On the Flame-Flow Interaction Under Distributed Combustion Conditions

2016 ◽  
Author(s):  
Ahmed E. E. Khalil ◽  
Ashwani K. Gupta
Keyword(s):  
Oxygen Concentration ◽  
Reaction Zone ◽  
Thermal Field ◽  
Combustion Efficiency ◽  
Combustion Noise ◽  
Lower Velocity ◽  
Significant Difference ◽  
Planar Laser ◽  
Reaction Region ◽  
Colorless Distributed Combustion

Colorless Distributed Combustion (CDC) has shown significant improvements in terms of high combustion efficiency, ultra-low pollutants emission, low combustion noise, uniform thermal field, and enhanced stability. Colorless distributed combustion is fostered through reduced oxygen concentration and high temperature oxidizer to result in distributed reaction over a larger volume of the combustor and uniform thermal field. In this paper, the interaction between fluid mechanics (velocity field, characterized through particle image velocimetry) and the reaction region (identified through hydroxyl planar laser induced fluorescence) is investigated with focus on swirl assisted distributed combustion. Nitrogen/Carbon Dioxide mixture was added to the normal air upstream of the burner to simulate the hot reactive gases. Comparing the PIV data for reacting conditions with OH-PLIF revealed significant difference between normal swirl and CDC flames. In swirl flame, the flame was located around the shear layer of the entry jet (with both the inner and outer recirculation zones) where the velocity fluctuations and OH-PLIF fluctuations coincided. Flame transitioning to CDC pushed the reaction zone further downstream to locate at a position of lower velocity than what was found for swirl flames. In addition, the reaction zone occupied a much larger volume with lower signal intensity to exhibit distributed reaction. Experiments performed at same flow rates and velocities but with no reduction in oxygen concentration confirmed that the change in reaction behavior is attributed to the lower oxygen concentration rather than the increased flowrates due to dilution.

Oxygen Enriched Air Effects on Combustion, Emission, and Distributed Reaction

2015 ◽  
Vol 137 (4) ◽  
Author(s):  
Ahmed O. Said ◽  
Ashwani K. Gupta
Keyword(s):  
Oxygen Concentration ◽  
Reaction Zone ◽  
Combustion Efficiency ◽  
Oxygen Enrichment ◽  
Novel Technologies ◽  
Nonpremixed Combustion ◽  
Exhaust Temperature ◽  
Lean Combustion ◽  
Colorless Distributed Combustion ◽  
Co Emission

A novel combustion technology which combines colorless distributed combustion (CDC) and oxygen enriched combustion (OEC) air is examined to achieve optimum benefits of both technologies and to foster novel technologies for cleaner environment. The influence of oxygen enriched air–methane flames under nonpremixed and premixed fuel-lean combustion conditions is examined with focus on emission of NO and CO, combustor exit temperature (Texit), and distribution of reaction zone in the combustor using OH* chemiluminescence intensity distribution. A cylindrical combustor was used at combustion intensity of 36 MW/m3·atm and heat load of 6.25 kW. Results are also reported with normal air (21% oxygen). Oxygen enrichment provided stable combustion operation at lower equivalence ratios than normal air and also reduced CO emission. Increase in oxygen concentration from 21% to 25% or 30% increased the NO and decreased CO emissions at all the equivalence ratios examined. Using 30% O2 enriched air in premixed case showed NO emissions of 11.4 ppm and 4.6 ppm at equivalence ratios of 0.5 and 0.4, respectively. Oxygen enrichment also reduced CO emission to 38 ppm at equivalence ratio of 0.5. Operating the combustor with normal air at these equivalence ratios resulted in unstable combustion. OH* chemiluminescence revealed increased intensity with the reaction zone to shift upstream at increased oxygen concentration. The exhaust temperature of the combustor increased with oxygen enrichment leading to lower CO concentration and increased combustion efficiency. The oxidizer injected at higher velocities moved the reaction zone to upstream location with simultaneous reduction of both NO and CO, specifically under nonpremixed combustion.

Impact of Oxygen Enriched Air on High Intensity Combustion and Emission

2015 ◽  
Author(s):  
Ahmed O. Said ◽  
Ahmed E. E. Khalil ◽  
Daniel Dalgo ◽  
Ashwani K. Gupta
Keyword(s):  
Oxygen Concentration ◽  
Reaction Zone ◽  
Combustion Efficiency ◽  
Oxygen Enrichment ◽  
Chemiluminescence Intensity ◽  
Exhaust Temperature ◽  
Lean Combustion ◽  
Unstable Combustion ◽  
The Impact ◽  
Co Emission

The influence of oxygen enriched air-methane flame under non-premixed and premixed fuel-lean combustion conditions is examined with focus on the emission of NO and CO, combustor exit temperature (Texit), and distribution of OH* chemiluminescence intensity. A cylindrical combustor was used at combustion intensity of 36MW/m3.atm and heat load of 6.25 kW. Results are also reported with normal air (21% oxygen). Oxygen enrichment provided stable combustion operation at lower equivalence ratios than normal air and also reduced CO emission. Increase in oxygen concentration from 21% to 25% and 30% increased the NO and decreased CO emissions at all equivalence ratios examined. Using 30% O2 enriched air in premixed case showed NO emissions of 11.4 ppm and 4.6 ppm at equivalence ratios of 0.5 and 0.4, respectively. Oxygen enrichment also reduced CO emission to 38 ppm at equivalence ratio of 0.5. Operating the combustor with normal air at these equivalence ratios resulted in unstable combustion. OH* Chemiluminescence revealed increased chemiluminescence intensity with the reaction zone to shift upstream at increased oxygen concentration. The exhaust temperature of the combustor increased with oxygen enrichment leading to lower CO concentration and increased combustion efficiency. The oxidizer injected at higher velocities mitigated the impact of reaction zone to move upstream that helped to reduce significantly both the NO and CO emission specifically under non-premixed combustion.

Dual-Location Fuel Injection Effects on Emissions and NO*/OH* Chemiluminescence in a High Intensity Combustor

2016 ◽  
Vol 138 (4) ◽  
Author(s):  
Ahmed O. Said ◽  
Ahmed E. E. Khalil ◽  
Ashwani K. Gupta
Keyword(s):  
Fuel Injection ◽  
Single Injection ◽  
Mixing Time ◽  
Fuel Mixture ◽  
Combustion Noise ◽  
Fuel Distribution ◽  
Mixture Preparation ◽  
Pollutants Emission ◽  
Fuel Mixing ◽  
Colorless Distributed Combustion

Colorless distributed combustion (CDC) has shown to provide ultra-low emissions of NO, CO, unburned hydrocarbons, and soot, with stable combustion without using any flame stabilizer. The benefits of CDC also include uniform thermal field in the entire combustion space and low combustion noise. One of the critical aspects in distributed combustion is fuel mixture preparation prior to mixture ignition. In an effort to improve fuel mixing and distribution, several schemes have been explored that includes premixed, nonpremixed, and partially premixed. In this paper, the effect of dual-location fuel injection is examined as opposed to single fuel injection into the combustor. Fuel distribution between different injection points was varied with the focus on reaction distribution and pollutants emission. The investigations were performed at different equivalence ratios (0.6–0.8), and the fuel distribution in each case was varied while maintaining constant overall thermal load. The results obtained with multi-injection of fuel using a model combustor showed lower emissions as compared to single injection of fuel using methane as the fuel under favorable fuel distribution condition. The NO emission from double injection as compared to single injection showed a reduction of 28%, 24%, and 13% at equivalence ratio of 0.6, 0.7, and 0.8, respectively. This is attributed to enhanced mixture preparation prior to the mixture ignition. OH* chemiluminescence intensity distribution within the combustor showed that under favorable fuel injection condition, the reaction zone shifted downstream, allowing for longer fuel mixing time prior to ignition. This longer mixing time resulted in better mixture preparation and lower emissions. The OH* chemiluminescence signals also revealed enhanced OH* distribution with fuel introduced through two injectors.

Acoustic Noise Reduction Under Distributed Combustion

2017 ◽  
Author(s):  
Ahmed E. E. Khalil ◽  
Ashwani K. Gupta
Keyword(s):  
Carbon Dioxide ◽  
Heat Release ◽  
Noise Reduction ◽  
Oxygen Concentration ◽  
Acoustic Signal ◽  
Thermal Field ◽  
Field Uniformity ◽  
Pollutants Emission ◽  
Reactive Gases ◽  
Swirl Flame

Colorless Distributed Combustion (CDC) has been shown to provide unique benefits on ultra-low pollutants emission, enhanced combustion stability, and thermal field uniformity. To achieve CDC conditions, fuel-air mixture must be properly prepared and mixed with hot reactive gases from within the combustor prior to the mixture ignition. The hot reactive gases reduce the oxygen concentration in the mixture while increasing its temperature, resulting in a reaction zone that is distributed across the reactor volume, with lower reaction rate to result in the same fuel consumption. The conditions to achieve distributed combustion were previously studied using methane and other fuels with focus on pollutants emission and thermal field uniformity. In this paper, the impact of distributed combustion on noise reduction and increased stability is investigated. Such reduced noise is critical in mitigating the coupling between flame and heat release perturbations and acoustic signal to enhance the overall flame stability and reduce the propensity of flame instabilities which can cause equipment failure. Nitrogen-carbon dioxide mixture is used to simulate the reactive entrained gases from with the combustor. Increasing the amounts of nitrogen and carbon dioxide reduced the oxygen concentration within the oxidizing mixture, fostering distributed combustion. Upon achieving distributed combustion, the overall flame noise signature decreased from 80 dB to only 63 dB, as the flame transitioned from traditional swirl flame to distributed combustion. The flow noise under these conditions was 54 dB, indicating that distributed combustion has only 9 dB increase over isothermal case as compared to 26 dB for standard swirl flame. In addition, the dominant flame frequency around 490Hz disappeared under distributed combustion. For the traditional swirl flame, both the acoustic signal and heat release fluctuations (detected through CH∗ chemiluminescence) had a peak around 150Hz, indicating coupling between the heat release fluctuations and pressure variation. However, upon transitioning to distributed combustion, this common peak disappeared, outlining the enhanced stability of distributed combustion as there is no feedback between the heat release fluctuations and the recorded acoustic signal.

Microstructure and Nanoindentation Studies of Hydridated Zircaloy-4 Claddings After High Temperature Oxidation

2020 ◽  
Vol 7 (2) ◽  
Author(s):  
Petra Gávelová ◽  
Patricie Halodová ◽  
Daniela Marušáková ◽  
Ondřej Libera ◽  
Jakub Krejčí ◽  
...  
Keyword(s):  
High Temperature ◽  
Oxygen Concentration ◽  
Hydrogen Content ◽  
Oxygen Solubility ◽  
Pressurized Water ◽  
Temperature Oxidation ◽  
Β Phase ◽  
Significant Difference ◽  
Water Reactors ◽  
Increasing Temperature

Abstract Zirconium-based alloys are one of the most significant materials in thermal-neutron reactor systems. With very low neutron capture cross section, good corrosion resistance, mechanical strength and resistance to neutron radiation damage, zirconium alloys are used for fuel claddings. Cladding materials are still improved and tested in normal as well as critical reactor conditions. Zircaloy-4 (Zr-1.5Sn-0.2Fe-0.1Cr) is used for west types of light-water reactors, Pressurized Water Reactors (PWR). In our study, Zircaloy-4 cladding tubes were high-temperature oxidized in steam at the series of temperatures from 950 up to 1425 °C to simulate PWR reaching severe accident conditions. To observe the influence of hydrogen (H) diffusing from the coolant water on oxidation process, the specimens with ∼1000 ppm H were compared to the specimens with almost no hydrogen content. Wave Dispersive Spectroscopy (WDS) and nanoindentation were performed in line profiles across the cladding wall. Both methods contributed to verify the pseudobinary Zircaloy-4/oxygen phase diagram with focus on determination of phase boundaries. The increase of oxygen concentration with increasing temperature was observed. Moreover, oxygen concentration profiles and related change in nanohardness and Young's modulus showed the effect of hydrogen on the cladding microstructure. Hydrogen dissolved in metallic matrix increases the oxygen solubility in prior β-phase, the specimens with 1000 ppm H showed the higher oxygen content at almost all temperatures. As well, material hardening was observed on specimens with 1000 ppm H with significant difference in β-phase, measured on specimens exposed to lowest and highest oxidation temperature. Thus, with increasing temperature and hydrogen content, increased oxygen solubility affects the cladding ductility.

Investigation of the Roles of Flame Propagation, Turbulent Mixing, and Volumetric Heat Release in Conventional and Low Temperature Diesel Combustion

2011 ◽  
Vol 133 (10) ◽  
Author(s):  
Sage L. Kokjohn ◽  
Rolf D. Reitz
Keyword(s):  
Low Temperature ◽  
Heat Release ◽  
Oxygen Concentration ◽  
Reaction Zone ◽  
Flame Propagation ◽  
Ignition Delay ◽  
Flame Structure ◽  
Combustion Model ◽  
Combustion Mode ◽  
Flame Structures

In this work, a multimode combustion model that combines a comprehensive kinetics scheme for volumetric heat release and a level-set-based model for turbulent flame propagation is applied over the range of engine combustion regimes from non-premixed to premixed conditions. The model predictions of the ignition processes and flame structures are compared with the measurements from the literature of naturally occurring luminous emission and OH planar laser induced fluorescence. Comparisons are performed over a range of conditions from a conventional diesel operation (i.e., short ignition delay, high oxygen concentration) to a low temperature combustion mode (i.e., long ignition delay, low oxygen concentration). The multimode combustion model shows an excellent prediction of the bulk thermodynamic properties (e.g., rate of heat release), as well as local phenomena (i.e., ignition location, fuel and combustion intermediate species distributions, and flame structure). The results of this study show that, even in the limit of mixing controlled combustion, the flame structure is captured extremely well without considering subgrid scale turbulence-chemistry interactions. The combustion process is dominated by volumetric heat release in a thin zone around the periphery of the jet. The rate of combustion is controlled by the transport of a reactive mixture to the reaction zone, and the dominant mixing processes are well described by the large scale mixing and diffusion. As the ignition delay is increased past the end of injection (i.e., positive ignition dwell), both the simulations and optical engine experiments show that the reaction zone spans the entire jet cross section. In this combustion mode, the combustion rate is no longer limited by the transport to the reaction zone, but rather by the kinetic time scales. Although comparisons of results with and without consideration of flame propagation show very similar flame structures and combustion characteristics, the addition of the flame propagation model reveals details of the edge or triple-flame structure in the region surrounding the diffusion flame at the lift-off location. These details are not captured by the purely kinetics based combustion model, but are well represented by the present multimode model.

Frequency-dependent velocity analysis and offset-dependent low-frequency amplitude anomalies from hydrocarbon-bearing reservoirs in the southern North Sea, Norwegian sector

Geophysics ◽  
2017 ◽  
Vol 82 (6) ◽  
pp. N51-N60 ◽  
Author(s):  
Sayyid Suhail Ahmad ◽  
R. James Brown ◽  
Alejandro Escalona ◽  
Børge O. Rosland
Keyword(s):  
North Sea ◽  
High Frequency ◽  
Spectral Decomposition ◽  
Low Frequency ◽  
Velocity Analysis ◽  
Frequency Dependent ◽  
Lower Velocity ◽  
The North ◽  
Significant Difference ◽  
The North Sea

Our aim was to identify some of the characteristics of low-frequency anomalies. Specifically, we have looked, in 3D broadband data from the North Sea, for any offset dependence in these anomalies and any frequency-related change in normal moveout (NMO) velocity that could influence stacking power over different frequencies. After high-resolution spectral decomposition, two types of low-frequency anomaly have been identified associated with hydrocarbon-bearing reservoirs: (1) at the reservoir top and (2) below the reservoir, with a time delay of approximately 100–200 ms. Both types of anomalies indicate offset dependence. On the near-offset stacks, they are relatively strong, but they tend to be absent on the far-offset stacks. In addition, horizon velocity analysis, which was performed along the horizons picked at the tops of reservoir and nonreservoir intervals, has revealed frequency-dependent NMO velocity. For nonreservoir events, we found no significant difference between the NMO velocities for the low-frequency and high-frequency filtered common-midpoint gathers. However, along the anomalously low-frequency events observed at the tops of, and below, oil-bearing reservoirs, lower velocity is observed for low-frequency and higher velocity for high-frequency filtered gathers. If these properties turn out to be universally typical, increased understanding and inclusion of them could lead to improved workflows and help increase the reliability of low-frequency analysis as a hydrocarbon indicator.

Investigation of the Roles of Flame Propagation, Turbulent Mixing, and Volumetric Heat Release in Conventional and Low Temperature Diesel Combustion

2010 ◽  
Author(s):  
Sage L. Kokjohn ◽  
Rolf D. Reitz
Keyword(s):  
Low Temperature ◽  
Heat Release ◽  
Oxygen Concentration ◽  
Reaction Zone ◽  
Flame Propagation ◽  
Ignition Delay ◽  
Flame Structure ◽  
Combustion Model ◽  
Combustion Mode ◽  
Multi Mode

In this work, a multi-mode combustion model, that combines a comprehensive kinetics scheme for volumetric heat release and a level-set-based model for turbulent flame propagation, is applied over the range of engine combustion regimes from non-premixed to premixed conditions. Model predictions of the ignition processes and flame structures are compared to measurements from the literature of naturally occurring luminous emission and OH planar laser induced fluorescence (PLIF). Comparisons are performed over a range of conditions from conventional diesel operation (i.e., short ignition delay, high oxygen concentration) to a low temperature combustion mode (i.e., long ignition delay, low oxygen concentration). The multi-mode combustion model shows excellent prediction of the bulk thermodynamic properties (e.g., rate of heat release), as well as local phenomena (i.e., ignition location, fuel and combustion intermediate species distributions, and flame structure). The results of this study show that even in the limit of mixing controlled combustion, the flame structure is captured extremely well without considering sub-grid scale turbulence-chemistry interactions. The combustion process is dominated by volumetric heat release in a thin zone around the periphery of the jet. The rate of combustion is controlled by transport of reactive mixture to the reaction zone and the dominant mixing processes are well described by the large scale mixing and diffusion. As the ignition delay is increased past the end of injection (i.e., positive ignition dwell), both the simulations and optical diagnostics show that the reaction zone spans the entire jet cross-section. In this combustion mode the combustion rate is no longer limited by transport to the reaction zone, but rather by kinetic timescales. Although comparisons of results with and without consideration of flame propagation show very similar flame structures and combustion characteristics, the addition of the flame propagation model reveals details of the edge or triple-flame structure in the region surrounding the diffusion flame at the lift off location. These details are not captured by the purely kinetics based combustion model, but are well represented by the present multi-mode model.

Interactions between AlN and SiAlON Ceramics

2008 ◽  
Vol 403 ◽  
pp. 97-98
Author(s):  
A. Kalemtas ◽  
Nurcan Calis Acikbas ◽  
Ferhat Kara ◽  
Hasan Mandal ◽  
Kristoffer Krnel ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Scanning Electron Microscopy ◽  
Aspect Ratio ◽  
Reaction Zone ◽  
High Aspect Ratio ◽  
X Ray ◽  
Plasma Sintering ◽  
Reaction Region ◽  
Spark Plasma ◽  
Scanning Electron

In the present study, interactions between AlN and SiAlON laminated couples were investigated after gas pressure (GPS) and spark plasma sintering (SPS) by scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX) with the aim to produce laminated composites. In the laminated couples sintered by GPS, a significant reaction zone (~100-150 μm), containing a high aspect ratio of elongated polytypoid grains, was observed at the interface. However, in the case of laminated couples sintered by SPS, a considerably thin reaction region (~2-3 μm) was observed, elongated polytypoid grain formations were also detected.

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