Hot Coflow Research Articles

Study of NO and CO Formation Pathways in Jet Flames with CH4/H2 Fuel Blends

The existing natural gas transportation pipelines can withstand a hydrogen content of 0 to 50%, but further research is still needed on the pathways of NO and CO production under moderate or intense low oxygen dilution (MILD) combustion within this range of hydrogen blending. In this paper, we present a computational fluid dynamics (CFD) simulation of hydrogen-doped jet flame combustion in a jet in a hot coflow (JHC) burner. We conducted an in-depth study of the mechanisms by which NO and CO are produced at different locations within hydrogen-doped flames. Additionally, we established a chemical reaction network (CRN) model specifically for the JHC burner and calculated the detailed influence of hydrogen content on the mechanisms of NO and CO formation. The findings indicate that an increase in hydrogen content leads to an expansion of the main NO production region and a contraction of the main NO consumption region within the jet flame. This phenomenon is accompanied by a decline in the sub-reaction rates associated with both the prompt route and NO-reburning pathway via CHi=0–3 radicals, alongside an increase in N2O and thermal NO production rates. Consequently, this results in an overall enhancement of NO production and a reduction in NO consumption. In the context of MILD combustion, CO production primarily arises from the reduction of CO2 through the reaction CH2(S) + CO2 ⇔ CO + CH2O, the introduction of hydrogen into the system exerts an inhibitory effect on this reduction reaction while simultaneously enhancing the CO oxidation reaction, OH + CO ⇔ H + CO2, this dual influence ultimately results in a reduction of CO production.

Numerical study on the effects of adding NH3 into syngas/NH3 mixtures on combustion characteristics and NOx emissions in diluted hot co-flows

NH3 is a carbon-free fuel with good storage and transport characteristics. Applying Moderate or Intense Low-oxygen Dilution (MILD) combustion to NH3/syngas blended fuel is expected to achieve clean and low-carbon combustion. This work explores the impacts of changing the NH3/syngas ratio and O2 content in co-flow on the combustion characteristics and the NOx formation and transformation mechanisms. As the proportion of NH3 increases, the flame is gradually lifted, while the rise of H2 in syngas reduces the lifted height. As the O2 proportion is increased to 15%, almost the entire computational domain is transformed from MILD to high-temperature combustion when the ratio of NH3 to syngas is 3:7. NH that is derived from the decomposition of NH3 participates more actively in the consumption process of NO and N2O. N radicals are more likely to generate N2 when O2 content in hot co-flow increases.

Differential diffusion modelling for LES of premixed and partially premixed flames with presumed FDF

Large eddy simulations (LES) with flamelet and presumed filtered density function closure are used to simulate turbulent premixed and partially premixed hydrogen flames. Different approaches to model differential diffusion are investigated and compared. In particular, two existing models are extended to the LES framework to correct the resolved diffusive flux of the controlling variables due to differential diffusion. A lean premixed turbulent hydrogen flame in a slotted burner configuration is simulated first to compare the capability of the considered models in capturing local mixture fraction redistribution, super-adiabatic temperatures and thermo-diffusive instabilities. Results show that both models describe the formation of cellular burning structures. Next, a partially premixed lifted hydrogen flame in vitiated hot coflow is simulated to gain insight on the relevance of differential diffusion modelling at a higher turbulence level, a different combustion mode and in the presence of a complex stabilisation mechanism. Good predictions of the turbulent mixing and temperature fields are observed. Moreover, results show that the flame lift-off height has an appreciable sensitivity to the differential diffusion model. When differential diffusion is included only in the thermochemistry database, only mild effects on the predicted temperature fields, mixing and flame height are observed. On the contrary, a considerable shift of the flame base is observed when corrections are applied in the LES at the resolved level, depending on what controlling variables are considered. Further analyses reveal how the corrections of diffusive fluxes in the thermochemistry and at the LES level affect differently the flame burning mode, whose details are given throughout the paper.

MILD combustion characteristics of a premixed CH4/air jet flame in hot coflow from a bluff-body burner

This numerical study is designated to comparatively investigate premixed CH4/air jet flames from bluff-body (BB) and non-BB (NBB) burners in the coflow of hot exhaust gas. Specifically, traditional combustion (TC) from BB burner (BB-TC), MILD combustion (MC) from BB burner (BB-MC), and NBB burner (NBB-MC) under varying coflow temperatures (TC) and oxygen concentrations (YO2, C) are considered. The flow field, combustion reactions, temperature rise, heat release, along with reaction zone, and pollutant emissions are thoroughly investigated. The findings underscore the feasibility of achieving MILD combustion using the BB burner. Remarkably, the small recirculation zone generated by the BB enhances entrainment and mixing, leading to superior combustion stability and reduced CO emission in the BB-MC case compared to the conventional NBB-MC case. Moreover, the BB-MC case exhibits a higher peak temperature, reaction rate, and heat release rate than the NBB-MC case, albeit with slightly higher NO emission. When compared to the BB-TC case, NO emission from both the BB-MC and NBB-MC cases are significantly lower, particularly under relatively low TC and YO2, C conditions.

Furnace MILD combustion versus its open counterpart in hot coflow

The open jet flame in hot co-flow (JHC) has been frequently utilized for fundamental investigations of Moderate or Intense Low-oxygen Dilution (MILD) combustion due to its controllable conditions and relatively easy measurement capabilities. However, practical MILD combustion must take place within a combustor that is enclosed. Therefore, it is necessary to examine the similarity and disparity of combustion characteristics between two flame configurations. This issue is addressed currently. Specifically, we investigate the flow mixing, ignition, and combustion, as well as emission characteristics of non-premixed and premixed JHC and cylindrical furnace (FUR) flames at various values of the environmental temperature (Te) and central jet Reynolds number (Re). For the open non-premixed flames, we employ both previous single-tube JHC (SJHC) combustor and presently modified JHC (MJHC) one that uses the same nozzle configuration as the FUR burner. It is revealed that significant differences occur in flow, combustion and emission characteristics between the SJHC and FUR cases. On the other hand, both non-premixed and premixed MJHC configurations exhibit high similarity to the corresponding FUR cases in terms of upstream flow mixing and combustion features. Moreover, different CO and NOx emissions result from open JHC and close furnace flames due to different post-combustion configuration and residence time. Accordingly, future experiments on non-premixed MJHC and premixed JHC flames are highly recommended for better understanding practical MILD combustion.

Influence of the H2 proportion on NH3/H2/air combustion in hot and low-oxygen coflows

This study investigated the effect of the H2 fraction on reaction dynamics and NOx formation in a premixed NH3/H2/air jet flame within a hot coflow (JHC) through numerical simulations. We found that even a small mole fraction of H2 (e.g., XH2, F = 5%) significantly enhances the ignition of the premixed NH3/air flame. However, the presence of H2 markedly intensifies the main combustion reactions only when XH2, F ≥ 30%, resulting in a sharp increase in temperature and heat release rate. Moreover, as XH2, F increases, the NOx emission first increases until XH2, F = 75% and then decreases. Without any H2 addition, a high emission of unburned NH3 occurs in the pure NH3 JHC flame. On the other hand, when XH2, F ≥ 50%, the emissions of unburned H2 and OH increase rapidly due to extremely high temperatures. Notably, maintaining XH2, F between 5% and 20% minimizes the emissions of NOx, NH3, H2, and OH. Furthermore, the preferential diffusion of H2 plays a key role in enhancing the production of active radicals (OH, O, and H), thereby significantly boosting the initiation and combustion reactions of NH3/H2 blended fuel.

Influence of velocity on the transient auto-ignition of ethylene jet in a hot coflow

The difficulty of ignition is one of the main problems in the combustion chamber of the air-breathing combined engine. Based on this, the transient auto-ignition in the turbulent non-premixed flames of ethylene in a high-speed jet in hot coflow was studied using a newly developed burner. High-speed hydroxyl (OH) planar laser-induced fluorescence (up to 10 kHz) and large eddy simulations (LES) were performed to investigate the auto-ignition process. The experimental conditions included injection velocities of 334–491 m/s, and the effect of jet velocity on the mixing process, ignition process, and flame stabilization mechanisms were investigated. The results showed that the numerical simulation results were consistent with the experimental data. The velocity could affect the auto-ignition position, whereas it had no effect on the auto-ignition time. The increase in fuel jet velocity is beneficial to the mixing of fuel and high-temperature gas and helps to increase the premixed combustion zone during the formation of fire kernels. Moreover, as the velocity increased, the flame status changed from a transitional flame to a lifted flame, the growth rates of the flame volume and total heat release increased, and the flame stabilization mechanism changed from flame propagation to auto-ignition.

Iron particles ignition in different hot coflow temperatures

Understanding the ignition of iron particle combustion in hot environments is critical for harnessing the use of metal fuels in clean energy production. In this research work, a new single particle burner is implemented to disperse and burn iron particles using hot air coflow. The burner disperses iron particles through Coulomb forces between two air-spaced capacitor electrodes. The oxidizer coflow is heated up as it passes through a newly designed induction gas heater. Electromagnetic induction is used to heat up multiple metallic and porous heat discs by Joule heating. When the coflow gas passes through the pores of the discs, it carries the heat generated within the porous discs and delivers it to the travelling iron particles which provides ultimate heat transfer effectiveness. Coflow temperature homogeneity is examined using Schlieren imaging and confirmed by thermocouple measurements at the outlet of the coflow tube. Particle ignition and combustion occur in open space, providing excellent conditions for optical diagnostics. Two synchronized high-speed cameras were used to simultaneously determine the particle size using green laser shadowgraphy and detect iron particles ignition. Iron particles ignition experiments were conducted for a sieved patch (45–53 μm) at different hot coflow temperatures starting at 700℃ up to 900℃. The burner showed extensive capabilities to maintains a stable and homogeneous hot coflow for a wide range of temperatures up to 900℃. Ignition experiments showed 5.67 % of ejected particles are burnt at 700 ℃, while full ignition (99.33 %) can be achieved at 900 ℃.

Numerical analysis of the enrichment of CH4/H2 in ammonia combustion in a hot co-flow environment

A numerical investigation is conducted using OpenFOAM to study the characteristics of ammonia flames enriched with methane and hydrogen, influenced by a diluted co-flow (exhaust gas). The study aims to explore various flame characteristics, including fuel consumption, ignition delay time, lift-off height and position, temperature, and CO and NO emissions while maintaining a constant energy input of 5 kW. Non-stabilized flames are identified by analyzing the OH peak in the computational domain among ammonia flames enriched with CH4/H2 for different dilutions of O2 (6 %, 9 %, and 12 %). The study is structured into three stages of numerical simulations corresponding to stabilized flames: (1) no reaction, (2) reaction in the absence of O2, and (3) reaction with O2 to see the sole impact of the hot co-flow and local turbulence mixing on the primary cracking of ammonia. Furthermore, the rates of H2 and N2 formation resulting from the utilization of ammonia in the presence of hot co-flow are thoroughly examined. Additionally, the behavior of ammonia flames and CH4/H2-enriched ammonia flames under preheated air conditions (23 % O2) is analysed. Stable flames are observed in the co-flow at 12 % O2, displaying characteristics of Moderate or Intense Low-oxygen Dilution (MILD) combustion. Conversely, ammonia flames under preheated air conditions exhibit conventional flame behavior. Pure ammonia flames exhibit stronger temperature gradients and higher levels of CO and NO emissions. As the enrichment of NH3 with CH4/H2 increases, peak temperatures decrease while lift-off heights increase, indicating a transition towards more distributed flame structures. Notably, the instance with 40 % H2 enrichment exhibits lower NO emissions and a higher rate of ammonia consumption along with more distributive flame characteristics.

Automated adaptive chemistry for Large Eddy Simulations of turbulent reacting flows

Large Eddy Simulations (LES) of turbulent reacting flows carried out with detailed kinetic mechanisms have a key role for the discovery of the physical and chemical processes occurring in combustion systems, and are essential for the development of efficient, stable, and non-pollutant technologies. Nevertheless, these simulations require a large amount of computational resources, making their utilization for large-scale systems, such as industrial burners and gas turbines, impractical. In this work, we combine state-of-the-art machine learning algorithms and model reduction methods to deliver a fully automated strategy for performing LES with adaptive chemistry. This strategy is based on the Sample-Partitioning Adaptive Chemistry (SPARC) algorithmic procedure, which consists of four steps: the generation of a training dataset, its partitioning in clusters, the generation of a set of reduced chemical mechanisms specifically tailored to each cluster and, lastly, the numerical simulation of the case of interest with adaptive chemistry enabled by an on-the-fly classification of every grid point.The SPARC approach has already been demonstrated to substantially reduce the computational effort of reactive flows simulations. However a non-negligible level of user interventions is needed, upon which the method’s success critically depend. Therefore, with the goal of boosting the performance of this workflow and minimise the user-specified degrees of freedom, we plug in and exploit the Local Principal Component Analysis augmented with an automated Bayesian-optimised search for optimal clustering solutions, and the Computational Singular Perturbation method with an additional layer of automation based on the Tangential Stretching Rate for minimally-sized reduced mechanisms. We employ a cheap and easy-to-generate 1-dimensional-flames training database and we demonstrate the efficiency, accuracy and robustness of this strategy with an application to LES of the Adelaide Jet in Hot Coflow (AJHC) burner, a turbulent reacting flow exhibiting intense turbulence-chemistry interactions.

Effects of coflow velocity on the lift-off characteristics of autoignited jet flame in hot air coflow

This paper aims to understand the effects of coflow velocity on the lift-off characteristics of autoignited jet flame and to develop a lift-off height model that considers such effects. A jet-in-coflow burner was used to conduct autoignition experiments. As the jet velocity increased, three distinctive flame types were observed: attached flame (AF), random spots (RS), and lifted flame (LF). The coflow velocity was found to have an interesting impact on the transition from one type to another. As the coflow velocity increases, the jet velocity needed to transition from AF to RS monotonically decreases, which aligns with our expectation. However, as the coflow velocity increases, the jet velocity required for RS to LF transition initially increases and then decreases. Such a shift implies that the dominant factor of the RS flame stabilization changes from the mixing to small-scale strain. The lift-off characteristics of RS under different coflow velocities were analyzed. With the increase of coflow velocity, the spots number increases non-monotonic, and the average lifetime decreases monotonically, further indicating that mixing and small-scale strain jointly determine the generation and extinction of RS. Moreover, the sensitivity of lift-off height to jet velocity is not only greater than that of coflow velocity but also affected by the coflow velocity. Therefore, considering the coflow velocity's effect, a modified mixing-strain model of RS was developed by improving the calculation of both strain rate and mixing time.

Numerical study on the ignition and combustion of a CH4 jet flame in diluted and undiluted coflows

This study numerically investigates the ignition and evolution of CH4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, YO2,C = 6 % and 9 %) and high temperature combustion (HTC, YO2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base.

Lift-off height model of hydrogen autoignited flame in turbulent hot air coflow

This study aims to develop a lift-off height model suitable for hydrogen autoignited flames in turbulent hot coflow. The differences in flame structure and lift-off characteristics between hydrogen and acetylene flames are compared through experiments. Due to hydrogen's high flame speed, the autoignition spots of hydrogen are undetectable, and the flame zone becomes regular and continuous. Therefore, the flame speed can be used to improve the calculation of the chemical time scale to incorporate the flame propagation factor into the lift-off height model. A novel model is established to consider the combined effects of flame propagation and autoignition on flame stabilization through the Karlovitz number and Damkohler number, respectively. Positive parameters β and 1-β represent the relative contributions of flame propagation and autoignition. The Ka/Da model successfully predicts the variations in the lift-off height of hydrogen flames at various coflow temperatures and fuel mole fractions. In addition, the good correlations based on the Ka/Da model to the lift-off heights of hydrogen, methane, and ethylene flames in the literature prove the model's generality. Unlike hydrocarbon flames (β < 0.5), hydrogen flames' β is bigger than 0.5, indicating that flame propagation plays a more significant role than autoignition in stabilizing autoignited flames.

MILD combustion of a premixed NH3/air jet flame in hot coflow versus its CH4/air counterpart

Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NOx emissions (ENOx) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH3/air jet flame in hot coflow (JHC) under different jet equivalence ratios (ΦJ), coflow temperatures (TC), and oxygen levels (XO2,C). For comparison, similar CH4/air MILD-JHC flame characteristics are calculated. The results show that the NH3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH4. This suggests that the NH3 flame can develop into a MILD regime more easily than CH4. ENOx of the NH3 flame is two orders of magnitude higher than that of CH4 flame at all TC and XO2,C values for ΦJ < 1. At ΦJ > 1, ENOx of the NH3 flame rapidly decreases to nearly zero, but unburned NH3 emissions (ENH3) and H2 emissions (EH2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, ENOx increases rapidly (gradually) with XO2,C (TC), whereas ENH3 and EH2 are considerably low at TC ≥ 1300 K and XO2,C ≥ 1% for the NH3 flame. ENOx primarily originates from NO; however, N2O is also important at low ΦJ and TC values. Therefore, considering ENOx, ENH3, EH2 and the burning efficiency, the optimal conditions for NH3 MILD combustion are ΦJ = 1, TC ≥ 1500 K, and XO2,C = 1%–3%. In addition, compared with conventional flames, NH3 MILD combustion can reduce ENOx by one to two orders of magnitude.

Instability Behaviors and Suppression of the Unsteady Autoignited Turbulent Jet Flame in Hot Coflow

ABSTRACT Autoignition widely exists in combustion devices that feature hot air. The unsteady autoignited flames caused by autoignition are potential hazards to the stable operation of combustion systems, often causing flameout or damage to the combustion devices. In this paper, we present our recent work in understanding the key factors determining the instability of autoignited turbulent jet flames and a novel method to suppress flame instability. Firstly, we studied the flame using a jet-in-hot-coflow burner. The instability behaviors were characterized using a pressure probe and a high-speed camera. When the fuel mole fraction was below a critical value, the amplitude spectrums of the unsteady autoignited flame had low-frequency and high-frequency peaks. The experimental results show that the low-frequency peak is related to the autoignition intermittency and can be eliminated by enhancing chemical reactions. The high-frequency peak is related to the autoignition frequency, and the peak amplitude can be reduced by improving reactants mixing. Considering that the heating rod can enhance the reaction as a heat source and improve mixing as a bluff body, we inserted an electric heating rod into the autoignition spatial region to suppress the flame instability. The suppression of flame instability by heating rod at different temperatures and locations were experimentally studied. The heating rod insertion can significantly reduce low-frequency and high-frequency instability. Moreover, increasing the rod temperature can also effectively reduce the amplitude of high-frequency pressure pulsation. The results show that the electric heating rod is an efficient method to suppress the instability of autoignited flame.

Effect of the primary oxidizer stream jet velocities on the ignition and combustion characteristics of pulverized coal under MILD oxy-coal combustion conditions

MILD oxy-coal combustion technology is a prospective technology with the potential for CO2 capture and storage, and it is characterized by the intense flue gas recirculation to preheat the fuel and to dilute the oxidizer due to the oxidizer or fuel jet with high velocity. The effect of the primary oxidizer stream jet velocities ranging from 1.7 m/s to 11.2 m/s on the coal ignition and combustion characteristics were experimentally investigated on a flat flame pulverized coal burner at various oxygen concentrations of 5%-21% and gas temperatures of 1473–1873 K under O2/CO2 and O2/N2 atmospheres. The coal flame images were recorded by the digital camera, and the flame structure evolution and ignition delay times of coal particles were analyzed at different primary oxidizer stream jet velocities and hot coflow conditions. The coal combustion radiation spectrum was monitored by an optical fiber spectrometer and the coal particle temperatures were derived with the two-color pyrometry. The results showed that the bright yellow soot flame gradually disappeared with the decrease of O2 concentration when CO2 replaced N2. The difference of ignition delay times between O2/CO2 atmosphere and O2/N2 atmosphere decreased from 2.8 ms to 0.2 ms when the primary air jet velocity increased from 1.7 m/s to 11.2 m/s at 1473 K and 5% O2. This meant the elevated jet velocity was beneficial to offset the delayed effect of the physicochemical properties of CO2 on coal ignition. The peak coal particle temperature increased by 68 K at 1873 K and 5% O2 under O2/CO2 atmosphere with the highest jet velocity owing to the accelerated diffusion of O2 into coal particle stream. Moreover, a dimensionless coal particle temperature parameter η was proposed to characterize the variation of the peak coal particle temperature caused by the improved heat transfer and mass transport between the coflow and coal particles under high jet velocity. The maximum value of η was 1.053 at 1873 K and 21% O2 under O2/CO2 atmosphere with the jet velocity of 11.2 m/s, indicating the increased jet velocity had the predominant effect on the increase of peak particle temperature at high gas temperature and O2 concentration.

Numerical study on pollutant emissions characteristics and chemical and physical exergy analysis in Mild combustion

This analytical study appraises the emission characteristics of different pollutants such as CO and NOX as well as conducting chemical and physical exergy analyses on conventional and Mild combustion regimes. The RANS turbulence approach and the k-ε RNG model were utilized in this study to simulate turbulence, whereas the eddy dissipation concept (EDC) model and the GRI–Mech 2.11 chemical mechanism were adopted to treat the turbulence-chemistry interaction. This study evaluated local and global pollutant emissions and exergy efficiency along the reacting jet and explored the effects of temperature and chemical species on the physical and chemical exergies in MILD combustion in comparison to conventional combustion. It is found that the overall exergy efficiency of MILD combustion rises by 1, 3, and 4% for oxygen mass fractions of 3, 6, and 9% in hot oxidizer coflow, with the NOx emissions are 81, 72, and 55% lower than conventional combustion, respectively. Chemical exergy is greater than physical exergy near the fuel nozzle due to high CH4 and H2 concentrations. Chemical exergy reduces along the burner due to chemical reactions, while physical exergy increases due to the increased temperature. However, owing to the trade-off between these two exergies along the flame jet, the total exergy approaches a constant value at large distances from the burner. The maximum total exergy occurs at a distance of 0.12 m from the fuel nozzle, with chemical exergy accounting for 83% of the total exergy. In the outlet (0.5 m from the fuel nozzle), chemical exergy accounts only for 30% of the total exergy. It can also be concluded that the exergy is stabilized at a distance of 0.3 m from the nozzle (60% of the domain length). Despite a higher local CO emission in conventional combustion than in MILD combustion, the CO emission substantially decreases at a smaller distance from the nozzle in conventional combustion due to the high O2 concentration.

Tabulated Chemistry Approach for the Simulation of MILD Combustion: Effects of Scalar Mixing and Chemistry Tabulation.

MILD (moderate and intense low-oxygen dilution) combustion is a highly promising technology to deliver clean and efficient thermal energy. However, because of its unconventional reaction nature, the optimization of the MILD combustion in various industrial burners is still challenging, for which the design tool based on accurate and cost-effective numerical simulation is most desirable. To this end, the tabulated chemistry approach (TCA) is thoroughly assessed for the modeling of MILD combustion by simulations of the Adelaide Jet in Hot Coflow (JHC) burner. The sensitivities to the submodel accounting for the scalar micromixing and the canonical flame configurations (i.e., flamelet and PSR-based reactors), being relevant for MILD regime characterization in TCA, are studied. It is found that the scalar mixing enhanced through the dynamic adjustment of model parameter C s leads to an improved prediction, and the optimal value of C s = 8 is identified for the current flames. The proper parametrization of the detailed chemical structures in TCA is found to affect the accurate prediction of the MILD flame profiles, especially the mass fraction of minor species (e.g., OH, CO). Furthermore, the endothermic reaction path of O + C2H2 ⇒ CO + CH2 is indicated as the main contributing step to the disparities in the CO predictions. This implies that the multiple reaction regimes in complex MILD burning should be accounted for by the use of either flamelet or PSR structures, depending on the local microscale diffusion/chemistry competitions. Overall, the results highlight the influential role of multiscale mixing and its intercoupling with the finite-rate chemistry, the accurate determination of which is important for MILD combustion modeling.

Characterisation of hydrogen jet flames under different pressures with varying coflow oxygen concentrations

The elevated temperature of hydrogen combustion increases the formation of thermal NOx. Moderate or intense low oxygen dilution (MILD) combustion is known to reduce NOx emissions and increase thermal efficiency. Pressure is often also used for increasing thermal efficiency. The impact that pressure has on fluid dynamics and chemical kinetics is especially relevant in MILD combustion conditions. Hydrogen jet flames issuing into a hot and vitiated coflow were imaged using OH∗ chemiluminescence at different pressures (1–7 bar) and oxygen levels (3–9% by vol.). Laminar flame simulations complemented the experiments. The observed mean radial OH∗ width increased with increased pressure, but only at O2 content less than 9%, suggesting that pressure has greater influence on kinetics when oxygen is reduced. The integrated OH∗ signal strength remained constant at 3% coflow O2, despite an apparent increase in flame width, suggesting a spatial broadening of the flame with pressure. Numerical results indicate that at 3–6% O2, conditions for MILD combustion of H2 are met across a wide range of strains and pressures, supporting the experimental observations for 3% O2.

Numerical and Chemical Kinetic Analyses on the Formation of CO and CO2 for C1–C4 Hydrocarbon Alkanes in a Hot Co-Flow under MILD Combustion

In this present study, computational fluid dynamics and chemical kinetic analysis are conducted to explore the formation of CO and CO2 for a fuel jet comprising C1–C4 hydrocarbon alkanes in a hot co-flow burner under a moderate or intense low-oxygen dilution (MILD) combustion environment. The fuel jet comprises three cases of fuel mixture (case 1: CH4 + H2, case 2: C3H8 + H2, and case 3: C4H10 + H2) having constant mixture density and operating under a fixed jet Reynolds number. The combustion and emission characteristics are analyzed in terms of radial profiles of temperature and mass fraction of OH, CH2O, HCO, CO, CO2, and C2H2 in the MILD combustion region. The formations of CO and CO2 are examined with the help of reacting flow analysis using the computational fluid dynamics (CFD) tool. The formation of CO is reported as maximum for case 3 and minimum for case 1. However, the reverse trend is observed in the case of CO2 formation. The reacting flow analysis from the CFD work suggests that the reaction C2H2 + O = CH2 + CO strongly influences CO formation in all three fuel mixtures cases. In the current study, acetylene is found to be the primary species that affects the CO formation in the adopted fuel mixtures and is mainly responsible for the increased CO concentration in case 3. Likewise, the formation of CO2 is majorly influenced by the reaction CO + OH = CO2 + H. The reacting flow analysis is assisted with the help of a zero-dimensional perfectly stirred reactor (PSR) model to elucidate the underlying chemistry and explore the significant reaction pathways influencing the formation of CO and CO2. The ethylene route in the reaction pathway substantially plays a vital role in the CO formation in all three fuel mixtures.