Ammonia Flame Research Articles

Ammonia and hydrogen blending effects on combustion stabilities in optical SI engines

Ammonia is a promising energy carrier and carbon-free fuel, which is getting more attention today when fossil energy is increasingly exhausted. However, ammonia combustion in the IC engine suffers from combustion instabilities and misfiring issues. Hydrogen is also carbon-free energy and can be directly obtained by ammonia catalytic reforming. The addition of a small amount of hydrogen can effectively alleviate the weakness of ammonia combustion, while ammonia and hydrogen blending effects on combustion stabilities and their critical conditions remain unclear. In this work, the effects of hydrogen addition on ammonia combustion characteristics were experimentally investigated in an optical SI engine with high compression ratios. The results show that the indicated thermal efficiency first increases and then decreases with the elevation of hydrogen-ammonia energy ratios (χ), with an optimal value at χ≈7.5 %. Further hydrogen addition can aggravate indicated thermal efficiency due to the large heat transfer loss caused by high-temperature hydrogen combustion. Combustion visualizations show that there is a critical threshold of hydrogen-ammonia energy ratio (χ=10.0–12.5 %), around which engine combustion characteristics are changed significantly. When χ < 10 %, the effect of hydrogen addition on the ammonia flame speed becomes more pronounced since the hydrogen substantially improves the stability of the early-stage combustion. When χ > 12.5 %, hydrogen shows an obvious influence on the initial flame formation of ammonia, which is attributed to the low flame stretch sensitivity, facilitating early flame development. In addition, flame propagation when χ < 10 % is more sensitive to temperature than flame propagation with χ > 12.5 %. The current research demonstrates that proper ammonia-hydrogen energy ratios are important for stable combustion and thermal efficiency improvement.

Combustion characteristics of steam-diluted decomposed ammonia in multiple-nozzle direct injection burner

In line with the decarbonisation of power sector, carbon-free fuels are currently being investigated. In particular, green ammonia or e-ammonia is a candidate fuel which will be playing a key role in many energy-intensive industries. It calls for an in-depth understanding of eFuels combustion characteristics in the fuel flexible combustors. Therefore, the present work for the first time numerically investigates the combustion regimes of steam-diluted, decomposed eNH3 in a novel multi-nozzle direct injection (MDI) burner. Although the MDI burner is not equipped with a conventional swirler, strong flow-flame interaction is observed. The two-layer, angled channels create swirling flows featuring swirl numbers larger than 0.9 in general. The centre recirculation region can help stabilise highly steam-diluted decomposed ammonia with a maximum steam-to-air ratio of 74%. This highest H2% containing, wettest ammonia flame case is found to emit the lowest total emission (NH3+NO + NO2+N2O) of ∼400ppmvd@15%O2 at stoichiometric conditions. The wall heat loss is confirmed responsible for the formation of N2O in distributed flame, suggesting the need of reducing pollution through good chamber wall insulation. However, for flames sitting in the conventional regimes, the impact of wall heat loss is found insignificant. Further, extensive data and flame regime analyses show that NNH can always accurately mark the high heat release region of all types of flames, while OH is only an effective marker for thin flames.

Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2Σ+−X2Π (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.

Using Ammonia as Future Energy: Modelling of Reaction Mechanism for Ammonia/Hydrogen Blends

To utilize ammonia-based fuels, it is fundamental to understand chemical mechanisms of combustion process, in which reaction characteristics of such a chemical are described in detail. Detailed chemical-kinetics mechanism of ammonia was modelled by an automatic reaction mechanism generation program to investigate characteristics of premixed combustion for ammonia/hydrogen fuel mixture. To develop an accurate model for practical combustion applications, validation of the reaction mechanism was carried out in terms of laminar flame speed under different conditions. Results suggested that the established mechanism model has satisfying performance under different ammonia/hydrogen ratio conditions. Moreover, comparison with other mechanism models demonstrated that the developed model can be used to describe flame propagation of ammonia/hydrogen fuels. Then characteristics of laminar flame speed were predicted under various ammonia concentration and equivalence ratio conditions. Sensitivity analyses showed that ammonia mole fraction has a prominent impact on kinetics of flame speed for ammonia/hydrogen blends. Flame structure analyses showed that hydrogen can enhance ammonia flames with higher light radical concentrations whilst deteriorate NOx emission in exhaust gases.

Study on combustion characteristics of hydrogen addition on ammonia flame at a porous burner

Due to the high hydrogen density and carbon-free characteristics of ammonia, porous media burners fueled by ammonia are currently attracting more and more attention in small and medium combustion/power equipment. In this study, the effects of pure ammonia combustion and hydrogen addition on ammonia combustion in a porous burner are investigated by numerical simulation and experiments to realize the practical application of ammonia as a fuel. The effects of porous burner parameters, hydrogen addition ratio, and equivalence ratio on ammonia flame characteristics, temperature distribution, and NO formation characteristics have been evaluated. The results show that stable ammonia flames can be obtained by this porous burner (Φ = 0.9–1.2, u0 = 3–7 m/s). When the equivalence ratio is 1.00, the peak temperature of the ammonia flame is obtained. It is worth noting that the physical parameters of porous media have a great influence on the combustion performance. As the thermal conductivity and pore density of the porous medium increase, the peak temperature of the ammonia flame decreases and the flame position moves upstream. Furthermore, the combustion temperature of the ammonia flame rises with the increase in H2 addition ratio. When the fuel is rich, element N in the fuel is rarely converted to NO in pure ammonia and ammonia/hydrogen flames. The main N element in the fuel is converted into N2, and the conversion rate is as high as 92.35%–98.57%. When the equivalence ratio is 1.20, the NO conversion rate is ideally less than 0.009%.

Turbulent flame speed and morphology of pure ammonia flames and blends with methane or hydrogen

Ammonia appears a promising hydrogen-energy carrier as well as a carbon-free fuel. However, there remain limited studies for ammonia combustion especially under turbulent conditions. To that end, using the spherically expanding flame configuration, the turbulent flame speeds of stoichiometric ammonia/air, ammonia/methane and ammonia/hydrogen were examined. The composition of blends studied are currently being investigated for gas turbine application and are evaluated at various turbulent intensities, covering different kinds of turbulent combustion regimes. Mie-scattering tomography was employed facilitating flame structure analysis. Results show that the flame propagation speed of ammonia/air increases exponentially with increasing hydrogen amount. It is less pronounced with increasing methane addition, analogous to the behavior displayed in the laminar regime. The turbulent to laminar flame speed ratio increases with turbulence intensity. However, smallest gains were observed at highest hydrogen content, presumably due to differences in the combustion regime, with the mixture located within the corrugated flamelet zone, with all other mixtures positioned within the thin reaction zone. A good correlation of the turbulent velocity based on the Karlovitz and Damköhler numbers is observable with the present dataset, as well as previous experimental measurements available in literature, suggesting that ammonia-based fuels may potentially be described following the usual turbulent combustion models. Flame morphology and stretch sensitivity analysis were conducted, revealing that flame curvature remains relatively similar for pure ammonia and ammonia-based mixtures. The wrinkling ratio is found to increase with both increasing ammonia fraction and turbulent intensity, in good agreement with measured increases in turbulent flame speed. On the other hand, in most cases, the flame stretch effect does not change significantly with increasing turbulence, whilst following a similar trend to that of the laminar Markstein length.

On nanosecond plasma-assisted ammonia combustion: Effects of pulse and mixture properties

In this study, the effects of nanosecond plasma discharges on the combustion characteristics of ammonia are investigated over a wide range of mixture properties and plasma settings. The results reveal that the impacts of the plasma on ammonia combustion change non-monotonically by altering the reduced electric field value. Within the studied range of the reduced electric field, i.e., 100–700 Td, it is shown that plasma is most effective in the medium range, e.g., 250–400 Td. At lower values, the main fraction of the plasma energy is consumed to excite the diluent to higher vibrational levels. At very high reduced electric field values, a substantial portion of the plasma energy is transferred into the ionization reactions of the diluent, which compromises the effective excitations of fuel and oxidizer species. In terms of the pulse energy density, results indicate that an increase in the range of 0–20 mJ/cm3, at a given reduced electric field, decreases the ignition delay time by five orders of magnitude, and increases the laminar flame speed up to an order of magnitude, depending on the mixture composition. The results show that the plasma discharge produces more radicals, electronically excited and charged species when He is used as the diluent in the oxidizer instead of N2, since NH3 and O2 ionization reactions are strengthened in NH3/O2/He. Moreover, plasma discharge is highly effective in assisting the combustion of preheated lean mixtures. The present study also indicates that ammonia flame thickness is minimum at a critical pulse energy density in the range of 12–14 mJ/cm3. Further increases in the pulse energy density can manipulate the inner structure of the flame, altering the pre-heat zone of the flame to include some levels of chemical reactions toward the flameless mode of combustion.

Ammonia as Green Fuel in Internal Combustion Engines: State-of-the-Art and Future Perspectives

Ammonia (NH3) is among the largest-volume chemicals produced and distributed in the world and is mainly known for its use as a fertilizer in the agricultural sector. In recent years, it has sparked interest in the possibility of working as a high-quality energy carrier and as a carbon-free fuel in internal combustion engines (ICEs). This review aimed to provide an overview of the research on the use of green ammonia as an alternative fuel for ICEs with a look to the future on possible applications and practical solutions to related problems. First of all, the ammonia production process is discussed. Present ammonia production is not a “green” process; the synthesis occurs starting from gaseous hydrogen currently produced from hydrocarbons. Some ways to produce green ammonia are reviewed and discussed. Then, the chemical and physical properties of ammonia as a fuel are described and explained in order to identify the main pros and cons of its use in combustion systems. Then, the most viable solutions for fueling internal combustion engines with ammonia are discussed. When using pure ammonia, high boost pressure and compression ratio are required to compensate for the low ammonia flame speed. In spark-ignition engines, adding hydrogen to ammonia helps in speeding up the flame front propagation and stabilizing the combustion. In compression-ignition engines, ammonia can be successfully used in dual-fuel mode with diesel. On the contrary, an increase in NOx and the unburned NH3 at the exhaust require the installation of apposite aftertreatment systems. Therefore, the use of ammonia seems to be more practicable for marine or stationary engine application where space constraints are not a problem. In conclusion, this review points out that ammonia has excellent potential to play a significant role as a sustainable fuel for the future in both retrofitted and new engines. However, significant further research and development activities are required before being able to consider large-scale industrial production of green ammonia. Moreover, uncertainties remain about ammonia safe and effective use and some technical issues need to be addressed to overcome poor combustion properties for utilization as a direct substitute for standard fuels.

Stabilization and Emission Characteristics of Gliding Arc-Assisted NH3/CH4/Air Premixed Flames in a Swirl Combustor

This paper investigates the effects of gliding arc (GA) discharge on the stabilization and emission characteristics of premixed NH3/CH4/air swirl flames under various ammonia contents, flow rates, and global equivalence ratios. First, the lean blowout (LBO) limits are measured. We find that although blending CH4 extends the LBO limit of the ammonia flame to 0.6–0.8, the GA discharge further remarkably extends it to 0.3–0.4. Then, the flow and flame structures are visualized by simultaneous OH planar laser-induced fluorescence and particle imaging velocimetry measurements. The results reveal that the discharge increases the OH radical concentration and expands the inner recirculation zone, leading to improved flame stability. Second, the NOx emissions are investigated over a wide range of global equivalence ratios and ammonia contents. It is seen that the GA discharge slightly increases the NOx emission by less than 7% at low NH3 contents (<0.6), which can be attributed to the thermal and OH-involved reaction pathway of NOx formation. However, as the NH3 content further increases (which is accompanied by the rapid growth of the NOx emission), the GA discharge effectively reduces the NOx emission by up to 30%. This effect might be due to the more intensive NOx-consuming reactions by plasma-induced NH2 radicals at a higher ammonia content, which is confirmed by the strengthened NH2* chemiluminescence under GA discharge conditions. Finally, a chemical reactor network analysis gives reasonable NOx predictions without GA discharge and highlights the NOx-reduction effects of NH2 radicals under high ammonia contents.

Experimental investigation of non-equilibrium plasma-assisted ammonia flames using NH2* chemiluminescence and OH planar laser-induced fluorescence

This study investigates the effect of nano-second pulsed non-equilibrium plasma on ammonia combustion in a model gas turbine combustor using NH2* chemiluminescence and OH planar laser-induced fluorescence (PLIF). Nano-second pulsed plasma discharge is generated at the exit of the premixed ammonia/air mixture injection tube. Broadband chemiluminescence observation shows that plasma can improve the stabilization of ammonia/air flame and extend the attached flame regime to a lower equivalence ratio. To understand the coupling effect between plasma kinetics and flame dynamics, NH2* chemiluminescence and OH PLIF of ammonia/air flames are measured at different conditions. With the increase of discharge voltage or discharge power, both NH2* chemiluminescence intensity and OH PLIF signal intensity increase, and this observation may explain the improved stabilization of ammonia flames. At very lean condition (equivalence ratio between 0.48 and 0.57, no visible flame existed), both NH2* chemiluminescence and OH PLIF signal are observed in the proximity of electrode region with plasma activation. If air is replaced by pure nitrogen (N2), NH2* chemiluminescence is measurable but extremely weak. As the oxygen (O2) concentration in the oxidizer stream gradually increases, NH2* chemiluminescence intensity increases linearly with O2 concentration under plasma activation. This finding indicates that the NH2* production in plasma-assisted ammonia oxidation process is possibly related to the production of OH or oxygen-related species. The direct electron impact on NH2* production might be secondary.

Numerical study on laminar burning velocity of ammonia flame with methanol addition

The combustion characteristics of ammonia/methanol mixtures were investigated numerically in this study. Methanol has a dramatic promotive effect on the laminar burning velocity (LBV) of ammonia. Three mechanisms from literature and another four self-developed mechanisms constructed in this study were evaluated using the measured laminar burning velocities of ammonia/methanol mixtures from Wang et al. (Combust.Flame. 2021). Generally, none of the selected mechanisms can precisely predict the measured laminar burning velocities at all conditions. Aiming to develop a simplified and reliable mechanism for ammonia/methanol mixtures, the constructed mechanism utilized NUI Galway mechanism (Combust.Flame. 2016) as methanol sub-mechanism and the Otomo mechanism (Int. J. Hydrogen. Energy. 2018) as ammonia sub-mechanism was optimized and reduced. The reduced mechanism entitled ‘DNO-NH3’, can accurately reproduce the measured laminar burning velocities of ammonia/methanol mixtures under all conditions. A reaction path analysis of the ammonia/methanol mixtures based on the DNO-NH3 mechanism shows that methanol is not directly involved in ammonia oxidation, instead, the produced methyl radicals from methanol oxidization contribute to the dehydrogenation of ammonia. Besides, NOx emission analysis demonstrates that 60% methanol addition results in the highest NOx emissions. The most important reactions dominating the NOx consumption and production are identified in this study.

Measurement and scaling of turbulent burning velocity of ammonia/methane/air propagating spherical flames at elevated pressure

This study reports the accurate laminar burning velocity, turbulent burning velocity and its correlations of ammonia/methane/air propagating spherical flames. The experiments were carried out on a medium-scale, fan-stirred cylindrical combustion chamber with ammonia molar content varying from 20% to 60% and the initial pressure up to 3 bar. The turbulent burning velocity decreases with the ammonia content due to the weakening effect of the laminar burning velocity under all turbulence intensities and pressures studied. Since the weakening of flame chemistry is dominated by the enhancement of turbulence eddies, the normalized turbulent burning velocity increases with the ammonia content. The turbulent expanding flame of ammonia/methane/air is self-similar under different ammonia content. This self-similar propagation follows the one-half power-law correlation between the normalized turbulent burning velocity, ST/SL, and the turbulent flame Reynolds number, which is quantitatively consistent with that of unity Lewis number methane/air flames (Chaudhuri 2012). The pressure dependence of turbulent burning velocity can be represented roughly as a 0.4 power law of ST/SL and (u′/SL)(P/P0). However, there is a quantitative gap between the pre-exponential factor of the present experimental data and the literature data based on this correlation, which could attribute to the difference in the turbulence eddy scales of different experimental apparatus. The integral length scale characterized the largest turbulence eddies is introduced to consider the turbulent length scale effect. A modified general correlation ST/SL∼(LI/L0)0.5[(u′/SL)(P/P0)]0.41 with the consideration of the integral length scale effect is obtained, which is able to predict a variety of spherical flame data regardless of temperatures, pressures, and fuel types. In addition, it is verified that turbulent burning velocity of ammonia flame could be expressed by the correlation of Karlovitz and Damköhler numbers: ST/SL∼Ka·Da=ReT,flow0.5. It can be seen that ammonia has similar turbulent combustion characteristics as hydrocarbon fuel. These findings indicate that it is feasible to simulate and optimize ammonia combustors utilizing previous turbulent burning velocity correlations based on hydrocarbon fuel.

Experimental and kinetic studies of laminar burning velocities of ammonia with high Lewis number at elevated pressures

The present study aims to expand the available database of laminar burning velocities to a higher initial pressure for ammonia kinetic model validation and to investigate the ammonia flame chemistry especially for the effect of N2 addition on N2 and NOx production. In the present study, a high-pressure constant-volume combustion vessel with the pressure release tank was developed and validated. The variations of Lewis number with diluent (He/N2, He/Ar) ratio and the relationships of Lewis number and adiabatic flame temperature with initial pressures were investigated to prevent the flame instability effects and create safe experiments at high pressures. The laminar burning velocities of NH3/O2/He/Ar mixtures with Lewis number around 1.5 and the fixed adiabatic flame temperature of 2250 K at elevated pressures (up to 1.5 MPa) were measured and the overall reaction order of about 1.6 was determined, besides, the effects of pressure on laminar burning velocities and NOx formations were investigated. Moreover, to provide more useful validation data at high pressure, the laminar burning velocities of NH3/O2/He/Ar mixtures at different adiabatic flame temperatures and initial pressure of 1.0 MPa were presented. Additionally, the predictions of four kinetic models on the experimental data were compared and the discrepancies between the calculated and measured results were found. Since ammonia contains nitrogen element, the effects of N2 addition on the laminar burning velocities and the production of N2 and NOx in ammonia combustion were investigated and the physical and chemical effects of N2 addition were separated. Results showed that the laminar burning velocities and NOx concentration for NH3/O2/N2 flames decreased with the increase of N2 dilution and a slight non-linear tendency was observed. Both physical and chemical effects inhibit the N2 production in the NH3/O2/N2 mixtures. The chemical effects play a dominant role in the inhibition of N2 production at the N2 dilution rates below 0.3. The main kinetic effect of N2 addition on NO production is that the addition of N2 shifts the reaction N + NO <=> N2 + O to the left-hand side.

Controllable NO emission and high flame performance of ammonia combustion assisted by non-equilibrium plasma

Owing to the high NOx emission and low combustion performance of ammonia fuel, many researchers are exploring new methods for enhancing ammonia combustion performance without the need for blending with other hydrocarbon fuels. In this study, a gliding arc plasma (GAP) reactor was combined with a swirl burner to simultaneously enhance combustion performance and reduce NO emission. The effects of the gas discharge medium, equivalence ratio (φ), and gas flow rate on flame stability, flame speed, and NO emission were investigated. Experimental results show that with air GAP and ammonia injected from the swirl ring of the burner, the optical emission spectrum of the ammonia flame was dominated by the atomic spectrum of O*, Hβ, and the molecular spectrum of the NH* and OH* components, which significantly promoted ammonia combustion performance, including combustion limitation and NO emission. In addition, with an NH3 gas flow rate of 10 SLM, optimal NO emission was reduced to approximately 100 ppm, even though the equivalence ratio φ was in a lean flame, and near zero at an equivalence ratio φ larger than 1.6. With ammonia GAP and the air injected from the swirl ring of the swirl burner, online hydrogen from ammonia decomposition was produced by ammonia GAP, and NO emission was always less than 100 ppm for various equivalence ratios φ with an NH3 gas flow rate of 10 SLM. These results could be used for ammonia combustion with low NO emission, which could be applied to the ammonia fuel industry.

Oxidizer effects on ammonia combustion using a generated non-premixed burner

In the present study, the pure ammonia combustion in a model combustor is performed to seek ammonia-fueled applications. To this aim, effects of the oxygen enrichment with an oxygen concentration of 100% in the oxidizer on flame characteristics, temperature profiles and NO profiles during the ammonia combustion were evaluated in terms of excess air/oxygen coefficients. Furthermore, in order to better understand the effect of an oxygen of 100% usage under the oxy-ammonia combustion conditions, the air-ammonia combustion has been studied as well and their results are compared and discussed each other. According to the results predicted, the oxidizer with an oxygen content of %100 provides better flame stability in the case of pure ammonia combustion. The most stable flame for oxy-ammonia combustion can be achieved when the excess oxygen coefficient is 1.0 or 1.2. Furthermore, the minimum NO levels emerge under the fuel-rich condition. Temperature and NO emissions decrease considerably under the air-ammonia combustion. However, except the fuel-rich conditions, flame stabilities are not satisfactory due to ammonia's flame speed under the air-ammonia combustion. Moreover, the air-ammonia combustion under the fuel-rich condition seems as a good option for obtaining the lowest NO levels. On the other hand, the oxy-enrichment condition is thought as a promising method for pure ammonia combustion provided that NO emissions should be optimized by using NO reduction methods. • Oxy- and air-ammonia combustion have been taken place. • Temperature and NO X values have been demonstrated. • Equivalence ratio effect on the flame has been studied. • It is revealed that oxy-ammonia combustion performance is favorable. • NO X values are more favorable for air-ammonia than oxy-.

Numerical study of heat release rate markers in laminar premixed Ammonia-methane-air flames

Ammonia has the potential of being a future fuel for carbon-free combustion to reduce greenhouse gases (GHG) emission. The heat release rate (HRR) information of ammonia flames is desired if the ammonia fuel is applied in real combustion devices, but direct measurement of HRR is almost impossible. In the present work, the HRR markers of ammonia-methane-air flames were investigated through the simulation of one-dimensional freely propagating flames (FPF) with a validated chemistry mechanism. Various HRR markers were proposed. The performance of the target HRR markers was tested with the defined criteria. It was found that the profiles of NH and HCO agree well with the HRR profiles, but only NH is a suitable HRR marker because the mole fraction of HCO could be too low for laser induced fluorescence (LIF) measurement. The products of two species, such as [OH][CH2O] and [NO][NH2], were then proposed to circumvent the low mole fraction issues. Product [OH][CH2O], which was usually used as HRR marker in hydrocarbon flames, was found to have the best performance in identifying the HRR location and be the only one that has a positive correlation with the amount of HRR among the proposed products. Moreover, the performance of [OH][CH2O] is valid over a wide range of conditions, including the very high ammonia fuel fraction (XNH3 = 95%), without suffering from the low mole fraction issues. The observations were further analyzed by looking into the reaction pathways and examining the reactions that contributes the largest amount to the total HRR. The present findings provide a meaningful reference for future indirect measurement of HRR in ammonia-methane-air flames.

Study on Combustion Characteristics of Hydrogen Addition on Ammonia Flame at a Porous Burner

Study on Combustion Characteristics of Hydrogen Addition on Ammonia Flame at a Porous Burner

Three-dimensional Measurements of Turbulent Ammonia Flames by Multi-Directional Schlieren CT (Computer Tomography) Technique

Three-dimensional Measurements of Turbulent Ammonia Flames by Multi-Directional Schlieren CT (Computer Tomography) Technique

Investigation into Thermal-Fluid interaction of ammonia turbulent swirling flames under various Non-Premixed burner conditions

This study deals with flow and turbulence interaction of an ammonia flame in a non-premixed burner. Numerical investigations have been performed on the velocity field, thermal field and the turbulence parameters for various oxidizers. In order to validate the simulations and select the most favorable turbulence model, some modeling have first been performed under non-reacting conditions, and the predictions are compared with the measurements. Although the predicted results obtained by using all k-epsilon models are in acceptable consistency with the experimental values, the predictions acquired through RNG k-epsilon model have showed more consistent results with the experimental measurements since there are swirling flows in the flow-field. The further modeling results under the reacting conditions showed the main flow and the external recirculation regions occurred in the flow-field. However, a central recirculation zone was not observed for all cases studied. It is revealed that the predicted axial velocity values for ammonia-air combustion are higher than those of oxy-conditions in the reaction zone. The radial and the tangential velocity values are lower than axial velocity values for all cases. Besides, the tangential velocity component is much lower in case of the ammonia-oxy combustion. It was observed that high turbulence intensity values (about % 350 and % 290 for oxy- and air-combustion) occurred in the mixing zone due to mean velocity affecting velocity fluctuations. Moreover, a consistency between the temperature and the velocity profiles has been predicted in the main flow zone.

Effects of initial mixture temperature and pressure on laminar burning velocity and Markstein length of ammonia/air premixed laminar flames

Ammonia is attractive not only as a hydrogen energy carrier but also as a carbon-free fuel. The goal of the present work is to study the laminar burning velocities and Markstein lengths of ammonia/air under a broad range of conditions including high-temperature and high-pressure because high-temperature and high-pressure conditions are relevant in internal combustion engines. The experiments were conducted in a constant volume chamber for equivalence ratios ranging from 0.8 to 1.2, initial mixture temperatures of 400 and 500 K, and initial mixture pressures ranging from 0.1 to 0.5 MPa. The temperature and pressure exponents were experimentally obtained, and it was clarified that the temperature exponents of the ammonia/air flame were larger than those of the methane/air flame. To evaluate the temperature and pressure effects on ammonia/air flames, these effects were compared with the effects on methane/air flames. Numerical simulations using the detailed reaction mechanisms showed that the effect of the initial mixture temperature on the reaction rate of H + O2 = O + OH was larger for ammonia/air flames than that for methane/air flames. This could be related to the difference in the effects of initial mixture temperature on the H2 and H radical formation reactions. The results also show that the pressure exponent of the ammonia/air flame was closer to zero compared with that of the methane/air flame. This observation can be explained from the standpoint of the effects of pressure on the reaction path of ammonia flames.