Towards Low-NOx Operation in a Complex Burner: Optimization of an Annular Trapped Vortex Combustor

2013 ◽  
Author(s):  
Pradip Xavier ◽  
Bruno Renou ◽  
Gilles Cabot ◽  
Mourad A. Boukhalfa ◽  
Michel Cazalens
Keyword(s):  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Chemical Parameters ◽  
Hot Gas ◽  
Highly Sensitive ◽  
Stable Flame ◽  
Mass Transfers ◽  
Gas Expansion ◽  
Trapped Vortex ◽  
Original Configuration

This paper focuses on optimizing an innovative annular Lean Premixed staged burner, following the Trapped Vortex Combustor concept. The latter consists of a lean main flame stabilized by passing past a rich cavity pilot flame. Unfortunately, this configuration is highly sensitive to combustion instabilities and the flame is not well stabilized. This work consists of adjusting aerodynamic variables, chemical parameters and burner geometry to reach a “low-NOx” operation while reducing other pollutants and getting a stable flame. Results show that stability is reached when mass transfers between main and cavity zones are reduced. Then, the main bulk velocity is increased to reduce the cavity thermal expansion, due to the hot gas expansion. In addition, the cavity flow rate is reduced to prevent from penetrating and disturbing the main flow. Re-arranging injections in the cavity also avoid local unsteady equivalence ratios, which creates an unsteady heat release and combustion with pulses. Regarding NOx, a leaner main flame combined with a sufficiently rich cavity mixture creates local stoichiometric zones at the interface between the cavity and the main zone. The latter point is found to be a good anchoring mechanism. Compared with the original configuration, a stable point of operation is found: acoustic energy is reduced by an order of 100, NOx level is less than 0.4 g/kgfuel, CO is cut by 93% with no more Unburned Hydro-Carbons.

Determination of Criteria for Flame Stability in an Annular Trapped Vortex Combustor

2015 ◽  
Author(s):  
Pradip Xavier ◽  
Mickael Pires ◽  
Alexis Vandel ◽  
Bruno Renou ◽  
Gilles Cabot ◽  
...  
Keyword(s):  
Driving Forces ◽  
Flux Ratio ◽  
Flame Stabilization ◽  
Operating Conditions ◽  
Pollutant Emissions ◽  
Combustion Instabilities ◽  
Stability Maps ◽  
The Rich ◽  
Acoustic Instabilities ◽  
Trapped Vortex

Development of lean premixed (LP) combustion is still a challenge as it results in considerable constraints for the combustor design. Indeed, new combustors using LP combustion are more prone to flashback, blow-off, or even thermo-acoustic instabilities. A detailed understanding of mechanisms leading to such extreme conditions is then crucial to reduce pollutant emissions, widen the range of operating conditions, and reduce design time. This paper reports the experimental study of an innovative LP trapped vortex combustor (TVC). The TVC concept uses a recirculating rich flow trapped in a cavity to create a stable flame that continuously ignites a main lean mixture passing above the cavity. This concept gave promising performances but some workers highlighted the existence of combustion instabilities for some operating conditions. Detailed studies have therefore been carried out in order to understand the occurrence of these drastic operating conditions. Results showed that the cavity flow dynamics in conjunction with the location of the interfacial mixing zone (between the cavity and the mainstream) were the driving forces to obtain stable combustion regimes. The goal of this work has been to take advantage of these detailed recommendations to determine stability maps, trends, and dimensionless parameters which could be easily used as early-design rules. For this reason, the study introduced a simple and robust criterion, based on the global pressure fluctuation energy. The latter was used to distinguish stable and unstable modes. An aerodynamic momentum flux ratio and a chemical stratification ratio (taken between the cavity and the mainstream) were defined to scale all measurements. Results indicated that the mainstream velocity was critically important to confine the cavity and to prevent combustion instabilities. Remarkably, this trend was verified and even more pronounced for larger cavity powers. In addition, flame stabilization above the cavity resulted in the existence of specific stratification ratios, in order to obtain a soft gradient of gas composition between the rich and lean regions. Finally, a linear relation between the mainstream and cavity velocities became apparent, thereby making possible to simply predict the combustor stability.

Influence of rarefaction on the flow dynamics of a stationary supersonic hot-gas expansion

2008 ◽  
Vol 77 (3) ◽  
Author(s):  
G. Abbate ◽  
C. R. Kleijn ◽  
B. J. Thijsse ◽  
R. Engeln ◽  
M. C. M. van de Sanden ◽  
...  
Keyword(s):  
Flow Dynamics ◽  
Hot Gas ◽  
Gas Expansion

Chemical trends and status of small lakes near Sudbury, Ontario, 1983-1995: evidence of continued chemical recovery

1998 ◽  
Vol 55 (1) ◽  
pp. 63-75 ◽  
Author(s):  
M L Mallory ◽  
D K McNicol ◽  
D A Cluis ◽  
C Laberge
Keyword(s):  
Base Cations ◽  
Chemical Recovery ◽  
Chemical Parameters ◽  
Emission Reductions ◽  
Gradual Improvement ◽  
Local Climate ◽  
Highly Sensitive ◽  
Local Emission ◽  
Chemical Conditions ◽  
Chemical Trends

We monitored 23 chemical parameters in 161 lakes northeast of Sudbury, Ontario, in most years between 1983 and 1995 to determine whether lake chemistries were responding to reduced local SO2 emissions. Lakes were typically small (median 4.0 ha, 4.5 m deep), rapid flushing, and acid stressed (median pH 5.58, acid-neutalizing capacity (ANC) 7.1 µequiv. ·L-1). Forty percent of the lakes declined significantly in SO4, base cations, and Al levels from 1983 to 1995, but only 12 and 16% increased in ANC and pH, respectively. Chemical trends were influenced by local climate; pronounced improvements occurred in response to drought conditions (1986 and 1987), but rapid deterioration followed the wet year in 1988. With more typical precipitation levels since 1989, a pattern of gradual improvement returned. Connected, rapid-flushing lakes, those on low or moderately sensitive bedrock, and peatlands showed the greatest improvements in pH and ANC whereas glacial headwaters and lakes on highly sensitive bedrock showed the least improvement. These results confirm that some recovery continues to occur following local emission reductions. However, many small Sudbury area lakes remain very degraded and unsuitable for acid-sensitive biota. Further emission reductions, including long-range sources, may be necessary to restore chemical conditions to critical levels sufficient to support further biological recovery.

scholarly journals Characterisation of an inlet pre-injector laser-induced fluorescence instrument for the measurement of atmospheric hydroxyl radicals

2014 ◽  
Vol 7 (10) ◽  
pp. 3413-3430 ◽  
Author(s):  
A. Novelli ◽  
K. Hens ◽  
C. Tatum Ernest ◽  
D. Kubistin ◽  
E. Regelin ◽  
...  
Keyword(s):  
Hydroxyl Radicals ◽  
Analytical Techniques ◽  
Laser Induced Fluorescence ◽  
High Reactivity ◽  
Measurement Unit ◽  
Atmospheric Measurements ◽  
Mass Spectrometers ◽  
Expansion Technique ◽  
Highly Sensitive ◽  
Gas Expansion

Abstract. Atmospheric measurements of hydroxyl radicals (OH) are challenging due to a high reactivity and consequently low concentration. The importance of OH as an atmospheric oxidant has motivated a sustained effort leading to the development of a number of highly sensitive analytical techniques. Recent work has indicated that the laser-induced fluorescence of the OH molecules method based on the fluorescence assay by gas expansion technique (LIF-FAGE) for the measurement of atmospheric OH in some environments may be influenced by artificial OH generated within the instrument, and a chemical method to remove this interference was implemented in a LIF-FAGE system by Mao et al. (2012). While it is not clear whether other LIF-FAGE instruments suffer from the same interference, we have applied this method to our LIF-FAGE HORUS (Hydroxyl Radical Measurement Unit based on fluorescence Spectroscopy) system, and developed and deployed an inlet pre-injector (IPI) to determine the chemical zero level in the instrument via scavenging the ambient OH radical. We describe and characterise this technique in addition to its application at field sites in forested locations in Finland, Spain and Germany. Ambient measurements show that OH generated within the HORUS instrument is a non-negligible fraction of the total OH signal, which can comprise 30 to 80% during daytime and 60 to 100% during the night. The contribution of the background OH varied greatly between measurement sites and was likely related to the type and concentration of volatile organic compounds (VOCs) present at each particular location. Two inter-comparisons in contrasting environments between the HORUS instrument and two different chemical ionisation mass spectrometers (CIMS) are described to demonstrate the efficacy of IPI and the necessity of the chemical zeroing method for our LIF-FAGE instrument in such environments.

Experimental Analysis of Simultaneous Non-Harmonically Related Unstable Modes in a Swirled Combustor

2011 ◽  
Author(s):  
Ammar Lamraoui ◽  
Franck Richecoeur ◽  
Se´bastien Ducruix ◽  
Thierry Schuller
Keyword(s):  
High Speed ◽  
Sound Pressure ◽  
Acoustic Pressure ◽  
Source Term ◽  
Low Frequency ◽  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Acoustic Response ◽  
Flame Region ◽  
Two Peaks

The present study investigates combustion instabilities generated in a turbulent swirled combustor featuring two non-harmonically related unstable modes. Sound pressure and chemiluminescence spectra show the presence of two peaks located around 180 Hz and 280 Hz during unstable operation. The low frequency acoustic response of the test-rig is then analyzed using a two-coupled-cavity model including a realistic impedance of the system at the premixer inlet. This analytical approach is used to link the two observed frequencies to the first chamber and premixer modes respectively. Analytical predictions are compared with acoustic pressure measurements to determine the structure of these modes. The Rayleigh source term in the energy balance is also computed and shows that the two modes feed acoustic energy simultaneously in the system. High-speed PIV data gathered under unstable operation are filtered around these two frequencies to obtain phase conditioned images. Results show that the unsteady flow in the flame region features distinct dynamics associated to a bulk longitudinal oscillation of the flow in the flame arms at 180 Hz and large wrinkles in the radial direction at 280 Hz.

An Acoustic Energy Approach to Modeling Combustion Oscillations

2006 ◽  
Author(s):  
Z. M. Ibrahim ◽  
F. A. Williams ◽  
S. G. Buckley ◽  
J. C. Y. Lee
Keyword(s):  
Operating Conditions ◽  
Energy Approach ◽  
Acoustic Energy ◽  
Amplification Coefficient ◽  
Design Phase ◽  
Combustion Instabilities ◽  
New Approach ◽  
New Perspective ◽  
Prediction Capability ◽  
Oscillatory Instabilities

A new approach for modeling combustion instabilities using acoustic energy conservation is proposed. This approach allows the flexibility to calculate contributions from amplification and/or attenuation across all frequencies, rather than predicting eigenmodes. It is likely that such a direct acoustic energy approach could be of benefit in determining the susceptibility of all frequencies to growth or decay as operating conditions in a gas turbine change. This approach may also give designers a method for improving passive damping in the initial design phase, with the ability to scan various frequencies and investigate the susceptibility to oscillatory instability at different operating conditions. A linear model, including linear approximations to nonlinear processes, is introduced whereby the mechanisms contributing to amplification and damping of acoustic energy are assessed independently to find a net amplification coefficient, a reciprocal time (or a rate). Stability for each frequency is assessed by examining a ratio between amplification and damping. It is anticipated that this effort may provide a useful new perspective on and enhanced prediction capability for combustion oscillatory instabilities.

scholarly journals Release characteristics of overpressurised gas from complex vents: implications for volcanic hazards

2020 ◽  
Vol 82 (11) ◽  
Author(s):  
Markus Schmid ◽  
Ulrich Kueppers ◽  
Valeria Cigala ◽  
Jörn Sesterhenn ◽  
Donald B. Dingwell
Keyword(s):  
Volcanic Eruptions ◽  
Volcanic Hazards ◽  
Gas Jet ◽  
The Novel ◽  
Slant Angle ◽  
Highly Sensitive ◽  
Explosive Volcanic Eruptions ◽  
Gas Expansion ◽  
Maximum Spreading ◽  
Expansion Behaviour

Abstract Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30° slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents.

Passive Damper LP Tests for Controlling Combustion Instability

2006 ◽  
Author(s):  
M. A. Macquisten ◽  
A. Holt ◽  
M. Whiteman ◽  
A. J. Moran ◽  
J. Rupp
Keyword(s):  
Combustion Instability ◽  
Vortex Formation ◽  
Helmholtz Resonator ◽  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Design Rules ◽  
Acoustic Damping ◽  
Passive Damper ◽  
Combustion Systems ◽  
Combustion Tests

The drive to low emissions from GT combustors has pushed manufacturers towards leaner combustion systems. Lean combustion systems are susceptible to thermo acoustic or combustion instabilities, which can significantly limit the operation of the GT in terms of performance and emissions. Combustion instability is the result of coupling between fluctuations in the heat release rate and pressure waves. The occurrence of instability dependent on (a) satisfying the Rayleigh criterion and (b) the growth must exceed the losses of acoustic energy. The growth of instability can be controlled by increasing the level of acoustic damping via a Helmholtz resonator and through viscous damping. Design rules for a passive damper have been developed through the EU funded project called PRECCINSTA (Prediction and control of combustion instabilities in tubular and annular combustion systems) by the University of Cambridge. These design rules are for a doubled-skinned perforated liner where a biasing flow is used to dissipated sound energy. The sound dissipation mechanism is via vortex formation. These design rules were then validated against atmospheric and intermediate pressure combustion tests at Rolls-Royce for self-excited and forced excited oscillations. This paper summaries these tests and gives the results for a simple perforated liner as a passive acoustic damper.

Effect of Cooling Liner on Acoustic Energy Absorption and Flame Response

2010 ◽  
Author(s):  
Umesh Bhayaraju ◽  
Johannes Schmidt ◽  
Karthik Kashinath ◽  
Simone Hochgreb
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Acoustic Waves ◽  
Bluff Body ◽  
Acoustic Energy ◽  
Combustion Instabilities ◽  
Acoustic Damping ◽  
Lean Combustion ◽  
Bias Flow ◽  
Flame Response

Gas turbine combustors with lean combustion injectors are prone to thermo-acoustic/combustion instabilities. Several passive techniques have been developed to control combustion instabilities, such as using Helmholtz resonators or viscous dampers using perforated liners that have potential for broadband acoustic damping. In this paper the role of single-walled cooling liners is considered in the damping of acoustic waves and on the flame transfer function in a sample bluff-body burner. Three liner geometries are considered: no bias flow (solid liner), normal effusion holes, and grazing effusion holes at 25° inclination. Cold flow experiments with speaker forcing are carried out to characterise the absorption properties of the liner and compared with an acoustic network model. The results show that whereas the bulk of the acoustic losses is due to the vortex recirculation zones, the liners contribute significantly to the absorption over a wide area of the frequency range. The flame transfer function gain is measured as a function of bias flow for a given operating condition of the burner. The experiments show that for the geometry considered, the global flame transfer function is little affected by cooling except in the case of the normal flow holes. Further analysis shows that whereas the total flame transfer function is not affected, the flame heat release becomes more spatially distributed along the axial length, and a 1D flame response shows distinct modes corresponding to the modal heat release locations.

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