Increasing Equivalence Ratio Research Articles

Study on premixed hydrogen-ammonia-air flame evolution in a horizontal rectangular duct

The hydrogen-ammonia mixture exhibits significant potential as a carbon-free fuel for internal combustion engines. To gain a deeper understanding of the combustion characteristics of hydrogen-ammonia-air flames in internal combustion, the flame evolution and pressure dynamics are systematically investigated in a closed duct by varying ammonia mixing ratio and equivalence ratio. Concerning flame dynamics, the distorted tulip flame disappears and the inclined flame is formed as the ammonia mixing ratio increases. Additionally, the depression and protrusion structure distributed on flame front is diminished. For pressure dynamics, as the equivalence ratio increases, both the maximum explosion overpressure and maximum pressure rise rate initially increase and then decrease. As the ammonia mixing ratio increases, both of them continue to decrease, and the equivalence ratio corresponding to the peak value transfers from Ф = 1.2 to Ф = 1.0. In terms of detailed chemistry analysis, the H and OH radical dominate when the ammonia mixing ratio is lower than Ω = 40%. NH2 and OH radical dominate at ammonia mixing ratio higher than Ω = 80% on the lean and stoichiometric side, but NH2 and H radical dominate at ammonia mixing ratio higher than Ω = 40% on the rich side.

Shock tube experiments and kinetic modeling of ignition of unsaturated [formula omitted] methyl esters

Ignition delay times (IDTs) of methyl crotonate (MC), methyl 3-butanoate (MB3D), and methyl methacrylate (MMA), were systematically measured to explore the influences of the position of unsaturated CC double bond and the structure of alkyl chain on ignition characteristics of unsaturated C5 methyl esters. Experiments were performed at 1108–1783 K and 4–16 atm with the equivalance ratio ranging from 0.25 to 2 in a shock tube. Reactivities of three unsaturated esters are similar: their IDTs increase with decreasing temperature and increasing equivalence ratio but are less sensitive to pressure. To gain the insight into differences in kinetics during fuel ignition, the literature MC, MB3D and MMA models were evaluated and the MMA model was updated. Sensitivity and reaction pathway analyses show that the hydrogen-abstraction, unimolecular decomposition, and radical addition reactions are important in controlling MC ignition. However, only hydrogen-abstraction and radical addition reactions show the significance during MB3D and MMA ignitions. Furthermore, the impact of saturation was also explored. The saturated methyl esters have slightly longer IDTs than MC, MB3D, and MMA under fuel-lean conditions with T> 1350 K, because the unsaturated methyl esters are the key intermediates and the hydrogen-abstraction reactions dominate the consumption of saturated esters.

Experimental and simulation study of NH3–H2-Air flame dynamics at elevated temperature in a closed duct

Ammonia allows for long-distance and large-scale storage transport of renewable energy sources. The reactivity of NH3-Air flame can be enhanced by doping with hydrogen and by increasing the initial temperature. The flame combustion characteristics of NH3–H2-Air are studied in a rectangular closed pipe with a mole fraction of 45% H2, an equivalence ratio (φ) of 0.8–1.2 and 0.1 MPa. The initial temperatures (Tu) of the mixture are 300 K, 375 K and 450 K. Experimental studies of flame front speed, flame tip structure evolution, and overpressure dynamics are performed. Laminar burning velocity (SL), peak mole fraction of key radicals, and flame instability of NH3–H2-Air flames are simulated in Chemkin Pro-software using the Mathieu and Petersen mechanism. The results show that the relatively low equivalence ratio and initial temperature promote the structural changes of the flame tip, forming a “tulip” flame and a distorted “tulip”. As the equivalence ratio increases, the flame front speed and overpressure increase and the flame oscillates slightly. The distortion in the lip of the “tulip” is more pronounced. The increase in temperature promotes flame propagation in the early stages and the pressure decreases. Reactive activity increases with increasing temperature, the peak mole fraction of NH2 and H radicals increases, and SL increases. At normal temperature, the flame is affected by thermo-diffusive instability only when lean mixture. The Darrieus-Landau instability increases as the equivalence ratio increases. Conversely, the Darrieus-Landau instability decreases with increasing temperature. Therefore, the current experimental and simulation results help to understand the laminar combustion characteristics of NH3–H2-Air at high temperatures.

Effect of 2-Butanone addition on laminar burning velocity of gasoline XP95 at higher mixture temperatures

2-Butanone, a product of pyrolysis of biomass, micro-biological fermentation of agricultural waste, possesses several advantages as a next generation biofuel for spark ignition engines. Very little is known about its combustion behavior due to non-availability of detailed combustion measurements. In this study, the laminar burning velocities (LBV) of 2-Butanone were measured and compared with benchmarked ethanol and gasoline using the externally heated diverging channel method. The measurements were conducted for 2-Butanone (B), premium gasoline (XP95) and their three different fuel blends (volume fraction: 10% B + 90% XP95, 30% B + 70% XP95, and 50% B + 50% XP95) for fuel-air equivalence ratios (ϕ) = 0.6–1.4, at elevated mixture temperatures of 300–620 K, and atmospheric pressure (1 bar). The present measurements of 2-Butanone show a good match with the values reported in literature and exhibit a good consistency with the predictions of Hemken 2017 reaction mechanism. The sensitivity analysis reveals that the inhibition effect of the key reaction R43: CH3 + H (+M) ⇔ CH4 (+M) is reduced by 34%, when the mixture temperature increased from 305 K to 620 K at ϕ = 1.1 and increased by 39% with an increase in mixture equivalence ratio from 0.7 to 1.3 at 620 K. The LBV of 2-Butanone shows a slight difference with ethanol and gasoline for leaner mixtures (ϕ = 0.6, 0.7). However, with increase in mixture strength (ϕ ≥ 0.8), the LBV becomes significantly higher than that of gasoline XP95 and slightly lower than ethanol regardless of the mixture temperature. With addition of 2-Butanone in gasoline XP95, the LBV improved by 14%, 24.6%, and 36.1% for 10% B + 90% XP95, 30% B + 70% XP95, and 50% B + 50% XP95 blends respectively, at ϕ = 1.0, 620 K. From the improved flame behavior of 2- Butanone under typical engine operating conditions against benchmarked ethanol, makes it a suitable biofuel substitute for gasoline in a sustainable transportation application.

Flame emission spectroscopy analysis of distributed liquid fuel combustion

Distributed combustion offers ultra-low pollutant emission, low susceptibility of thermoacoustic instabilities, and is highly fuel flexible, being an ideal choice for, e.g., boilers, furnaces, and gas turbines. It was recently demonstrated that Mixture Temperature-Control could also achieve distributed combustion and not restricted to Moderate or Intense, Low-oxygen Dilution combustion. The former approach is employed in the present paper. Online flame control is required by the need for dynamic operation, which is often performed via spectral flame measurements. Presently, the distribution of OH*, CH*, C2*, O2*, and two H2O* chemiluminescent emission peaks were evaluated; the first two represent ignition, while the latter four appear later in the reaction zone. Characteristics of distributed flames are compared to those of straight and V-shaped flames, which are well-known in the literature. The following observations were made. Distributed combustion is lifted and occupies the entire cross-section of the combustion chamber without featuring a characteristic shape. The OH*/CH* ratio shows a low dependence on the equivalence ratio, and the reactions start only 50 mm downstream of the burner lip. The dominant radical was CH* for all distributed flames. As the equivalence ratio increases, the reaction zone moves slightly upstream.

Experimental studies on the OH∗ chemiluminescence and structure characteristics in NH3/H2 and NH3/cracked gas swirl flames

Ammonia (NH3) has been gaining popularity as a carbon-free alternative fuel in recent years. OH∗ chemiluminescence is a promising method to study the structure of ammonia flames, but research on this aspect is still lacking. In this study, the OH∗ chemiluminescence distributions and structure characteristics in NH3/H2 and NH3/cracked gas swirl flames are experimentally investigated. The OH∗ distributions of NH3/H2 and NH3/cracked gas under different addition levels and equivalence ratios are compared. The results show that the increase of equivalence ratio and hydrogen doping level significantly increased the OH∗ chemiluminescence peak of NH3/air swirling flame. Especially in fuel-lean flames, the equivalence ratio effect is greater than the NH3 dilution effect, while the two effects are comparable in equivalence and fuel-rich conditions. The stretching effect of NH3/cracked gas is stronger, and the flame width is narrower than that of NH3/H2 flame, but the peak value of OH∗ radiation is higher.

Onset of flame acceleration in propane–oxygen mixtures

In this study, the onset of flame acceleration because of Darrieus–Landau and diffusive–thermal instability in propane–oxygen mixtures was investigated using experiments and analysis. Using the soap bubble method, the unstretched laminar burning velocity Su0 and critical flame radius rc at which flame acceleration occurs of the propane-oxygen mixture were evaluated. The experimental values of Su0 and critical Péclet number Pec agreed qualitatively with the calculated values, and Pec tended to decrease with increasing equivalence ratios. In addition, the dependency of Pec on the Markstein number Mab and the critical Karlovitz number Kac on Mab for the propane–oxygen flame was compared with that for other fuel–air mixtures. The comparison revealed that the decreasing trend of Kac with Mab for the propane-oxygen flame was qualitative consistent with other fuel-air mixtures, in contrast to the different slopes of the Pec-Mab relationship.

Effects of Ammonia Substitution on Explosion Limits of Methane

As a hydrogen-rich and carbon-free fuel, ammonia is regarded as a promising carrier and storage medium for clean energy. By mixing methane with ammonia, the emission of carbon dioxide is also significantly reduced, which is of great significance for reducing greenhouse gas emissions and protecting the environment. However, ignition studies of ammonia/methane mixtures are still limited. In this paper, by means of numerical simulation with detailed chemical reaction mechanism, the effect of ammonia replacing methane on combustion was analyzed. Characteristics of explosion limit under different temperature (750-850K), equivalent ratio (0.5, 1.0 and 2.0) and ammonia mixing ratio (0-90%) were studied. The results show that the explosion limit decreases with the increase of temperature and equivalence ratio. When the proportion of NH3 is around 10%, the explosion limit shows a turning point. When the mole fraction of NH3 is higher than 50%, the explosion limit shows obvious increasing tendency with ammonia addition. Moreover, sensitivity and rate of production were also analyzed to expand the understanding of explosion limit for premixed ammonia-methane fuel blends.

The turbulent flame structure in a steam diluted H2/Air micromix flame

In order to safely organize hydrogen flame with low NOx emission in traditional gas turbines and industrial burners, an attractive combustion technology is the steam diluted micromix combustion technology. The effects of equivalence ratio (φ), steam dilution ratio (D) and nozzle diameter (d) on the turbulent flame structure in a steam diluted H2/air micromix flame were investigated experimentally with a 10 kHz high-speed OH∗ chemiluminescence imaging system to detect the instantaneous flame zone. With the increase of equivalence ratio (φ) or the decrease of steam dilution ratio (D), the overall OH∗ signal intensity and the area of OH∗ signal increase. The maximum OH∗ signal intensity along the axial direction (Z) is distributed far from the nozzle outlet with the increase of nozzle diameter (d). Two types of flame were found for time-averaged images, namely the anchored flame and the lifted flame. And the increase of hydrogen velocity could lead to the changing of flame from anchored to lifted due to lower uniformity of the fuel-air mixture at the nozzle outlet. The instantaneous flames for larger nozzle diameter appear as “M” or “V” shape, and the changing of flame shape indicates an unstable combustion. This was further discussed with the OH∗ spatial distribution uniformity (SDU) and its variation determined by the coefficient of variation (CV) parameter.

Experimental research of the performance and pressure gain in continuous detonation engines with aerospike nozzles

Several continuous detonation engines (CDEs) with aerospike nozzles were designed, and nearly 100 effective experimental tests with methane/GOX were carried out. All tests exhausted to sea level backpressure and were conducted using a calibrated thrust stand and Coriolis mass flow meters. Throughout testing, three key parameters (area ratio, equivalence ratio, and mass flux) affecting CDE operability are discussed in detail. Three normalized parameters were proposed and analyzed to evaluate the performance of CDEs, called the normalized characteristic velocity, the normalized thrust coefficient, and the normalized specific impulse. A new evaluation method for total pressure gain is proposed and implemented. The experimental results showed that lower area ratio (less than 20%) and lower mass flux contribute to form a stable detonation and enjoys a wider range of equivalence ratio. The studied CDEs presented higher performance in the range of area ratio 12.8% - 18.8% and equivalence ratio 1.0 - 1.2. The pressure gain of detonation mode ranged between 15% and 25%, and decreased slightly with the increase of equivalence ratio (0.7 - 1.2). A suitable area ratio is necessary to improve the pressure gain.

Improved reactivity and energy release performance of core-shell structured fuel-rich Si/PTFE energetic composites

Si is an attractive fuel in pyrotechnic applications due to its high gravimetric and volumetric energy density. In this article, core-shell structured fuel-rich Si/PTFE (CS-PSi) energetic composites were prepared through high-power ultrasonic mixing. The energy release performance of the CS-PSi composites with different equivalence ratios (ϕ=1.5, 2.5, and 3.5) is characterized, and the magnetic stirring mixed Si/PTFE (MS-PSi, ϕ=3.5) composite is also studied for comparison. For the CS-PSi composites, with the increase in equivalence ratio, the relative intensity and burning rate both increase. Evaluated in a closed vessel, the CS-PSi (ϕ=2.5) exhibits the optimum pressure performance in terms of peak pressure and pressurization rate. By modifying the equivalence ratio, the reactivity, as indicated by the pressurization rate, of the CS-PSi energetic composites can be tuned accordingly. Having the same equivalence ratio (ϕ=3.5), the relative intensity, burning rate, peak pressure, and pressurization rate of the CS-PSi composite are 6.1, 4.3, 1.3, and 4.1 times larger than those of MS-PSi counterpart, respectively, while the apparent activation energy of the CS-PSi composites is reduced by 23.6%. The improved reactivity and superior energy release performance of the CS-PSi composites can be attributed to the higher degree of intimacy between the reactants as a result of core-shell configuration. Our approach provides a facile and efficient way to elevate the performance of the Si/PTFE energetic composites.

Experimental study on propagation characteristics of rotating detonation wave of liquid kerosene with oxygen-enriched air

In order to study the propagation characteristics of rotating detonation wave (RDW) of liquid fuel, an experimental study was carried out with liquid kerosene at room temperature as fuel and oxygen-enriched air with oxygen volume fraction of 38.5% as oxidant in this paper. RDW with equivalence ratio in the range of 0.65~0.9 was successfully obtained by controlling kerosene mass flow, and a detailed analysis of propagation characteristics of RDW was carried out. The results show that with the increase of equivalence ratio, the propagation state of RDW gradually changes from the premature flameout of discontinuous detonation to the complete operation of continuous detonation. The flow blockage of the injectors will further worsen the mixing effect under low equivalence ratio conditions, resulting in the discontinuous detonation phenomenon of repeated flameout-initiation process in the combustion chamber. In this study, almost all detonation waves operate in two-counter RDW mode, except that there is a single wave mode in a short period after flameout-initiation process.

Flexural behaviour of steel–basalt fibre composite bar-reinforced concrete beams

The utilisation of steel–basalt fibre composite bars (SBFCBs) can effectively solve the corrosion problem of steel-reinforced concrete beams and avoid the poor ductility problem of pure FRP-reinforced concrete beams. However, the research on flexural performance of SBFCB-reinforced concrete beams is still rare. Eleven beams with different reinforcement methods were tested to study the flexural behaviour of SBFCB-reinforced concrete beams and provide reference for its design and application. The failure mode, ultimate bearing capacity and strain of concrete and reinforcement, as well as the crack distribution during loading, were studied. The effects of different reinforcement methods and equivalent reinforcement ratios on the flexural performance of the beams was discussed. In addition, the equivalent reinforcement ratio range of under-reinforced failure was determined, and the theoretical calculation formulas of the yield load and ultimate bearing capacity of the beams were deduced. Results demonstrated that the failure mode of SBFCB-reinforced concrete beams was affected by the equivalent reinforcement ratio; the cracking load values of the beams were close to each other, and the yield load and ultimate load increased with the increase in equivalent reinforcement ratio. The SBFCB-reinforced concrete beams have obvious secondary stiffness. Furthermore, the deflection of the beams at the late loading stage increased faster than that before the yield of the steel bar, which indicates that the beams have good ductility. Moreover, increasing the equivalent reinforcement ratio increased the crack number in the pure bending section of beams, reduced the average crack spacing and reduced the maximum crack width. The ultimate bearing capacity calculated by the deduced formulas were in good agreement with the measured values. The maximum strain of SBFCBs is recommended to be limited to 0.75 times the ultimate strain to prevent sudden brittle failure.

Experimental study on combustion and thermal characteristics of impinging premixed flames for low heating value gas (LHVG) fuels

The combustion and thermal characteristics of impinging premixed flames for low heating value gases (LHVGs) composed of methane, propane, and nitrogen are experimentally investigated. For the experiment, a combustor with an impingement plate is used to study the flame stability regions based on flame shapes and visualize OH radical distribution. The OH radical distribution is observed via OH* chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF). Thermal characteristics are investigated by measuring the temperature distribution of the plate and the heat transfer rate into the cooling water. The results show that the flame stability region narrows as the heating value of the fuels decreases. Also, as the equivalence ratio increases (i.e., »1), the flame stability region is expanded. The OH radical intensity of LHVGs is observed to be lower than that of methane. The OH-PLIF images show where OH radicals are more distributed in the flame. The temperature distribution of the plate is determined by the flame shape according to the heating value, and the heat transfer rate into the water has a linear tendency to the fuel's thermal power. Based on these findings, LHVGs can be utilized without system improvement within the flame stability range in the impinging flame system.

Investigation on geometric throat matching characteristics of variable geometry rocket-based combined cycle combustor

One of the most effective ways to realize multi-mode and high-performance matching operation of rocket-based combined cycle (RBCC) engine is to adopt a variable geometry combustor. In this study, direct-connect experiments and three-dimensional numerical simulations were used to investigate the matching characteristics between the isolator and the geometric throat of the variable geometry rocket-based combined cycle combustor under Ma 6 condition. The influences of equivalence ratio and geometric throat height variations on the combustor matching characteristics were obtained. The study results show that: (1) The pressure in the combustor and the outlet pressure of the isolator increase gradually with the increase of equivalence ratio, which causes the pre-combustion shock train position in the isolator to move forward gradually. Meanwhile, the combined injection of secondary fuel with the isolator and the pylon is beneficial to the improvement of combustor pressure, thus further improving the engine performance. (2) Different geometric throat heights have a substantial impact on the isolator and combustor matching when the equivalent ratio of the combustor is fixed. Under the conditions of Ma 6 operating at an altitude of 24 km inflow, the starting position of the pre-combustion shock train in the isolator is precisely at the isolator entrance, and the corresponding geometric throat height is 2.10H. The isolator and the combustor can complete matching operation at this point which the geometric throat height is the minimal throat height for Equivalent ratio = 1.0. (3) The reduction in geometric throat height can result in an increase in combustor pressure but a loss in combustor heat release and combustion efficiency of fuel combustion, which are reduced by 21.1% and 7.2% respectively. (4) The heat release and combustion efficiency in the combustor will decrease with a decrease in equivalent ratio at the minimal throat height condition where the isolator and the combustor can complete matching operation. This happens when the equivalent ratio and the geometric throat height change synergistically.

Simulation Investigation on the Effects of EGR Ratio and Nozzle Angle on In-Cylinder Combustion and Pollutant Generation of PODE/Methanol Blends

ABSTRACT In order to explore effects of coal-based oxygenated fuels polyoxymethylene dimethyl ether (PODE) and methanol on pollutant emissions of diesel engines, in-cylinder combustion numerical model was developed by coupling PODE/methanol reaction mechanism with CONVERGE software. The influences of exhaust gas recirculation (EGR) and nozzle angle on in-cylinder combustion and pollutant generation of PODE/methanol blends were studied. Results show that with the increase of EGR ratio, the fuel-air equivalence ratio increases, the in-cylinder pressure decreases, and the ignition delay period is prolonged. The generation range and rate of NOx gradually decrease, and the final NOx generation decreases by 85.1% at EGR = 20%. However, the peak soot generation rate and the final soot generation increase. With the increase of nozzle angle, the peak in-cylinder pressure and the overall heat release rate increase. When the nozzle angle is adjusted from 148° to 164°, the rich area of fuel-air equivalent ratio in the piston pit decreases, the high temperature area in the piston pit almost disappears. The final in-cylinder NOx generation has been reduced by 29.3%. The soot distribution gradually moves to the upper layer of combustion chamber, and the peak soot generation rate decreases, while the final soot generation increases by almost 49.5%.

Influence of hydrogen and methane addition in laminar ammonia premixed flame on burning velocity, Lewis number and Markstein length

The use of Ammonia (NH3) and blends with either Methane (CH4) or Hydrogen (H2) obtained by in-situ NH3 cracking, seem to be promising solutions to partially or fully decarbonise our energy systems. To strengthen understanding of fundamental combustion characteristics of these NH3 blends, the outwardly propagating spherical flame configuration was employed to determine the flame speeds and Markstein lengths. The air/fuel mixtures were varied across a large range of compositions and equivalence ratios. In general addition of CH4 or H2 results in a linear and exponential increase in measured laminar burning velocity, respectively. Of the appraised mechanisms, Stagni and Okafor kinetics mechanisms yielded best agreement with NH3/H2 and NH3/CH4 flame speed measurements. With respect to measured Markstein length, for a fixed equivalence ratio, addition of CH4 to NH3 resulted in a linear reduction in stretch sensitivity for the tested conditions. For lean NH3/H2 flames, an initial decrease in Markstein length is observed up to 30–40% H2 addition, at which point any further addition of H2 results in an increase in Markstein Length, with a non-linear behaviour accentuated as conditions get leaner. Above stoichiometry similar stretch behaviour is observed to that of NH3/CH4. Different theoretical relationships between the Markstein length and Lewis Number were explored alongside effective Lewis Number formulations. For lean NH3/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat release model resulted in better agreement at richer conditions. For NH3/CH4 mixtures, a volumetric based Lewis Number formulation displayed best agreement for all evaluated equivalence ratios. For NH3/H2, changes in measured Markstein Length were demonstrated to potentially be the result of competing hydrodynamic and thermo-diffusive instabilities, with the influence of the thermo-diffusional instabilities reducing as the equivalence ratio increases. On the other hand, the addition of CH4 to NH3 results in the propensity of moderating hydrodynamic instabilities, resulting in a stabilising influence on the flame, reflected by increasing positive Markstein number values. Finally, a systematic analysis of the flame speed enhancements effects (kinetic, thermal, diffusive) of CH4 and H2 addition to NH3 was undertaken. Augmented flame propagation of NH3/CH4 and NH3/H2 was demonstrated to be principally an Arrhenius effect, predominantly through the reduction of the associated activation energy.

Combustion Performance of the Premixed Ammonia-Hydrogen-Air Flame in Porous Burner

ABSTRACT Flame stability and pollutant emission performances of a porous burner fueled with ammonia/hydrogen blend were investigated numerically in this study. In this regard, a 2D solver based on finite volume method was developed in order to simulate reacting flow and heat transfer modes through a porous medium. Combustion characteristics in terms of porous structure, equivalence ratio and fuel component were studied and the pollutant emission trends were discussed. Flame stability limit was observed to be extended with increasing equivalence ratio regardless to ammonia fraction. Results proved that addition of ammonia in the fuel blend reduced the flame stability limits and thermal flame thickness but led to the enhanced NO emission. Increasing equivalence ratio led to a decrease in thermal flame thickness under fuel-lean conditions while it caused that the thickness of flame zone to be increased slightly under fuel-rich conditions. It was found that flame stability limits were extended as the mean pore diameter of the porous medium increased. The maximum amounts of NO concentration were achieved at stoichiometric conditions while, NO emission increased as equivalence ratio increased at fuel-lean conditions and it decreased as equivalence ratio increased at rich combustion regimes.

Study on the effect of fuel injection on combustion performance and NOx emission of RQL trapped-vortex combustor

As a combustor that can reduce pollutant emissions, the trapped-vortex combustor can not only reduce the length of the combustor because of its radial classification technology, but also its design concept is inseparable from RQL (Rich burn, Quench, and Lean burn). In this paper, the effects of cavity equivalence ratio and fuel injection cone angle on combustion performance and NOx emission were studied by numerical simulation. It is found that the trapped-vortex combustor achieves RQL combustion mode. The increase of the cavity equivalence ratio results in the non-uniform fuel distribution. However, the fuel injection cone angle in the primary region has little effect on the fuel distribution. With the increase of the cavity equivalence ratio, the combustion efficiency and NOx emission first decrease and then increase, and outlet temperature distribution factor is positively correlated with the cavity equivalence ratio. The difference is that the outlet temperature distribution factor and combustion efficiency show almost the same law with the increase of injection cone angle in the primary region, and the NOx emission decreases, but the reduction is not significant.

NO formation and reduction during methane/hydrogen MILD combustion over a wide range of hydrogen-blending ratios in a well-stirred reactor

Hydrogen-blending methane or even hydrogen moderate or intense low-oxygen dilution (MILD) combustion has attracted much interest due to its advantages of low CO2 and NOx emissions. This paper reports a comprehensive parametric study through kinetic modeling with the Glarborg2018 mechanism in a well-stirred reactor to evaluate the formation and reduction of NO during MILD combustion for CH4/H2 co-firing fuels over a wide range of hydrogen-blending ratios from no hydrogen to pure hydrogen. In particular, NO formation via thermal, prompt, NNH, and N2O-intermediate routes as well as NO reduction not only by hydrocarbon radicals but also by non-hydrocarbon radicals are separately investigated. The results show that, at a fixed residence time of 1 s, hydrogen addition cannot change the dominant role of the pathway via N2O under fuel-lean conditions, whereas it can change the major NO formation pathway from prompt to NNH under fuel-rich conditions. In hydrogen MILD combustion, the predominant pathway taken to convert N2 to NO is via N2O when the equivalence ratio is below 0.8, and with increasing equivalence ratio, the NNH pathway eventually prevails over the N2O-intermediate pathway to become the dominant one under reducing conditions. The NNH pathway at lower mixture temperatures and short residence times contributes more to NO production. On the other hand, NO reduction by non-hydrocarbon radicals, which proceeds primarily through the route H + NO → HNO → NH → N2, is effective in high-percentage hydrogen co-firing, while that by hydrocarbon radicals is weakened with hydrogen addition.