Search

This work presents the design and testing of a seeder based on laser ablation capable of introducing gallium particles into a gaseous flow at elevated pressures typical of that found in practical combustion devices. The ability to seed such a flow with gallium particles is required to apply Ga-TLAF, a spatially and temporally resolved thermometry imaging technique well suited to harsh combustion environments. The design criteria for this gallium particle seeder are first listed and all the necessary details required to understand and replicate it are then provided. Next, the efficiency of gallium ablation is verified as a function of the ablation laser's fluence and repetition rate and of the pressure using gallium laser induced fluorescence and laser scattering measurements at ∼403 nm. For the conventional 355-nm nanosecond, Nd:YAG laser used for ablation in this study, data show that the quantity of gallium seeded into the flow can be conveniently modulated by varying the fluence of the ablation laser and/or its repetition rate. Data also show that the efficiency of ablation is marginally better for solid gallium than for liquid gallium, but that ablation of liquid gallium should be preferred to avoid a loss of ablation efficiency after some time. The temperature of the liquid gallium feedstock is found to be unimportant. SEM, EDX, and SPMS analyses show that laser ablation yields pure gallium particles with a characteristic size ranging at least from 50 nm to 10 μm and that the size distribution is insensitive to the pressure and to the ablation laser's fluence. Also important for future applications of Ga-TLAF in high-pressure flames, data show that the gallium LIF intensity recorded at room temperature or in the hot products of a flame is not significantly affected by pressure.

This study reports on the performance of the Ansaldo AE-T100 commercial micro gas turbine (mGT) when fueled by ammonia-methane blends instead of its design fuel, natural gas. This micro gas turbine was used as the experimental platform to understand effects of ammonia addition, and its hardware was only marginally updated. Ammonia was added to the main fuel line in varying proportions, but the pilot flame fuel line was only fed with methane. Experiments were performed for constant electrical output power and turbine outlet temperature of 60 kWe and 645°C, respectively. Important operational parameters and exhaust emissions were continually monitored during the turbine's operation. Results show that stable operation on the mGT was possible until the volume fraction of ammonia in the fuel blend reached XNH3 = 0.63. To reach this ammonia fraction, it was necessary to increase the power of the pilot flame that provides a continuous ignition source at the base of the main flame. Concentrations of carbon monoxide and unburned hydrocarbon measured in the exhaust did not increase significantly until XNH3 = 0.22. However, above this value, concentrations increased rapidly, which is indicative of a drop in the combustion efficiency and, in turn, in the thermal efficiency. For XNH3 = 0.63, the measured thermal efficiency was ∼0.23, which is significantly lower than that found for operation with pure methane, ∼0.27. Although the CO2 concentration was found to decrease linearly when the ammonia fraction was increased, measurements reveal that NOx emissions increased rapidly, with a maximum NOx concentration of 2161 ppmvd. The concentration of N2O also increased rapidly when the ammonia fraction was increased. Due to N2O's very large global warming potential, this more than canceled the benefits associated with the reduction of CO2 emissions. Consequently, results showed that, even though stable operation of the Ansaldo AE-T100’s mGT in its original configuration is possible with ammonia-methane blends at least up to 60 kWe, hardware modifications will be required to comply with current NOx regulations and ensure sufficiently low N2O emissions.

Information about molecular collisions is encoded in the shapes of collision-perturbed molecular resonances. This connection between molecular interactions and line shapes is most clearly seen in simple systems, such as the molecular hydrogen perturbed by a noble gas atom. We study the H2–Ar system by means of highly accurate absorption spectroscopy and ab initio calculations. On the one hand, we use the cavity-ring-down-spectroscopy technique to record the shapes of the S(1) 3-0 line of molecular hydrogen perturbed by argon. On the other hand, we simulate the shapes of this line using ab initio quantum-scattering calculations performed on our accurate H2–Ar potential energy surface (PES). In order to validate the PES and the methodology of quantum-scattering calculations separately from the model of velocity-changing collisions, we measured the spectra in experimental conditions in which the influence of the latter is relatively minor. In these conditions, our theoretical collision-perturbed line shapes reproduce the raw experimental spectra at the percent level. However, the collisional shift, δ0, differs from the experimental value by 20%. Compared to other line-shape parameters, collisional shift displays much higher sensitivity to various technical aspects of the computational methodology. We identify the contributors to this large error and find the inaccuracies of the PES to be the dominant factor. With regard to the quantum scattering methodology, we demonstrate that treating the centrifugal distortion in a simple, approximate manner is sufficient to obtain the percent-level accuracy of collisional spectra. [ABSTRACT FROM AUTHOR]

Lean premixed combustion mode has become attractive for utilization in industrial gas turbines due to its ability to meet strict emissions regulations without compromising engine efficiency. In this combustion mode, the mixing process is the key player that affect the flame structure and stability, as well as the generated emissions. Many studies have investigated the aspects that influence premixed flames, including the effects of turbulence, combustor geometry, and level of partial premixing, while mostly using conventional natural gas fuel represented by methane. Recently, ammonia, a sustainable energy source, has been considered in gas turbines due to its carbon-free fuel producing no CO2. Utilizing 100% ammonia or a blend of methane and ammonia alters the combustion performance of a premixed flame due to the variation associated with the physical and chemical properties of ammonia. Thus, investigating the coupling between blend ratios and mixing length of methane-ammonia on flame stability and emissions is an essential step toward implementing ammonia in industrial gas turbines. In this study, the influence of various methane-ammonia blends, from 0 (pure methane) to XNH3 = 75%, and mixing lengths on the flame performance were studied. The mixing length was altered by delaying the injection (i.e., partially premixing) of the ammonia while using a fixed injection location for the reference methane-air mixture. This was done by using three fuel ports located at three different heights upstream of the combustion chamber. The results showed that the flame stability is negatively influenced by increasing (decreasing) ammonia fraction (mixing length ratio) and is more sensitive to the ammonia fraction than to the mixing length. At a constant equivalence ratio, the CO and NOx performances improved positively by increasing the ammonia volume fractions (especially at XNH3 = 75% compared to XNH3 = 25% and 50%) and the mixing length.

Context. The hydrogen deuteride (HD) molecule is an important deuterium tracer in astrophysical studies. The atmospheres of gas giants are dominated by molecular hydrogen, and the simultaneous observation of H2 and HD lines provides reliable information on the D/H ratios on these planets. The reference spectroscopic parameters play a crucial role in such studies. Under the thermodynamic conditions encountered in these atmospheres, spectroscopic studies of HD require not only the knowledge of line intensities and positions but also accurate reference data on pressure-induced line shapes and shifts. Aims. Our aim is to provide accurate collision-induced line-shape parameters for HD lines that cover any thermodynamic conditions relevant to the atmospheres of giant planets, namely any relevant temperature, pressure, and perturbing gas composition (the H2–He mixture). Methods. We performed quantum-scattering calculations on our new, highly accurate ab initio potential energy surface (PES), and we used scattering S matrices obtained in this way to determine the collision-induced line-shape parameters. We used cavity ring-down spectroscopy to validate our theoretical methodology. Results. We report accurate collision-induced line-shape parameters for the pure rotational R(0), R(1), and R(2) lines, the most relevant HD lines for investigations of the atmospheres of the giant planets. Besides the basic Voigt-profile collisional parameters (i.e., the broadening and shift parameters), we also report their speed dependences and the complex Dicke parameter, which can influence the effective width and height of the HD lines up to almost a factor of 2 for giant planet conditions. The sub-percent-level accuracy reached in this work is a considerable improvement over previously available data. All the reported parameters (and their temperature dependences) are consistent with the HITRAN database format, hence allowing for the use of the HITRAN Application Programming Interface (HAPI) for generating the beyond-Voigt spectra of HD. [ABSTRACT FROM AUTHOR]

As a fueling vector, ammonia is gaining more interest for future power generation, transportation and heating systems. Compared to typical hydrocarbons, ammonia's low reactivity characteristics and higher NOx emissions limit its practical implementation. However, the current need for decarbonization of the energy sector puts ammonia as an energy vector and carrier at the far front, making overcoming the challenges of ammonia utilization necessary. In late 2018, Kobayashi et al. presented a review article on the science and technology of ammonia combustion. In continuing their effort, this review is aimed towards giving the most recent updates in ammonia combustion. The article starts with the importance of ammonia as an energy carrier and covers the fundamentals of ammonia combustion. We then present recent activities for addressing the low ammonia reactivity and its impact on NOx emissions. In addition, the chemical kinetics of ammonia oxidation is presented through a discussion of recently proposed chemical kinetics mechanisms. This section emphasizes the role of crucial reaction differences, the updates in the NOx and De-NOx pathways, NHi recombination reactions and H2NO chemistry. Towards utilising ammonia in gas turbines and industrial applications, this review discusses recent efforts and various NO mitigation strategies. This work systematically summarizes the fundamentals of ammonia-based combustion and its application. It serves as a reference for the transition of ammonia into future practical applications.

This work presents an experimental study on the influence of the pilot flame characteristics on the flame morphology and exhaust emissions of a turbulent swirling flame. A reduced-scale burner, inspired by that fitted in the AE-T100 micro gas turbine, was employed as the experimental platform to evaluate methane (CH4) and an ammonia-methane fuel blend with an ammonia (NH3) volume fraction of 0.7. The power ratio (PR) between the pilot flame and the main flame and the fuel composition of the pilot flame was investigated. The pilot power ratio was varied from 0 to 20% for both fuel compositions tested. The NH3 volume fraction in the pilot flame ranged from pure CH4 to pure NH3 through various NH3–CH4 blends. Flame images and exhaust emissions, namely CO2, CO, NO, and N2O were recorded. It was found that increasing the pilot power ratio produces more stable flames and influences most of the exhaust emissions measured. The CO2 concentration in the exhaust gases was roughly constant for CH4-air or NH3–CH4–air flames. In addition, a CO2 concentration reduction of about 45% was achieved for XNH3 = 0.70 compared with pure CH4, while still producing stable flames as long as PR ≥ 5%. The pilot power ratio was found to have a higher relative impact on NO emissions for CH4 than for NH3–CH4, with measured exhaust NO percentage increments of about 276% and 11%, respectively. The N2O concentration was constant for all pilot power ratios for CH4 but it decreased when the pilot power ratio increased for NH3–CH4. The pilot fuel composition highly affected the NO and N2O emissions. Pure CH4 pilot flames and higher power ratios produced higher NO emissions. Conversely, the NO concentration was roughly constant for pure NH3 pilot flames, regardless of the pilot power ratio. Qualitative OH-PLIF images were recorded to further investigate these trends. Results showed that the pilot power ratio and the pilot fuel composition modified the flame morphology and the OH concentration, which both influence NO emissions.

The CO2 molecule is of great interest for astrophysical studies since it can be found in a large variety of astrophysical media where it interacts with the dominant neutral species, such as He, H2, or H2O. The CO2–He collisional system was intensively studied over the last two decades. However, collisional data appear to be very sensitive to the potential energy surface (PES) quality. Thus, we provide, in this study, a new PES of the CO2–He van der Waals complex calculated with the coupled-cluster method and a complete basis set extrapolation in order to provide rotational rate coefficients that are as accurate as possible. The PES accuracy was tested through the calculations of bound state transition frequencies and pressure broadening coefficients that were compared to experimental data. An excellent agreement was globally found. Then, revised collisional data were provided for the 10–300 K temperature range. Rate coefficients were compared to previously computed ones and are found to be up to 50% greater than previously provided ones. These differences can induce non-negligible consequences for the modeling of CO2 abundance in astrophysical media. [ABSTRACT FROM AUTHOR]

In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O 2 , CO 2 , CO, NO, NO 2 , and N 2 O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O 2 concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO 2 , and N 2 O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO 2 and N 2 O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO 2 and N 2 O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities. [ABSTRACT FROM AUTHOR]

This work investigates the UV and visible chemiluminescence signatures of laminar ammonia-hydrogen-air flames. A counterflow burner was used to stabilize flat and steady premixed ammonia-hydrogen-air twin-flames for wide ranges of ammonia fuel fractions (0.55 ≤ X NH 3 ≤ 0.90), equivalence ratios (0.35 ≤ ϕ ≤ 1.70), and strain rates (50 ≤ a ≤ 300 / s). The chemiluminescence spectra in the UV and visible regions were measured. The important contributors to the chemiluminescence of ammonia-hydrogen-air flames were first identified and their sensitivities to equivalence ratio and ammonia fuel fraction were then quantified. Promising ratios of chemiluminescence intensities for the non-intrusive optical sensing of equivalence ratio and ammonia fuel fraction in premixed ammonia-hydrogen-air flames were also identified. Among the 36 ratios that could be formed with the 9 contributors to chemiluminescence that were defined, 3 are particularly promising, namely, OH*/NO*, OH*/ violet (violet refers to the 350 to 400 nm range), and red/blue (blue and red refer to the 450 to 500 nm and 650 to 750 nm ranges, respectively). Ratios OH*/NO* and OH*/ violet were found to provide suitable measures of equivalence ratio for lean and rich ammonia-hydrogen-air flames, respectively, because these ratios are very sensitive to equivalence ratio, are insensitive to ammonia fuel fraction, and are marginally sensitive to strain rate. Once equivalence ratio is known, the ammonia fuel fraction can then be measured with ratio red/blue because it is sensitive to the ammonia fuel fraction and it is globally insensitive to strain rate. [ABSTRACT FROM AUTHOR]