Stratified Combustion Research Articles

CFD unified approach under Eulerian–Lagrangian framework for methanol and gasoline direct injection sprays in evaporative and flash boiling conditions

Innovative synthetic fuels for advanced propulsion systems, such as methanol and ammonia, and synthetic blended fuels (E00, E10, and E30), known for their high volatility, are often injected directly into combustion chambers. It follows that Eulerian–Lagrangian spray models need to accurately capture the spray collapse as a consequence of flash boiling onset and be capable of proficiently handling the preferential evaporation of multi-component fuels in evaporative scenarios.So, we performed the assessment of an Eulerian–Lagrangian CFD code for simulating methanol and E00 gasoline blend sprays in both early and late injection conditions involving flash boiling conditions and preferential evaporation. The adoption of an effervescent breakup model and of a non-equilibrium phase transition model for the discrete phase allows the adoption of a setup that is almost completely free from specific constant tuning, especially for what concerns the breakup model. We validated the simulations using experimental PLV maps of methanol and E00 sprays issued from the ECN Spray M injector. The results highlight a significantly different morphology of the methanol spray compared to the E00 one under late injection conditions. Under stratified combustion, low-volatile fuels are likely to be ignited first, and the flame propagates toward the high-volatile fuels. The spray collapse was also correctly reproduced, inducing the presence of a low-pressure zone and modifying the spray morphology.

Exploring energy utilization of the methanol/dimethyl ether dual-fuel engine under the methanol reforming strategy: A comparison of different low-temperature combustion modes

Numerous studies have explored methanol and dimethyl ether (DME) as alternative engine fuels to enhance thermal efficiency and lower emissions. In this study, based on methanol reforming, a new potential strategy for the realization of the methanol/DME dual-fuel compression-ignition engine by fueling only methanol is proposed. The methanol/DME dual-fuel engine under three low-temperature combustion modes (i.e., reactivity controlled compression ignition (RCCI), reverse-reactivity controlled compression ignition (R-RCCI), and homogenous charged compression ignition (HCCI)) is computationally optimized, and the methanol reforming ratio is considered. The results demonstrate that the R-RCCI mode displays enhanced performance in both combustion efficiency and emissions, achieving an equivalent indicated specific fuel consumption (EISFC) of 152.3 g/kWh and nitrogen oxide (NOx) emissions far below the Euro VI regulatory threshold. A double injection of methanol with a minor secondary injection in the R-RCCI mode facilitates more pronounced stratified combustion, and the ringing intensity (RI) can be well controlled. The mode-specific optimal DME energy ratio was determined, approximately 40 % for RCCI and R-RCCI, and around 14 % for HCCI. The demand for the DME energy ratio can be realized by the onboard methanol reforming system, which demonstrates the feasibility of the combination of the fuel reforming strategy with low-temperature combustion modes.

Numerical Study on the Potential of Stratified Mixture to Improve Thermal Efficiency and Reduce Carbon Emissions in High-Speed Gasoline Direct Injection Engine

Abstract Stratified combustion improves the indicated thermal efficiency (ITE) of gasoline direct injection (GDI) engines, but the mechanism of its impact on unregulated emissions remains unclear. In this simulation-based study, double injection strategies were used to create stratified mixtures in the cylinder. The results indicated that as the second fuel injection quantity (FIQ) was increased or as the second fuel injection timing (FIT) was delayed, the oil-film mass increased, leading to an increase in soot emissions. The formation of a large area of stoichiometry (STO) region at the spark plug and at its right side increases the laminar flame velocity and improves the ITE. At 4000 rpm, the ITE of case2-2 (with a second FIT of −220 °CA after top dead center (ATDC) and a second FIQ of 65.5 mg) increased by 1.6% compared to the original scheme. With the increase in STO area, NOx emissions and the content of CH3OH and CH2O increased, while carbon monoxide (CO) and greenhouse gas emissions showed a decreasing trend. Compared to the original scheme, CO and greenhouse gas emissions decreased by 1.97% and 6.7%, respectively, in case2-2. This study provides guidance for the development of GDI engines with high ITE and low carbon emissions.

Study of Efficient and Clean Combustion of Diesel–Natural Gas Engine at Low Loads with Concentration and Temperature Stratified Combustion

The approach for achieving efficient and clean combustion in a diesel–natural gas (NG) heavy-duty engine at low loads was studied by computational fluid dynamics simulation. This study proposed the concentration and temperature-stratified combustion technology and clarified its mechanism. The results revealed that different stratified combustions can be organized by controlling the pressures, timings, and durations of diesel and NG injections, and stratified combustion can be classified into moderate, lean, and rich stratified combustion modes. Efficient and clean combustion can be realized simultaneously at low engine loads: the gross indicated thermal efficiency (ITEg) of engine breakthrough was improved to 47.9%, and the indicated-specific emissions of unburned hydrocarbon (ISUHC) were greatly reduced to 1.6 g/kWh, while the indicated-specific emissions of nitrogen oxide (ISNOx) remained at 0.6 g/kWh. Moreover, the detailed analysis of three typical stratified combustion modes demonstrates that coupling control of the concentration and temperature of the charge is the key to obtaining excellent engine performance. Most of the NG-stratified mixture should burn in the react ratio range of 0.4 to 0.8 for low unburned hydrocarbon emissions, low nitrogen oxides emissions, and rapid combustion. The proper temperature stratification should ensure that a high-temperature charge is around the over-lean NG mixture. This study can provide the fundamentals of stratified combustion control and feasible solutions for commercial applications of NG engines.

Comparative study on combustion and emission of ternary-fuel combined supply SI engine with oxyhydrogen/ethanol/gasoline by different injection modes of fuel

The combination of renewable energy and multifuel combined supply is an efficient way to deal with the energetic crisis and the pollution of environment. Ethanol, gasoline and oxyhydrogen participate in combustion by means of three independent supply paths, which can improve engine power and reduce gasoline consumption and gaseous pollutant emission at the same time. However, the optimization of ternary-fuel combined supply mode is a problem that must be clarified. In this study, the engine performance at four direct injection pressure (DIP), six direct injection timing (DIT), five intake manifold absolute pressures (MAP) and four speeds is studied under the condition of equal ethanol injection ratio and gasoline injection ratio. The experimental results show that ethanol direct injection forms a mixture with a better stratified combustion state than gasoline direct injection. Moreover, oxyhydrogen negatory pressure inhalation (ONPI) can effectually increase Pmax and IMEP, reduce APmax, CA 10–90, CA 0–10, CO emission, HC emission and particulate emission, but increase NOx emission. However, ethanol direct injection (EDI) can reduce NOx emission produced by ONPI, and oxyhydrogen can attenuate the negatory effect on mixture combustion due to the excessive latent heat of vaporization of ethanol. NO emission in EDI mode is reduced by an average of 28.39 % compared to gasoline direct injection (GDI) mode under different MAPs. In conclusion, the combustion and emission characteristics of the engine in EDI + gasoline port injection (GPI) + ONPI mode are better than those in ethanol port injection (EPI) + GDI + ONPI mode. And the control strategy of "9–11 MPa DIP+250–300°CA BTDC DIT +16 L/min oxyhydrogen negative pressure inhalation volume (ONPIv)" can improve the performance of EDI + GPI + ONPI engine while reducing emission.

Improved combustion stability of biogas at different CO2 concentrations using inhomogeneous partially premixed stratified flames

Biogas is a renewable and clean energy source that plays an important role in the current environment of low- carbon transition. If high-content CO2 in biogas can be separated, transformed, and utilized, it not only realizes high-value utilization of biogas but also promotes carbon reduction in the biogas field. To improve the combustion stability of biogas, an inhomogeneous, partially premixed stratified (IPPS) combustion model was adopted in this study. The thermal flame structure and stability were investigated for a wide range of mixture inhomogeneities, turbulence levels, CO2 concentrations, air-to-fuel velocity ratios, and combustion energies in a concentric flow slot burner (CFSB). A fine-wire thermocouple is used to resolve the thermal flame structure. The flame size was reduced by increasing the CO2 concentration and the flames became lighter blue. The flame temperature also decreased with increase in CO2 concentration. Flame stability was reduced by increasing the CO2 concentration. However, at a certain level of mixture inhomogeneity, the concentration of CO2 in the IPPS mode did not affect the stability. Accordingly, the IPPS mode of combustion should be suitable for the combustion and stabilization of biogas. This should support the design of highly stabilized biogas turbulent flames independent of CO2 concentration. The data show that the lower stability conditions are partially due to the change in fuel combustion energy, which is characterized by the Wobbe index (WI). In addition, at a certain level of mixture inhomogeneity, the effect of the WI on flame stability becomes dominant.

Implications of Using Scalar Forcing to Sustain Reactant Mixture Stratification in Direct Numerical Simulations of Turbulent Combustion

A recently proposed scalar forcing scheme that maintains the mixture fraction mean, root-mean-square and probability density function in the unburned gas can lead to a statistically quasi-stationary state in direct numerical simulations of turbulent stratified combustion when combined with velocity forcing. Scalar forcing alongside turbulence forcing leads to greater values of turbulent burning velocity and flame surface area in comparison to unforced simulations for globally fuel-lean mixtures. The sustained unburned gas mixture inhomogeneity changes the percentage shares of back- and front-supported flame elements in comparison to unforced simulations, and this effect is particularly apparent for high turbulence intensities. Scalar forcing does not significantly affect the heat release rates due to different modes of combustion and the micro-mixing rate within the flame characterised by scalar dissipation rate of the reaction progress variable. Thus, scalar forcing has a significant potential for enabling detailed parametric studies as well as providing well-converged time-averaged statistics for stratified-mixture combustion using Direct Numerical Simulations in canonical configurations.

Application of semi-direct fuel injection system to free piston engine generator for better performance: Simulation approach with validation results

This paper explores methods to improve the fuel economy of a two-stroke free piston engine generator (FPEG) with no additional cost. In order to solve the problem of low fuel capture and poor fuel economy of two-stroke FPEG using port fuel injection (PFI), the fuel injection method is optimised in this paper. This fuel injection system is semi-direct injection (SDI). In contrast to PFI, SDI also uses a low-pressure injector, which introduces fuel directly into the ports of the engine. This study explores the feasibility of SDI on a free-piston engine generator through numerical simulation. The results show that SDI can effectively avoid short-circuit losses and improve the fuel capture rate by more than 60 % relative to PFI. Meanwhile, the ISFC is reduced by 42.3 %, and the lowest fuel consumption rate is 259.7 g/(kW·h). The combustion efficiency of SDI is maintained at over 95 % under warm-up conditions. Compared with PFI, the stagnation period is shortened by up to 10.9 % and the centre of gravity of combustion is advanced by up to 16.7 %. SDI effectively reduces HC emissions and has a lower NOx emission level than PFI. At the same time, SDI produces more CO due to stratified combustion.

Multiple-objective optimization of direct dual fuel stratification (DDFS) combustion at different loads

The operating parameters of the direct dual fuel stratification (DDFS) strategy in a heavy-duty engine are optimized across a full load range by utilizing a combination of three-dimensional computational fluid dynamics simulation and genetic algorithm. After obtaining the optimized results, sensitivity analyses were conducted on the operating parameters at various loads using the Pearson method. The results show that the DDFS strategy can attain stable and efficient combustion across the entire full-load range after optimization. At low-to-medium loads, the engine’s performance is predominantly influenced by initial operating parameters, while both initial and injection parameters play critical roles at high loads. The sensitivities of operating parameters increase as load increases, with the operating parameters having higher sensitivities having more concentrated distributions, while those with lower sensitivities have more dispersed distributions. The optimal conditions for low-to-medium load combustion generally involve a premixed-dominated combustion regime with some degree of reactivity stratification, which is strongly influenced by charge thermodynamics. Increasing the proportion of high-reactivity diesel fuel can improve combustion efficiency and stability, particularly under low-load conditions. Under high-load conditions, the optimal combustion strategy involves using a significant amount of direct-injected gasoline to achieve a more distinct stratified and diffusion combustion regime, which helps mitigate excessive heat release rates. However, this approach may result in reduced fuel economy compared to the optimal strategy for low-to-medium loads. As a consequence, the role of charge thermodynamics becomes less significant while the injection strategy becomes more critical for achieving optimal combustion at high loads.

Study on the Effect of Secondary Injection on the Combustion and Emission of n-Butanol Engines.

n-Butanol, as a biological alternative fuel containing oxygen, has similar physical and chemical properties to gasoline and has a wide range of sources, which has attracted more and more attention and research. Direct injection technology has been widely used in the field of internal combustion engine due to its advantages of flexibility and control ability. In this paper, the secondary injection of n-butanol engine under the mode of in-cylinder direct injection is discussed to organize stratified combustion of the mixture, optimize combustion to improve the thermal efficiency, and reduce emission. A four-cylinder four-stroke spark ignition (SI) engine was selected to carry out the secondary injection experiment of n-butanol under the excess air ratio (λ) of 1, an engine speed of 1500 r/min, and a low load, and the variables were the second injection ratio and timing. The results show that the secondary injection of n-butanol can achieve stratified combustion of the mixture, but only at a specific second injection timing such as 100°CA before compression top dead center (BTDC) or 125°CA BTDC, the combustion effect is the best. A small second injection ratio can optimize combustion, improve brake thermal efficiency, and reduce hydrocarbon and carbon monoxide emissions. When the second injection ratio is greater than 60%, it results in incomplete fuel combustion, a 3 to 4% reduction in thermal efficiency, and an increase in emissions. Coefficient of variation (COV) was increased by secondary injection, but the effect was insignificant in the small injection ratio, and it will increase with the increase of the second injection ratio. The change of particle number is mainly affected by the nuclear particle number, and with the increase of the second injection ratio, the total particulate number is more affected by the second injection timing. The second injection ratio of 40% can reduce the total particle number under the mixed-gas stratification condition.

Combustion Process of the Compound Supply CNG Engine

Objective: In order to study the lean combustion process of a natural gas engine by separating the combustor, a spark ignition natural gas engine with separated combustors was retrofitted from a S195 single-cylinder diesel engine. Methods: The electronic control system controlled the gas supply and the spark plug ignition. A low pressure injection valve was set in the inlet pipe to form a lean mixture while a high pressure injection valve was placed in the subsidiary chamber to create a rich mixture, which was then ignited and injected into the main combustor, where the lean mixture was subsequently ignited again to achieve stratified combustion. Results: The test results showed that steady ignition is feasible in the system and verified the impact of the shape of the main combustor on HC, the impact of channel diameter on NOX production, and the impact of the ratios of high-pressure gas and low-pressure gas on HC and NOX. The combustion conditions of high-pressure gas and low-pressure gas in the engine combustor vary greatly. Our results signify that the shape of the main combustor has a great impact on the performance of the engine, that is, a shorter propagation distance can reduce the generation of HC. Conclusion: The best ignition advance angle under different conditions was determined using a spark ignition natural gas engine. The ratios of high-pressure gas and low-pressure gas greatly impact the performance and emission of the engine. The reduced diameter of the channels between the main and subsidiary combustors can enhance the stratification and facilitate the secondary ignition.

The Lean-Burn Limit Extending Experiment on Gasoline Engine with Dual Injection Strategy and High Power Ignition System

In the context of the energy crisis and global warming, improving thermal efficiency is the most important issue in research on gasoline engines, and lean mixture combustion strategy is becoming the most promising method. Thus, a high compression ratio, a high-power interval ignition system, and a stratified combustion scheme achieved via dual injection were novelly adopted in a single cylinder gasoline engine in this study. The results show that the lean combustion limit could be literally extended and improved thermal efficiency was observed under the ultra-lean condition. Meanwhile, reverse combustion performance trends were observed by altering the second injection proportion from 30% to 45% under the lean condition (λ = 1.6) and ultra-lean condition (λ = 1.9). This was related to a combustion velocity change caused by great concentration gradient at the middle and end combustion stage. Finally, according to research on the effects of altering the timing of the second injection, it is clear that the dual injection strategy is an ideal method for realizing operation under the lean condition (λ = 1.6). But for the operation under the ultra-lean condition (λ = 1.9), more injection times and suitable air flow organization are needed to enhance the robustness of mixture distribution.

Effects of fuel-hydrogen levels on combustion, operability, and emission parameters of CH4/H2/O2/CO2 stratified flames in a dual-swirl gas turbine burner

Towards the development of emission-free hydrogen gas turbines with wider operability limits, the combined effects of lean premixed combustion, dual combustion (combustion staging), hydrogen enrichment, and oxy-fuel combustion on the combustion and emission characteristics of CH4/H2/O2/CO2 flames are investigated in a dual annular counter-rotating swirl burner. The central stream is of a higher equivalence ratio (∅) of 0.9 for stable flame ignition, whereas the annular stream is of a lower ∅ of 0.65 to maintain the burner’s environmental performance. The stratified flame reactions are modelled using the partially premixed combustion model, which has been validated using the available experimental data. The inner recirculation zone becomes smaller as the hydrogen fraction (HF) becomes higher until it vanishes and there exists only an outer recirculation zone when the secondary hydrogen fraction approaches 100%. For 100% HF in both primary and secondary flow, the maximum combustion temperature within the combustor was observed to be as high as 2350 K. Increasing HF makes the flame more compact and anchored to the burner centre body with faster kinetics, less ignition delay time, and, accordingly, with increased potential for flashback. The product formation rate was found to increase in the inner shear layer at higher HF, implying a higher flame intensity and a more compact flame. The hydrogen fraction of the primary stream (maintained at higher equivalence ratio) is observed to be dominant in regulating CO mole fractions. Stable well-anchored flames are obtained over the considered fuel mixtures, indicating the role of stratified combustion in widening the operability of gas turbine combustors under stratified combustion conditions.

Combustion Characteristics of a Novel Ammonia Combustor equipped with Stratified Injection for Low Emissions

Ammonia – a carbon-free fuel- is being considered by various research groups and organizations as a potential alternative fuel to tackle some global warming issues of the 21st century. However, ammonia faces some challenges as a fuel, particularly, the production of unwanted NOx and unburned ammonia traces at the back end of combustion systems accompanied by unstable flame regimes. Therefore, new concepts are currently being developed to address these challenges whilst also providing stable, high-power operation. This work presents a newly designed stratified combustion system for gas turbines capable of operating with both premixed and stratified modes. The system has been initially evaluated using these two modes employing solely hydrogen and ammonia. Increase of hydrogen stratification resulted in the reduction of NO and NO2, while N2O slightly increased due to the reduction of fuel in the premixed system. Further tests were conducted to evaluate the potential of the systems to replace fossil fuels. Previous work done using methane as the fuel-to-be-replaced has shown interesting trends in terms of emissions and stability. Thus, to progress on the subject, the work conducted here attempted propane/ammonia/hydrogen using this innovative strategy. Results denoted how stratification has a strong impact on NOx production at the flame front, whilst the mechanism of flame stabilization of the new burner enables large operability regions under a great variety of fuel combinations. Strategic injection setups were identified, whilst the best propane/ammonia/hydrogen blends for stability and emissions reduction are also detailed for further implementation in larger systems.

Study on flame propagation characteristics of deposited coal dust explosion induced by pressure waves of different intensities

AbstractIn the hope of studying the laws of secondary explosion induced by gas explosion shock waves lifting deposited coal dust (CD) in coal mines, the paper discusses the explosion overpressure, composite flame propagation characteristics and the acceleration mechanism of composite flame with the aid of a self‐built gas explosion experiment pipeline. The experimental results demonstrate that under the methane concentrations of 7.5%, 9.5% and 11.5%, the explosion time‐overpressure curves present same variation trend at different measuring points. Specifically, they all surge first, then decrease to a negative value and increase in oscillations, ultimately stabilizing at around 0 MPa. The presence of deposited CD in the reaction has an insignificant impact on the explosion overpressure at the measuring points, and the maximum overpressures all appear near the P2 (0.75 m away from the ignition source) measuring point. However, the deposited CD exerts a considerable influence on the flame's instantaneous velocity, especially on the flame structure during flame propagation. In stark contrast, when the methane concentration is 7.5%, the single flame has a larger instantaneous velocity than the composite flame and it exhibits finger flame, flat flame and tulip flame successively. Compared with the blue flame produced by pure methane combustion, the composite flame shows a bright white light strip that is distributed along the pipeline axis, accompanied by more obvious stratified combustion. The high‐pressure wave intensity and the flame front temperature jointly promote the overall turbulence intensity in the area where CD is swept up, resulting in continuous acceleration of the composite flame.

Investigation of the polystyrene combustion completeness increasing method in the environmentally safe waste disposal installation

The article presents the study results of the solid waste components (on the example of polystyrene) combustion optimizing based on traditional solutions used in the furnaces organization (stratified combustion, combustion in a vortex flow, combustion products recirculation), as well as due to the electrostatic field impact. Relative changes in the CO and NO emissions values were obtained for various methods of air supply to the combustion chamber and electrostatic field parameters. The use of an electrostatic field with a potential difference of 3–4 kV made it possible to increase the combustion efficiency, which led to a decrease in the formation of CO by up to 40%, and NO to almost zero.

NUMERICAL STUDY OF DIRECT INJECTION SPRAY BEHAVIOR OF GASOLINE AND METHANOL-GASOLINE BLENDS UNDER SPLIT-INJECTION STRATEGY IN ENGINE-LIKE CONDITIONS

Several studies have shown that the split-injection strategy has improved the shortcomings of the high particulate matter emissions and cycle-to-cycle variations in homogeneous and stratified combustion modes of gasoline direct injection (GDI) engines. However, the spray behavior under the split injection strategy is poorly understood. This study uses computational fluid dynamics (CFD) simulations in Converge software to investigate the spray characteristics of gasoline and methanol-gasoline blends under a split-injection strategy. The simulation studies were performed for a multihole GDI injector in a constant volume spray chamber, replicating the ambient conditions similar to a GDI engine's homogeneous and stratified combustion modes. Appropriate models for simulating different spray phenomena, such as spray breakup, collision, and coalescence, were used. The CFD model was validated using the experimental spray penetration length provided in the engine combustion network database. The results showed that the split-injection reduced the Sauter mean diameter (SMD) of fuel spray droplets and liquid fuel mass content than a single injection case. The dwell time of 2 ms was found suitable for homogeneous mode conditions, while its effect was insignificant in stratified mode conditions. Further, a split ratio of 80:20 resulted in smaller SMD during the injection period and a higher fuel evaporation rate. The effect of methanol addition was also explored under the split-injection mode. M85 showed higher deviations in the liquid spray penetration, SMD, and liquid spray mass content than G100.

Large eddy simulation of multi-regime combustion with a two-progress variable approach for carbon monoxide

Simulations of two cases in a novel multi-regime burner configuration are undertaken using a presumed joint probability density function (PDF) approach with tabulated chemistry. The flame conditions are varied by changing the central jet equivalence ratio, which produces different multi-regime combustion modes in the non-premixed inner flame. An outer premixed flame and recirculation zone behind a bluff body are present to supply heat and combustion products to stabilise the inner flame. A two-progress variable approach is tested to improve predictions of carbon monoxide (CO) in the post-flame regions, where CO oxidation occurs. The large eddy simulation set-up and sub-grid combustion model are assessed through comparisons with time-averaged measurements for radial profiles at different streamwise locations. The jet break-up length, the shear layers and the mixture fraction distribution are well captured in both cases. The temperature distribution is well captured for the inner flame in each case but the temperature and mixture fraction are over predicted in the downstream regions of the outer premixed flame, which is due to increased dilatation that suppresses air entrainment. Improved predictions of the CO mass fraction are obtained for the outer premixed flames with the two-progress variable approach. Over predictions are seen in the upstream regions of the inner flame when the CO mass fraction is obtained from a look-up table, suggesting that the CO mass fraction should be transported to include the convection/diffusion balance in regions where there is no flame. Furthermore, transporting the CO mass fraction with a one-progress variable approach produces over predictions in the burnt regions, suggesting a two-progress variable model is needed to capture the consumption region of CO. The multi-regime combustion characteristics are observed to be stronger in flame MRB26b, where non-premixed and rich premixed combustion is present. For flame MRB18b, the non-premixed contribution is smaller and weak stratified combustion is observed.

Effect of inhomogeneous methane-air mixtures on combustion characteristics in a constant volume combustion chamber

The inhomogeneous mixture can improve the combustion performance under certain conditions. In this study, an experimental study of turbulent stratified combustion under different background equivalent ratio conditions (0.6, 0.8, 1.0) was carried out based on a constant volume combustion chamber. Injection of pure methane to achieve stratification of gas mixture concentration, and the ignition timing was adjusted to control the mixture stratification degree. Optical diagnosis of the flame using a high-speed camera and the optical schlieren system to understand the morphological changes in flame development. Based on the combustion pressure and ion current signals to explore the effect law of stratification degree on the turbulent stratified combustion characteristics. The results of the study show that the concentration stratification of the mixture can accelerate the flame propagation speed. The leaner the background mixture concentration, the stronger the promotion effect of the mixture concentration stratification on the propagation speed. The appropriate degree of stratification is conducive to improving the stability of the flame. Turbulent stratified combustion can increase the maximum combustion pressure, shorten the combustion duration. The degree of stratification has a large effect on the ion current signal curve during the background equivalent ratio of 0.6 mixture combustion, and almost no effect when the background equivalent ratio is 1.0. The peak moment of the combustion pressure rises rate and the peak moment of the ion current signal occur almost simultaneously under different combustion conditions. The correlation between the maximum of the combustion pressure and the maximum of the ion current is approximately linear and the correlation is greater than 95%.

Laminar flame speed of methane/air stratified flames under elevated temperature and pressure

Flame propagation under mixture stratification is relevant to a wide range of applications including gas turbine combustors and internal combustion engines. One of the local stratification effects is known as the back-support effect, where the laminar flame speed is modified when a premixed flame propagates into gradually richer or leaner mixtures. A majority of previous studies have focused on the propagation of methane/air stratified flames under standard temperature and pressure. However, stratified combustion often occurs under elevated temperature and pressure in practical applications, which may influence the characteristics of the back support effect through modified reaction pathways. This study performs numerical simulations of stratified laminar counterflow flames under an Atmospheric Temperature and Pressure (ATP) condition and an Elevated Temperature and Pressure (ETP) condition and examines the influence of elevated temperature and pressure on the back-support effect. Reaction flow analyses were extensively conducted to elucidate the difference in the primary reaction pathway between the two conditions. When scaled by the stratification Damköhler number, the back-support effect on the rich-to-lean stratified flame is weaker under the ETP condition than the ATP condition in the stoichiometric to lean region. This is due to increased contribution from reactions involved with OH radicals under the ETP condition, which leads to lower H2 reproduction in the reaction zone than under the ATP condition. The contribution from OH radicals is increased under the ETP condition because the conversion of H into OH is enhanced. These results suggest that the back-support effect may become negligibly small in practical combustors operating under elevated temperature and pressure due to (1) the flame being less sensitive to stratification because of the thinner flame, and (2) the lower H2 reproduction that deteriorates the radical production that drives the back-support effect.