Conventional Combustion Regimes Research Articles

Open-Source Energy, Entropy, and Exergy 0D Heat Release Model for Internal Combustion Engines

Internal combustion engines face increased market, societal, and governmental pressures to improve performance, requiring researchers to utilize modeling tools capable of a thorough analysis of engine performance. Heat release is a critical aspect of internal combustion engine diagnostic analysis, but is prone to variability in modeling validity, particularly as engine operation is pushed further from conventional combustion regimes. To that end, this effort presents a comprehensive open-source, zero-dimensional equilibrium heat release model. This heat release analysis is based on a combined mass, energy, entropy, and exergy formulation that improves upon well-established efforts constructed around the ratio of specific heats. Furthermore, it incorporates combustion using an established chemical kinetics mechanism to endeavor to predict the global chemical species in the cylinder. Future efforts can augment and improve the chemical kinetics reactions for specific combustion conditions based on the radical pyrolysis of the fuel. In addition, the incorporation of theoretical calculations of energy and exergy based on the change in chemical species allows for cross-checking of combustion model validity.

Recent advances in low-gradient combustion modelling of hydrogen fuel blends

Low-gradient combustion (LGC) proved to be an effective alternative technology to reduce pollutant emissions and carbon footprint, specifically when combined with hydrogen as a fuel or blend component. This novel technology offers several advantages over conventional combustion regimes, including a more effective control of emissions and providing greater flexibility in fuel application. The impact of hydrogen on this regime is still not well-known, especially when it comes to the combustion of pure hydrogen and fuels with a high hydrogen content. In the last two decades, numerical simulations have become a powerful tool that facilitates the research and design of LGC, particularly in terms of stability of the process and the emission of pollutants. This article provides an up-to-date review of recent trends and theoretical knowledge in low-gradient combustion. This includes the guidelines and recommendations applied to LGC modelling. Comparisons have been made between the recently published modelling approaches presented by the authors, including a detailed assessment of the discrepancies in the temperature predictions. The challenges and limitations associated with the LGC combustion modelling of conventional fuels (i.e., natural gas, methane, syngas) blended with hydrogen are also discussed. The review demonstrated that the Eddy Dissipation Concept (EDC) is the most common turbulent-chemical interaction model employed in LGC combustion simulations. The performance of the EDC can be significantly improved by variable constants Cγ and Cτ based on local turbulent Reynolds and Damköhler numbers. However, the most recent publications indicate that the flamelet-based approach can be considered as a promising (and more cost-effective) alternative to the EDC. Furthermore, the chemical kinetic studies considered in this review confirm that there is no detailed reaction mechanism capable of accurately predicting the temperature profile along with the emissions of the main species of interest, i.e., NOx, CO, CO2, and OH. Although GRI-Mech 2.11 is the most widely used mechanism in LGC simulations, providing satisfactory overall accuracy.

Numerical investigation behavior of reacting flow for flameless oxidation technology of MILD combustion: Effect of fluctuating temperature of inlet co-flow

To grow the knowledge to the moderate or intense low-oxygen dilution combustion, namely MILD combustion, the present paper examines the CH4H2 MILD combustion under various working conditions in a jet in hot coflow burner which called Dally burner using a finite volume code. This burner can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. For this end, detailed reaction mechanism (GRI-Mech 2.11), the k-ε RNG by the kinetic energy of turbulence of 53 m2/s2 for fuel inlet and 3 m2/s2 for hot coflow inlet as well as the eddy dissipation concept model for treating the interaction between the turbulence and the chemistry in flames are used. At first, the results are compared with the experimental data and after validating the results of the solution obtained, the MILD combustion is investigated by using sinusoidal pattern temperature between 1580 K and 1180 K, respectively. The results show that by using fluctuation temperature of inlet co-flow, the MILD combustion regime develop in a wider area. reaction zone and the release energy have increased during the initial oscillation at the incoming temperature, and along with this amount of Water vapor and CO2 have increased, while the amount of CO has also been slightly reduced.

Kinetic simulation of flameless burners with methane/hydrogen blended fuel: Effects of molecular diffusion and Schmidt number

The newly developed concept of MILD combustion has paved the way through achievement of high thermal efficiencies with low levels of pollutants and greenhouse gases. High fidelity numerical models play key role in design and optimization of these burners. The present research aims to assess the effect of molecular diffusion and deviations in the amount of different species Schmidt number on the precision of the model. To this end, a previously investigated MILD burner is opted as case-study. From the results it is evident that in contrast to conventional combustion regimes problems, the impact of the often-neglected laminar diffusion is comparable to turbulent diffusion. On the other hand, consideration of molecular diffusion in the species transport equation significantly improves the model accuracy only if proper Schmidt number for species are considered. Suitable Sutherland coefficients and Schmidt numbers for each species are found based on relevant data in the literature and reported.

Study of the low-temperature reactivity of large n-alkanes through cool diffusion flame extinction

The low-temperature oxidation of hydrocarbon fuels has received increasing attention as advanced engines seek to operate in less conventional combustion regimes. Large n-alkanes are a notable component of many real transportation fuels and possess strong reactivity in this important low-temperature range. These n-alkanes have been studied extensively in various canonical kinetic experiments but seldom in systems with strong coupling between low-temperature chemistry, transport, and heat release. To address this issue, the present study investigates self-sustaining n-alkane cool diffusion flames in a counterflow burner. The extinction limits of both hot diffusion flames and cool diffusion flames are measured at atmospheric pressure for a range of n-alkanes from n-heptane to n-tetradecane. It is observed that while these fuels behave similarly for hot flames, the larger n-alkanes are substantially more reactive in the low-temperature cool flame regime. Moreover, ozone addition strongly enhances the low-temperature chemistry to the point where the differences in fuel reactivity are nearly suppressed.The experimental measurements are compared with numerical simulations employing both detailed and reduced chemical kinetic models of various sizes. Although the different kinetic models adequately predict the extinction limits of the hot flames, a large scatter is present in the model results for cool flames, and a general overprediction of the measured cool flame extinction limit is observed for all of the fuels studied. This implies that the cool flame heat release is not well agreed upon by the current chemical kinetic models, despite their capability to reproduce many homogeneous reactor experiments at low temperatures. Furthermore, it is observed that the cool flame heat release is spread over a substantial number of reactions involving large molecules, a trait that makes it particularly difficult to create reduced kinetic models that can accurately describe cool flame behavior. The results of this study suggest that the cool flame platform can provide crucial validation of the coupling between chemistry, transport, and heat release in flames at low temperatures.

Flame Characteristics of CH4/H2 on a Jet-in-Hot-Coflow Burner Diluted by N2, CO2, and H2O

To enhance the knowledge to the combination of oxy-fuel combustion and MILD (moderate or intense low-oxygen dilution) combustion, namely oxy-MILD combustion, the present article examines the effects of different diluents (i.e., N2, CO2, and H2O) on the flame characteristics of CH4/H2 blended fuel in a JHC (jet-in-hot-coflow) burner, which can produce MILD combustion regime in the upstream and conventional combustion regime in the downstream. CFD modeling method considering RANS equations and detailed reaction mechanism (GRI-Mech 2.11) are used for such purpose. Results show that, in MILD combustion regime, peak temperature under oxy-fuel condition (diluted by CO2 and H2O) is reduced as compared to air-fuel condition (diluted by N2) from both physical and chemical effects of the diluents. However, in the conventional combustion regime, chemical effect of the diluents on suppressing the peak temperature is weakened, resulting in the predominance of its physical effect. As for the flame ignition behavior, th...

Application of jet propellant-8 to premixed charge ignition combustion in a single-cylinder diesel engine

The aviation fuel, jet propellant-8, was applied to premixed charge ignition combustion as well as conventional combustion in a single-cylinder diesel engine and was compared with diesel fuel. The engine performance and emissions were tested with and without exhaust gas recirculation under two operating conditions. The liquid- and vapor-phase penetration of diesel fuel and jet propellant-8 were also compared by Mie-scattering and Schlieren method under evaporating conditions using a constant-volume chamber. It was observed that jet propellant-8 exhibited slightly longer vapor-phase penetration and evidently shorter liquid-phase penetration compared with diesel fuel due to the lower distillation temperature and density of jet propellant-8. However, despite the faster evaporation rate of jet propellant-8, it was found that the ignition delay with jet propellant-8 was 2–3 crank angle degrees longer than that with diesel fuel due to its lower cetane number. This result was consistent for all operating conditions and combustion regimes. Jet propellant-8 also showed lower nitrogen oxides (NOx) and smoke emissions for both combustion regimes under the low-load condition because of locally leaner air–fuel mixture caused by longer ignition delay, higher volatility, and lower aromatic contents. As the engine load increased, jet propellant-8 emitted more NOx under conventional combustion regime due to the more vigorous premixed burn phase compared with diesel fuel. With exhaust gas recirculation, jet propellant-8 showed an improved trade-off relationship between NOx and smoke emissions for both combustion regimes due to its better evaporation characteristics and lower aromatic contents. Premixed charge ignition combustion with jet propellant-8 emitted lower smoke than conventional combustion with jet propellant-8 under near-zero NOx level.

Optimization of EGR and split injection strategy for light vehicle diesel low temperature combustion

Low temperature combustion was developed using a four-cylinder light vehicle diesel engine. Operating conditions considered were 1600 rpm, 1bar and 3bar IMEP. Both EGR and split injection strategy were optimized in order to obtain the lowest BSFC accompanied with a low level of emissions. It was found that a late injection strategy with high levels of EGR rate was required for simultaneous reduction of NOx and soot. However, the fuel consumption remains higher than the conventional combustion regime. Thus, the optimization study of injection parameters to improve the trade-off between NOx and soot emissions while maintaining good fuel efficiency was performed. Several injection pressures were tested. The results showed that as injection pressure increased, NOx emissions increased slightly, soot initially decreased sharply, but further increase of injection pressure on soot was not obvious at low temperature atmosphere, and might lead to increased BSFC. Next, split injection strategy was adopted, optimized pilot injection conditions for minimizing fuel consumption were found at late pilot injection timing with big injection quantity, but little amount of pilot injection could make better comprehensive performance of diesel engine. Through multi-parameter collaborative optimization, the emission reduction path was proposed at operating condition of partial load. Low NOx and soot emissions could be obtained with slightly increase of fuel consumption.

Effect of air–fuel mixing quality on characteristics of conventional and low temperature diesel combustion

A comparative study on the effects of air–fuel mixing quality on combustion characteristics was carried out in both conventional and low temperature diesel combustion (LTC) regimes. The injection pressure and intake pressure were considered as variables as they are important factors which influence the air–fuel mixing process. The intake O2 concentration was varied to realize different combustion regimes. Improved air–fuel mixing with a higher injection pressure enhanced the combustion process in both conventional combustion and LTC regimes, resulting in higher peaks of in-cylinder pressure and heat release rate. The combustion phase in the LTC regime was more influenced by injection pressure due to longer premixing time than that of conventional combustion. A higher injection pressure reduced CO and HC emissions over a wide range of intake O2 concentrations. The reduction of CO and HC emissions in the conventional combustion regime was due to higher combustion temperature, while that in the LTC regime was due to decreased under-mixed fuel by improved air–fuel mixing. Soot emissions at a higher injection pressure were reduced, particularly, in the conventional combustion regime where the soot formation rate is high. The increase of intake pressure was also advantageous in reducing CO, HC and soot emissions due to improved air–fuel mixing as well as enrichment of absolute amount of oxygen, which lead to enhanced combustion process. A direct flame image was taken to observe the flame structure of two different combustion regimes to correlate with the exhaust emission results and combustion characteristics. High flame luminosity was observed around the periphery of the spray jet in the conventional combustion regime, which was a direct indication of soot formation and high temperature combustion; while low luminosity was observed around the piston bowl in the swirl direction in the LTC regime, which indicated a longer air–fuel mixing period and low temperature combustion.