling mechanisms are active and can be found in different sections in the chamber. In the region close to the injector, the convective transport of periodically oscillating vortices is dominant, which modulates the heat release by periodic intensification of propellant mixing. In the remaining region further downstream baroclinic effects due to strong density gradients in combination with axial acoustic pressure gradients govern the periodic generation of vortices, which, in turn, lead to a fluctuating heat release rate. These two mechanism provide unequal contributions to the total fluctuating heat release rate, leading to different driving potentials for thermoacoustic instability. OH* chemiluminescence measurements provided by the DFG SFB TRR 40 project partner DLR Lampoldshausen allowed for a validation of the injector-near FTF in terms of the phase. As it can be seen in Figure 2, an excellent agreement between experimental data and numerical findings could be achieved around the frequency range of interest of approximately 10 – 11 kHz. The rocket engine stability analysis method and results are reported in [2].
In the gas turbine research, the group established a theoretical model with which the distribution and even the severity of high-frequency thermoacoustic source terms in flame tubes can be calculated based on steady combustor parameters and the geometrical acoustic eigenmodes. This model is built upon the sheer experimental observation of a moving flame, which periodically displaces from its mean position in accordance with the acoustic pressure field at the instability frequency as illustrated in Figure 3. This displacement alone represents a thermoacoustic driving mechanism, and additionally induces a deformation of the flame shape, which generates further sound. Dynamic high-speed camera diagnostics were then utilized to visually capture the flame behavior during the instability. Employing tomographic reconstruction techniques yielded thermoacoustic source term distributions, which served as the validation benchmarks for the postulated models. Figure 4 compares the experimental and calculated source terms, and reveals excellent quantitative and qualitative agreement, which implies the correctness of the postulated theoretical model. This validation was carried out for several operation points, all of which consistently revealed matching (experiment vs. model) outcomes. Consequently, the postulated models can be confidentially considered eligible to be used for predictive thermoacoustic analyses (e.g. for novel gas turbine combustors or refurbishment activities of existing engines). This work was published in corresponding papers [3-4].
The numerically determined FTF for the BKD rocket test chamber have also been used in the developed linear stability assessment tool for the prediction of the occurrence of an instability. Single flame considerations for the generation of an adequate mean flow and for the calibration of feedback models are employed within the procedure. The application of a realistic mean flow field including combustion and real gas effects explains the spatial separation of transverse modes into a near face plate mode, which is found linearly unstable under certain operation conditions for the first transverse, and a rear part mode, see Figure 5. The axial mode shape length as well as eigenfrequencies are affected by propellant injection specifications and, in consequence, decisively influence pressure and transverse velocity sensitive dynamic flame response. Finally, it could be shown that the implemented stability prediction tool is capable of predicting the experimentally observed instabilities in the BKD test chamber for four exemplary load points, which differ in chamber pressure, equivalence ratio and total power output. A detailed description can be found in [5].
Furthermore a low-order analysis tool for high-frequency thermoacoustic systems was developed and applied to calculate the temporal performance of the gas turbine model combustor in Figure 1 (right). Methodologically, these models are low-order state space systems, which are capable of modeling nonlinear and stochastic thermoacoustic processes in a straightforward and computationally efficient manner. Experimentally observed high-frequency mode dynamics – i.e. whether the pressure mode (cf. Figure 3) rotates, stands or exhibits a mix of both with respect to the azimuthal coordinate – was effectively reproduced as shown in Figure 6. These results gave physical insight in the dynamics of high-frequency thermoacoustic systems, and underlined the importance of stochastic effects as these proved crucial for reproduction of the experimental observations. The work was published in 2015 and 2016 in [6-9].
Selected Publications
- Gröning, S., Hardi, J., Suslov, D., and Oschwald, M., 2015. "Injector-Driven Combustion Instabilities in a Hydrogen/Oxygen Rocket Combustor". Journal of Propulsion and Power 32(3), 2015.
- Schulze, M., 2016. "Linear Stability Assessment of Cryogenic Rocket Engines". Dissertation, Lehrstuhl für Thermodynamik, Technische Universität München.
- Berger, F., Hummel, T., Hertweck, M., Schuermans, B.,and Sattelmayer, T., 2017. “High-Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors, Part I: Measurement of Non-Compact Flame Transfer Functions”. Journal of Engineering for Gas Turbines and Power, 2017.
- Hummel, T., Berger, F., Hertweck, M., Schuermans, B.,and Sattelmayer, T., 2017. “High-Frequency Thermoacoustic Modulation Mechanisms in Swirl-Stabilized Gas Turbine Combustors, Part II: Modeling and Analysis”. Journal of Engineering for Gas Turbines and Power, 2017.
- Schulze, M., Sattelmayer, T., 2016. "Linear Stability Assessment of a Cryogenic Rocket Engine". Proceedings of the International Symposium on Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry meets Academia, 2016.
- Hummel, T., Temmler, C., Schuermans, B., and Sattelmayer, T., 2016. “Reduced Order Modeling of Aeroacoustic Systems for Stability Analyses of Thermoacoustically Non-Compact Gas Turbine Combustors”. Journal of Engineering for Gas Turbines and Power, 138 / 051502-1.
- Hummel, T., Schulze, M., Schuermans, B., and Sattelmayer, T., 2015. “Reduced Order Modeling of Transversal and Non-Compact Combustion Dynamics”. 22nd International Congress on Sound and Vibration, July 12 - 16, Florence, Italy.
- Hummel, T., Hammer, K., Romero, P., Schuermans, B., and Sattelmayer, T., 2016. “Low-Order Modeling of Nonlinear Transversal Thermoacoustic Oscillations in Gas Turbine Combustors”. Journal of Engineering for Gas Turbines and Power, 2016.
- Hummel, T.; Berger, F.; Schuermans, B.; Sattelmayer, T.: "Theory and Modeling of Non-Degenerate Transversal Thermoacoustic Limit Cycle Oscillations". Proceedings of the International Symposium on Thermoacoustic Instabilities in Gas Turbines and Rocket Engines: Industry meets Academia, 2016.