Keith Robert McManus

Modeling and Control of Combustion Dynamics in Industrial Gas Turbines

The thermoacoustic response of an industrial-scale gas turbine combustor to fuel flow perturbations is examined. Experimental measurements in a laboratory combustor along with numerical modeling results are used to identify the dynamic behavior of the combustor over a variety of operating conditions. A fast-response actuator was coupled to the fuel system to apply continuous sinusoidal perturbations to the total fuel mass flow rate. The effects of these perturbations on the combustor pressure oscillation characteristics as well as overall operability of the system are described. The results of this work suggest that persistent excitation of the fuel system may present a viable means of controlling combustion dynamics in industrial gas turbine and, in turn, enhance their performance.Copyright

Rig and Gas Turbine Engine Testing of MI-CMC Combustor and Shroud Components

GE is continuing work on the development of Melt-Infiltrated Ceramic Matrix Composites (MI-CMC) for use in industrial gas turbine engine components. Long-term environmental degradation of test samples under realistic engine conditions is being determined using a unique high-pressure combustion rig apparatus. Rig testing is also being used to evaluate an F-class 1st stage shroud system incorporating an MI-CMC inner shroud component. While large, advanced engines, such as the F and H classes, offer the greatest benefits for using MI-CMC components, initial engine tests have been done using a GE-2 (2MW) machine to reduce costs and risk. Long term (1000 hours) engine testing results for single piece GE-2 shrouds are also described.Copyright

Dynamics Suppression in Liquid-Fueled Combustors Using Fuel Modulation

In the present work, an atmospheric pressure combustor using a modern aviation gas turbine fuel nozzle was used to demonstrate active combustion control. The combustor exhibited a low-frequency dynamics mode under fuel-rich conditions. A fast-response fuel valve was adapted as an in-line valve upstream of the nozzle for fuel modulation. Large fuel modulation amplitudes were achieved with the combination of the valve and the engine nozzle at frequencies exceeding 200 Hz. Open-loop flame response to fuel modulation was first examined when the instability mode was absent; for a range of inlet air temperatures, fuel flow rates, and combustor pressure drops. Simple open-loop control at discrete off-resonance frequencies was found ineffective in suppressing the instability mode. An advanced, fast algorithm was developed to enable closed-loop control. In this scheme, the entire fuel supply to the combustor was modulated with the control valve and injected through the fuel nozzle. The control algorithm commanded the fuel injector to produce a steady fuel flow, when the combustion was stable, or to modulate the fuel when the level of pressure oscillations in the combustion chamber became unacceptable. With an optimized control algorithm, an 88% reduction in the amplitude of the low-frequency dynamics mode was achieved.Copyright

Flame Transfer Function Measurements in a Single Nozzle Combustor

This paper describes an experimental investigation of Flame Transfer Function (FTF) behavior where the response of a swirl-stabilized, natural gas fueled combustor is measured for partially premixed conditions. Controlled perturbations of the combustor inlet flows are produced using a siren and the combustor response is observed using several measurement techniques. The fuel/air equivalence ratio fluctuation is measured using a diode laser absorption sensor operating near 1.6 μm. Measurements of global heat release perturbations are obtained using a three-color optical emission technique and velocity measurements are obtained using the two-microphone method. FTFs are derived from these measurements for a frequency range commensurate with field-observed tones. Typical experimental investigations of dynamic signals involve the use of Fourier methods to obtain average signal amplitudes and phase. In this investigation, standard Fourier techniques are used to verify the driving frequencies, but they are coupled with a homodyne detection algorithm to measure time-dependent gain and phase behavior of a FTF.Copyright

EXPERIMENTAL EVALUATION OF A TWO -STAGE PULSE DETONATION COMBUSTOR

A two -stage pulsed detonation device was assembled and examined in an experimental program. The device consisted of a precombustor, used to mix and burn a fuel-rich mixture, and a resonator designed to sustain high-frequency repeating detonations. The device was evaluated over a range of operating conditions and with a variety of fuels including natural gas, ethylene and liquid Jet-A. It was found that the addition of an initiator, consisting of a small tube PDE firing directly into the resonator, was necessary to produce high-speed wave combustion processes in the device. The addition of the initiator enabled the device to produce supersonic combustion waves at the resonator exit, however, the resulting wave speeds were not as high as calculated detonation speeds for the mixtures.

Effect of Mode of Operation on Gas Turbine Combustor Performance

The primary goal of this study is to quantify the differences in combustor performance under different modes of operation as a function of the combustor exit temperature. The main focus is the effect on liner heat loading since liner durability is strongly dependent on the metal peak temperature as well as temperature gradients. The study also includes the effect on emissions and acoustics. Three modes of operation are discussed: non premixed natural gas operation, dry liquid fuel operation and wet liquid fuel. Limited, premixed, natural gas data are also discussed when appropriate. Experiments are conducted on a Single Nozzle Rig (SNR) at conditions representative of different power levels of F class gas turbines. The results are analyzed to show the natural gas vs. dry liquid fuel performance, followed by the effect of water injection and effect of load variation. The results show that heat loading impact is amplified in the head end region since this is where the flame stabilizes. The combustor head end average temperature is 55.6 K (100 F) higher for dry fuel oil when compared to natural gas. However, the tail section of the liner is 25.3 K (50 F) lower for fuel oil. The difference is mainly attributed to the change in flame shape and flame radiation. CO emissions were roughly 80 ppm lower for natural gas. NOx is higher for liquid fuel (35–80% depending on flame temperature). Emissions profiles at the combustor exit are similar for both fuels with a variability of 40% indicating [the profile] is controlled by flow-flame interaction. Acoustics are lower in amplitude for non-premixed natural gas, but peak frequency is similar. Liquid fuel wet mode of operation results in significant reduction in the head end temperature due to the water impingement on the liner 166.8 K (∼300F). However, the maximum temperature over the tail section is 25.3 K (50F) higher than dry operation. Yet, the average tail temperature is similar under both modes of operation with higher variability under wet operation (70%). This strong temperature variability can influence the liner life. More than 80% reduction in NOx emissions is achieved with water injection. Water injection impact on CO is dictated by CO generation mode (quenching vs. dissociation). More important is the significant improvement in emissions profile at the combustor exit with water injection (60% reduction in variability). At higher load, the maximum liner temperature is 44.5–83.5 K (80–150 F) higher than part load. This is mainly attributed to the increase in air inlet temperature and increase in radiation heat flux which is dependent on both flame temperature and pressure. More investigations are required to determine the impact on complicated engine configurations as well as to isolate the effect of flame shape.Copyright

Combustion Dynamics Diagnostics and Mitigation on a Prototype Gas Turbine Combustor

Combustion dynamics have detrimental effects on hardware durability as well as combustor performance and emissions. This paper presents a detailed study on the impact of combustion dynamics on NOx and CO emissions generated from a prototype gas turbine combustor operating at a pressure of 180 psia (12.2 bars) with a pre-heat temperature of 720 F (655.3 K) (E-class machine operating conditions). Two unstable modes are discussed. The first is an intermittent mode, at 750 Hz, that emerges at flame temperatures near 2900°F (1866.5 K), resulting in high NOx and CO emissions. With increasing fuel flow, NOx and CO emissions continue to increase until the flame temperature reaches approximately 3250°F (2061 K), at which point the second acoustic mode begins to dominate. Flame images indicate that the intermittent mode is associated with flame motion which induces the high NOx and CO emissions. The second mode is also a 750 Hz, but of constant amplitude (no intermittency). Operation in this second 750 Hz mode results in significantly reduced NOx and CO emissions. At pressures higher than 180 psia (12.2 bars), the intermittent mode intensifies, leading to flashback at flame temperatures above 2850°F (1839 K). In order to mitigate the intermittent mode, a second configuration of the combustor included an exit area restriction. The exit area restriction eliminated the intermittent mode, resulting in stable operation and low emissions over a temperature range of 2700–3200°F (1755–2033 K). A comparison of the NOx emissions, as function of flame temperature, with previous published data for perfectly premixed indicates that, while the low amplitude 750 Hz oscillations have little effect, the intermittent mode significantly increases emissions. Mode shape analysis shows that the 750 Hz instability corresponds to the 1/4 wave axial mode. In the current research a ceramic liner is used while the previous published data was collected with a quartz liner. Typically, quartz is avoided due to reductions in effective flame temperature by radiation losses. Experiments showed that NOx emissions were not affected by the combustor liner type. This agreement between the quartz and ceramic liners data indicates limited effect from the radiation heat losses on NOx emissions.Copyright