Tim Lieuwen

Flow Field Characterization in a Premixed, Swirling Annular Flow

This paper presents measurements and large eddy simulations of the flowfield in an annular, swirling, reacting flowfield. Depending upon operating conditions, the flame can exhibit four different configurations, depending upon whether it is stabilized in the vortex breakdown bubble, inner shear layer, and/or outer shear layer. Flow field characteristics such as vortex breakdown bubble length, vortex breakdown zone topology, annular jet spreading angle, and outer recirculation zone topology vary substantially between these different configurations. For the most case, the LES captures these different topological flow features and flow bifurcations, although some quantitative differences exist for the reacting cases, principally in strength of the recirculation zone and jet spreading angle.

Fuel-Flexible Combustion System for Co-production Plant Applications

Future high-efficiency, low-emission generation plants that produce electric power, transportation fuels, and/or chemicals from fossil fuel feed stocks require a new class of fuel-flexible combustors. In this program, a validated combustor approach was developed which enables single-digit NO{sub x} operation for a future generation plants with low-Btu off gas and allows the flexibility of process-independent backup with natural gas. This combustion technology overcomes the limitations of current syngas gas turbine combustion systems, which are designed on a site-by-site basis, and enable improved future co-generation plant designs. In this capacity, the fuel-flexible combustor enhances the efficiency and productivity of future co-production plants. In task 2, a summary of market requested fuel gas compositions was created and the syngas fuel space was characterized. Additionally, a technology matrix and chemical kinetic models were used to evaluate various combustion technologies and to select two combustor concepts. In task 4 systems analysis of a co-production plant in conjunction with chemical kinetic analysis was performed to determine the desired combustor operating conditions for the burner concepts. Task 5 discusses the experimental evaluation of three syngas capable combustor designs. The hybrid combustor, Prototype-1 utilized a diffusion flame approach for syngas fuels with a lean premixed swirl concept for natural gas fuels for both syngas and natural gas fuels at FA+e gas turbine conditions. The hybrid nozzle was sized to accommodate syngas fuels ranging from {approx}100 to 280 btu/scf and with a diffusion tip geometry optimized for Early Entry Co-generation Plant (EECP) fuel compositions. The swozzle concept utilized existing GE DLN design methodologies to eliminate flow separation and enhance fuel-air mixing. With changing business priorities, a fully premixed natural gas & syngas nozzle, Protoytpe-1N, was also developed later in the program. It did not have the diluent requirements of Prototype-1 and was demonstrated at targeted gas turbine conditions. The TVC combustor, Prototype-2, premixes the syngas with air for low emission performance. The combustor was designed for operation with syngas and no additional diluents. The combustor was successfully operated at targeted gas turbine conditions. Another goal of the program was to advance the status of development tools for syngas systems. In Task 3 a syngas flame evaluation facility was developed. Fundamental data on syngas flame speeds and flame strain were obtained at pressure for a wide range of syngas fuels with preheated air. Several promising reduced order kinetic mechanisms were compared with the results from the evaluation facility. The mechanism with the best agreement was selected for application to syngas combustor modeling studies in Task 6. Prototype-1 was modeled using an advanced LES combustion code. The tools and combustor technology development culminate in a full-scale demonstration of the most promising technology in Task 8. The combustor was operated at engine conditions and evaluated against the various engine performance requirements.

Measurement of Flame Characteristics of a Low Swirl Burner at High Pressures and Velocities

A Low Swirl Burner (LSB) has been experimentally examined at pressures and temperatures relevant to ground power gas turbine engines. The LSB configuration was tested with a premixed methane-air mixture, preheated to about 500 K, at a lean equivalence ratio, φ = 0.56. CH* chemiluminescence images were acquired and used to analyze specific flame characteristics, such as flame shape and location. The combustor operating conditions (e.g., combustor pressure and flow velocity) and the LSB geometry (swirler vane angle) were varied to examine the dependence of the flame characteristics on these parameters. The weak dependence of flame location on reference velocity previously observed is confirmed at low to moderate values of u � /SL. At high pressures and velocities, turbulent flame speed effects and variations in the mass flow split within the LSB injector are suggested to influence the flame position.

Characterization of Aerodynamically Stabilized Flames Using Simultaneous Analysis of Planar and Line-of-Sight Images

High swirl flows are frequently used to create recirculating flow regions for flame stabilization. Aerodynamically stabilized flames anchor in the flow interior, not at welldefined separating shear layer locations, and are subject to significant variations in spatial location. The conditions under which high swirl flows with vortex breakdown enable (or do not enable) aerodynamically stabilized flames are not well understood, and are the focus of this study. This paper presents analysis from 10 kHz, simultaneous stereo Planar Image Velocimetry (sPIV), OH radical Planar Laser Induced Fluorescence (PLIF) and OH* chemiluminescence data. Conditioned frames in which the flame’s most upstream point was captured in the OH-PLIF plane were used to determine the coordinates, local flow velocity and full, three-dimensional, hydrodynamic strain component of flame stretch at the dynamically evolving flame stabilization point. Key observations from these data about the location/conditions at the stabilization point are: (1) the mean hydrodynamic strain rate at the flame stabilization point is positive and 3.3 times higher than the corresponding laminar flame extinction stretch rate calculated from detailed kinetics, (2) it is, on average, positioned in a region of reverse flow, (3) there is no correlation between instantaneous stretch rate and local flow velocity, and (4) there is no correlation between the instantaneous radial and axial location of the stabilization point.

Stability Analysis of Reacting Wakes: Flow and Density Asymmetry Effects

This paper explores the hydrodynamic stability of bluff body wakes with non-uniform mean density, and with asymmetric mean density and velocity profiles. This work is motivated by recent experimental bluff body combustor studies by Tuttle et al. [1], which investigated reacting wakes with equivalence ratio stratification, and hence asymmetry in the base flow density profiles. They showed that highly stratified cases exhibited strong, narrowband oscillations, suggestive of global hydrodynamic instability. In this paper, we present a hydrodynamic stability analysis for non-uniform density wakes that includes base flow asymmetry. The results show that increasing the degree of base density asymmetry is generally a destabilizing effect, and that increasing base velocity asymmetry tends to be stabilizing. Furthermore, we show that increasing base density asymmetry slightly decreases the absolute frequency, and that increasing the base velocity asymmetry slightly increases the absolute frequency. In addition, we show that increasing the degree of base density asymmetry distorts the most absolutely unstable hydrodynamic mode from its nominally sinuous structure. This distorted mode exhibits higher amplitude pressure and velocity oscillations near the flame with the lower density jump, than near the flame with the higher density jump. This would then be anticipated to lead to strongly non-symmetric amplitudes of flame flapping, with much stronger flame flapping on the side with lower density ratio. These predictions are shown to be consistent with the experimental data measured by Tuttle et al. [1]. These comparisons support the analytical predictions that increased base density is destabilizing, and that hydrodynamic velocity fluctuation amplitudes should be greatest at the flame with the lowest density jump.

Comparative Study of Harmonically Forced, High Frequency Premixed Flame Response Mechanisms

This paper describes a comparative analysis of the role of pressure, velocity, and fuel/air ratio oscillations in exciting high frequency heat release oscillations. Each of these mechanisms has multiple pathways by which they excite heat release oscillations in premixed flames, where the relative roles of the different paths change with frequency and operating conditions. Consistent treatment of high frequency phenomena necessitates accounting for various physical processes such as interference (phase cancellation), unsteady stretch, non quasi-steady internal flame processes, and decay of flow disturbances through molecular transport. This analysis suggests that fuel/air ratio coupling and velocity coupling processes are dominant at low frequencies, but pressure coupling becomes the most important