• Energy Research

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame–acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so-called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the measurement of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are measured for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adhere well to a Strouhal number scaling for different mean flow velocities.

Axisymmetric and helical instabilities modes were identified in an experimental combustor. The low-frequency instabilities were associated with the external recirculation zone downstream of the dump plane and the central recirculation zone formed by vortex breakdown. High-frequency helical instabilities were excited by the small-scale vortices that were shed at the initial separating shear layer at high-power levels. Miniature vortex generators were installed at the circumference of the burner's exit to interfere with the rollup of these vortices through the induction of streamwise vorticity. The tests showed that, in addition to the effect on the initial vortices, the process that leads to the formation of large-scale vortices through pairing and vortex merging was disrupted. Thermoacoustic instabilities that are excited by the periodic heat release due to the presence of coherent vortices were, thus, avoided in both the high- and low-frequency ranges. The effect was particularly significant in the high-frequency oscillations that reached high-amplitude level in the baseline burner and were suppressed by up to 28 dB by the miniature vortex generators. At the same time, low-frequency instabilities were reduced by 50%. Emissions of NO x were reduced by a factor of two in a wide range of operating conditions. The results obtained in the laboratory combustor operating at atmospheric pressure were also confirmed in high-pressure combustion tests.

In modern gas turbines operating with premix combustion flames, the suppression of pressure pulsations is an important task related to the quality of the combustion process and to the structural integrity of engines. High pressure pulsations may occur when the resonance frequencies of the system are excited by heat release fluctuations independent of the acoustic field (“loudspeaker” behavior of the flame). Heat release fluctuations are also generated by acoustic fluctuations in the premixed stream. The feedback mechanism inherent in such processes (“amplifier” behavior of the flame) may lead to combustion instabilities, the amplitude of pulsations being limited only by nonlinearities. In this work, the application of Helmholtz resonators for damping low-frequency pulsations in gas turbine combustion chambers is discussed. We present a nonlinear model for predicting the acoustic response of resonators including the effect of purging air. Atmospheric experiments are used to validate the model, which is employed to design a resonator arrangement for damping low-frequency pulsations in an ALSTOM GT11N2 gas turbine. The predicted damper impedances are used as the boundary condition in the three-dimensional analysis of the combustion chamber. The suggested arrangement leads to a significant extension of the low-pulsation operating regime of the engine.

Abstract Energy storage and its combination with electric network services is a major issue in networks with a high percentage of variable renewables. Among the many energy storage options very few are commercially available and economically viable. The current work presents a method to combine electrolytic hydrogen energy storage with steam power plants, with the aim to provide primary control reserve. The key component is a steam generator that burns stoichiometric mixtures of H2 and O2 to produce steam. The product gas is directly injected in the steam cycle of a coal power plant to increase its power. The integration of this particular steam generator in the steam cycle is evaluated thermodynamically and the economic performance of the whole system (electrolysis, gas storage and steam generator) is subsequently presented. It is demonstrated that the proposed system is a viable alternative for energy storage and primary control reserve. Especially when the levelized cost of electricity of a coal power plant is paid for the electrolysis electricity consumption, the proposed system becomes a viable option for large scale energy storage. In conclusion, it is shown that electricity prices and the price of primary control reserve have the strongest influence on the economic viability of the system.

Abstract The hydrodynamic instabilities in the flow field of a swirl-stabilized combustor are investigated theoretically. These instabilities give rise to large-scale flow structures that interact with the flame front causing unsteady heat release rate fluctuations. The streamwise growth of these coherent structures depends on the receptivity of the shear layer, which can be predicted numerically by means of linear stability analysis. This analysis is applied to the reacting flow field of a perfectly premixed swirl-stabilized combustor that is subjected to strong axial forcing mimicking thermoacoustic oscillations. The linear stability analysis reveals a clear correlation between the shear layer receptivity and the measured amplitude dependent flame transfer function. The stability analysis based on the natural flow predicts the distinctive frequency dependent flame response to low amplitude forcing. At these conditions, the flow reveals strong spatial amplification near the nozzle, causing the inlet perturbations to be significantly amplified before they reach the flame. At higher forcing amplitudes, the flow instabilities saturate, which manifests in a saturation of the flame response. The saturation of the shear layers predicted from the linear stability analysis is compared to phase-locked measurements of the forced flow field revealing good qualitative agreement. The analysis of the mean flow stability offers a powerful analytical tool to investigate the impact of shear flow instabilities on the flame describing function.

Humidified gas turbines (HGT), operating with humidified air, promise a higher efficiency compared to dry gas turbines, as well as reduced NOx emissions. In this study, premixed, near-stoichiometric combustion was investigated for gas turbine applications at ultra wet conditions with steam levels up to 35w%. Based on a generic burner, an experimental study was conducted for the combustion of natural gas at atmospheric pressure. The influence of humidity on emissions and flammability limits was investigated for different inlet temperatures, swirl numbers, and fuel injection strategies. At near-stoichiometric conditions, a degree of humidity of 35w% was achieved with a stable flame and single-digit NOx and CO emissions. In experiments conducted in a water tunnel, the flow field of the combustor was investigated using Laser Doppler Anemometry and Laser-induced Fluorescence. In addition to the experiments, a kinetic assessment of the emission formation was conducted with a reactor network, using a combination of several perfectly stirred reactors. The results of these simulations confirm the experimental findings.

Thermoacoustic interactions in industrial combustion systems are difficult to model because they involve complex interactions between several physical mechanisms. In order to obtain dynamic models of such systems, a hybrid approach is used: numerical, experimental and analytical techniques are combined to describe the system. The system is modeled as a modular network, where the input–output relation of the modules can be based on analytic models, experimental data or numerical analysis. The modules are represented as state-space realizations. A modal expansion technique is used to obtain a state-space representation of the acoustic propagation through complex 3-dimensional geometries. The modal expansion can be based on an analytic model (for relatively simple volumes), or on a finite element analysis (for geometries of any complexity). Modules that are very complex, such as the acoustic behavior of the combustion process itself, are modeled using a combined experimental and analytic approach. The method is not restricted to symmetries of any kind: configurations with geometrically or operationally different burners are simulated. The state-space network approach allows for time domain simulations, including non-linearities. An active controller has been synthesized for an (hypothetical) annular multi burner combustion system. The controller uses spatial filtering to decompose the acoustic field to its individual modes. The modes are then controlled using an H∞ control algorithm. Time domain simulations of this control system demonstrate the effectiveness of this method, even in the presence of non-linear saturation and parametric errors.

Thermoacoustic instability in annular combustors involves feedback interaction between individual (acoustically compact) flames and – predominantly – azimuthal acoustic modes of the annular combust...

An experimental method to determine the thermoacoustic properties of a gas turbine combustor using a lean-premixed low emission swirl stabilized burner is presented. To model thermoacoustic oscillations, a combustion system can be described as a network of acoustic elements, representing for example fuel and air supply, burner and flame, combustor, cooling channels, suitable terminations, etc. For most of these elements, simple analytical models provide an adequate description of their thermoacoustic properties. However, the complex response of burner and flame (involving a three-dimensional flow field, recirculation zones, flow instabilities and heat release) to acoustic perturbations has — at least in a first step — to be determined by experiment. In our approach, we describe the burner as an active acoustical two-port, where the state variables pressure and velocity at the inlet and the outlet of the two port are coupled via a four element transfer matrix. This approach is similar to the “black box” theory in communication engineering. To determine all four transfer matrix coefficients, two test states, which are independent in the state vectors, have to be created. This is achieved by using acoustic excitation by loudspeakers upstream and downstream of the burner, respectively. In addition, the burner might act as an acoustic source, emitting acoustic waves due to an unsteady combustion process. The source characteristics were determined by using a third test state, which again must be independent from the two other state vectors. In application to a full size gas turbine burner, the method’s accuracy was tested in a first step without combustion and the results were compared to an analytical model for the burner’s acoustic properties. Then the method was used to determine the burner transfer matrix with combustion. An experimental swirl stabilized premixed gas-turbine burner was used for this purpose. The treatment of burners as acoustic two-ports with feedback including a source term and the experimental determination of the burner transfer matrix is novel.