The Effects of Exit Boundary Condition on Precessing Vortex Core Dynamics

Many industrial combustion systems, especially power generation gas turbines, use fuel-lean combustion to reduce NOx emissions. However, these systems are highly susceptible to combustion instability, the coupling between combustor acoustics and heat release rate oscillations of the flame. It has been shown in previous work by the authors that a precessing vortex core (PVC) can suppress shear layer receptivity to external perturbations, reducing the potential for thermoacoustic coupling. The goal of this study is to understand the effect of combustor exit boundary condition on the flow structure of a swirling jet to increase fundamental understanding of how combustor design impacts PVC dynamics. The swirling jet is generated with a radial-entry, variable-angle swirler, and a quartz cylinder is fixed on the dump plane for confinement. Combustor exit constriction plates of different diameters are used to determine the impact of exit boundary condition on the flow field. Particle image velocimetry (PIV) is used to capture the velocity field inside the combustor. Spectral proper orthogonal decomposition, a frequency-resolved eigenvalue decomposition that can identify energetic structures in the flow, is implemented to identify the PVC at each condition in both energy and frequency space. We find that exit boundary diameter affects both the structure of the flow and the dynamics of the PVC. Higher levels of constriction (smaller diameters) force the downstream stagnation point of the vortex breakdown bubble upstream, resulting in greater divergence of the swirling jet. Further, as the exit diameter decreases, the PVC becomes less energetic and less spatially defined. Despite these changes in the base flow and PVC coherence, the PVC frequency is not altered by the exit boundary constriction. These trends will help inform our understanding of the impact of boundary conditions on both static and dynamic flame stability.

Synchronization During Multi-Mode Thermoacoustic Oscillations in a Liquid-Fueled Gas Turbine Combustor at Elevated Pressure

This paper analyzes intermittent self-excited thermoacoustic oscillations in which the pressure (P′) and heat release rate (q̇′) fluctuations are harmonically coupled. That is to say, P′ and q̇′ do not oscillate at the same frequencies, but rather at frequencies in integer ratios. Thus, this system represents a case dominated by nonlinear cross-mode coupling. The measurements were obtained in an optically-accessible combustor equipped with an industrial gas turbine fuel injector operating with liquid fuel under partially-premixed conditions at elevated pressure. High-speed chemiluminescence (CL) imaging of OH* was used as an indicator of the heat release rate. The data was processed using spectral proper orthogonal decomposition (SPOD) to isolate the dominant heat release and pressure modes. Synchronization theory was used to determine when the modes are coupled and how their interaction manifests in the measurements, particularly how it relates to the observed intermittency. The results show three distinct intervals of synchronized oscillation shared by all the mode pairs analyzed. The first interval exhibits the same characteristics as a pair of noisy, phase-locked self-oscillators, with phase-slipping and frequency-pulling. While the behaviour of the second interval differs among mode pairs, strong frequency-pulling is observed during the third interval for all pairs.

Impact of Flame Lifting on Nitrogen Oxide Emissions From Premixed Reacting Jets in a Vitiated Crossflow

This paper describes measurements of nitrogen oxide (NOx) emissions from reacting jets in crossflow (RJICF). Primary factors that influence RJICF NOx emissions are: jet stoichiometry, mixing between jet and crossflow before combustion, and mixing of the remainder of the crossflow with the combustion products of the secondary combustion region. The aforementioned mixing is controlled by shear layer vortices and the counter-rotating vortex pair, as well as flame lifting. The coupled effects of bulk averaged temperature rise as a result of the RJICF (ΔT), jet stoichiometry (ϕJet), and momentum flux ratio (J) present a challenge in understanding critical factors controlling NOx production as it is difficult to vary them independently. Therefore, significant attention was paid to designing a test matrix that differentiated these effects. The data reported herein were obtained from the injection of premixed ethane/air or ethane/methane/air mixtures into a vitiated crossflow at one of two temperatures (1350K and 1410K). Varying the ethane/methane ratio allowed for systematic variation of flame lifting independent of ϕjet and J. The jets contained sufficient fuel to create an adiabatic bulk temperature rises from 75K–350K, with J values from 8–40, and ϕJet values from 0.8–8.0. The reported measurements confirm that NOx emissions increase monotonically with ΔT, as discussed in literature, but also indicates that the lifting of the flame significantly impacts NOx production. Lifting itself is a function of the variables described above and was quantified with chemiluminescence imaging. In fact, flame lifting is the dominant factor influencing NOx emissions, including ΔT.

An Assessment of the Implicit Non-Iterative PISO Solution Procedure for the Prediction of Combustor Performance

Large Eddy Simulation (LES) of gas turbine combustors has gained traction as a key tool in the design process. Accurate prediction of the multiphysics of reacting flows — evaporating fuel spray, turbulent mixing, turbulent chemistry interaction, radiation, and conjugate heat transfer to name a few — is key to the accurate prediction of combustor performance. The overall solution time for a standard LES simulation on an industrial system can be burdensome because of the small time and length scales required to capture the aforementioned multiphysics to an acceptable level. Any performance improvements are therefore welcomed. In this paper, we compare the implicit non-iterative PISO solution procedure with the implicit iterative SIMPLE method for the Large Eddy Simulation of a Honeywell combustor using the commercial software, Simcenter STAR-CCM+ v13.04. Time averaged simulation results are validated against rig data. Results show that the PISO solution method provides results which are similar to those found using the SIMPLE method, and accurate when compared to rig data, but at up to a 3.4X speed-up for this liquid fueled gas turbine combustor.

Impact of Wall Temperature in Large Eddy Simulation of Light-Round in an Annular Liquid Fueled Combustor and Assessment of Wall Models

The process of ignition in aero-engines raises many practical issues that need to be faced during the design process. Recent experiments and simulations have provided detailed insights on ignition in single-injector configurations and on the light-round sequence in annular combustors. It was shown that Large Eddy Simulation (LES) was able to reliably reproduce the physical phenomena involved in the ignition of both perfectly premixed and liquid spray flames. The present study aims at further extending the knowledge on flame propagation during the ignition of annular multiple injector combustors by focusing the attention on the effects of heat losses, which have not been accounted for in numerical calculations before. This problem is examined by developing Large Eddy Simulations of the light-round process with a fixed temperature at the solid boundaries. Calculations are carried out for a laboratory-scale annular system. Results are compared in terms of flame shape and light-round duration with available experiments and with an adiabatic LES serving as a reference. Wall heat losses lead to a significant reduction in the flame propagation velocity as observed experimentally. However, the LES underestimates this effect and leads to a globally shorter light-round. To better understand this discrepancy, the study focuses then on the analysis of the near wall region where the velocity and temperature boundary layers must be carefully described. An a-priori analysis underlines the shortcomings associated to the chosen wall law by considering a more advanced wall model that fully accounts for variable thermophysical properties and for the unsteadiness of the boundary layer.

Atmospheric Tests of a Full Scale Gas Turbine Burner Fed With Blast Furnace Gas and Coke Oven Gas

The present paper describes atmospheric experimental tests of a new Ansaldo Energia full scale burner which was designed to burn fuels byproduct of steel making processes (mixtures of Blast-Furnace Gas (BFG) and Coke-Oven Gas (COG)), characterized by very low heating values (LHV∼2–3.5 MJ/kg) and very low stoichiometric air/fuel ratios (∼0.5–1 kg/kg). In particular, flame stability and blow-out margins were assessed for different burner variants and fuel compositions such as pure BFG, blends of BFG with increasing content of COG, and also a synthetic mixture of natural gas, hydrogen and nitrogen (NG/H2/N2). Except for pressure, all burner inlet conditions were simulated as in the actual gas turbine engine. The best performing burner among those tested demonstrated an excellent burning stability behavior over a wide operating range and stably burned pure BFG without any supplementary fuel. Furthermore, considering that in most operating concepts gas turbine engines for Ultra-Low BTU applications require a back-up fuel (such as oil, propane or natural gas) to ignite and ramp up or to perform load-rejections, the present atmospheric tests also assessed maneuvers to switch from natural gas operation to syngas operation. Also in this type of dual-fuel operation the burner demonstrated a wide flame stability range.

Comparison of Machine Learning Algorithms in the Interpolation and Extrapolation of Flame Describing Functions

This paper examines and compares commonly used Machine Learning algorithms in their performance in interpolation and extrapolation of FDFs, based on experimental and simulation data. Algorithm performance is evaluated by interpolating and extrapolating FDFs and then the impact of errors on the limit cycle amplitudes are evaluated using the xFDF framework. The best algorithms in interpolation and extrapolation were found to be the widely used cubic spline interpolation, as well as the Gaussian Processes regressor. The data itself was found to be an important factor in defining the predictive performance of a model, therefore a method of optimally selecting data points at test time using Gaussian Processes was demonstrated. The aim of this is to allow a minimal amount of data points to be collected while still providing enough information to model the FDF accurately. The extrapolation performance was shown to decay very quickly with distance from the domain and so emphasis should be put on selecting measurement points in order to expand the covered domain. Gaussian Processes also give an indication of confidence on its predictions and is used to carry out uncertainty quantification, in order to understand model sensitivities. This was demonstrated through application to the xFDF framework.

Simultaneous TR-PIV and CH* Chemiluminescence During Combustion-Instability Triggered Flame-Flashback in a Backward Facing Step Combustor

The present study is an experimental investigation of the nature of acoustically induced flashback in a lab-scale dump combustor. The control parameters varied include the inlet Reynolds number (Re) and the inlet turbulence intensity. The primary bifurcation plots of the combustor from stable to the unstable condition are seen to be significantly altered by the inlet turbulence intensity, with the latter delaying the onset of combustion instability to higher Re. The analysis of multivariate high-speed data acquisition and processing (viz. unsteady pressure, flame imaging and velocity field by means of PIV) reveals the role of low-frequency high amplitude acoustics in modulating the flame. It is seen that high amplitude oscillations are sustained by two mechanisms 1. Modulation of the flame by coherent structures shedding at the step and 2. The bulk flame motion in-and-out at the edge of the step. It is seen that flow reversal at sufficiently low frequencies provide enough duration for the hot products to ignite fresh reactants upstream of the duct, which in-turn reinforces the coherent unsteadiness in the system, thereby increasing the propensity of the mixture to be ignited more upstream with every cycle. This ultimately leads to the flame flashing back till the point of premixing. This work thus addresses and reforms the occurrence of flashback being an example of loss of static stability, whereby the overriding presence of dynamic combustion instability results in a flashback to behave in a dynamic manner.

Combustion Instabilities Damping System Development for Dry Low NOx Emissions Operability Enhancement of a Heavy-Duty Gas Turbine

This paper describes the development phases of a damping system for combustion instability reduction in an annular type combustor for heavy-duty gas turbine applications. As reported by the authors in a previous paper, the full scale annular test rig allowed for an extensive characterization of the combustor with realistic acoustic boundaries at engine-relevant conditions. Emissions and operability assessment over a wide range of load conditions was performed, allowing the evaluation of the response of the system near the thermo-acoustic instability onset. The instability is quantified by its acoustic growth rate. This quantity is a crucial input in the design process of dampers. A methodology has been used to extract these growth rates form measured pulsation data. Experimentally determined growth rates have been evaluated for different fuel flow rate split between the main and the pilot injections, providing input to dampers preliminary design. Given current combustor architecture constraints, a first attempt configuration has been proposed and performances evaluated in the full annular rig. Dampers have been equipped with dynamic sensors and thermocouples with the purpose of measuring the growth rate abatement and the consequent NOx emissions reduction. A dedicated numerical toolbox, in-house developed by GE Power, has been used for both dampers preliminary design and growth rate reduction evaluation. Fine tuning of dampers elements as well as design assumptions adjustments required additional experimental evaluations and design iterations. Encouraged by the successful test in the concept phase, an optimized design for engine implementation was defined, that featured a significant increased damper volume, involving combustor parts re-design. The optimized configuration was finally tested in full annular rig and results demonstrated an important enhancement of operability while maintaining NOx emissions below the target levels.

Study of Dynamical Instabilities in Siemens Liquid Spray Injectors Using Complementary Modal Decomposition Techniques

Liquid fuel spray characterisation is essential for understanding the mechanisms underlying fuel energy release and pollutant formation. Careful selection of operating conditions can promote flow instabilities in the fuel spray which can enhance atomisation and fuel mixing, thereby resulting in more efficient combustion. However, the inherent instabilities present in the spray could have adverse effect on the combustor dynamics. Hence, it is important to better understand the dynamical behaviour of the spray, and particularly at representative operating conditions. This work describes an experimental investigation of dynamical behaviour of pressure-swirl atomisers used in Siemens industrial gas turbine combustors, at a range of chamber pressures and fuel injection pressures, using high speed laser planar imaging. Two modal decomposition techniques — Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) — are applied and compared to assess the spray dynamics. Results indicate that both POD and DMD are able to capture periodic structures occurring in the spray at different spatial length scales. The characteristic frequencies estimated from both the methods are in good agreement with each other. Both techniques are able to identify coherent structures with variable size, shape and level of staggering, which are observed to be dependent on the pressure difference across the atomiser and the chamber pressure. The spatio-temporally resolved data and the results could be used for spray model development and validation. Furthermore, the methods employed could be applied to other fuel atomisers, and more complicated conditions involving cross flow and higher chamber temperatures.