Ianko Chterev

FLAME AND FLOW TOPOLOGIES IN AN ANNULAR SWIRLING FLOW

This article describes an investigation of flame shapes and flow configurations in a premixed, swirl-stabilized dump combustor. High swirl, annular nozzle flows of this nature enable a variety of different flame configurations and heat release distributions with their associated flow fields. These differences are significant, since each of these configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle lifetime, and liner heating. These different configurations arise because multiple flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. We present results from high-speed luminosity imaging, particle image velocimetry (PIV), and OH-planar laser induced fluorescence (PLIF) to illustrate time-averaged and instantaneous flame shapes and flow fields associated with the different configuration “families.” Selected cases are compared with large eddy simulations (LES). Particular emphasis is given to the distinctly different flame and flow topologies that exist in these flows, and their sensitivity to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel/air ratio) parameters. We particularly emphasize the importance of the centerbody shape, and its associated impact on the structure of the central recirculating flow, as differentiating between two different families of flame shapes.

Simultaneous High Speed (5 kHz) Fuel-PLIE, OH-PLIF and Stereo PIV Imaging of Pressurized Swirl-Stabilized Flames using Liquid Fuels

This paper describes implementation of simultaneous, high speed (5 kHz) stereo PIV, OH and fuel-PLIF in a pressurized (up to 5.2 atm), liquid fueled, swirl stabilized flame, representative of a gas turbine combustor. The experiments were performed to characterize the flowfield, qualitative heat release and fuel spray distributions, and flame dynamics. Acquiring high speed OH-PLIF in pressurized, liquid fuel systems is difficult due to the fuel’s absorption and emission spectra strongly overlapping that of the OH fluorescence spectrum. To overcome the fuel emission polluting the OH signal, the OH and fuel fluorescence signals were partially separated by using two cameras with differing spectral filters and data acquisition timing, as the emission from OH and fuel differ both in spectral width and time. The first camera captured only fuel-PLIF, while the second captured fuelPLIF and OH-PLIF. The fuel-PLIF images were used to compute two intensity thresholds, separating each image into regions of no fuel, fuel only and an intermediate region. In the region of no fuel, OH was detected in the second camera. In the intermediate region there was a mix of fuel and OH. Instantaneous and time-averaged results are discussed showing the flow field, flame position and dynamics, and spray distribution from the fuel signal for two different multi-component liquid fuels (Jet-A and C-5), at two inlet temperatures of 450 and 570 K, and three pressure of 2.1, 3.5 and 5.2 bar. The flame shape in some cases is described as M-shaped, existing both inside and outside of the annular swirling jet produced by the nozzle, while in other cases no reaction is apparent on the inside. The spray penetration and distribution, and flame position are sensitive to the various conditions, while the flow field topology is qualitatively insensitive. Furthermore, elevated pressure as expected sharpens all spatial gradients in the data.

High Resolution Particle Image Velocimetry and CH-PLIF Measurements and Analysis of a Shear Layer Stabilized Flame

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical toward the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence of CH radicals (CH-PLIF) measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.

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.

Precession Effects on the Relationship Between Time-Averaged and Instantaneous Reacting Flow Characteristics

ABSTRACT High swirl number flows with vortex breakdown exhibit a number of unsteady flow features, including shear-induced coherent structures and precessing recirculation zones. This article analyzes how precession influences the relationship between the reacting flows’ time-averaged and instantaneous features. Its objective is to provide interpretive insights into high fidelity computations or experimental results. It shows how precession influences three significant topological features in the time-averaged flow: (1) centerline axial jets, (2) centerline stagnation points, and (3) symmetry of the flow about the centerline. It also discusses the extent to which these first two features provide insight into the actual instantaneous flow topology. A particularly significant result of this work is in regards to aerodynamically stabilized flames. Stabilization of such flames requires a low velocity interior stagnation point(s), presumably in the vortex breakdown region. We show how precession causes systematic differences between the location of the time-averaged position of the instantaneous stagnation point, and the stagnation point of the time-averaged velocity. An important implication of this point is that a perfect prediction of the time-averaged flow field could still lead to a completely erroneous time-averaged flame position prediction. Finally, we discuss the influence of precession and coherent motion on convergence of estimated averaged quantities.

High Resolution PIV and CH-PLIF Measurements and Analysis of a Shear Layer Stabilized Flame

Understanding the mechanisms and physics of flame stabilization and blowoff of premixed flames is critical towards the design of high velocity combustion devices. In the high bulk flow velocity situation typical of practical combustors, the flame anchors in shear layers where the local flow velocities are much lower. Within the shear layer, fluid strain deformation rates are very high and the flame can be subjected to significant stretch levels. The main goal of this work was to characterize the flow and stretch conditions that a premixed flame experiences in a practical combustor geometry and to compare these values to calculated extinction values. High resolution, simultaneous PIV and CH-PLIF measurements are used to capture the flame edge and near-field stabilization region. When approaching lean limit extinction conditions, we note characteristic changes in the stretch and flow conditions experienced by the flame. Most notably, the flame becomes less critically stretched when fuel/air ratio is decreased. However, at these lean conditions, the flame is subject to higher mean flow velocities at the edge, suggesting less favorable flow conditions are present at the attachment point of the flame as blowoff is approached. These measurements suggest that blowoff of the flame from the shear layer is not directly stretch extinction induced, but rather the result of an imbalance between the speed of the flame edge and local tangential flow velocity.© 2015 ASME

Shear Layer Flame Stabilization Sensitivities in a Swirling Flow

A variety of different flame configurations and heat release distributions can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper focuses on the sensitivities of the outer shear layer stabilization point to bulkhead temperature, flow velocity, swirl number, preheat temperature, and fuel/air ratio. It also characterizes the hysteresis that is present in conditions where the outer shear layer locally re-attaches and blows off. The sensitivities to bulkhead temperature, preheat temperature and fuel/air ratio follow the expected trends. Moreover, the strong bulkhead temperature sensitivities show that computations must include heat transfer to combustor hardware in order to capture flame stabilization correctly. The preheat temperature and fuel/air ratio sensitivities are captured with detailed kinetics calculations of the extinction stretch rate of the mixture. Somewhat counter intuitively, there is little variation in transition conditions with swirl number for the Sm∼0.6 and 0.8 swirlers analyzed here. Finally, velocity sensitivities are in many cases much weaker than what would be predicted assuming that the fluid mechanic straining time scales as 1/u.Copyright

Shear layer flame stabilization sensitivities in a swirling flow

A variety of different flame configurations and heat release distributions exist in high swirl, annular flows, due to the existence of inner and outer shear layers as well a vortex breakdown bubble. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle durability, and liner heating. This paper presents findings on the sensitivities of the outer shear layer- stabilized flames to a range of parameters, including equivalence ratio, bulkhead temperature, flow velocity, and preheat temperature. There is significant hysteresis for flame attachment/detachment from the outer shear layer and this hysteresis is also described. Results are also correlated with extinction stretch rate calculations based on detailed kinetic simulations. In addition, we show that the bulkhead temperature near the flame attachment point has significant impact on outer shear layer detachment. This indicates that understanding the heat transfer between the edge flame stabilized in the shear layer and the nozzle hardware is needed in order to predict shear layer flame stabilization limits. Moreover, it shows that simulations cannot simply assume adiabatic boundary conditions if they are to capture these transitions. We also show that the reference temperature for correlating these transitions is quite different for attachment and local blow off. Finally, these results highlight the deficiencies in current understanding of the influence of fluid mechanic parameters (e.g. velocity, swirl number) on shear layer flame attachment. For example, they show that the seemingly simple matter of scaling flame transition points with changes in flow velocities is not understood.

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.

Flame and Flow Topologies in an Annular Swirling Flow

A variety of different flame configurations and heat release distributions, with their associated flow fields, can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle life, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region.This paper discusses the flame and flow topologies that exist in these flows. These results illustrate the importance of the sensitivity of flame configurations to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel air ratio) parameters. We particularly emphasize the centerbody shape as differentiating between two different families of flame shapes. Results are shown illustrating the time averaged and instantaneous flame shape and flow fields, using high speed PIV, OH-PLIF, and luminosity imaging.Copyright