Swirl-stabilized Combustion Systems Research Articles

A Novel Large-Eddy Simulation-Based Process for NOx Emission Assessment in a Premixed Swirl Stabilized Combustion System

Abstract This paper presents a new computational fluid dynamics (CFD) approach for the assessment of NOx emission. The methodology is validated against the experimental data of a heavy-duty gas turbine annular combustor. Since the NOx formation involves time scales that are different from the fuel oxidation time, this work defines the transport equation source terms for NOx basis on a dedicated NOx-Damköhler number. The latter parameter allows to properly distinguish the “in-flame” contribution from the “postflame” one. While the former is a mix of several mechanisms (prompt, N2O-pathway, thermal), the latter is dominated by the thermal contribution. The validation phase is developed in a large-eddy simulation (LES) framework where the extended turbulent flame speed model is implemented to consider the influence of both heat loss and strain rate on the progress variable source term. The accuracy of the model against the most important operability parameters of the combustor is verified. A strong focus on the fuel composition effect onto NOx is presented as well. For any simulated operating condition, the present methodology is able to provide a limited percentage error if compared with the data, considering also different combustion regimes. Leveraging this alignment, the last portion of the paper is dedicated to detailed postprocessing highlighting the role of some key factors on NOx formation. In particular, the focus will be dedicated to the impact of the fuel gas composition and the pilot split.

Impact of the Precessing Vortex Core on NOx Emissions in Premixed Swirl-Stabilized Flames—An Experimental Study

Abstract The reduction of NOx emissions remains a driving factor in the design process of swirl-stabilized combustion systems, to meet legislative restrictions. In reacting swirl flows, hydrodynamic coherent structures, such as periodic large-scale vortices in the shear layer, induce zones with increased heat release rate fluctuations in connection with temperature peaks, which lead to an increase of NOx emissions. Such large-scale vortices can be induced by the helical coherent structure known as precessing vortex core (PVC), which influences the flow and flame dynamics under certain operating conditions. We developed an active flow control system, allowing for a targeted actuation of the PVC, to investigate its impact on combustion properties such as NOx emissions. In this work, a perfectly premixed flame, which slightly damps the PVC, is studied in detail. Since the PVC is slightly damped, it can be precisely excited by means of open-loop flow control. In connection with time-resolved OH*-chemiluminescence and stereoscopic particle image velocimetry (PIV) measurements, the impact of the actuated PVC on flow and flame dynamics is characterized. It turns out that the PVC rolls up the inner shear layer, which results in an interaction of PVC-induced vortices and flame. This interaction considerably influences the measured level of NOx emissions, which grows with increasing PVC amplitude in a perfectly premixed flame. Nearly, the same increase is measured for partially premixed conditions. This is in contrast to previous studies, where the PVC is assumed to reduce the NOx emissions due to vortex-enhanced mixing.

Modeling and Quantification of Acoustic Damping Induced by Vortex Shedding in Noncompact Thermoacoustic Systems

Abstract This paper presents a methodology to compute acoustic damping rates of transversal, high-frequency modes induced by vortex-shedding. The acoustic damping rate presents one key quantity for the assessment of the linear thermoacoustic stability of gas turbine combustors. State-of-the-art network models—as employed to calculate damping rates in low-frequency, longitudinal systems—cannot fulfill this task due to the acoustic noncompactness encountered in the high-frequency regime. Furthermore, it is yet unclear, whether direct eigensolutions of the linearized Euler equations (LEE), which capture the mechanism of vortex shedding, yield correct damping rate results constituted by the implicit presence of acoustic as well as hydrodynamic contributions in these solutions. The methodology's applicability to technically relevant systems is demonstrated by a validation test case using a lab-scale, swirl-stabilized combustion system.

Twin-Fluid Atomized Spray Combustion of Straight Vegetable Oil at Elevated Pressures

The effect of the chamber pressure on combustion of a twin-fluid-atomized spray of straight vegetable oil (VO) in a swirl stabilized combustion system is experimentally studied. A system with high pressure capabilities was developed, and flame and emissions characteristics of VO are investigated at elevated pressures up to about 5 bars, different heat release rates (HRRs), and atomizing air to liquid ratios (ALR) by mass. An image analysis technique was developed to infer flame and soot characteristics from visual images acquired by a digital camera. An increase in the ALR resulted in improved combustion of VO, characterized by blue flames, lower CO and NOx emissions, and minimal soot formation. For a given fuel flow rate, an increase in the chamber pressure resulted in smaller volume flames with lower CO levels but higher NOx emissions. Compared to diesel, as pressure increased, straight VO flames produced lower NOx and more voluminous flames characterized by distributed combustion with less soot formation. Overall, straight VO could be atomized and combusted at elevated pressures using the twin-fluid atomizer of the present study, and the resulting VO flames exhibited less sensitivity to chamber pressure variations.

Nonlinear Interactions Between the Fundamental and Higher Harmonics of Self-Excited Combustion Instabilities

ABSTRACTMeasurements of self-excited combustion instability were performed in a lean-premixed, swirl-stabilized, gas turbine combustor in order to characterize the interaction between the fundamental and higher harmonics of dynamic pressure, heat release rate, and acoustic velocity perturbations. The formation of higher harmonics, their interactions with the fundamental, and the influence of these interactions on nonlinear combustion processes are highlighted in this article. A detailed analysis of the fundamental and higher harmonics of the pressure signal reveals that for a small amplitude disturbance the second harmonic of the pressure signal is governed by a quadratic function. For high amplitude disturbances, the measured data deviate from the power law dependence; this is a manifestation of nonlinear energy transfer toward higher harmonic components. The disturbance amplitude at which the inception of the non-quadratic dependence occurs coincides with the transition from linear to nonlinear heat release response. In particular, the higher harmonic characteristics are significantly altered by varying swirler location, which is known as a primary source of vortical disturbance in swirl-stabilized combustion systems. As a result, for certain inlet conditions, the amplitude of the higher harmonic of acoustic velocity is even higher than the amplitude of the fundamental, and this leads to multiple-frequency excitation. The system then evolves toward a periodic state that is completely different from that of a system with single frequency excitation.

Combustion instability feedback mechanisms in a lean-premixed swirl-stabilized combustor

Constructive interference between heat release rate and acoustic perturbation is responsible for the growth of acoustic pressure amplitudes, leading to high-amplitude combustion instabilities in combustion systems. This is referred to as the Rayleigh criterion. Even though the knowledge of the heat release-acoustic pressure coupling processes is critical in the description of self-excited combustion instabilities, little is known about how the unsteady coupling processes are determined in response to the variation of time and length scales of acoustic and convective waves present in turbulent reacting flow fields. To address this issue, we performed a large number of measurements of self-excited instabilities, using two different tunable gas turbine combustors, over a wide range of operating conditions. The initiation, evolution, and saturation of pressure disturbances in swirl-stabilized combustion systems were systematically investigated by integrated analyses of pressure-heat release-velocity feedback coupling processes. It was found that self-excited combustion instabilities are governed by the Rayleigh criterion over the entire parameter space. We also observed that at certain inlet conditions, high amplitude velocity and heat release fluctuations coexist even without pressure-heat release coupling, meaning that the Rayleigh criterion is not necessarily associated with the onset of the self-excited instability. The Rayleigh criterion, however, does play an essential role in the linear to nonlinear transition that is necessary for the system to evolve toward a final state of stable limit cycles. We also found that the presence of equivalence ratio nonuniformities exerts a profound influence on instability feedback mechanisms, and consequently the Rayleigh criterion becomes a weak necessary condition for the occurrence of self-excited instabilities. A feedback mechanism that controls the relationship between velocity and acoustic pressure fluctuations was investigated to understand the effect of swirl number on limit cycle behavior. Acoustic admittance analysis reveals that at sufficiently high disturbance amplitude, nonlinear gas dynamic processes become so significant that the nonlinear saturation behavior cannot be defined by the acoustic velocity amplitude normalized by mean flow velocity.

Time-resolved PIV of lean premixed combustion without and with porous inert media for acoustic control

In the past, we have documented the efficacy of the ring-shaped porous inert media (PIM) to passively mitigate combustion noise and thermo-acoustic instabilities in lean premixed (LPM) swirl-stabilized combustion systems, although the underlying mechanisms involved have not been understood. The present study utilizes time-resolved particle image velocimetry (PIV) to measure the turbulent flow field in a LPM combustion system without and with PIM placed on the dump plane. Measurements of instantaneous and average flow fields, vorticity, and turbulent kinetic energy are presented to gain insight into the flow structure. Results are analyzed using the proper orthogonal decomposition (POD) to identify the effect of PIM on the coherent turbulent structures in the flow field. The addition of PIM removes the recirculation zones and the dominant coherent turbulent structures formed in the shear layers are convected downstream of the reaction zone. Harmonic reconstruction of the flow field is performed at the frequency of thermo-acoustic instability to identify the relationship between the flow and acoustic fields of the system. Without PIM, the vortical modes in the corner recirculation zone are shown to be the driving source for the instability. PIM alters the flow field from global instability to convective instability, which effectively eliminates the feedback mechanism for thermo-acoustic instabilities in the present LPM swirl-stabilized combustion system.

Experimental and numerical investigation of a swirl-stabilized premixed methane/air flame

Purpose – The purpose of this paper is to assess the validity of Weller's b-ω flamelet model for practical swirl-stabilized combustion applications. Design/methodology/approach – Swirl-stabilized premixed flame behavior is investigated utilizing an atmospheric combustor test rig. Swirl number of the flow is 0.74 with a cold flow Reynolds number of 19,400 based on the hydraulic diameter at the inlet pipe. Operating condition corresponds to an equivalence ratio of 0.7 at a thermal load of 20.4 kW. Reacting flow was seeded with TiO2 particles, and velocity distribution at the center plane was measured utilizing particle image velocimetry. These results serve as a validation dataset for numerical simulations. An open-source computational fluid dynamics (CFD) code library (OpenFOAM) is used for numerical computations. These unsteady Reynolds averaged Navier Stokes (RANS) computations were performed at the same load condition corresponding to experimental data. Parallel numerical simulations were carried out on 128 processor cores. To resolve turbulence, Menter's k-ω shear stress transport model was utilized; flame behavior, on the other hand, was described by Weller's b-ω flamelet model. A block-structured all-hexahedral mesh was used. Findings – It is observed that two counter-rotating vortices in the main recirculation zone are responsible for flame stabilization. Weak secondary recirculation zones are also present at the sides above the dump plane. Flame front location was inferred from Mie scattering images. Experimental findings show that the flame anchors both on the tip of the center body and also at the rim of the outlet pipe. Numerical simulations capture the complex interactions between the flame and the turbulent flow. These results qualitatively agree with the flame structure observed experimentally. Practical implications – Swirl-stabilized combustion systems are used in many practical applications ranging from aeroengines to land-based power generation systems. There are implications regarding the understanding of these combustion systems. Social implications – Better understanding of combustion systems contributes to better performing turbine engines and reduced emissions with implications for the entire society. Originality/value – The paper provides experimental insight into the application of a combustion model for a flame configuration of practical interest.

Large-Eddy Simulation and Acoustic Analysis of a Swirled Staged Turbulent Combustor

The analysis of self-excited combustion instabilies encountered in a laboratory-scale, swirl-stabilized combustion system is presented. The instability is successfully captured by reactive large-eddy simulation (LES) and analyzed by using a global acoustic energy equation. This energy equation shows how the source term due to combustion (equivalent to the Rayleigh criterion) is balanced by the acoustic fluxes at the boundaries when reaching the limit cycle. Additionally, an Helmholtz-equation solver including flame-acoustics interaction modeling is used to predict the stability characteristics of the system. Feeding the flame-transfer function from the LES into this solver allows to predict an amplification rate for each mode. The unstable mode encountered in the LES compares well with the mode of the highest amplification factor in the Helmholtz-equation solver, in terms of mode shape as well as in frequency.