Investigation of the Combustion Performance in a Three-Nozzle RQL Combustor

RQL (Rich-burn/Quick-quench/Lean-burn) is a candidate to support fuel flexible stationary power generation. The equivalence ratio of rich-burn zone (Φr) and the quench air flow are paramount for implantation of the whole process. In this paper, an experimental test stand with multi-sector model combustor was established. Rich premixed combustion were used in rich zone. The experiments which pay attention to the impacts of Φr and quench air flow on the combustion performance and emission are conducted. The results show that the flame in RQL combustor is segmented when Φr >1.4, presenting flameless combustion in rich zone and a pale blue flame in lean zone. Axial temperature distribution is M-type. Two peaks appear at the head and tail of the combustion chamber, and the valley is located in the quench zone. The concentration of CO decreases rapidly in quench zone because of the injection of quench air. However, the concentration of NOx increases quickly at the same time. The outlet emissions of CO and NOx in RQL combustor are maintained at low level (<20ppm@15%O2). With a decrease of Φr from 1.4 to 1.2, the emission of NOx increases, and the emission of CO decreases. With jet-to-mainstream mass-flow ratio increases from 1.28 to 2.22., the concentration of NOx in outlet declines gently, but the CO emission increase. The average exhaust temperature depresses gradually, and the uniformity coefficient of exhaust temperature increases.

Control of the Precessing Vortex Core by Open and Closed-Loop Forcing in the Jet Core

The precessing vortex core (PVC) is the dominant coherent structure of swirling jets, which are commonly applied in gas turbine combustion. It stems from a global hydrodynamic instability that is caused by internal feedback mechanisms in the jet core. In this work, we apply open and closed-loop forcing in a generic non-reacting jet to control this mechanism and the PVC. Control is exerted by two oppositely facing, counter-phased zero-net mass flux jets, which are introduced radially into the flow through a thin lance positioned on the jet center axis. By using this type of forcing, the instability mode m = 1, corresponding to the PVC, can either be excited or damped. This markedly affects the PVC oscillation frequency and amplitude. The passive influence of the actuation lance on the mean flow field properties and the coherent flow dynamics is studied first without forcing. PIV and hot-wire measurements reveal an effect on the mean flow, but no qualitative changes of the PVC dynamics. Lock-in experiments are conducted, in which the synchronization behavior of the PVC with the forcing is determined. Here, two different cases are considered. First, actuation is applied at different streamwise positions in order to identify the region of highest receptivity towards external forcing. This region of lowest lock-in amplitude is shown to coincide with the location of the wavemaker, shortly upstream of the vortex breakdown bubble. Second, the lock-in behavior at a fixed axial position and various forcing frequencies ff is studied. A linear correlation between the lock-in amplitude and the deviation of the forcing frequency from the natural oscillation frequency |ff – fn| is observed. Closed-loop control is then applied with the aim to suppress the PVC. The actuator lance is positioned in the wavemaker region, where the flow is most receptive. Magnitude and phase of the natural flow oscillation associated with the PVC are estimated from four hot-wire signals using an extended Kalman filter. The estimated PVC signal is phase-shifted and fed back to the actuator. PIV measurements reveal that feedback control achieves a reduction of the PVC oscillation energy of about 40%.

Investigation of Ducted Inverse Nonpremixed Flame Using Dynamic Systems Approach

Gas turbine combustion has a number of practical applications, including aviation engines, ocean vessels, and tanks. The various advantages of normal diffusion flames, such as increased flame stability and reduced susceptibility to dynamic instabilities, has made it the de facto industrial standard. However, high NOx emission and sooting from such flames is a major problem, particularly for heavier hydrocarbons fuels. In that regard, the inverse diffusion flame offers a feasible alternative; but the dynamic response of such a flame, particularly in ducted conditions — where the unsteady heat release interacts with the duct acoustics — is relatively less researched. In the present work, an experimental investigation of a laboratory-scale inverse diffusion flame has been carried out. The inverse diffusion flame is found in applications like rocket motors, gas turbine combustors, and furnaces. In the present study, inverse diffusion flame from a coaxial burner inside a quartz tube was studied. The position of the duct with respect to the flame was kept fixed, while the global equivalence ratio was varied by keeping the air flow rate constant and changing the fuel flow rate. Various tools of nonlinear dynamics such as phase space reconstruction and recurrence quantification have also been used for dynamic characterization of such flames. The results show that the dynamics of the flame strongly depends on the global equivalence ratio.

Application of a Turbulent Jet Flame Flashback Propensity Model to a Commercial Gas Turbine Combustor

Because flashback is a key operability issue associated with low emission combustion of high hydrogen content fuels, design tools to predict flashback propensity are of interest. Such a design tool has been developed by the authors to predict boundary layer flashback using non-dimensional parameters. The tool accounts for the thermal coupling between the flame and burner rim and was derived using detailed studies carried out in a test rig at elevated temperature and pressure. The present work evaluates the applicability of the tool to a commercial 65 kW micro turbine generator (MTG). Two sets of data are evaluated. One set is obtained using the combustor, removed from the engine, which has been configured to operate like it does in the engine but at atmospheric pressure and various preheat temperatures. The second set of data is from a combustor operated as it normally would in the commercial engine. In both configurations, studies are carried out with various amounts of hydrogen added to either natural gas or carbon monoxide. The previously developed model is able to capture the measured flashback tendencies in both configurations. In addition, the model is used to interpret flashback phenomena at high pressures and temperatures in the context of the engine conditions. An increase in pressure for a given preheat temperature and velocity reduces the equivalence ratio at which flashback occurs and increases the tip temperature due to lower quenching distance. The dependency of the flashback propensity on the injector tip temperature is enhanced with an increase in pressure. The variation of critical velocity gradient with equivalence ratio for a constant preheat temperature is more pronounced at higher pressures. In summary, the model developed using the high pressure test rig is able to predict flashback tendencies for a commercial gas turbine engine and can thus serve as an effective design tool for identifying when flashback is likely to occur for a given geometry and condition.

Low-Order Modeling of Nonlinear High-Frequency Transversal Thermoacoustic Oscillations in Gas Turbine Combustors

This paper analyzes transversal thermoacoustic oscillations in an experimental gas turbine combustor utilizing dynamical system theory. Limit cycle acoustic motions related to the first linearly unstable transversal mode of a given 3D combustor configuration are modeled, and reconstructed by means of a low order dynamical system simulation. The source of nonlinearity is solely allocated to flame dynamics, saturating the growth of acoustic amplitudes, while the oscillation amplitudes are assumed to always remain within the linearity limit. First, a Reduced Order Model (ROM), which reproduces the combustor’s modal distribution and damping of acoustic oscillations is derived. The ROM is a low-order state-space system, which results from a projection of the Linearized Euler Equations (LEE) into their truncated eigenspace. Second, flame dynamics are modeled as a function of acoustic perturbations by means of a nonlinear transfer function. This function has a linear and a nonlinear contribution. The linear part is modeled analytically from first principles, while the nonlinear part is mathematically cast into a cubic saturation functional form. Additionally, the impact of stochastic forcing due to broadband combustion noise is included by additive white noise sources. Then, the acoustic and the flame system is interconnected, where thermoacoustic non-compactness due to the transversal modes’ high frequency is accounted for by a distributed source term framework. The resulting nonlinear thermoacoustic system is solved in frequency and time domain. Linear growth rates predict linear stability, while envelope plots and probability density diagrams of the resulting pressure traces characterize the thermoacoustic performance of the combustor from a dynamical systems theory perspective. Comparisons against experimental data are conducted, which allow the rating of the flame modes in terms of their capability to reproduce the observed combustor dynamics. Ultimately, insight into the physics of high-frequency, transversal thermoacoustic systems is created.

Numerical Analysis of the Dynamic Flame Response and Thermo-Acoustic Stability of a Full-Annular Lean Partially-Premixed Combustor

A thermo-acoustic stability of a full-annular lean partially-premixed heavy-duty gas turbine combustor is carried out in the present paper. A sensitivity analysis is performed, varying the flame temperature for two operating conditions. The complex interaction between the system acoustics and the turbulent flame is studied in Ansys Fluent, using Unsteady-RANS simulations with Flamelet-Generated Manifolds combustion model. Perturbations are introduced in the system imposing a broadband excitation as inlet boundary condition. The flame response is then computed exploiting system identification techniques. The identified flame transfer functions are compared each other and the results analysed in order to give more physical insight on the coupling mechanisms responsible for the flame dynamic response. The effect of fuel mass flow fluctuations is then introduced as further driving input, describing the flame as a Multi-Input Single-Output system. Further in-depth studies are carried out on pilot flames aiming at replicating the dynamic response of the real flame and understanding the driving mechanism of thermo-acoustic instability onset as well. The obtained results are implemented into a finite element model of the combustor, realized in COMSOL Multiphysics, to analyse the system stability. Numerical model affordability has been assessed through comparisons with results from full-annular combustor experimental campaign carried out by GE Oil & Gas since the early phases of the design and development of a heavy-duty gas turbine. This allowed the discussion of the model ability to describe the stability properties of the combustor and to catch the instabilities onset as detected experimentally. Valuable indications for future design optimization were also identified thanks to the obtained results.

Shockless Explosion Combustion: Experimental Investigation of a New Approximate Constant Volume Combustion Process

Approximate constant volume combustion (aCVC) is a promising way to achieve a step change in the efficiency of gas turbines. This work investigates a recently proposed approach to implement aCVC in a gas turbine combustion system: shockless explosion combustion (SEC). The new concept overcomes several disadvantages such as sharp pressure transitions, entropy generation due to shock waves, and exergy losses due to kinetic energy which are associated with other aCVC approaches like, e.g., pulsed detonation combustion. The combustion is controlled via the the fuel/air mixture distribution which is adjusted such that the entire fuel/air volume undergoes a spatially quasi-homogeneous autoignition. Accordingly, no shock waves occur and the losses associated with a detonation wave are not present in the proposed system. Instead, a smooth pressure rise is created due to the heat release of the homogeneous combustion. An atmospheric combustion test rig is designed to investigate the autoignition behavior of relevant fuels under intermittent operation, currently up to a frequency of 2Hz. Application of OH*- and dynamic pressure sensors allows for a spatially- and time-resolved detection of ignition delay times and locations. Dimethyl ether (DME) is used as fuel since it exhibits reliable autoignition already at 920K mixture temperature and ambient pressure. First, a model-based control algorithm is used to demonstrate that the fuel valve can produce arbitrary fuel profiles in the combustion tube. Next, the control algorithm is used to achieve the desired fuel stratification, resulting in a significant reduction in spatial variance of the auto-ignition delay times. This proves that the control approach is a useful tool for increasing the homogeneity of the autoignition.

Analysis of Combustion Noise Using Locally Resolved Density Fluctuations and a Microphone Array

For turbulent swirl-stabilized flames combustion noise can be directly calculated, if density fluctuations as a function of time and space are known. It is however not easily possible to assess the density fluctuations directly. Therefore, in the past, combustion noise has been expressed as a function of chemiluminescence, an approach bringing in more assumptions. Now, by using interferometry, density fluctuations in the flame can be measured quantitatively. The advantage of this technique is that it measures the time derivative of density fluctuations directly. In this work laser interferometric vibrometry (LIV) was used to scan a two dimensional field in the flame in order to calculate the sound power emitted by the flame. Sound intensity was measured in a half-hemisphere by pressure-pressure-probes in order to record the total sound power of the direct combustion noise emitted by the unconfined flame. The goal of this study was to compare the measured sound power exhibited by the flame with the sound power predicted due to fluctuations of density within the flame. By using a siren to generate linear excitation, it was possible to qualitatively predict combustion noise with good agreement in trend. A quantitative comparison between both measurement techniques showed a deviation of a factor of six.

Noise-Induced Dynamics in the Subthreshold Region in Thermoacoustic Systems

This article is a report of experiments conducted in order to investigate the role of noise on thermoacoustic systems. In contrast to most studies in this direction, in the present work, the role of noise in the subthreshold region, prior to the (subcritical) Hopf bifurcation and the associated saddle-node bifurcation is considered. Although, in this regime, a thermoacoustic system is stable and does not undergo transition to self-excited thermoacoustic oscillations, the system can feature dynamics that arise due to the proximity of the system to the approaching Hopf bifurcation in response to noise. Experiments were performed on a model thermoacoustic system featuring a laminar flat flame. Noise was introduced in a controlled manner and the effect of increasing levels of noise intensity was studied. Results presented here show that noise addition induces coherent oscillations. The induced coherence can be quite significant, and is dependent on the noise amplitude and proximity to the Hopf bifurcation. Furthermore, this noise-induced behavior is characterized by a well-defined ‘resonance-like’ response of the system: An optimum level of coherence is induced for an intermediate level of noise. For practical thermoacoustic systems (e.g. combustors), which are inherently noisy due to factors such as flow turbulence and combustion noise, these results can have important implications.

Comparison of Flame Transfer Functions Acquired by Chemiluminescence and Density Fluctuation

The goal of this study was to measure the Flame Transfer Function of a perfectly and a partially premixed turbulent flame by means of Laser Interferometric Vibrometry. For the first time, this technique is used to detect integral heat release fluctuations. The results were compared to classical OH*-chemiluminescence measurements. Effects of equivalence ratio waves and vortex rollup were found within those flames and were then investigated by means of time resolved planar CH*/OH*-chemiluminescence and Frequency modulated Doppler global velocimetry. This work is motivated by the difficulties chemiluminescence encounters when faced with partially premixed flames including equivalence ratio waves and flame stretching. LIV, recording the time derivative of the density fluctuations as line-of-sight data, is not affected by these flame properties.