Application of diffuse background illumination to lean direct injection combustion undergoing oscillations

Effects of noise intensity on early warning indicators of thermoacoustic instability: An experimental investigation on a lean-premixed combustion system

Due to growing energy demands and the need for increased fuel consumption efficiency, environmental protection agencies are imposing more stringent emissions regulations on gas turbine combustion systems with emphasis on NOx emissions reduction. Emerging technological combustion schemes to reduce NOx commonly employ lean premixed combustion. Decreases in NOx are globally obtained by flame temperature decrease and locally improved by homogeneity of the reacting mixture. A new micro fuel injection swirler, capable of providing efficient and rapid mixing over a short distance, is presented in this thesis. The conception of the Micro Fuel Injection Swirler (MFIS) was motivated by the need for enhanced mixing devices in lean premixed combustion and the capabilities of a micro manufacturing technique developed at Louisiana State University in conjunction with Mezzo Technologies. The MFIS uses a circular array of porous panels manufactured with an internal fluid cavity which allows for micro scale fuel distribution. The fuel is injected perpendicular to the blade opposing the oncoming stream of air which produces a highly turbulent swirling flow to enhance combustion stability at ultra lean operation necessary to reduce NOx emissions. A process was developed to fabricate and assemble the MFIS economically and reliably while ensuring dimensional stability. A benchmark swirler was also manufactured with similar dimensions as the MFIS but none of the inherent geometry characterizing the advantages of the MFIS. A combustion chamber was designed and fabricated to provide testing infrastructure for verifying the performance of the MFIS. Combustion results indicated that the MFIS was capable of achieving relatively lower equivalence ratios at LBO compared to benchmark cases tested. At a set equivalence ratio, the MFIS produced higher flame temperatures, higher heat release rates, and comparable NOx emissions. At equivalent operating temperatures, the MFIS produced nearly equal NOx emissions compared to the perfectly premixed case. Hydrogen testing showed that the lean blowout limits could be extended with hydrogen addition, providing further reduction in NOx while allowing stable combustion. In summary, the MFIS was capable of providing efficient air/fuel mixing over a short premixing distance, affirming its effectiveness in lean premixed combustion systems.

The purpose of this study is to compare the part-load performance of a lean burn catalytic combustion gas turbine (LBCCGT) system in three different control modes: varying fuel, bleeding off the fuel mixture flow after the compressor and varying rotational speed. The conversions of methane species for chemical process are considered. A 1D heterogeneous plug flow model was utilized to analyze the system performance. The actual turbomachinery components were designed and predicted performance maps were applied to system performance research. The part-load characteristics under three control strategies were numerically investigated. The main results show that: the combustor inlet temperature is a significant factor that can significantly affect the part-load characteristics of the LBCCGT system; the rotational speed control mode can provide the best performance characteristics for part-load operations; the operation range of the bleed off mode is narrower than that of the speed control mode and wider than that of the fuel only mode; with reduced power, methane does not achieve full conversion over the reactor at the fuel only control mode, which will not warrant stable operation of the turbine system; the thermal efficiency of the LBCCGT system at fuel only control strategy is higher than that at bleed off control strategy within the operation range.

In order to achieve low levels of pollutants modern gas turbine combustion systems operate in lean and premixed modes. However, under these conditions self-excited combustion oscillations due to a complex feedback mechanism between pressure and heat release fluctuations can be found. These instabilities may lead to uncontrolled high pressure amplitude oscillations which can damage the whole combustor. The flame induced acoustic source terms are still analytically not well described and are a major topic of thermo-acoustic investigations. For the analysis of thermo-acoustic phenomena in gas turbine combustion systems flame transfer functions can be utilized. The purpose of this paper is to introduce and to investigate modeling parameters, which could influence a novel computational approach to reconstruct flame transfer functions known as the CFD/SI method. The flame transfer function estimation is made by application of a system identification method based on Wiener-Hopf formulation. Varying acoustic boundary conditions, combustion models and time resolutions may strongly affect the reconstructed flame response characterizing overall system dynamics. The CFD/SI approach has been applied to a generic gas turbine burner to derive a flame response. 3D unsteady simulations excited with white noise have been performed and the reconstructed flame transfer functions have been validated with experimental data. Moreover, the impact on the reconstructed flame transfer functions because of different boundary condition configurations has been examined.

Thermoacoustic instabilities in gas turbine combustion systems, caused by a feedback loop between acoustic fluctuations and the flame, can be a major factor in determining the durability of the combustor. Of interest here are helical modes caused by a Kelvin-Helmholtz instability emanating from a region of high shear close to the outlet of the fuel injector. A liquid fuelled lean burn fuel injector, containing three air flow passages is studied in the present work using non-reacting compressible unsteady RANS CFD simulations. An acoustic wave is injected at the downstream boundary with excitation frequencies of 300Hz and 450Hz to compare to an unforced case. Analysis of the flow response is carried out using linear stability analysis, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD). The linear stability analysis required interpolation of the solution from the unstructured CFD grid onto a uniform cylindrical polar mesh. The analysis found an absolute instability in the shear region between two passages. This m = −2 mode is unstable over frequencies from 400Hz to 1000Hz with wavelengths of 1.08 to 1.41 of the injector outer diameter. For the unforced case the POD identifies the first two modes with azimuthal wave number m = 1 and these are seen to spiral from the splitter plate inwards to disturb the pilot and outwards to the main. The dominant frequency is around 450Hz which is consistent with measurements and close to the linear stability analysis value. For the 300Hz forced case POD identifies the first four modes as being helical but has difficulty determining the dominant azimuthal wave number. There is shown to be a significant interaction between the acoustic and helical modes and double the total resolved kinetic energy as compared to the unforced case. The 450Hz forced case shows the asymmetric m = 1 mode to be damped and the m = 2 helical mode is relatively unchanged. The resolved kinetic energy was only marginally higher than the unforced case and significantly lower than the 300Hz forced case. The DMD analysis showed how, as the forcing increased the flow through the injector, the flow is simultaneously pushed radially inwards and accelerated azimuthally. It also identified the region downstream of the splitter plate with significant fluctuations and is likely to be the wavemaker region responsible for the generation of helical instabilities. This work improves understanding of how helical modes of different azimuthal wave numbers react to acoustic forcing. The ability to manipulate the strength of these modes through alteration of the fuel injector geometry gives designers an additional tool to control thermo-acoustic instabilities.

Gas turbine combustion systems are prone to thermo-acoustic instabilities, and this is particularly the case for new low emission lean burn type systems. The presence of such instabilities is basically a function of the unsteady heat release within the system (i.e., both magnitude and phase) and the amount of damping. This paper is concerned with this latter process and the potential damping provided by perforated liners and other circular apertures found within gas turbine combustion systems. In particular, the paper outlines experimental measurements that characterize the flow field within the near field region of circular apertures when being subjected to incident acoustic pressure fluctuations. In this way the fundamental process by which acoustic energy is converted into kinetic energy of the velocity field can be investigated. Experimental results are presented for a single orifice located in an isothermal duct at ambient test conditions. Attached to the duct are two loudspeakers that provide pressure fluctuations incident onto the orifice. Unsteady pressure measurements enable the acoustic power absorbed by the orifice to be determined. This was undertaken for a range of excitation amplitudes and mean flows through the orifice. In this way regimes where both linear and nonlinear absorption occur along with the transition between these regimes can be investigated. The key to designing efficient passive dampers is to understand the interaction between the unsteady velocity field, generated at the orifice and the acoustic pressure fluctuations. Hence experimental techniques are also presented that enable such detailed measurements of the flow field to be made using particle image velocimetry. These measurements were obtained for conditions at which linear and nonlinear absorption was observed. Furthermore, proper orthogonal decomposition was used as a novel analysis technique for investigating the unsteady coherent structures responsible for the absorption of energy from the acoustic field.

Operators of heavy duty gas turbines desire more flexibility of operation in compliance with increasingly stringent emissions regulations. Delivering low NOx at base load operation, while at the same time meeting aggressive startup, shutdown, and part load requirements for NOx, CO, and unburned hydrocarbons is a challenge that requires novel solutions in the framework of lean premixed combustion systems. The DLN2.6+ combustion system, first offered by the General Electric Company (GE) in 2005 on the 9F series gas turbines for the 50 Hz market, has a proven track record of low emissions, flexibility, and reliability. In 2010, GE launched a program to incorporate the DLN2.6+ into the 7F gas turbine model. The primary driver for the introduction of this combustion system into the 60 Hz market was to enable customers to capitalize on opportunities to use shale gas, which may have a greater Wobbe range and higher reactivity than traditional natural gas. The 7F version of the DLN2.6+ features premixed pilot flames on the five outer swirl-stabilized premixing fuel nozzles (“swozzles”). The premixed pilots have their roots in the multitube mixer technology developed by GE in the US Department of Energy Hydrogen Gas Turbine Program. A fraction of air is extracted prior to entering the combustor and sent to small tubes around the tip of the fuel nozzle centerbody. A dedicated pilot fuel circuit delivers the gas fuel to the pilot tubes, where it is injected into the air stream and given sufficient length to mix. Since the pilot flames are premixed, they contribute lower NOx emissions than a diffusion pilot, but can still provide enhanced main circuit flame stability at low-load conditions. The pilot equivalence ratio can be optimized for the specific operating conditions of the gas turbine. This paper presents the development and validation testing of the premixed pilots, which were tested on E-class and F-class gas turbine combustion system rigs at GE Power & Water’s Gas Turbine Technology Lab. A 25% reduction in NOx emissions at nominal firing temperature was demonstrated over a diffusion flame pilot, translating into more than 80% reduction in CO emissions if increased flame temperature is employed to hold constant NOx. On the new 7F DLN2.6+, the premixed pilots have enabled modifications to the system to reduce base load NOx emissions while maintaining similar gas turbine low-load performance and bringing a significant reduction in the combustor exit temperature at which LBO occurs, highlighting the stability the pilot system brings to the combustor without the NOx penalty of a diffusion pilot. The new combustion system is scheduled to enter commercial operation on GE 7F series gas turbines in 2015.

Chapter 8 - Lean Hydrogen Combustion

This paper describes the low–emissions capability of lean direct injection, liquid–fueled burners for gas turbine combustors. The emissions characteristics of direct injection and premixed–prevaporized combustion systems were evaluated at inlet air temperatures of 450, 550 and 650 K and at atmospheric pressure. The equivalence ratio was varied in a range from about 1.2 to values close to flammability limits. The effects of fuel nozzle capacity and mixing tube length were also investigated. In the direct injection system, a double swirler/fuel nozzle module was used to inject fuel spray and air into combustion chamber. Fuel was atomized into the circular swirling jet using a pressure fuel nozzle placed 15 mm upstream of the inner swirler channel exit. In the lean premixed–prevaporized combustion system, the same double swirler/fuel nozzle module was used also for fuel–air mixing and a tube was inserted between the double swirler/fuel nozzle module and the end wall of the combustor chamber for fuel prevaporization. At lean conditions, combustion was more complete for the direct injection combustion system than premixed–prevaporized combustion system, though both were capable of very low NOx emissions.

Premixed or partially premixed swirling flames are widely used in gas turbine applications because of their compactness, high ignition efficiency, low NOx emissions and flame stability. A typical annular combustor consists of about eighteen to twenty-two swirling flames which interact (directly or indirectly) with their immediate neighbors even during stable operation. These interactions significantly alter the flow and flame topologies thereby bringing in some discrepancies between the single nozzle (SN) and multi nozzle (MN), ignition, emission, pattern factor and Flame Transfer Functions (FTF) characteristics. For example, in MN configurations, application of a model based on SN FTF data could lead to erroneous conclusions. Due to the complexities involved in this problem in terms of size, thermal power, cost, optical accessibility etc., a limited amount of experimental studies has been reported, that too on scaled down models with reduced number of nozzles. Here, we present a detailed experimental study on the behavior of three interacting swirl premixed flames, arranged in-line in an optically accessible hollow cuboid test section, which closely resembles a three-cup sector of an annular gas turbine combustor with very large radius. Multiple configurations with various combinations of swirl levels between the adjacent nozzles and the associated flame and flow topologies have been studied. Spatio-temporal information of the heat release rate obtained from OH* chemiluminescence imaging was used along with the acoustic pressure signatures to compute the Rayleigh index so as to identify the regions within the flame that pumps energy into the self-excited thermoacoustic instability modes. It was found that the structure of the flame-flame interaction regions plays a dominant role in the resulting thermoacoustic instability. To resolve the flow and reactive species field distributions in the interacting flames, two-dimensional, three component Stereoscopic Particle Image Velocimetry (SPIV) and Planar Laser Induced Fluorescence (PLIF) of hydroxyl radical was applied to all the test conditions. Significant differences in the flow structures among the different configurations were observed. Simultaneous OH-PLIF and SPIV techniques were also utilized to track the flame front, from which the curvature and stretch rates were computed. Flame surface density which is defined as the mean surface area of the reaction zone per unit volume is also computed for all the test cases. These measurements and analyses elucidate the structure of the interaction regions, their unique characteristics and possible role in thermoacoustic instability.

New combustion systems for gas turbines (NGT)

Gas turbine combustion systems are prone to thermo-acoustic instabilities, and this is particularly the case for new, low emission, lean burn type systems. The presence of such instabilities is basically a function of the unsteady heat release within the system (i.e. both magnitude and phase), and the amount of damping. This paper is concerned with this latter process and the potential damping provided by perforated liners and other circular apertures found within gas turbine combustion systems. In particular the paper outlines experimental measurements that characterise the flow field within the near field region of circular apertures when being subjected to incident acoustic pressure fluctuations. In this way the fundamental process by which acoustic energy is converted into kinetic energy of the velocity field can be investigated. Experimental results are presented for a single orifice located in an isothermal duct at ambient test conditions. Attached to the duct are two loudspeakers that provide pressure fluctuations incident onto the orifice. Unsteady pressure measurements enable the acoustic power absorbed by the orifice to be determined. This was undertaken for a range of excitation amplitudes and mean flows through the orifice. In this way regimes where both linear and non-linear absorption occur along with the transition between these regimes can be investigated. The key to designing efficient passive dampers is to understand the interaction between the unsteady velocity field, generated at the orifice, and the acoustic pressure fluctuations. Hence experimental techniques are also presented that enable such detailed measurements of the flow field to be made using PIV. These measurements were obtained for conditions at which linear and non-linear absorption was observed. Furthermore, Proper Orthogonal Decomposition was used as a novel analysis technique for investigating the unsteady coherent structures responsible for the absorption of energy from the acoustic field.

Confinement has direct influence on the dome reference velocity and indirect influence on the combustor’s performance, emissions, operability, liner and dome temperature levels and gradients. Understanding the effects of the confinement is crucial to conventional combustor design. Recently, lean direct injection (LDI) combustor has been popularly employed to reduce nitrogen oxides (NOx) emissions from gas turbine. Compared to conventional combustor, LDI combustor has a much larger amount of air entering into the dome. Therefore, combustion characteristics and confinement effects in LDI combustor are quantitatively or even qualitatively different from those in conventional combustor. As a result, some design criteria for conventional combustor are no longer applicable for LDI combustor and it is essential to investigate the effects of confinement level in LDI combustor. In the literature, several studies have been conducted to understand how the confinement levels affect the characters of swirling non-reactive flow. However, there are few studies on the reactive flow and thereby the effects of confinement on combustion characteristics are not well understood. Due to its important role in combustor design, the confinement effect on spray combustion in LDI mode is systematically investigated in the present study. The experimental setup is established to study the confinement effects. Experimental data on flame characteristics, centerline temperature distribution, and pollutant emissions are obtained. Experiments at five different confinement ratios, 2.8, 6.8, 10.6, 19.5, and 28.5, are conducted. The results show that the confinement level has an important impact on the flame characteristics, centerline temperature, and pollution emissions. The optimal confinement ratio is determined, at which the pollution emissions become the lowest.

Dynamic processes in gas turbine (GT) combustors play a key role in flame stabilization and extinction, combustion instabilities and pollutant formation, and present a challenge for experimental as well as numerical investigations. These phenomena were investigated in two gas turbine model combustors for premixed and partially premixed CH4/air swirl flames at atmospheric pressure. Optical access through large quartz windows enabled the application of laser Raman scattering, planar laser-induced fluorescence (PLIF) of OH, particle image velocimetry (PIV) at repetition rates up to 10 kHz and the simultaneous application of OH PLIF and PIV at a repetition rate of 5 kHz. Effects of unmixedness and reaction progress in lean premixed GT flames were revealed and quantified by Raman scattering. In a thermo-acoustically unstable flame, the cyclic variation in mixture fraction and its role for the feedback mechanism of the instability are addressed. In a partially premixed oscillating swirl flame, the cyclic variations of the heat release and the flow field were characterized by chemiluminescence imaging and PIV, respectively. Using phase-correlated Raman scattering measurements, significant phase-dependent variations of the mixture fraction and fuel distributions were revealed. The flame structures and the shape of the reaction zones were visualized by planar imaging of OH distribution. The simultaneous OH PLIF/PIV high-speed measurements revealed the time history of the flow field–flame interaction and demonstrated the development of a local flame extinction event. Further, the influence of a precessing vortex core on the flame topology and its dynamics is discussed.

<div class="section abstract"><div class="htmlview paragraph">This study offers an overview of the impact of lean burn technology in two-wheeler vehicles, specifically concentrating on enhancing the fuel economy and addressing the challenges associated with its adoption. Lean burn systems, characterized by a fuel-air mixture with a higher air content than stoichiometric ratio. The study focuses on technology which meets stringent emission standards while enabling the optimization of fuel efficiency.</div><div class="htmlview paragraph">The lean burn system employs strategies to optimize air-fuel ratio using electronic fuel injection, ignition timing control, and advanced engine control algorithms like - updated torque modulation control algorithm for drivability, lambda control algorithm for rich and lean switch and NOx modelling algorithm for LNT catalyst efficiency tracking. The challenges related to lean burn systems, includes issues related to combustion stability, nitrogen oxide (NOx) emissions, and their impact on drivability, is summarized in the study. Mitigation strategies, ranging from after-treatment systems to catalyst technologies, are discussed as means to address these challenges while preserving the benefits of lean burn operation.</div><div class="htmlview paragraph">Furthermore, this study sheds light on the Lean NOx Trap (LNT) catalyst which is a critical component in modern emission control systems, particularly in the context of lean burn engines. Designed to reduce nitrogen oxides (NOx) emissions, the Lean NOx Trap catalyst operates efficiently in oxygen-rich environments, such as those found in lean burn systems. It captures and stores NOx during fuel-lean conditions and subsequently releases and converts them to harmless nitrogen and oxygen when the engine switches to a fuel-rich state.</div><div class="htmlview paragraph">In conclusion, this study synthesizes the current knowledge on lean burn technology in two-wheelers, offering valuable insights for researchers, engineers, and industry stakeholders. By addressing technological advancements, challenges, and future directions, it aims to contribute to the ongoing efforts to enhance the efficiency and sustainability of two-wheeled transportation systems.</div></div>