Annular Gas Turbine Research Articles

Isothermal Flow Field Characterization of a Full-Scale Sector Combustor at Elevated Pressures

Abstract An experimental investigation in a sector (20 deg) of full-scale annular gas turbine combustor is performed. The sector combustor is optically accessible for the flow and flame visualization of the primary and exit zones of the combustor. The distinctive feature of the experimental setup is that it preserves the geometrical details of an annular combustor that includes the casing, dome and combustor liner. The combustor design features a series of primary and secondary dilution holes with multiple film cooling strips on the outer and inner liner. In the present study, the combustor is operated at inlet Mach numbers of 0.02–0.3 at operating absolute pressures of 1–5 bar. Static pressure measurements are performed at multiple locations in the rig to characterize the pressure drop across the combustor. Two-dimensional particle image velocimetry (PIV) is performed to measure the velocity fields of the primary and exit zones of the combustor simultaneously. The results show the presence of a central recirculation zone (CRZ), high-velocity annular jets, and a pair of dilution jets in the primary zone of the combustor. The steady-state flow structures are invariant of inlet Mach number and pressures. The relationship between the relative pressure drop across the combustor and the combustor inlet condition is obtained. Mass flowrate and momentum flux are calculated for the flow through the swirler, central recirculation zone, the primary dilution jets, and the exit zone. The paper shows how the flow structures in a realistic combustor change with variations in global combustor parameters.

Experimental investigation on the effects of transverse injection distribution scheme on dual flame dynamics subjected to flow disturbances

Combustion instability has become a major problem in low-emission annular gas turbine combustors. It is of practical importance to investigate the effects of transverse nozzle distribution scheme on multi-flame dynamics subjected to external acoustic perturbations. In this study, the responses of heat release rate perturbations from dual propane/air laminar premixed flames to axial acoustic perturbations have been experimentally measured. Specifically, the velocity disturbance upstream the flame base has been measured by two-microphone method and the flame heat release rate perturbation is obtained by the photomultiplier tube with CH* filter lens. The dynamic evolution of dual flames is captured by the high-speed camera and then analyzed with the proper orthogonal decomposition (POD) method. Results indicate that there is a critical distribution distance, where the peak gain of the dual flame transfer function is the largest. And the direct injection dual flame transfer function can behave as the low-pass filter. Furthermore, the dual flame transfer function behaves nonlinearly with the increase of the injection angle. The dual flame transfer function for large injection angle can behave as the hybrid mode of the low-pass filter and the band-pass filter. The POD analysis reveals that the coupling effect of gas thermal expansion and stretch effect would contribute to the interaction between dual flame front dynamics. The transverse injection distribution scheme (dual flame distance and injection angle) has a significant effect on the dual flame front dynamics, which in turn affects the flame heat release rate disturbance. The bulk mean flow velocity can enhance the interaction effect between dual flame front dynamics.

Effect of Diffuser Geometry on the Performance of an Annular Aero Gas Turbine Combustor

Combustor is an important component of a gas turbine engine and its performance governs the overall performance of the engine. This paper describes the effect of diffuser geometry on the performance parameters of short annular combustors. Full-scale combustors having similar liner configurations but different diffuser geometry were tested in the combustor test facility for this investigation. The combustor aerothermal characteristics such as light-up behavior, exit temperature pattern factors and combustion efficiency are found to be inde- pendent of diffuser geometry. But the major aerodynamic parameters such as diffuser pressure loss, static pressure recoveries etc are significantly affected by the diffuser geometry.

Hysteresis and Bi-Stability in Transversely Excited Swirling Flows

Abstract We investigate the effect of transverse acoustic excitation on non-reacting swirling jets. The work is motivated by the azimuthal instabilities in annular gas turbine combustor which are one of the major challenges in aero-engines. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features a dual co-rotating radial swirler configuration. Two compression drivers placed on either side of the combustion chamber are used to generate acoustic fields in the direction transverse to the flow. Simultaneous 2D PIV and high-frequency pressure measurements are conducted to measure the time-averaged velocity field and the chamber acoustics, respectively. It is observed that once the swirling jet is excited with a transverse acoustic forcing, it instantaneously transitions to a wall-jet state. In wall-jet state, the flow moves radially outwards and remains attached to the walls on either side of the nozzle, and is characterised by a recirculation zone with a strong negative axial velocity. In our experiments we demonstrate that transverse acoustic excitation can lead to a bistable state in swirling flows. We investigated the acoustic response to low-amplitude forcing on the combustion chamber by performing a forced acoustic response analysis using COMSOL. It is observed that acoustic forcing leads to a peak response of the radial and azimuthal velocities at the flare of the swirler, which could induce an axisymmetric deflection of the jet shear layer pushing it into a wall-jet state.

Impact of dynamic modelling of the flame subgrid scale wrinkling in large-Eddy simulation of light-round in an annular combustor

Ignition of annular gas turbine combustors, also called light-round, is a challenging problem for aeronautic reliability and safety. Large Eddy Simulation (LES), based on massively parallel computations, has been already assessed as a reliable tool to analyse and predict such a phenomenon. The present study focuses on the effect of the flame subgrid scale wrinkling factor entering the combustion model. Large Eddy Simulations of the annular MICCA-Spray laboratory-scale chamber are performed with constant and dynamic strategies for the wrinkling parameter and are compared against experimental data. A bi-modal distribution of the model exponent during light-round is obtained with a different behaviour between the flames stabilized around each injector, and the flame fronts propagating in a circumferential direction. The impact of such locally defined and time-dependent evolution is analysed by studying several quantities associated to the flame propagation mechanisms. It is shown that the dynamic procedure entails a reduction of the wrinkling factor that is not balanced by an increase of the resolved flame surface. After considering the exhaust of burnt gases that affects the ignition in such a semi-confined system, the dynamic formulation leads to a moderate slowing down of the flame. Surprisingly, the sounder dynamic model does not yield better predictions of the experimental data, revealing the need to address other modelling improvements in the future. However, this does not temper the need for a dynamic combustion model in numerical simulation of light-round.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

Abstract This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

Evaluation of exit pattern factors of an annular aero gas turbine combustor at altitude off-design conditions

Abstract This paper presents the experimental results of an annular combustor. A full scale full annular combustion chamber is tested at ground test stand simulating altitude off-design operating conditions. Traversing thermocouple rakes are used to measure temperatures over the entire annulus at combustor exit. The radial temperature profile is found to be a strong function of operating parameters while the circumferential pattern factor is within the design goal. Increase in reference Mach number at these off-design conditions has caused the redial temperature profiles to deviate and increase from the intended profile. These results will be used for benchmarking the computational model. The computational model will be used for detail study of the temperature non-uniformity over the entire flight envelope and also for modification of combustor liner for performance improvement.

Stochastic properties of thermoacoustic oscillations in an annular gas turbine combustion chamber driven by colored noise

Large-amplitude thermoacoustic oscillations are unwanted in gas turbines due to the detrimental damage to combustors. Thus predicting the onset of such oscillations and a better understanding of the unstable behaviors are important. In this work, the stochastic properties of thermoacoustic oscillations in the subthreshold region of thermoacoustic systems are investigated theoretically and numerically. The energy conversion from the unsteady heat release to sound is mainly achieved via two ways: one is caused by inherent turbulent fluctuations, and the other is due to the flame response to the acoustic waves. The turbulence-induced non-coherent heat release fluctuation is characterized by colored noise, and the coupling between the unsteady heat release and acoustic pressure is characterized by a 3rd order polynomial. Both stochastic averaging and stochastic normal form are utilized to approximate the responses of the system as a Markov process. The result shows that the correlation time τc and intensity D of the colored noise have significant but opposite effects on the dynamics of thermoacoustic oscillations, including the most probable amplitude, autocorrelation of the system as well as the correlation time. In addition, the resonance-like behaviors in signal-to-noise ratio (SNR) are observed, which denotes the emergence of coherence resonance (CR) in this annular combustion system. The optimal noise intensity, at which SNR is maximized, becomes sensitive to the variation of τc, as τc is larger than certain value. Lastly, the extent of the system coherent motions relies significantly on the proximity to the supercritical Hopf bifurcation point. Being closer to the stability boundary is found to increase the strength of system association. Meanwhile, SNR of the colored noise-induced motion becomes more distinguished, and the optimal noise intensity is shifted to a smaller value. This variation can serve as a precursor to predict the onset of thermoacoustic instability, as the correlation time remains unchanged.

Volumetric PIV measurement for capturing the port flow characteristics within annular gas turbine combustors

The three-dimensional flows within a full featured, unmodified annular gas turbine combustor have been investigated using a scanned stereoscopic PIV measurement technique. Volumetric measurements have been achieved by rigidly translating a stereoscopic PIV system to scan measurements around the combustor, permitting reconstruction of volumetric single-point statistics. Delivering the measurements in this way allows the measurement of larger volumes than are accessible using techniques relying upon high depth of field imaging. The shallow depth of field achieved in the stereoscopic configuration furthermore permits measurements in close proximity to highly detailed geometry. The measurements performed have then been used to assess the performance of the combustor port flows, which are central to the emissions performance and temperature/velocity profile at turbine inlet. Substantially differing performance was observed in the primary ports with circumferential position, which was found to influence the behaviour of the second secondary port jets. The measurements indicated that the interaction between the primary and secondary jets occurred due to variations in the external boundary conditions imposed by the annular passages in which the combustor is located.Graphic abstract

On the Interaction of Swirling Flames in a Lean Premixed Combustor

Abstract 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 twenty 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 multinozzle (MN), ignition, emission, pattern factor and flame transfer function (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 is used along with the acoustic pressure signatures to compute the Rayleigh index (RI) so as to identify the regions within the flame that pumps energy into the self-excited thermoacoustic instability modes. It is 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 distributions in the interacting flames, two-dimensional (2D), three component stereoscopic particle image velocimetry (SPIV) and planar laser-induced fluorescence (PLIF) of hydroxyl radical is 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 (FSD) 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.

Analysis of the Self-Excited Dynamics of a Heavy-Duty Annular Combustion Chamber by Large-Eddy Simulation

Abstract This paper discusses the use of large eddy simulation (LES) to predict the performance of an annular combustion chamber in stable operating conditions and in the presence of self-exited dynamics. The availability of high-accuracy data taken in a full-scale combustion test facility allowed an extensive validation of the prediction capability. The analysis focuses on a small size heavy duty annular gas turbine whose size allows to test and compute the entire 360 deg combustion chamber. The comparison with measurements confirms that, if the correct operating conditions are implemented, LES is capable to discern between stable and unstable operating conditions, as well as predict several other engineering relevant parameters, although the model is sometime affected by a limited shift in frequency.The post processing of LES results in presence of combustion dynamics is not a trivial task. Here, the results of the simulations have been postprocessed by means of a triple decomposition method to determine a mean flow, a deterministic unsteady flow at the main instability frequency, and a turbulent stochastic flow. Such decomposition indicated the instability triggering mechanism together with the cross-talk mechanism between different components. This approach is currently used for design phase, while further validation is on-going to include different geometries and operating conditions with the goal of reducing both risks and number of tests.

Assessment of Exit Temperature Pattern Factors in an Annular Gas Turbine Combustor: An Overview

Abstract The present paper overviews the works carried out on achieving desired temperature pattern factors at combustor exit in gas turbine engines. These pattern factors are very important from the point of engine performance and life of turbine blades and vanes. They are controlled by a number of geometrical parameters such as liner front-end air passages, primary air holes, atomizer characteristics and air swirl number and dilution zone geometrical configuration. Combustor inlet pressure, Mach number, velocity profile and fuel-air ratio are the major operating parameters that influence the pattern factors. Due to the design uniqueness and importance of pattern factors, it is always a challenge to assess the pattern factors over a wide range of mission points for a gas turbine combustor designed for combat aircraft.

A systematic study of nonlinear coupling of thermoacoustic modes in annular combustors

Thermoacoustic instabilities in annular gas turbine combustors often involve modes which vary in both the longitudinal and circumferential directions. Recent experimental studies show that during limit cycle oscillations, different thermoacoustic modes may be uncoupled, as is the case in purely longitudinal or circumferential spinning modes. They may also be coupled, for example two counter-rotating circumferential modes combining to give standing or mixed modes, and coupling between circumferential and longitudinal modes giving rise to the slanted mode. Accurately predicting such modal couplings and the resulting spatial pattern of limit cycle oscillations remains an open challenge. This work uses a 2-D low-order network model based on modal expansions, validated against a full 3-D Helmholtz solver, to systematically investigate these couplings. For the first time, low-order network modelling is shown to capture limit cycle oscillations exhibiting both uncoupled and nonlinearly coupled modes, the latter including coupling between counter-rotating circumferential modes and between longitudinal and circumferential modes. It is shown that limit cycle solutions with totally different mode patterns (longitudinal, circumferential spinning, circumferential standing and slanted) can all exist in a given thermoacoustic system, with switches between modal patterns arising from slight changes in parameters such as the flame time delay.

Large eddy simulation of CH4-air and C2H4-air combustion in a model annular gas turbine combustor

Combustion instabilities are one of the major challenges in developing and operating propulsion and power generating gas-turbine engines. More specifically, techniques for managing the increasingly stringent emissions regulations and efficiency demands have often given rise to thermo-acoustic instabilities, particularly for annular combustors operating in a lean premixed mode. In this paper, we combine experimental and computational methods to examine unsteady gas turbine combustion in a full annular model gas turbine combustor installed at NTNU, operating both methane- and ethylene-air blends. The experimental data consists of flame images, high-speed OH* chemiluminescence images, as well as pressure and heat-release time-series at discrete locations for the ethylene-air case. The computational set-up consists of the 18 inlet tubes and swirlers, and the full annular combustor placed in a large external domain. The computational model consists of a compressible finite rate chemistry LES model using skeletal methane-air and ethylene-air combustion chemistry. The combustor is simulated in its self-excited state, without external forcing. From the experiments and simulations the methane and ethylene cases are found to behave differently: The ethylene-air flames are much smaller than the methane-air flames, resulting in different interaction between adjacent flames. The LES predictions show good qualitative agreement with the measurements in terms of instantaneous and time-averaged flame structure. Comparing measured and predicted time-series of pressure and heat-release also shows good quantitative agreement with respect to the dynamics and structure for the ethylene-air case. Investigating the predicted combustion dynamics using Proper Orthogonal Decomposition (POD) confirms the importance of the self-excited azimuthal mode on the behavior of the flame: the presence of nodes and anti-nodes of pressure induced fluctuations of the swirler mass-flow, which then, in turn, influence the heat-release. These events occur shifted in time.

Acoustic Coupling between Longitudinal and Transverse Modes in an Annular Gas Turbine Combustor

Acoustic Coupling between Longitudinal and Transverse Modes in an Annular Gas Turbine Combustor

Numerical Investigation of the Stochastic Behavior of Light-Round in Annular Non-Premixed Combustors

ABSTRACTThe ignition behavior of a non-premixed multiple-burner annular combustion chamber was investigated numerically, focusing on the stochasticity and average speed of the light-round mechanism ensuring flame propagation from burner to burner that have been observed experimentally. During the propagation sequence, the flame expansion process is tracked by a previously developed stochastic low-order ignition model adapted to full combustor ignition. A stochastic model based on the probability that a flame fragment coming from an ignited burner leads to successful ignition of the next un-ignited one is developed in order to explain and quantify the global ignition behavior of the combustor. The stochastic behavior of the rig, highlighted through the experimentally observed variability of the burner-to-burner propagation times during the ignition sequence, was clarified and quantified. The lean light-round ignition limiting conditions and the mean light-round speed measured experimentally are explained and reasonably accurately predicted, demonstrating the validity of the use of the probabilistic model together with the low-order ignition model for the combustor considered. The results presented in this paper can be used to predict the ignition envelope of annular gas turbines combustors at the design stage.

Swirl Flame Response to Simultaneous Axial and Transverse Velocity Fluctuations

In this experimental study, we investigate the impact of transverse acoustic velocity fluctuations on flame response to axial velocity fluctuations. Such a scenario where a flame is under the influence of a 2D acoustic field occurs in annular gas turbine combustors during thermoacoustic instability. A generic premixed swirl flame is exposed to simultaneous transverse and axial acoustic forcing. The amplitude of axial forcing was kept constant, while the amplitude and relative phase (with respect to axial forcing) of the transverse forcing was systematically varied. Results obtained indicate that transverse velocity affects flame response, and that both the magnitude of transverse velocity and its phase with respect to axial forcing are important factors.

Progress in analytical methods to predict and control azimuthal combustion instability modes in annular chambers

Longitudinal low-frequency thermoacoustic unstable modes in combustion chambers have been intensively studied experimentally, numerically, and theoretically, leading to significant progress in both understanding and controlling these acoustic modes. However, modern annular gas turbines may also exhibit azimuthal modes, which are much less studied and feature specific mode structures and dynamic behaviors, leading to more complex situations. Moreover, dealing with 10–20 burners mounted in the same chamber limits the use of high fidelity simulations or annular experiments to investigate these modes because of their complexity and costs. Consequently, for such circumferential acoustic modes, theoretical tools have been developed to uncover underlying phenomena controlling their stability, nature, and dynamics. This review presents recent progress in this field. First, Galerkin and network models are described with their pros and cons in both the temporal and frequency framework. Then, key features of such acoustic modes are unveiled, focusing on their specificities such as symmetry breaking, non-linear modal coupling, forcing by turbulence. Finally, recent works on uncertainty quantifications, guided by theoretical studies and applied to annular combustors, are presented. The objective is to provide a global view of theoretical research on azimuthal modes to highlight their complexities and potential.

Effect of Spray Cone Angle on Flame Stability in an Annular Gas Turbine Combustor

AbstractEffect of fuel spray cone angle in an aerogas turbine combustor has been studied using computational fluid dynamics (CFD) and full-scale combustor testing. For CFD analysis, a 22.5° sector of an annular combustor is modeled and the governing equations are solved using the eddy dissipation combustion model in ANSYS CFX computational package. The analysis has been carried out at 125 kPa and 303 K inlet conditions for spray cone angles from 60° to 140°. The lean blowout limits are established by studying the behavior of combustion zone during transient engine operation from an initial steady-state condition. The computational study has been followed by testing the practical full-scale annular combustor in an aerothermal test facility. The experimental result is in a good agreement with the computational predictions. The lean blowout fuel–air ratio increases as the spray cone angle is decreased at constant operating pressure and temperature. At higher spray cone angle, the flame and high-temperature zone moves upstream close to atomizer face and a uniform flame is sustained over a wide region causing better flame stability.

Lean Blow-out Studies in a Swirl Stabilized Annular Gas Turbine Combustor

AbstractLean blow out characteristics in a swirl stabilized aero gas turbine combustor have been studied using computational fluid dynamics. For CFD analysis, a 22.5° sector of an annular combustor is modeled using unstructured tetrahedral meshes comprising 1.2 × 10