CH4 Flames Research Articles

Ultra-lean dynamics of holder-stabilized hydrogen-enriched flames in a preheated mesoscale combustor near the laminar critical limit

This work experimentally investigates the ultra-lean dynamics of a 40% H2–60% CH4 flame near the laminar critical limit in a preheated mesoscale combustor with a flame holder. These experiments are conducted to verify a conjecture we proposed in a previous publication and reveal the ultra-lean flame dynamics under the synergistic effects of heat and flow recirculation. Notably, not only is our conjecture confirmed, but also some novel flame behaviors are found. As the equivalence ratio ϕ is decreased from 0.500 to 0.320, the conventional stable flame, stable residual flame, periodic residual flame with repetitive local extinction and re-ignition (periodic RFRER), and periodic oscillating residual flame are observed in sequence. For the stable residual flame (0.370 ≥ ϕ ≥ 0.355), the left and right flame roots reside directly behind the flame holder, and the flame tip stays near the combustor exit. For the periodic RFRER (0.350 ≥ ϕ ≥ 0.340), observed experimentally for the first time, the flame roots reside at almost the same location, but the flame tip oscillates up and down over time with pinch-off events. For the periodic oscillating residual flame (0.335 ≥ ϕ ≥ 0.320), found for the first time, the stable flame roots also reside at almost the same location, but the residual flame tip oscillates up and down over time without a pinch-off event. When ϕ decreases to 0.315, the oscillating residual flame extinguishes, and its blow-off dynamics are revealed in detail.

Co-firing hydrogen and dimethyl ether in a gas turbine model combustor: Influence of hydrogen content and comparison with methane

To promote the utilization of hydrogen (H2) in existing gas turbines, dimethyl ether (DME) was used to co-fire with H2 in a model combustor. The swirl combustion characteristics of DME/H2 mixtures were measured under the varying H2 content up to 0.7. The results show that the flow velocity elevates as the H2 content increases, which is associated with the increased flame temperature. The OH level firstly increases and subsequently keeps nearly unchanged as the H2 content increases. Meanwhile, the OH area nonlinearly increases with the increasing H2 content. Moreover, the increasing H2 content induces almost linearly decreased lean blowout limit (LBO), increased NO emission, and intensified combustion acoustics. Furthermore, the combustion characteristics of the 0.46DME/0.54H2 mixture and CH4 with the same volumetric heat value were compared. The 0.46DME/0.54H2 flame displays lower LBO and higher NO emission than the CH4 flame, which mainly results from the higher reactivity of 0.46DME/0.54H2 mixture.

The role of volatile organic compounds in the oxidation-driven fragmentation of soot particles

In this work, the role of volatile organic compounds (VOCs) in soot oxidation and fragmentation was investigated in a lean, premixed CH4 flame. An aerosol generator was used to homogeneously disperse soot with and without VOCs (termed Soot-W and Soot-WO, respectively) into the flame. Because minimal soot was formed in the flame itself, information on the fragmentation of the Soot-W and Soot-WO during the oxidation process was readily captured to determine the role of VOCs in soot fragmentation. The integrated number concentration of fine mode particles and the degree of particle aggregation show that the presence of VOCs suppresses soot aggregate fragmentation, primarily by the mitigating consumption of the neck sites between primary particles due to the decreased specific surface area. The actual oxygen diffusion route and effective pore width between the crystallite structures were determined using the image analysis technique. To determine the characteristic regime of oxygen diffusion, the distribution of effective pore width was studied using high-resolution transmission electron microscopy and image analysis. The estimated effectiveness factor indicates that the presence of VOCs lowers the propensity of internal burning via intra-particle oxygen diffusion, and reduces the extent of primary particle fragmentation, as evidenced by a relatively smaller peak of ultrafine particles. The analysis of the nanostructure of neck sites vs. the bulk discovers significantly more disordered and thus reactive structure at neck sites, explaining the prevalence of aggregate fragmentation over primary particle fragmentation. The finding of the role of VOCs in oxidation-driven fragmentation is beneficial to the optimization of existing models for predicting the number concentration of fine and ultrafine particles. Furthermore, a better understanding of this role may aid in the control of the most harmful fine and ultrafine particle emissions.

Experimental and numerical investigation of soot growth and inception in an ammonia-ethylene flame

The effect of ammonia (NH3) addition on soot growth and inception is investigated in laminar co-flow NH3-ethylene (C2H4) diffusion flames. By comparing C2H4 flames with increasing% of NH3 by volume for the same carbon flow and flame height, the impact of NH3 addition on soot formation is identified. Experimental measurements of flame temperature, soot volume fraction, and primary particle sizes and number densities are compared with a numerical two-dimensional co-flow flame model to evaluate the contribution of NH3 to soot reduction in C2H4 flames. Experimental and numerical results show a significant reduction in the amount of soot formed and the diameter and the number of the primary particles suggesting reduced rates of soot growth and inception due to NH3 addition. The numerical model provides the understanding of how NH3 inhibits soot formation in the flame by analyzing the soot formation pathways and investigating the concentration of relevant gas phase species in the flame. While the impact of NH3 addition on the growth of soot particles is not fully captured by the model, the NH3’s reductive effect on the number of soot particles formed is well predicted. The results suggest that NH3 reduces soot by suppressing the concentration of methyl radicals which are responsible for forming odd numbered carbon species such as propargyl and cyclopentadiene. These species are found to be important contributors to the formation of large polycyclic aromatic compounds which are precursors of soot inception.

Experimental and theoretical study on the suppression effect of water mist containing dimethyl methylphosphonate (DMMP) on hydrogen jet flame

This work investigates the suppression effect of water mist containing dimethyl methylphosphonate (DMMP) on H2 jet flame. The flame appearances are analyzed and the flame temperature is recorded by a high-speed infrared camera. Results show that the suppression effect of water mist containing DMMP is better than that of pure water mist. To better understand the suppression effect of water mist containing DMMP, the burning velocity, heat production and free radicals are analyzed. Results indicate that suppressant could scavenge lots of free radicals to reduce the burning velocity in a wide range of equivalence ratios. Moreover, the influence of DMMP on lean CH4 and H2 flames is compared. Different with that of lean CH4 flame, the combustion parameters of lean H2 flame decline with DMMP addition. There is no combustion enhancement effect of DMMP on lean H2 flame. Because of the higher combustion intensity of lean H2 flame, the hydrocarbon in DMMP has little impact on the combustion. Furthermore, the reaction path of DMMP decomposition is analyzed. Results demonstrate that the rates of most phosphorus reactions in H2 flame are larger than that in CH4 flame. This work could provide foundation for improving the safety of hydrogen energy application.

A numerical investigation on the formation of NO2 and N2O in laminar counterflow methane/n-heptane dual fuel flames

To understand the fundamental mechanisms of nitrogen dioxide (NO2) and nitrous oxide (N2O) formation in natural gas-diesel dual fuel combustion, a numerical study on NO2 and N2O formation in laminar counterflow methane (CH4)/n-heptane (n-C7H16) dual fuel flames is conducted. The results indicate that NO2 accounts for a small part of total NOx formation. The NO2 emission index first increases and then decreases with increasing CH4 addition and increases monotonically with increasing flame strain rate. The NO2 emission indices by different reactions are identified and analyzed. It is revealed that a small amount of CH4 addition increases the NO2 emission index because of increased HO2 generation while a larger amount of CH4 addition decreases the NO2 emission index due to the reduced NO production. Besides, it is concluded that decreasing flame temperature is beneficial to the formation of NO2, which explains the monotonic increase of NO2 emission index with increasing flame strain rate. Little N2O is formed in CH4/n-C7H16 dual fuel flames compared to NO and NO2. The N2O emission index decreases with increasing CH4 addition and flame strain rate. However, the relatively change of N2O emission index is quite small compared to that of NO2 emission index.

Hydrogen-Enriched Natural Gas Swirling Flame Characteristics: A Numerical Analysis

Increasing the amount of hydrogen (H2) in natural gas mixtures contributes to gas turbine (GT) decarbonisation initiatives. Hence, the swirling flame characteristics of natural gas mixtures with H2 are investigated in the current work using a numerical assessment of a single swirl burner, which is extensively employed in GT combustors. The baseline numerical and experimental cases pertained to natural gas compositions largely consisting of methane (CH4). The results show that the numerical model adequately describes the swirling component of the flame observed in the experiment. Altogether, the findings show that hydroxyl (OH) radical levels increase in H2-enriched CH4 flames, implying that greater OH pools are responsible for the change in flame structure caused by considerable H2 addition. The addition of 10 % H2 is predicted to raise the peak flame temperature by 4 % compared to the baseline CH4 flame. Therefore, adding 10 % H2 into a GT combustor without any flowrate tuning raises the risk of turbine material deterioration and increased thermal NOx emission. Due to the lower volumetric Lower Heating Value (LHV) of H2, which needs a higher volumetric fuel flow rate than burning natural gas/CH4 at the same thermal output, the addition of 2 % H2 is predicted to reduce the peak flame temperature by 4 % compared to the baseline CH4 flame. Hence, if 2 % H2 is fed into a GT combustor without any flowrate tuning, the required load may not be obtained. When compared to the baseline CH4 case, the addition of 5 % H2 is predicted to provide almost identical peak flame temperature, which can be postulated that the addition of 5 % H2 can produce roughly the same peak flame temperature as the pure CH4 flame because the Wobbe Index is comparable. Therefore, it reveals that incorporating 5 % H2 in the natural gas-fired GT combustor with nearly no modification is viable. More research, however, is required to fully capture the flame structure and strain for assessing transient-related phenomena such as flashback and blow off by raising the H2 proportion and utilising a higher precision turbulence model.

A comparative study on PAH characteristics of ethanol and ammonia as fuel additives in a premixed flame

Due to issues towards polycyclic aromatic hydrocarbons (PAH) emissions and energy security, the renewable and non- or low-sooting tendency fuels are growing increased attention as alternatives to conventional fossil fuels. One feasible approach is to co-fire ethanol and ammonia with hydrocarbons, thus improving their combustion properties and pollutant emissions. This study presents a comparative study about the influence of ethanol and ammonia addition on PAH formation with the same blending ratios up to 30% by mole fraction in premixed C2H4 flames. The axial fluorescence intensities in three wavelength bands were measured to represent concentrations of 2- and 3-ring, 4-ring, 5-membered-ring aromatic species using the laser-induced fluorescence (LIF) technique. Experimental results indicated that both ethanol and ammonia addition could cause PAH reductions in premixed ethylene flames as their blending ratios increase. Further, ammonia was found more efficient than ethanol at inhibiting PAH formation under the same blending ratio. A chemical kinetic analysis was conducted to reveal the inhibiting role of ethanol and ammonia as substituents to ethylene on PAH formation. It was found that both ethanol and ammonia addition chemically suppress benzene formation and subsequent PAH growth due to the low yield of C2H2 and C3H3. Meantime, the additional C1 and C2 supply from ethanol decomposition could explain the relatively lower inhibiting effects on C6H6 and PAH formation as compared to the NH3 doped flames.

Lewis number effects on lean premixed combustion characteristics of multi-component fuel blends

Variation in natural gas composition, alongside the potential for H2 enrichment, creates the potential for significant changes to premixed flame behaviour. To strengthen fundamental understanding of lean multi-component alternative fuel blends, an outwardly propagating spherical flame was employed to measure the flame speeds and Markstein lengths of C1C4 hydrocarbons, alongside precisely mixed blends of CH4/C2H6, CH4/C3H8 and CH4/H2. Theoretical relationships between Markstein length and Lewis Number are explored alongside effective Lewis number formulations. Under lean conditions, equal volumetric additions of H2 and C3H8 (30% vol.) to CH4 resulted in similar augmentation of burning velocity, however, opposite susceptibility to preferential diffusional instability was noted. At a fixed equivalence ratio of 0.65, limited changes in composition provide a marked change in the premixed flame response with the addition of C2H6 and C3H8 to CH4. For lean CH4/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat-release model resulted in better agreement for lean CH4/C3H8 blends. Modelling work suggests that measured enhancement of lean CH4 flames upon H2 or C3H8 is strongly correlated to changes in volumetric heat release rates and production of H radicals. Furthermore, a systematic analysis of the flame speed enhancement effects (thermal, kinetic, diffusive) of H2 and C3H8 addition to methane was undertaken. Augmented flame propagation of CH4/H2 and CH4/C3H8 was demonstrated to be principally an Arrhenius effect, predominantly through reduction of associated activation energy. Finally, plausible short-term variations in composition with hydrogen-enriched multi-component natural gas flames were investigated experimentally and numerically. At the leanest conditions, small variations in CH4:C3H8 content at a fixed H2 fraction resulted in discernible changes in stretch related behaviour, a reflection of the thermo-diffusive behaviour of each fuel's response.

A computational comparison of NH3/O2 and CH4/O2 non-premixed laminar flames

The structure of non-premixed, laminar, counterflow NH3/O2 flames was studied and compared with the structure of CH4/O2 flames. A commercially available computational tool was utilized through the introduction of ammonia and methane chemistry in order to compute the flow fields of strained flames. The tool was validated by comparing with previously published results for CH4 flames and by employing two different mechanisms for NH3 oxidation kinetics. It was shown that NH3 flames achieve lower maximum temperature and narrower high-temperature area compared to CH4 flames, which was attributed to much less heat release from the NH3 oxidization process. This is due to the fact that NH3 oxidation proceeds through a chemical path drastically different than the one of CH4 and is completed with the formation of N2 as an equilibrium product, without substantial formation of nitrogen oxides. The structure of the CH4 and NH3 flames were compared for mildly strained flames and it was shown that, despite its much slower kinetics, ammonia can sustain near-equilibrium flames, even for relatively small values of the Damköhler number.

Quenching distance, wall heat flux and CO/NO thermochemical states in the wall vicinity of laminar premixed biogas-hydrogen impinging flame

The quenching distance, wall heat flux and pollutant thermochemical states of the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2 were investigated using the numerical simulation with detailed chemical kinetics and transport parameters. With the 10% H2 addition, the biogas composed of 75% CH4 and 25% CO2 was used as fuel in the study. Results show that PeQ ≈5 is obtained for the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2. The smaller PeQ than that of CH4 flame is resulted from the CO2 increasing the flame thickness, as well as the decelerated flame propagation due to the strong stretch effects. The quenching distance of biogas-hydrogen flame is codetermined by intrinsic characteristics of heat transfer process and effects of equivalence ratio and flame stretch on the flame propagation. The wall heat flux of laminar premixed biogas-hydrogen impinging flame is maximum at ϕ = 1.0, followed by ϕ = 1.2 and 0.8. At ϕ = 0.8 and 1.2, the location difference between the quenching point and peak wall heat flux of premixed biogas-hydrogen impinging flame is ascribed to the oxidization of escaped unburned fuel near the wall, and the worse field synergy of temperatue and velocity near the quenching point. Additionally, it is found that the fast NO accumulation near the wall at ϕ = 0.8 is ascribed to the transportation of NO generated by NNH route and the effective prompt NO production, while that at ϕ = 1.2 is resulted from the prompt NO production primarily. In the wall vicinity, the low temperature shift of NO production are resulted from the much faster characteristic time scale of heat transfer, while the NNH route and prompt route play the dominant roles in this low-temperature shifts of NO production at ϕ = 0.8 and 1.2, respectively. This means the increased importance of prompt and NNH routes on the NO emission in the downsized combustor.

Flame flow field interaction in non-premixed CH4/H2 swirling flames

Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH4 and H2–enriched CH4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH4 or CH4+H2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H2% in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H2-enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H2%, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen.

Study on characteristics of methane explosion flame and pressure wave propagation to the non‐methane area in a connected chamber

SummaryThis study is aimed at investigating the characteristics of CH4 explosion diffusion and propagation to the non‐methane area. The CH4 explosion pressure and flame were tested with the aid of a self‐built horizontal pipeline system. On this basis, the flame images, photoelectric signals, and pressure propagation laws during explosions of methane with different volume fractions in the pipeline were obtained. The experimental research results indicate that an obvious secondary explosion pressure occurs at L/D = 3.5. After the explosion pressures at different positions along the entire pipeline reach the peak values, they experience varying degrees of oscillations. The maximum explosion pressure in the non‐methane zone may occur at different length‐to‐diameter ratios. When the volume fraction of CH4 is larger, the maximum explosion pressure occurs farther. However, the explosion pressure at the end of the pipeline is lower than that at the explosion source. The flame intensity and flame duration of methane explosion are remarkably enhanced at L/D = 17.5 and then become gradually weakened in the non‐methane area. The variations of explosion pressure are always prior to those of flame signal. CO and CO2 are the main toxic and harmful gases produced by methane explosions, and high‐concentration (13% in this experiment) methane explosions produce a small amount of hydrocarbon gases such as C2H4 and C2H6. The volume fractions of the gases produced decrease with the increase in the distance from the explosion source. The research results boast great significance for suppressing methane explosions and their propagation in the pipeline.

Experimental and kinetic study on the laminar burning velocities of NH3 mixing with CH3OH and C2H5OH in premixed flames

Ammonia (NH3) is a promising carbon-free alternative fuel, and blending NH3 with other more active fuels can enhance the low burning rate of pure NH3 flames, which makes it more compatible with existing combustors. There are laminar burning velocity data of NH3+H2, NH3+CO/syngas, and NH3+CH4 flames in literature, however, blending NH3 with the oxygenated alternative fuel alcohols have not yet been checked. In the present work, the laminar burning velocities of NH3+CH3OH/air and NH3+C2H5OH/air flames were measured using the heat flux method at 1 atm with varied equivalence ratios and mixing ratios. Measurements were carried out at 298 K and elevated temperatures until 448 K unburnt temperatures, in order to check the data consistency via the temperature coefficient α in SLSL0=(TuTu0)α. A new updated kinetic mechanism, based on our previous mechanism named CEU-NH3, was proposed and validated based on the present and published experimental data for laminar burning velocity, ignition delay times and species profiles. This mechanism contains 91 species and 444 reactions, which is small in size and thus advantageous to be used in the modeling of complex combustion fields. By numerical analyses using the CEU-NH3 and the detailed Konnov mechanisms, the interactions between C- and N- containing species are found to be insignificant for the laminar burning velocities of the various NH3 blending mixtures.

Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Previous studies showed that adding hydrogen (H2) can have an opposite chemical effect on soot formation: its chemical effect enhances and suppresses soot formation in methane (CH4) and ethylene (C2H4) diffusion flames, respectively. Such opposite chemical effect of H2 (CE-H2) remains unresolved. The different CE-H2 is studied numerically in the two laminar coflow diffusion flames. A detailed chemical mechanism with the addition of a chemically inert virtual species FH2 is used to model the gas-phase combustion chemistry in this study. Particularly, a reaction pathway analysis was performed based on the numerical results to gain insights into how H2 addition to fuel affects the pathways leading to the formation of benzene (A1) in CH4 and C2H4 flames. The numerical results show that the CE-H2 in CH4 diffusion flame to prompt soot formation is ascribed that the higher mole fraction of H atom promotes the formation of A1 and Acetylene (C2H2) and leads to higher nucleation rate and eventually higher soot surface growth rate. In contrast, adding H2 to C2H4 diffusion flames decreases soot nucleation and surface growth rate. The lower soot nucleation rate is due to the lower mole fractions of pyrene (A4), while the lower soot surface growth rate is due to the lower mole fractions of H atom and C2H2, higher mole fraction of H2 and lower soot nucleation rate. Furthermore, the CE-H2 in C2H4 diffusion flames promotes the formation of A1, but suppresses the formation of A4.

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

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.

Effects of superheated steam on combustion instability and NOx emissions in a model lean premixed gas turbine combustor

This study experimentally explored the effects of preheated steam on combustion instabilities and NOx emissions in a lean-premixed model gas turbine combustor. Three variables were investigated to confirm the effectiveness of preheated steam injection on combustion instability and NOx emissions—the steam flow rate, the injection heights and the inner diameter of injectors. Experimental results denote that optimal the damping ratio of sound pressure can reach 91.73% with preheated steam dilution, the amplitude of sound pressure reduced from 46 Pa to 3.8 Pa. The optimal damping ratio of NOx emissions can reach 83.74% with preheated steam dilution, concentration of NOx emissions reduced from 24.6 ppm to 4.0 ppm. Mode shifting of thermoacoustic instability is appeared as the inner diameter of the injector varied from 1.5 mm to 4.0 mm. The oscillation frequency of the flame CH* chemiluminescence suddenly drops from 256 Hz to 74 Hz as the inner diameter changes from 2.0 mm to 2.5 mm. The lower the relative height of the superheated steam injectors, the better the control effect of combustion instability and NOx emission. However, unreasonable design of the steam injector will trigger thermoacoustic instability. The flame roots, fronts and recirculation zones are all changed under preheated steam, the whole flame become short and compact. The flame length reduced from 75 mm to 37 mm while the preheated steam flow increases. This study explored the dilution effects of preheated steam on lean premixed combustion instability and NOx emissions, the results could be conducive to the design of clean, safe and high-efficient gas turbine combustors.

Study on the Effects of Cone Height on the Turbulent Nonpremixed Flames Downstream of a Conical Bluff Body

Abstract Flow, thermal, and emission characteristics of turbulent nonpremixed CH4 flames were investigated for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 and 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO2, CO, NOx, and O2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely fuel jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer when compared with the flames of the coflow-dominated regime. In the latter regime, emissions were largely reduced due to the dilution by the excess air, which also surpasses their production.

Effects of gas preheat temperature on soot formation in co-flow methane and ethylene diffusion flames

This study reports the effects of gas preheat temperature on soot formation in a series of methane (CH4), ethylene (C2H4) and nitrogen-diluted C2H4 co-flow diffusion flames with a constant mass flow rate of carbon element. The flames were established in a Santoro-type burner and the temperatures of co-flow air and fuel were varied simultaneously from 293 K to 723 K. Planar distributions of soot volume fraction and soot temperature were measured using laser-induced incandescence and two-color pyrometry based on incandescence, respectively. The results show that the preheat temperature of gases has no significant effect on the shape of the CH4 and nitrogen-diluted C2H4 flames, but significantly influences that of the C2H4 flames with high soot loadings. With an increase in gas preheat temperature, both soot formation and oxidation are enhanced and soot loading increases for all flames. The magnitude of enhancement correlates with the soot loading, while the effect is more significant in the flames with lower soot loading. Soot temperature in the CH4 and nitrogen-diluted C2H4 flames also increases but is not as much as the change in the gas preheat temperature. However, for the C2H4 flames, an inverse tend is found in the central region of the flames, i.e., that lower soot temperatures are found in the flame with the higher gas preheat temperature.

The blow-off and transient characteristics of co-firing ammonia/methane fuels in a swirl combustor

Recent studies have demonstrated that ammonia could be one of the most promising hydrogen carrier candidates which can be used in large-scale power plants. However, it is challenging to burn ammonia in gas turbines due to its narrow flame stabilization limits. This study investigates the blow-off characteristics and flame macrostructure transition behavior of ammonia/air flame (i.e. NH3 flame) and ammonia/methane/air flame (i.e. 50%NH3 flame) in a swirl combustor. Methane/air flame (i.e. CH4 flame) is also demonstrated for comparative purposes. The flow field and instantaneous OH profile are measured with PIV and OH-PLIF technique, respectively. Large eddy simulation (LES) is conducted to extend understandings of the experimental findings. The results show that the NH3 flame possesses a poor lean flame stability limit which can be largely extended by adding CH4 in the fuel. Moreover, changing swirl number (S) shows no apparent effect on the lean blow-off limit (ϕb) for the NH3 flame. On the contrary, a clear extension on ϕb is found for the 50%NH3 flame when increasing S. Four flame macrostructure modes can be identified when decreasing equivalence ratio (ϕ). The transition from flame II to flame III (ϕt describes the transition equivalence ratio) can be considered as the early warning of blow-off for a swirl stabilized flame. It is found that for the NH3 flame, there is no clear flame macrostructure transition at small inlet velocities (U < 3.8 m/s), i.e., ϕb ≈ ϕt, while the difference between ϕb and ϕt will be observed as the inlet velocity increases. However, for the 50%NH3 and CH4 flames, a clear flame macrostructure transition from flame II to flame III is observed even for a lower inlet velocity. The LES results show that the NH3 flame has a faster blow-off process compared to the CH4 flame, which is mainly attributed to the excessive stretch causing local extinction during the blow-off process.