| 1. |
- Guo, Shilong, et al.
(författare)
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Investigation on bluff-body and swirl stabilized flames near lean blowoff with PIV/PLIF measurements and LES modelling
- 2019
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311. ; 160
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Tidskriftsartikel (refereegranskat)abstract
- Lean premixed combustion (LPC) is a promising technology for low-NOx emission, while it increases the risk of blowoff at the same time. Experiments and Large Eddy Simulations (LES) on swirl stratified lean-premixed CH4/air flames were performed to study the differences between the stable and near blowoff flame. The flow fields and instantaneous flame structures were measured by simultaneous Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (OH-PLIF). The Thickened Flame (TF) model coupled with a two-step reduced chemical mechanism was used in LES modelling. The non-dynamic formulation of sub-grid flame wrinkling model is performed well for stable condition while it cannot predict the near lean blowoff flame reasonable. Compared with the stable flame, several significant differences can be observed in the near lean blowoff flame. The height of high-temperature-zone is relatively low and the heat loss of flame attachment can be easily enhanced by the low temperature spot induced by flame-vortex interaction. The flame attachment is subject to higher excess strain rate and turbulence fluctuation. Meanwhile, a Processing Vortex Core (PVC) appears downstream of the centerline. It is concluded that lean blowoff is the result of interactions between the fuel/air mixture ignition, PVC instability and flame attachment lift-off.
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| 2. |
- Guo, Shilong, et al.
(författare)
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Numerical simulation of premixed combustion using the modified dynamic thickened flame model coupled with multi-step reaction mechanism
- 2018
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 233, s. 346-353
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Tidskriftsartikel (refereegranskat)abstract
- Thickened flame (TF) model is one of the effective methods to resolve the flame front in turbulent premixed combustion modeling. The multi-step reaction mechanism is becoming increasingly important for combustion simulations such as pollutant formation, ignition and extinction. The effect of TF model on flame structures when coupling with multi-step reaction mechanism was investigated. The simulation results show that, no matter in laminar or turbulent condition, the global TF model coupling with multi-step reaction mechanism results in an incomplete combustion, which is mainly due to the enhanced species diffusion. Although Durand and Polifke's dynamic thickened flame (DTF) sensor performs well for predicting laminar flame structure when coupling with multi-step reaction mechanism, it underestimates the effective thickening factor. In turbulent premixed flame simulation, the underestimated thickening factor leads to a faster local fuel consumption speed because of the over-predicted sub-grid flame wrinkling factor. A modified DTF sensor suitable for multi-step reaction mechanism is proposed. This sensor using the hyperbolic tangent function of progress variable to calculate thickening factor dynamically. It ensures that both the preheated and reaction zones are thickened effectively. The sub-grid wrinkling factor is hence estimated corresponding to the calculated flame thickness. Results of 1D laminar and 3D turbulent flame show that this method performs well for predicting both burned gas temperature and species concentration in burnt gas, which is important for predicting emissions.
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| 3. |
- Hodzic, Erdzan, et al.
(författare)
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Numerical and experimental investigation of the cecost swirl burner
- 2018
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Ingår i: ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition : Combustion, Fuels, and Emissions - Combustion, Fuels, and Emissions. - 9780791851050 ; 4A
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Konferensbidrag (refereegranskat)abstract
- Clean technology has become a key feature due to increasing environmental concerns. Swirling flows, being directly associated with combustion performance and hence minimized pollutant formation, are encountered in most propulsion and power-generation combustion devices. In this study, the development process of the conceptual swirl burner developed at the Swedish National Centre for Combustion and Technology (CeCOST), is presented. Utilizing extensive computational fluid dynamics (CFD) analysis, both the lead time and cost in manufacturing of the different burner parts were significantly reduced. The performance maps bounded by the flashback and blow-off limits for the current configuration were obtained and studied in detail using advanced experimental measurements and numerical simulations. Utilizing high speed OH-chemiluminescence, OH/CH2O-PLIF and Large Eddy Simulation (LES), details of the combustion process and flame-flow interaction are presented. The main focus is on three different cases, a stable case, a case close to blow-off and flashback condition. We show the influence of the flame on the core flow and how an increase in swirl may extend the stability limit of the anchored flame in swirling flow burners.
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| 4. |
- Liu, Xin, et al.
(författare)
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Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor
- 2021
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 46:24, s. 13377-13388
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Tidskriftsartikel (refereegranskat)abstract
- Methane and hydrogen-enriched (25 vol% and 50 vol% H2-enriched CH4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H2 concentrations. The effects of equivalence ratio (Φ), Re, and H2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH4/air cases, the flame was stabilized with an M shape; while for H2-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH4/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape, whereas the BLF occurs for all H2-enriched flames with П shape.
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| 5. |
- Subash, Arman Ahamed, et al.
(författare)
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Flame investigations of a laboratory-scale CECOST swirl burner at atmospheric pressure conditions
- 2020
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 279
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Tidskriftsartikel (refereegranskat)abstract
- Experimental and numerical studies were performed to understand the stabilization of lean premixed natural gas/air flames in a gas turbine model combustor which was equipped with a swirl burner, known as the CECOST burner, designed to replicate the flow and flame structures in an industrial gas turbine engine. The operability range, flame stabilization, and flashback were investigated employing simultaneous OH– and CH2O-PLIF, and high-speed chemiluminescence imaging. Large eddy simulation (LES) was carried out for analysis of the vortex breakdown structures under non-reacting conditions. It was found that the vortex breakdown structures under isothermal conditions were insensitive to the Reynolds number (Re) for Re ≥ 10000; however, the stability of the flames and operability range of the burner were highly sensitive to Re as well as to equivalence ratio (ϕ). The equivalence ratio was varied at various Reynolds numbers to observe different regimes of the flame ranging from the lean blowout (LBO) limit to the flashback limit. The LBO limit was found to be mainly a function of equivalence ratio while being nearly independent of the Reynolds number, whereas the occurrence of flashback showed distinct characteristics for different ranges of the Reynolds number. At low and moderate Reynolds numbers, (Re ≤ 17000), flashback occurred when increasing ϕ from lean towards stoichiometric conditions. The coupling between the flow field and heat release induces vortex breakdown in the mixing tube and initiates flashback. In contrast, at higher Reynolds numbers (Re > 17000) no flashback was observed even when ϕ was increased to stoichiometric conditions. At these conditions with high Re, the increase in the bulk flow velocity affects the vortex breakdown structure, pushing the vortex breakdown downstream, which in turn prevents the flame from flashing back into the mixing tube.
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| 6. |
- Tong, Yiheng, et al.
(författare)
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Experimental study on dynamics of a confined low swirl partially premixed methane-hydrogen-air flame
- 2017
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 42:44, s. 27400-27415
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Tidskriftsartikel (refereegranskat)abstract
- The addition of hydrogen to swirl stabilized methane-air flame in gas turbine has gained more and more attentions in recent years. In the current study, flame structures, flame dynamics and lean blowout limits of partially premixed hydrogen-methane-air flames were investigated. The swirling flow, which was generated from the tangential flow injection, was utilized to stabilize the flame. The flow swirl number was kept low varying from S ≈ 0.28 to S ≈ 0.34 while the thermal power of the burner ranged from 10.8 kW to 13.8 kW. Two different fuel injection strategies were investigated and compared with each other. Long exposure CH* chemiluminescence from the flame was captured to visualize the time averaged flame shapes. In addition, an intensified high speed camera was adopted to study the flame dynamics. A high speed PIV system was utilized to investigate the interaction of flame dynamics and flow fields oscillations. Based on the experimental results, it can be concluded that: in the current experimental cases, fuel injection strategy plays an important role in determining the flame macro-structures and thus strongly affects the flame dynamics and lean blowout limits. Flame with fuel injected through the axial flow has lower lean blowout limits. The flashback and vortex breakdown were observed when fuel was injected in the tangential flow near lean blowout. High frequency flame oscillations (f ≈ 170 Hz) were observed when the global equivalence ratio Φ g > 0.72 while lower frequency oscillations (f ≈ 50 Hz and f ≈ 20 Hz) were found near lean blowout limits, Φ g < 0.55. Combustion dynamic and its interaction with the pressure oscillation, flow fields alternation and mass flow rate oscillation are proposed. The differences on fuel concentration at the burner exit are proposed as the main reason for different flame instabilities and flame structures.
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| 7. |
- Wang, Zhenkan, et al.
(författare)
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Structure and burning velocity of turbulent premixed methane/air jet flames in thin-reaction zone and distributed reaction zone regimes
- 2019
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 37:2, s. 2537-2544
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Tidskriftsartikel (refereegranskat)abstract
- A series of turbulent premixed methane/air jet flames are studied using simultaneous planar lase diagnostic imaging of OH/CH/temperature and CH/OH/CH2O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH2O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH2O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor.
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| 8. |
- Xu, Shijie, et al.
(författare)
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Effects of ambient methanol on pollutants formation in dual-fuel spray combustion at varying ambient temperatures : A large-eddy simulation
- 2020
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Ingår i: Applied Energy. - : Elsevier BV. - 0306-2619. ; 279
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulation with a finite-rate chemistry model is carried out to investigate the formation of soot and nitrogen oxides (NOx) in the dual-fuel spray combustion. Liquid n-heptane is injected into a constant volume chamber, filled with a premixed methanol–air mixture with an equivalence ratio (ϕm) of 0.3. Three dual-fuel cases are simulated under initial temperatures of 900, 950 and 1000 K. Three single-fuel cases, with the same configurations, but with pure air being used as ambient gas composition are also simulated and used as baselines for comparison purposes. The paper aims to identify the main mechanisms of soot and NOx reduction in dual-fuel spray combustion under conditions relevant to internal combustion engines. The numerical model is validated using the Engine Combustion Network n-heptane fuel experimental data. It is found that soot emission has a strong non-linear dependence on ambient temperature in dual-fuel combustion. At high temperatures, soot emission is enhanced whereas at lower temperatures, it is suppressed. For the presently studied cases, the results show that the enhanced mixing is the primary reason for soot reduction in the 900 and 950 K cases, whereas the onset auto-ignition in ambient methanol/air mixture, which leads to a shortened lift-off length of the n-heptane spray flame and reduced ambient oxygen concentration, is the mechanism behind the enhanced soot emission in the 1000 K case. The methanol dilution effects of oxygen concentration and heat capacity on ignition delay time and the maximum flame temperature are minor in the current dual-fuel configurations. Regarding NOx emission in dual-fuel combustion, it is found that the effect of methanol on NOx formation also depends on the ambient temperatures. The NOx formation rate in the dual-fuel case is lower than that of the single-fuel case at 900 K. However, an opposite trend of NOx formation rate is observed in the 1000 K cases. The main reason for the increased NOx emission is the larger high temperature region resulted from the interaction of the spray flame and the ambient mixture ignition.
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| 9. |
- Xu, Shijie, et al.
(författare)
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LES/TPDF investigation of the effects of ambient methanol concentration on pilot fuel ignition characteristics and reaction front structures
- 2021
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361. ; 287
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Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulations with a transported probability density function model coupled with a finite-rate chemistry is applied to study the ignition process of an n-heptane spray in a constant volume chamber with a premixed methanol-air atmosphere under conditions relevant to reactivity controlled compression ignition (RCCI) engines. Three reacting spray cases with initial methanol-air equivalence ratio (ϕm) ranging from 0 to 0.3 are investigated at an initial temperature of 900 K. The case setup is based on the Engine Combustion Network Spray-H configuration, where n-heptane fuel is used. The effects of the ambient methanol-air equivalence ratio on the ignition characteristics and the reaction front structures in n-heptane/methanol RCCI combustion are studied in detail. It is found that the ambient methanol affects the low temperature chemistry of n-heptane, which results in a change of spatial distribution of key species such as heptyl-peroxide, and therefore the cool flame structure. With the presence of methanol in the ambient mixture cool flame is found in the entire fuel-rich region of the n-heptane jet, while when methanol is absent in the ambient mixture, the cool flame is established only around the stoichiometric mixture close to the n-heptane injector nozzle. In general, both low- and high-temperature ignition stages of n-heptane ignition are retarded by the methanol chemistry. An increase in ϕm leads to a decrease of the peak heat release rate of the n-heptane first-stage ignition. The chemistry of methanol inhibits the n-heptane ignition by decreasing the overall hydroxyl radicals (OH) formation rate and reducing the OH concentration during the transition period from the first-stage ignition to the second-stage ignition. As a result, the transition time between the two ignition stages is prolonged. Under the present lean methanol/air ambient mixture conditions, the impact of methanol on n-heptane ignition has a tendency of reducing the high-temperature, fuel-rich region, which is in favor of soot reduction.
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| 10. |
- Yu, Senbin, et al.
(författare)
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Numerical studies of flame extinction and re-ignition behaviors in a novel, ultra-lean, non-premixed model GT burner using LES-ESF method
- 2019
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Ingår i: Fuel. - : Elsevier BV. - 0016-2361.
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Tidskriftsartikel (refereegranskat)abstract
- The flame dynamics in a novel, ultra-lean non-premixed model gas turbine (GT) burner flame was numerically studied using large eddy simulation (LES) coupled with probability density function (PDF) based on the Euler stochastic field (ESF) method. One non-reacting case and three reacting cases with global equivalence ratio ϕglob=1.0,0.6,0.3 were simulated. Comparison of mean flow fields and OH distributions between numerical and experimental results was conducted. Flame dynamics including flame stabilization, structures and transitions of combustion modes were investigated. The simulation results were in close agreements with the experimental measurements showing the capability of PDF-ESF LES model in predicting the flame behaviors. At higher ϕglob, the fuel jet velocity was higher, which yielded higher scalar dissipation rate, χ, near the burner exit, leading to local extinction and flame lifted-off. In the extinction region, a series of relatively low to medium temperature reactions were active, providing favorable conditions for re-ignition downstream where χ is lower than the critical scalar dissipation rate for flame extinction, χcrt. In addition, with ϕglob decreasing, the flame height decreased due to a smaller jet velocity and χ, and thus the mechanism of flame stabilization changed from the swirl-stabilized to the bluff-body stabilized.
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