Laminar Flame Propagation Research Articles

Three-dimensional dynamics of unstable lean premixed hydrogen-air flames: Intrinsic instabilities and morphological characteristics

The 3D swinging laser sheet technique was employed to study the development and morphological characteristics of premixed hydrogen-air unstable flames in a spherical explosion vessel. Pressure dependencies for laminar flame propagation were sought to exploit the role of the Darrieus-Landau (DL) and Thermal-diffusive (TD) instabilities in the unstable self-accelerating flame regime. A sufficiently low Markstein number, as a consequence of the increased pressure, leads to more cracking and smaller cells over the flame surface. The degree of wrinkling on the flame surface is proportional to the increase in flame burning velocity, a relationship that holds true for low pressures but is not applicable under high pressures. External turbulence can significantly alter the extent of flame surface wrinkling even at low root mean square velocities, producing a more wrinkled flame surface compared to intrinsic cellularity, and distinctly affecting flame dynamics. The increased wrinkling and flame speed due to external turbulence can be attributed to the synergistic effects between thermo-diffusive instabilities and turbulence, resulting in higher fuel consumption rates per flame surface area and the formation of finger-like structures that enhance flame displacement speed in curved segments. The parameters, ϵ, deviation of the Lewis number from a critical value, and ω2, obtained through classical linear stability analysis, display a clear linear relationship with the ratio of the wrinkled surface area observed in planar flames. This study enhances the understanding of hydrogen flame instabilities, which is crucial for preventing explosions in hydrogen storage and utilization, and provides valuable insights into flame dynamics, supporting the design of safer and more efficient hydrogen-fueled engines and turbines.

COMBUSTION OF HYDROGEN IN OXYGEN-STEAM MIXTURE FOR INCREASING THE STEAM TEMPERATURE OF POWER PLANTS

In the work the problems that arise at combustion of hydrogen in oxygen-steam mixture for the purpose of combustion products (steam) mixing for heating steam, which is planned to be used in steam turbines of power plants were researched. The main problem is the formation of underburning H2 and, accordingly, the presence of O2, which have a negative effect on metals, and it will also prevent steam condensation in the condenser was determined. For solve this problem, calculations of the equilibrium concentrations of chemical reaction products in the combustion zone depending on the amount of ballast steam added to the oxidizer (oxygen) for the initial conditions: T0=528 K, p=0.1 MPa; T0=​​528 K, p=3.1 MPa; T0=​​584 K, p=10 MPa were made. The corresponding adiabatic temperatures were calculated. The dilution of the oxidant (oxygen) with steam significantly decreases the adiabatic temperature (Tb) and reduces the equilibrium concentrations of other substances in the combustion zone, but at the same time the laminar flame propagation velocity (SL) also significantly decreases was established. It is important when a certain concertation of ballast steam is achieved (the final percentage is determined by the design of the burner) there will be a sharp deterioration of combustion or even the formation of a flame will be impossible. The principal design of the hydrogen-oxygen-steam combustion chamber was proposed. The necessity of heating oxygen and hydrogen and the principle of determining the pressure under which it is advisable to supply oxygen and hydrogen to ensure the maximum intensification of mixing first of oxygen and steam, and then of the formed mixture and hydrogen, were substantiated. Bibl. 20, Fig. 5, Table 3.

Laminar burning velocity, radiative heat loss and cellular instability for NH3/biogas/H2/air premixed flame

This study presents, for the first time, a comprehensive investigation into the laminar flame propagation of ammonia/biogas/hydrogen/air mixtures. The laminar burning velocities (LBVs) were determined using the outwardly propagating spherical flame method under atmospheric temperature and elevated pressures across various equivalence ratios. The Okafor and CEU–NH3–Mech-1.1 mechanisms showed the best agreement with flame speed measurements. The influence of equivalence ratio, hydrogen ratio, and initial pressure on LBV was examined. Results indicate that LBV varies non-monotonically with the equivalence ratio, decreases with increasing pressure, and increases with higher hydrogen ratios. The enhancement of LBV due to hydrogen addition is mainly attributed to chemical effects, with thermal effects also playing a role. Given the significant CO2 content in biogas, which is a key radiative species, the study also analyzed the radiative heat loss of ammonia/biogas/hydrogen mixtures. It was found that as the equivalence ratio nears the flammability limit, LBV is more affected by radiative heat loss, with a lower hydrogen ratio increasing this effect. Additionally, higher CO2 mole fractions in biogas increase radiative heat loss. Detailed analysis of cellular instability, considering factors such as thermal expansion ratio, flame thickness, effective Lewis number, the logarithmic growth rate of perturbations and in conjunction with schlieren images, revealed that flame instability is enhanced with higher initial pressure and hydrogen ratios, while higher equivalence ratios inhibit instability.

Numerical Simulation and Field Experimental Study of Combustion Characteristics of Hydrogen-Enriched Natural Gas

For the safe and efficient utilization of hydrogen-enriched natural gas combustion in industrial gas-fired boilers, the present study adopted a combination of numerical simulation and field tests to investigate its adaptability. Firstly, the combustion characteristics of hydrogen-enriched natural gas with different hydrogen blending ratios and equivalence ratios were evaluated by using the Chemkin Pro platform. Secondly, a field experimental study was carried out based on the WNS2-1.25-Q gas-fired boiler to investigate the boiler’s thermal efficiency, heat loss, and pollutant emissions after hydrogen addition. The results show that at the same equivalence ratio, with the hydrogen blending ratio increasing from 0% to 25%, the laminar flame propagation speed of the fuel increases, the extinction strain rate rises, and the combustion limit expands. The laminar flame propagation speed of premixed methane/air gas reaches the maximum value when the equivalence ratio is 1.0, and the combustion intensity of the flame is the highest at this time. In the field tests, as the hydrogen blending ratio increases from 0% to nearly 10% with the increasing excess air ratio, the boiler’s thermal efficiency decreases as well as the NOx emission. This indicates that there exists a tradeoff between the boiler thermal efficiency and NOx emission in practice.

Turbulent partially cracked ammonia/air premixed spherical flames

The combustion of ammonia requires, for most energy conversion systems, a combustion promoter such as hydrogen to guarantee the start-up, stability and combustion efficiency. Partially cracked ammonia (PCA) can provide sufficient hydrogen concentrations to enhance the burning velocity in comparison with pure ammonia. However, little work exists on the use of PCA blends operating under relevant turbulent conditions. To that end the outwardly propagating spherical flame configuration was employed to determine the laminar and turbulent flame propagation characteristics of PCA (NH3/(H2+N2)) and corresponding binary (NH3/H2) mixtures across various turbulent combustion regimes. First, PCA and ammonia-hydrogen blends exhibit similar flame propagation rates under various turbulent intensities, even for the laminar case. The highest turbulent burning velocity was observed at leanest conditions, as opposed to laminar flames which exhibited highest flame speed at conditions above stoichiometry. Under rich conditions, no substantial flame enhancement due to turbulence was measured irrespective of the hydrogen content. This lack of flame enhancement under turbulent conditions is attributed to the effect of preferential diffusion with good agreement observed with trends in measured Markstein numbers. The normalized turbulent flame speed is dominated by the enhanced molecular diffusivity afforded by the presence of hydrogen up to 15 % enrichment, prior to decreasing upon further hydrogen addition under lean and stoichiometric conditions. This ‘bending’ phenomenon may be the contribution of several factors including; the transitioning between combustion regimes associated with low Damköhler numbers (Da) and flame thickening; merging of flamelets due to the presence of ammonia enhancing wrinkling; and combined changes in laminar burning velocity and preferential diffusional behavior. Furthermore, good agreement for turbulent flame speed is observed with a correlation that includes the influence of turbulent stretch (Ka) and non-equidiffusion (Le), with the agreement reducing with decreasing chemical to turbulent time scale ratios (Da << 1).

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Co-firing ammonia and hydrogen with butane under methane-equivalent calorific value and Wobbe index: Insights into transition in flame propagation and swirl flame characteristics

Fuel flexibility stands as a critical requirement for the development of combustors in future carbon-free energy systems. Carbon-free fuels including ammonia (NH3) and hydrogen (H2) are substantial supports for achieving this target, raising strong needs to counterbalance their distinct fuel thermochemical properties from methane (CH4), such as volumetric calorific value (CV) and Wobbe index (WI). In this work, butane (C4H10) which is an emerging renewable synthetic fuel is proposed to co-fire NH3 and H2 under methane-equivalent calorific value (MECV) and Wobbe index (MEWI). Laminar flame propagation of NH3/H2/C4H10 mixtures under MECV and MEWI are investigated in a cylindrical constant-volume combustion vessel. C4H10 co-firing can simultaneously tune the fuel thermochemical properties to CH4 levels and regulate the laminar flame propagation of NH3 and H2. Kinetic simulation and modeling analysis are performed to provide insights into the dominant kinetics and transition in laminar flame propagation. CV-compensated transition variables can generally well constrain the transition profiles of laminar burning velocity (LBV), while WI-compensated transition variables can linearize the transition profile from NH3 to C4H10 but meet challenges in constraining the transition profiles of H2-blending mixtures. Chemical effects are found to play the dominant role in the transition of laminar flame propagation, while thermal effects also have positive contributions. Test experiments in a gas turbine model combustor are also conducted to assess the swirl flame characteristics of NH3/H2/C4H10 mixtures under MECV and MEWI. The measured swirl flame morphologies and lean blowout limits indicate enhanced fuel reactivity and flame stability with the transition in fuel content, which is in accordance with the observed trends in LBV. Furthermore, comparison with CH4 provides a comprehensive evaluation of NH3/H2/C4H10 mixtures from the aspects of combustion characteristics and thermochemistry.

Hydrogen concentration measurements using spark induced breakdown spectroscopy in a real engine

This study develops spark induced breakdown spectroscopy (SIBS) for measurements of direct injected hydrogen concentration in a multi-cylinder engine operated at real conditions. To this end, calibration is performed in the air excess ratio (λ) range of 2.1 ∼ 2.9 and simultaneously, high-speed schlieren imaging of laminar hydrogen flame propagation is performed in an optical high-pressure constant volume combustion chamber (CVCC). To achieve real engine applications, the SIBS is further developed using a modified conventional spark plug for sapphire window insertion onto the positive electrode and guided arc formation via a newly designed nipple type ground electrode. Specifically, the peak spectra intensity ratio of Hα (656 nm) to O (777 nm) is used for calibration, which shows a linear correlation with λ. For the first time, the calibrated SIBS is applied to a real engine operated at 46.2 ∼ 63.9 Nm load and 2000 rpm with 3.5 MPa hydrogen direct injection. The results show lower local λ than the global λ despite early injection timing of 165 crank angles before top dead centre, indicating a significant influence of in-cylinder flow motion on the spatial distribution of hydrogen-air mixtures.

Comparative analysis and optimisation of hydrogen combustion mechanism for laminar burning velocity calculation in combustion engine modelling

Hydrogen stands out as a compelling alternative to fossil fuels for combustion engines. Predictive combustion models are instrumental in developing hydrogen-fuelled engines. A fundamental metric of these models is the laminar burning velocity (LBV), which can be precisely determined through laminar flame propagation simulations. In this context, the selection of an appropriate combustion mechanism is critical. This paper aims to propose the appropriate combustion mechanism for calculating LBV in predictive combustion models of hydrogen-fuelled engines. 15 state-of-the-art combustion mechanisms were applied to reproduce the LBV measurements in engine-like conditions, especially considering the application of lean combustion and water injection. The FFCM 1.0 mechanism was identified from them and further optimised to improve its prediction accuracy at elevated pressures for the lean mixture. The maximum deviation of LBV was reduced from 17.6 % to 8.7 % by this optimisation, in comparison to 10.5 % for its closest competitor mechanism, ELTE (Varga et al., 2015).

Exploration of NH3 and NH3/DME laminar flame propagation in O2/CO2 atmosphere: Insights into NH3/CO2 interactions

Cofiring with reactive hydrocarbon and oxygenated fuels is an effective approach to enhance ammonia (NH3) combustion, which raises growing interest in understanding CN interactions. Among these fuels, dimethyl ether (DME, CH3OCH3) has attracted extensive attention due to its renewable nature and potential for large-scale synthesis from CO2 feedstock. This work reports an experimental and modeling investigation on the laminar flame propagation of NH3 and NH3/DME in O2/CO2 atmosphere, with an emphasis on evaluating the enhanced NH3/CO2 interactions under flame conditions. Laminar burning velocities (LBVs) were measured at elevated pressures up to 5 atm in a constant-volume combustion vessel. A kinetic model was developed and can well reproduce the measured LBVs in this work, as well as the LBVs and speciation data in literature. Modeling analyses and the modified fictitious diluent gas method were used to explore the effects of DME cofiring, pressure and CO2 dilution on LBVs. The results reveal that the chemical effect plays a crucial role in promoting the laminar flame propagation of NH3 with DME cofiring, while the reduction of LBVs with CO2 dilution is predominantly influenced by the thermal effect. LBVs of NH3/DME present a stronger pressure dependency than that of pure NH3, which is attributed to the enhanced pressure-sensitive three-body reactions, including CH3 + H (+M) = CH4 (+M) and H + O2 (+M) = HO2 (+M). Phenomenological analysis of LBVs was conducted by varying the diluent gas from pure N2 to pure CO2, complemented by modeling analyses with adjusted rate constants, to provide insights into NH3/CO2 interactions. The results show that CO2 reduction reactions, including NH + CO2 = HNO + CO and CO + OH = CO2 + H, compete for reactive radicals like H and NH with chain-branching reactions H + O2 = OH + H and consequently prohibit the laminar flame propagation.

Effect of Impurities on Lean Laminar Hydrogen–air Flames

Simulations of the effect of addition of H, O, OH, HO2, and H2O2 on the structure and propagationof laminar flames in lean (12 and 15%) hydrogen-air flames are performed at pressures of 1 and 6 bar. Itis found that impurities in concentrations of no more than 0.1% do not have any significant effect on laminarburning velocity. When initial temperature is increased to 400 K, the effect of impurities becomes evenweaker. Among the impurities under study, only the addition of OH reduces the laminar flame velocity. Theweak effect of the impurities is attributed to fast formation of intermediate products via reactions involving Oand H atoms without noticeable change in heat release rate. An increase in initial pressure to 6 bar does notchange the effect of impurities.

On accelerative propagation of premixed hydrogen/air laminar and turbulent expanding flames

The accelerative propagation of laminar and turbulent premixed hydrogen/air flames are investigated using a constant volume combustion chamber. The relative flame accelerative characteristics are analyzed under different φ, P, and u’. The results show that three distinct stages, namely quasi steady, transition acceleration and saturation acceleration stages are distinguished in laminar flame propagation process, and the onsets of saturation acceleration and transition acceleration nearly satisfy the relationship of Pecr2/Pecr1≈2. The temporal acceleration exponents (αR) of laminar flames increase quickly in transition acceleration stage and nearly remain as a constant (1.1–1.25) in saturation acceleration stage. The acceleration exponents (α) of laminar flames are weakly dependent on flame conditions once the cellular structures are settled. The turbulent flame propagation follows a continuous acceleration law as SF/SL,b∼Pedt and no evident transition point is observed. The temporal acceleration exponents of turbulent flames continuously increase as flame propagates outwardly and are susceptible to turbulence intensity. The acceleration parameters (dt) of turbulent flames continuously increase with u’/SL in different flame regimes and the growth trend becomes slow in corrugated flamelets regime. Besides, the dt of turbulent flames are promoted in the fuel-lean conditions. All of these indicate that the accelerative propagations of turbulent H2/air flames are dominated by turbulent stretch and the diffusive-thermal instability still plays a promotion role.

Flammability Properties of the Pyrolysis Gas Generated from Willow Wood

Willow wood presents a real interest for biomass pyrolysis due to its fast growth, renewability, and high energy density. Following the pyrolysis of willow wood in an inert atmosphere, a multi-fuel gaseous mixture was obtained, with the following composition (by volume): 38.20% CO/21.87% H2/17.44% CH4/1.15% O2/17.15% CO2/4.19% N2. The propagation of laminar premixed flames in these multifuel mixtures with air was investigated numerically for initial temperatures from 298 to 500 K, initial pressures from 1 to 20 bar, and fuel equivalence ratios between 0.60 and 2.00. The combustion of these gaseous mixtures as free laminar premixed flames was simulated by means of the INSFLA package and an extended reaction mechanism with 592 elementary reactions and 53 species. The modelling of the gas-phase combustion delivered several important parameters: the laminar burning velocities, Su, the maximum flame temperatures, Tfl,max, the flame front thicknesses, dfl, and the peak concentrations of the main reaction intermediates. The obtained parameters, discussed in correlation with the initial pressure and temperature, afforded the determination of the overall activation parameters of multifuel oxidation with air.

Self-Acceleration and global pulsation of unstable laminar Hydrogen-Air flames

The self-acceleration and global pulsation of spherically expanding laminar hydrogen-air flames were studied in a spherical explosion vessel, over a wide range of equivalence ratios (0.4–2.0), initial temperatures (300–400 K), and initial pressures (0.1–0.7 MPa). Comprehensive quantitative data on the self-acceleration propagation of unstable laminar hydrogen-air flames were obtained. The results show that the self-similarity appears after the onset of instability, and the previous assertion of acceleration exponent α = 1.5 is not valid for hydrogen-air flames. The derived values of α for such flames vary from 1.125 and 1.39 in the present work. For the self-accelerating flame, the temporal evolution of the flame radius is described by ru=Atα, and temporal flame propagation speed is described by Sn=Aαtα-1. A modified theoretical expression of the constant A is proposed and validated against the present experimentally derived results. The acceleration phase after flame instability presents a global pulsating acceleration pattern. The frequency of global pulsation rises as the pressure or temperature of the flame increases. A pulsing flame speed equation can be used to reliably estimate flame speed after the onset of cellular instability. Self-acceleration observed in small laboratory explosions can serve as a predictive indicator for flame behaviour on a larger atmospheric scale.

Optical experiment study on Ammonia/Methanol mixture combustion performance induced by methanol jet ignition in a constant volume combustion bomb

Ammonia as an alternative fuel has shown great potential in reducing carbon dioxide emissions. However, the issues of high ignition temperature and unstable combustion of ammonia limit its extensive application. As an ignition energy enhanced method, the ignition chamber (pre-chamber) seems to be an effective solution to improve the combustion performance and extend the ammonia lean flammability limit. In this study, the methanol was provided as relatively high activity fuel to improve the mixture activity in the ignition chamber and main chamber. The mixture activity control method was introduced to accelerate ammonia premixed combustion. The visualization experiment compounded with shadow method was applied to investigate. Results showed that the methanol fueled ignition chamber as jet ignition model reduced the combustion duration by almost half compared to spark ignition method. The methanol jet flame propagation formatted multiple zones of ignition and combustion in the main chamber. The ammonia combustion speed shifted from being determined by laminar flame propagation speed to being determined by jet velocity. The methanol jet velocity is related to the methanol energy substitution ratio and nozzle diameter of the ignition chamber. A 2 % methanol energy substitution ratio is optimized for the lean condition of Φ = 0.8 compared to 1 % methanol energy substitution ratio achieve for the condition Φ = 1.0 in the main chamber. When 50 % methanol (energy) was added in the main chamber, the activity thermo-atmosphere improvement induced the combustion duration to decrease by 43.8 % and 51.9 % at equivalent ratios of 1.0 and 0.8, respectively, compared to pure ammonia. The small nozzle diameter of ignition chamber can improve the jet velocity, however it caused the methanol combustion flame quenching leading to unstable combustion phenomenon. A 3 mm nozzle diameter achieved a short combustion duration compared to 4.5 mm and 6 mm nozzle diameter.

Effect of diluent N2 addition on NH3/H2/air combustion characteristics

As a clean energy source with high-energy hydrogen carriers, ammonia has a promising application prospect in gas turbine power production. The existence of N2 as an ammonia combustion product and as an ammonia decomposition product at high temperatures influences ammonia combustion characteristics. Accordingly, this paper measured the combustion characteristics of the NH3/H2/air flame by the method of a constant-pressure spherical flame under different N2 dilution ratios and different equivalent ratios (0.7–1.5), with initiating temperatures of 303 K and 500 K and an initiating pressure of 1 atm, respectively. The experimental results show that the influence of diluent N2 leads to a downward trend in the laminar burning velocities (LBVs) of the NH3/H2/N2/air mixture, and the dilution effect inhibits the propagation of the NH3/H2/N2/air flame. The laminar flame propagation of the mixture is aided by the initiating temperature increase. The existence of diluent N2 reduces the major branching reactions’ net reaction and net heat production rates while enhancing inhibition of the H/OH radical pool and the rate-limiting effect of the primitive reactions. The initiating temperature reduces the impact of the radical reactions on the flame rate and enhances the chemotaxis of NHi (i = 1, 2). The diluent N2 changed the NH3 consumption and effectively suppressed the NOx production, showing the NO emission potential of NH3/H2/N2/air mixtures combustion under fuel-rich conditions.

Experimental study of laminar and turbulent spherical flame propagation characteristics of hydrogen-rich syngas

Hydrogen-rich syngas, produced from biomass-based feedstocks, is a promising alternative energy source. However, its combustion characteristics are completely different from those of typical hydrocarbon fuels due to the presence of hydrogen. As a result, the kinetic mechanism of hydrogen-rich syngas premixed flame (HRSPF) propagation is very complex. In this work, the propagation of HRSPF was investigated employing different fan speeds (0–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0 bar–3.0 bar), and equivalence ratios (0.6–1.0). It was found that the propagation speed of turbulent HRSPF increased when the mixing process of turbulent flow was enhanced or the stretching effect of flame front was weakened. When the turbulence intensity increased, the effect of flame inherent instability on the propagation speed of HRSPF was weakened. When the relative changes of thermal diffusion and mass diffusion of flame front were considerable, the effect of diffusional-thermal instability on flame acceleration process was more significant. On the contrary, the effect of hydrodynamic instability on flame acceleration process was more significant.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.

A comprehensive experimental and kinetic modeling study of laminar flame propagation of ammonia blended with propene

Enhancement of ammonia reactivity is crucial for potential applications of ammonia as an engine and gas turbine fuel. A common strategy for improving ammonia's poor reactivity is blending it with more reactive fuels like hydrogen and methane. However, fundamental studies of ammonia combustion with higher hydrocarbons and key intermediate oxidation species of higher alkanes such as propene do not exist. Thus, this work presents an effort to study the laminar flame propagation of ammonia blended with propene. Laminar burning velocity (SL) of NH3/C3H6/air mixtures was measured at 298 K, pressures up to 5 bar, equivalence ratios of 0.7 to 1.3, and various propene to ammonia ratios (i.e.,% propene to ammonia mole fraction, xC3H6 = 10 to 50) in a high-pressure spherical propagating flame vessel. A kinetic model was developed based on our previous work to characterize the combustion behavior of NH3/C3H6/air mixture. The model reasonably agrees with the experimental data and follows the observed trends very well. The results showed that blending NH3 with C3H6 positively enhanced SL of NH3 by promoting the formation of key radicals e.g., O, OH, and H. Relative to a neat ammonia/air mixture, co-firing ammonia with propene leads to a reduced pressure dependence of the laminar burning velocity. However, the reaction H + O2(+M)=HO2(+M) leads to strong pressure dependency of lean NH3/C3H6 mixtures compared to rich mixtures. The model reveals that besides fuel-NO coming from NH3, prompt NO also actively contributes to NO formation. It is seen that N2O formation is significantly suppressed with increasing pressure or increasing C3H6 content in the fuel blend. In contrast to NO and N2O, NO2 concentration increases slightly with an increase in pressure. The reported experimental data and model will be useful in understanding the interaction between NH3 and alkenes.

Experimental and theoretical investigation of ethyl methyl carbonate/air flames: Laminar burning velocity and cellular instability at elevated pressures

Ethyl methyl carbonate (EMC) gained great attention because of its advantages shown in practical applications both as a blend with conventional oil-derived liquid fuels, and as a solvent being used in Li-ion cell electrolyte compositions. Thus, a better knowledge of the combustion characteristics of EMC/air premixed flames is necessary, which has not yet been well-explored. Towards this goal, the laminar flame propagation of EMC/air mixture was investigated using a constant-volume combustion chamber at the initial pressures of 1, 2, 5 and 8 atm, with the unburned temperature of 423 K and the equivalence ratios ranging from 0.7 to 1.5. A recently established chemical kinetic mechanism of EMC by Takahashi et al. (Combust. Flame 2022) was adopted for the theoretical prediction, which accurately reproduced the measured LBVs at 1–5 atm; however, it underpredicted the LBVs for the lean flame condition at 8 atm. According to the flame instability analysis, the increase in pressure leads to the elevated hydrodynamic instability and the decreased critical radius of spherical premixed flame. Meanwhile, the rich flames of EMC/air had the smaller critical Peclet numbers and suffered the more severe thermal-diffusive instabilities.