Search

• Combustion and emission characteristics of NH 3 /H 2 blends are studied in an optical engine. • Effects of energy ratio and equivalence ratio on engine performance of NH 3 /H 2 are investigated. • Critical energy ratios for optimal combustion stabilities and thermal efficiency are obtained. • Variations of unburned NH 3 , NO, NO 2 , and N 2 O emissions with key parameters are measured. Ammonia is a promising alternative carbon-free fuel, but its applications in engines encounters great challenges from poor combustion and heavy emissions. Hydrogen can effectively enhance ammonia reactivity as a carbon–neutral solution. Hydrogen energy ratio and equivalence ratio exhibit significant impacts on engine performance, but the comprehensive characteristics and underlying mechanisms remain not fully understood. In this work, we investigated the engine performance, combustion evolutions, and nitrogen-based emissions of ammonia/hydrogen blends in an optical engine, addressing energy ratio and equivalence ratio. Six levels of hydrogen energy ratio and three groups of equivalence ratio were conducted under different spark-ignition timing conditions. Results show that hydrogen addition significantly improve ammonia combustion stability, particularly for low-level hydrogen energy ratios at stoichiometric conditions. Optimal thermal efficiency is achieved at a hydrogen energy ratio of 10% at stoichiometric conditions and higher hydrogen addition under leaner or richer conditions. Combustion evolutions show that the initial flame kernel and flame development become intensified at higher levels of hydrogen energy ratio, resulting in advanced combustion phasing. The tendency becomes more pronounced at the stoichiometric conditions. Regarding emissions characteristics, unburned NH 3 , NO 2 , and N 2 O emissions decrease as hydrogen energy ratio is raised, particular for low-level hydrogen addition, whereas NO emissions show an opposite tendency. Unburned NH 3 , NO 2 , and N 2 O emissions achieve the optimal at stoichiometric conditions, but NO emissions become lower at rich-burned conditions. [ABSTRACT FROM AUTHOR]

• Experiments of syngas explosion in the parallel narrow channels were carried out. • Flame propagation with various equivalent ratio and ignition distance was got. • The pressure and flame temperature in the channels were obtained. • The relation between pressure and temperature and flame was elaborated. Experiments of premixed syngas/air explosion in parallel narrow channels(PNCS) were carried out under the condition of equivalent ratio (Φ) varying from 0.6 to 2.5 and ignition distance away from the entrance to PNCS changing from 190 mm to 590 mm. The flame propagation process, explosion pressure and temperature in PNCS were obtained. The result showed that the ignition distance had a significant effect on flame propagation in PNCS by affecting the inlet flame structure and velocity. The equivalent ratio had a significant effect on the explosion dynamics propagation in PNCS, which changed not only the inlet flame structure and velocity, but also explosion reactive rate in PNCS. The flame in the PNCS inherited the characteristic of inlet flame at the entrance to PNCS and developed further into a multi-headed tulip shape at Φ = 1.2, into a tulip shape at Φ = 1.0 and 1.5, and into a finger-shaped flame at Φ = 0.8 and 2.0. While the flame quenched inside the parallel narrow channels at lower (Φ = 0.6) and higher (Φ = 2.5) equivalent ratios. After the flame passed through the PNCS, the flame came back into and leaved from the PNCS several times. The flame velocity, pressure maximum (P max) and temperature maximum (T max) in PNCS reached a maximum at ignition distance S 11 (490 mm away from the entrance to the PNCS) for Φ = 1.2 due to the greatest inlet flame velocity at S 11. As the equivalent ratio increased, the pressure and temperature increased and then decreased which reached the maximum at Φ = 1.2 because of the chemical reaction rate of syngas being the fastest at Φ = 1.2. The pressure and temperature were greatly affected by the flame back and forth entry the PNCS from the buffer pipe. The front-end pressure was about half of the back-end pressure in PNCS. The temperature had more than one peak in the PNCS. The experimental results were important for insight into the kinetic mechanism of flame propagation in parallel narrow channels. [ABSTRACT FROM AUTHOR]

• Parametric studies on combustion and emissions of ammonia engines were numerically studied. • NH 3 combustion is promoted at elevated intake temperature and pressure and compression ratios. • Equivalence ratios have significant impacts on the shortening of late-stage combustion duration. • Fuel-NO emissions are pronounced and thermal-NO emissions are minimal for ammonia combustion. Ammonia is an attractive hydrogen carrier and zero-carbon fuel for achieving de-carbonization targets. However, realistic engines with pure ammonia often suffer from serious issues like ignition instabilities, combustion variations, and heavy emissions. It has shown that ammonia combustion is highly sensitive to combustion boundary conditions. However, comprehensive parametric studies on combustion and emissions of ammonia engines are still lacking currently. In this work, ammonia combustion and emissions were numerically investigated in a spark-ignition engine under various operating conditions, emphasis on pressure characteristics, combustion flow field, and NOx and unburned NH 3 emissions. The results show that ammonia combustion can be promoted significantly at elevated intake temperatures and pressures and compression ratios, manifesting increased peak pressure, accelerated flame propagation, and shortened combustion duration. Indicated power increases at elevated compression ratios and intake pressures while decreasing at elevated intake temperatures. Equivalence ratios affect ammonia combustion nonmonotonically, and the highest peak pressure and thermal efficiency are achieved at Φ = 1.2 and 1.1, respectively. Besides, equivalence ratios have large impacts on the shortening of late-stage combustion duration (i.e., CA50-CA90). Regarding emission characteristics, it shows that fuel-NO is dominant while thermal-NO is minimal. Equivalence ratios can result in pronounced NO emissions under lean-burning conditions. Unburned NH 3 decreases while NO 2 and N 2 O remain low levels at an elevated compression ratio, intake temperature and pressure and equivalence ratio of around 1.1. This work demonstrates the significance of active thermo-atmosphere in the optimization of ammonia engine performance. [ABSTRACT FROM AUTHOR]

• H 2 /CH 4 /Air combustion in burners with/without dual-inlet are compared. • Characteristics of the combustion with varied reactants ratio in the two inlets are analyzed. • The appropriate fuel/air ratio in each inlet and dual-inlet setting is selected. Combustion characteristics of premixed H 2 /CH 4 /Air in a combustor with dual-inlet are investigated under various operating conditions. Effects of reactants ratio in the two inlets, dual-inlet setting and flow rates on the combustion characteristics and heat transmission are compared and analyzed. Results demonstrate that the setting of dual-inlet strongly affects burning such as OH generation and consumption, and the weak flame is formed in the second-inlet, modifying the species distribution and gas temperature. For the seven conditions with 28 cases under a kept H 2 /CH 4 blended ratio m C H 4 = 5%, the case of 20% H 2 /CH 4 /Air imported via the second-inlet achieves a better thermal performance, where reactions around the second-inlet contribute to the sufficient burning, heat release, heat transfer of gas-wall and gases. It also reduces the heat loss through exhaust gases, resulting in a higher burner radiation temperature, and the mean radiation temperature of case 19 is 55 K higher than that of case 21. Moreover, the radiation power of the combustor can be further improved by optimizing the second-inlet setting. The combustor with the distance of the two inlets L = 14 mm obtains a higher thermal performance, where the T m reaches 1254 K and is 16 K higher than the stepped tube and the radiation power reaches 44.1 W under condition of m f = 3.339 × 10-5kg/s and m C H 4 = 5%. [ABSTRACT FROM AUTHOR]

• The effects of the equivalence ratio on mazut gasification features are explored experimentally. • An equivalence ratio of 0.37 provides the optimum performance for air-mazut gasification. • The consumption of solid char and CCE increases with the increased gasifying agent. • An innovative approach is presented to specify the minimum length of the gasifier. In order to utilize heavy fuel oil in an effective gasification system, it is necessary to comprehend the phenomena within the gasifier. This study provides a performance evaluation and feasibility study for the gasification of specific heavy fuel oil in an entrained flow gasifier. The experiments are performed in the air equivalence ratio range of 0.29–0.41 to assess gasification characteristics by measuring the gas composition at the reactor outlet and the temperature distribution along the gasifier. The characteristic of fuel spray and burner flame is also discussed based on the temperature and pressure of heavy fuel oil. Moreover, the performance parameters are compared with those calculated from previous studies on heavy fuel oil gasification using equilibrium and kinetics models. The results show that optimum gasification performance is obtained at an equivalence ratio of 0.37 so that syngas yield (CO + H 2), cold gas efficiency, and carbon conversion efficiency achieve 76%, 41%, and 63%, respectively. The experiment outcomes give a deep insight into heavy oil gasification in an entrained flow gasifier and indicate its potential for industrial application. [ABSTRACT FROM AUTHOR]

• Three Fe/Ca based oxygen carriers were used for biomass char CLG. • Effect of ER on the evolved gas composition of three oxygen carriers was studied. • Different oxygen carriers exhibit different performances with different ER. • The CO selectivity of CaFe 2 O 4 had little changes with a wide range of ER. • The CO selectivity of Fe 2 O 3 decreased markedly with increasing ER. Equivalence ratio is one of the important operating parameters for biomass gasification. The oxygen carrier possesses high producer gas production in a wide range of equivalence ratio (oxygen carrier/biomass ratio), which is exceedingly desirable for biomass chemical looping gasification (CLG). In this study, the effect of different equivalence ratios on the CO selectivity (the evolved gas composition) of Fe 2 O 3 , Fe 2 O 3 /CaO and CaFe 2 O 4 oxygen carriers in biomass char CLG was investigated in a fixed bed reactor. The results showed that the CO selectivity of CaFe 2 O 4 had little changes and decreased from 70.39% to 59.79% and 73.40% to 63.98% with increasing equivalence ratio from 0.5 to 3.0 at 900 °C and 1000 °C, respectively. In sharp contrast, the CO selectivity of Fe 2 O 3 changed markedly, which declined respectively from 65.86% to 31.13% (900 °C) and 69.68% to 47.27% (1000 °C). Fe 2 O 3 /CaO illustrated the similar tendency as Fe 2 O 3 at 900 °C but as CaFe 2 O 4 at 1000 ℃. The three oxygen carries exhibited a high CO production at lower equivalence ratio, and CaFe 2 O 4 displayed the best performance of CO selectivity in the given equivalence ratio range. X-ray diffraction (XRD) spectrums showed that the oxygen carriers were transformed to different iron phases with difference equivalent ratios. The Fe (III) in the three oxygen carries was reduced completely to Fe (0) at the equivalence ratios of 0.5 and 1.0, while FeO and Ca 2 Fe 2 O 5 were the dominant species in the spent oxygen carriers as equivalence ratio was further increased. The trends of CO selectivity obtained by thermodynamic analysis for three oxygen carries were consistent with the experimental results. [ABSTRACT FROM AUTHOR]

• A novel pilot scale bubbling fluidized bed gasification system was developed. • Air, oxygen-enriched air, air-steam, and oxygen-steam was used as gasifying agents. • Gasifying agents had a significant effect on gasification performance. • The maximum value 1.44 of the H 2 /CO ratio was found. Effect of gasifying agents, such as air, oxygen-enriched air, air-steam, and oxygen-steam, on sawdust gasification was investigated in a novel pilot scale bubbling fluidized bed gasification system. The results showed that gasifying agents had a significant effect on gasification performance and producer gas properties. H 2 content in the producer gas increased as equivalence ratio (ER) increased from 0.20 to 0.30 for all gasifying agents used. CO, CH 4 , and C n H m decreased and CO 2 presented two different trends as ER increased from 0.20 to 0.30, resulting in lower heating value decreasing, gas yield increasing, carbon conversion efficiency and cold gas efficiency change. Compared to sawdust gasification using air only, oxygen-enriched air increased lower heating value due to reducing N 2 dilution, air-steam favored H 2 production because of enhancing water gas shift reaction, and oxygen-steam took both advantages of N 2 free and steam addition. The maximum value of the H 2 /CO ratio was 1.44 by using air-steam and oxygen-steam as the gasifying agents at the ER of 0.30. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Explosion hazard and dynamic evolution of the LPG/DME mixtures were studied. • Propagation of the shock waves along the pipeline was investigated. • Effects of DME content and equivalence ratio on explosion hazard were researched. • The effecting mechanism of DME on the mixture was revealed from combustion heat. Abstract Experiments were performed to study the explosion characteristics of DME-blended LPG mixtures with different DME contents and equivalence ratios using a closed vessel. The maximum explosion pressure (P max), the maximum rate of pressure rise (d P /d t) max and the propagation velocity of shock wave (v) were investigated. The results indicated that both the peak overpressure and peak rate of pressure rise were raised with increasing DME content, reaching peak values at the equivalence ratio of 1.2. Furthermore, it was found that slight incorporation of DME had a small effect on the explosion hazard, but as the ratio of DME to LPG/DME mixture exceeded 30%, the explosion hazard would increase dramatically. The propagation of the shock wave behind the obstacle was divided into three stages in this paper: initial acceleration, stable acceleration and decaying. In the propagation process, P max , (d P /d t) max and v first increased and then decreased along the pipeline, and the peak values often appeared in the stable acceleration stage. The peak overpressure and the maximum value of v also appeared in the same location approximately, indicating that the occurrence of maximum overpressure risk was often accompanied with the appearance of the most severe dynamic evolution. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Characterization of Rapeseed Methyl Ester spray via 2D visible and infrared imaging. • Penetration and cone angle of Diesel and Rapeseed Methyl Ester spray. • Analysis of injection delay time for Diesel and Rapeseed Methyl Ester fuel. • Modeling of liquid/vapor phase penetration of pure and blended biodiesel fuels. • Modeling of jet equivalence ratio and fuel vapor mass. Abstract This paper provides a study of injection process in a research compression ignition engine fueled with Rapeseed Methyl Ester pure biodiesel and its blends by advanced optical diagnostics and 1D model. Experimental tests have been performed in an optical direct injection single-cylinder engine equipped with the Common Rail injection system of a real compression ignition engine. The injection strategies consisted of two events per cycle, the pilot and the main. The investigated conditions were representative of two engine operating points of the WLTC corresponding to two different engine speed and brake mean effective pressure values. The 2D visible imaging and the novel infrared (IR) technique have been applied to evaluate fuel spray parameters (i.e. penetration and cone angle) for liquid and vapor phases, respectively. Experimental results have been compared to data obtained with commercial diesel fuel available from previous authors' works, underlining the main differences. Furthermore, to improve biodiesel spray knowledge, 1D model for diesel jets has been suitably modified to analyze RME spray. The model has been set up via collected experimental data. Pure fuel and blends properties have been evaluated using REFPROP® software. Once the model has been validated, it has allowed to calculate the equivalence ratio along both the jet axis and radial direction, and the amount of vapor fuel over time. The modified 1D model presented in this study could be an advanced and effective instrument to study injection process in the modern engines fed with biofuel, supporting experimental activities in order to reduce time and costs. [ABSTRACT FROM AUTHOR]

Highlights • Low pressure DME/O 2 /Ar flames were investigated experimentally and numerically. • Effects of equivalence ratio on flame structure and reaction path of DME were illustrated. • Methoxy-methyl radical is easily and mainly consumed through β-scission reaction. Abstract Low pressure premixed laminar dimethyl ether/oxygen/argon flames with different equivalence ratios (0.8, 1.0, and 1.5) were investigated experimentally and numerically. The major intermediates were identified, and their mole fractions were measured using molecular-beam mass spectrometry combined with synchrotron radiation photoionization. Simulations were performed using PREMIX CODE and a representative chemical reaction model for dimethyl ether. The influence of the equivalence ratio on the flame structure and reaction path of dimethyl ether was analysed. The results show that formaldehyde and methyl are the main C1 intermediates in dimethyl ether flames. Dimethyl ether is mainly consumed through H-abstraction reactions by H, OH, CH 3 , and O. The effect of these radicals on the H-abstraction reactions varies at different equivalence ratios. Most of the methoxy-methyl radicals formed by the H-abstraction reactions promptly undergo β-scission into methyl and formaldehyde, with only a small amount reacting with oxygen under extremely lean conditions. [ABSTRACT FROM AUTHOR]

To evaluate the effect of hydrogen fraction and equivalence ratio on the competitive relationship between flame and crack development, combustion experiments of H 2 /CO/air mixtures at various equivalence ratios and hydrogen fractions were conducted in a constant volume combustion chamber. The crack length and growth rate were defined. The results indicate that the crack length and the number of the cells increase with the flame development. With the increase (decrease) in hydrogen fraction (equivalence ratio), the disturbances caused by flame inherent instabilities increases, and the crack length at the same radius increases. The appearance and generation of the new cracks dominate the cellular structure process, clear increasing the crack growth rate. The effect of hydrogen fraction on the development of cracks is more obvious at lower equivalence ratios. Flame stretch affects the competitive relationship between crack growth rate and flame propagation speed, exhibiting two completely different trends. The critical crack growth rate decreases with the increase of the equivalent ratio. [ABSTRACT FROM AUTHOR]

Micro-thermophotovoltaic is facing challenge caused by small size, large area-volume proportion and heat loss ratio, reducing the burning stability and output power. So, the dual-inlet combustors with exit-step and porous medium are proposed for micro-thermophotovoltaic system. Effects of equivalence ratio, exit-step and porous medium setting on combustion characteristic and thermal performance are conducted and analyzed. Results demonstrate that 1% air in the second-inlet is conducive to releasing heat of combustion and optimizing wall temperature distribution, where the total equivalence ratio of H 2 /air keeps 1.0. Furthermore, the exit-step with applicable length and channel height can generate heat recirculation and enhance working performance of micro-combustor, while the employment of porous medium can further enhance heat transmission of gas-wall and boost the radiation temperature T w. Combustor CP13 with porosity ε = 0.85 obtains the highest average T w of 1273.9 K at m f = 3.376 × 10-5 kg/s. With the increase of m f , system efficiency of the combustor with porous medium is increased, while that with exit-step reaches the maximum at m f = 4.501 × 10-5 kg/s. The highest output electrical power and efficiency are appeared in combustor CP15 with ε = 0.95 at m f = 5.626 × 10-5 kg/s, that is, 4.2 W and 2.2%. [ABSTRACT FROM AUTHOR]

This study presents a method for the prediction of equivalence ratio in pulse combustor from the ion current amplitude spectrum measured by a multi-frequency ion current sensor supplemented with principal component analysis (PCA) and support vector machine (SVM) technologies. An experimental study was conducted on a Helmholtz-type combustor to validate the proposed method. During the experiment, a multi-frequency excitation source whose crest factor was pre-optimized was applied on the ion current sensor to obtain ion current signals with high SNR. The excitation source has ten frequency components from 500 Hz to 5 kHz. The multi-frequency ion current signals were acquired under six different operation conditions. Under each condition, three typical positions with different distances from the gas inlet of the combustor were selected. The amplitudes of all frequency components in the multi-frequency ion current signals were extracted and projected into two subspaces via PCA to reduce the data dimensions and attenuate the noise corruption. An SVM model was built based on the amplitude spectrum features in the first two principle components subspace to predict the equivalence ratio. Experimental results show that the ion current amplitude spectrum relies on the equivalence ratio and the position where the sensor is installed. The equivalence ratio can be predicted with the PCA-SVM model with an average determination coefficient higher than 98%. This study validates the feasibility of the amplitude spectrum obtained by the ion current sensor for prediction of the equivalence ratio in the pulse combustor. [ABSTRACT FROM AUTHOR]

The goal of this work is to investigate the potentiality of energetic recovery by sewage sludge gasification. Experimental gasification tests were carried out in a bench scale rotary kiln under simulated autothermal condition. The mass rate of sludge was in the range 170–260 g/h. The gasification temperature was varied from 750 to 850 °C, equivalence ratio was increased up to 0.24. The tests run for about 3 h. Input and output streams of the process were quantified and analyzed as to assess the cold gas efficiency. Tar content in the raw gas was determined as well. The main drawbacks occurred during the tests were highlighted to have a realistic frame of the experimented technology. [ABSTRACT FROM AUTHOR]

• The impact of an air dilution by N 2 on the nitrogen oxides (NOx) produced by a swirled-stabilized magnesium flame is investigated. • For each dilution conditions (0, 30 and 50 %), the NOx (NO + NO 2) mole fraction versus the equivalence ratio follows a parabolic shape. • The maximal NOx emissions decrease by a factor 2 with a dilution of 50%. • No N 2 O is produced from the magnesium flame. • The combustion efficiency is higher than 98%, whatever the dilution conditions. New sources of clean energy with reduced greenhouse gas emissions must be considered because of global warming. Among these alternative sources, metal fuels such as aluminum or magnesium are promising. The present study focuses on the impacts of an air dilution by N 2 and of the equivalence ratio on the amount of nitrogen oxides (NOx) produced by a swirled-stabilized magnesium flame. A polydispersed aerosol of pure magnesium in the size fraction 50–70 µm was injected in the combustion chamber. Different air–fuel ratios – from 0.14 to 1.0 – were obtained by changing the mass flowrate of the injected fuel. Thermocouples and a pyrometer were implemented to monitor the gas and flame temperatures. On-line paramagnetic analyzers were used to measure the oxygen (O 2) and NOx concentrations. For each dilution condition (0, 30 and 50 %), the NOx mole fraction versus the equivalence ratio follows a parabolic shape. The amount of NOx first increases when increasing the air–fuel ratio as the power dissipated by the flame increases, leading to the rise of the gas temperature. For the highest air–fuel ratios, the NOx production is disfavored in the rich mixture. The maximal NOx emissions are shifted to higher equivalent ratios when increasing the dilution level and it decreases by a factor 2 with a dilution of 50 %, for a constant value of the air–fuel ratio. The gas temperature in the flame decreases due to the N 2 dilution. The NOx emissions become negligible for an equivalence ratio of 1 at a 50 % dilution. Nevertheless, under the tested experimental conditions, the level of NOx never exceeded 7 gNOx/kWh, which is lower than the maximal value of NOx emissions from a gasoline engine. [ABSTRACT FROM AUTHOR]

The flame stabilization within a two-layer porous burner was investigated numerically in the present study. Two-dimensional model was implemented to simulate the premixed methane-air combustion process. The governing equations including the radiative transport equation and separate energy equations for the solid and gas phases were solved by using the finite volume method. In order to validate the utilized model, the gas and solid temperature profiles within the burner were compared to the experimental temperature distribution and good agreement was observed. The results proved that the stability limit and flame temperature within a porous burner can be controlled by the equivalence ratio of the incoming mixture. Heat recirculation study showed that high convective heat recirculation occurred at low equivalence ratios caused higher stable inlet velocities. The radiative recirculation efficiency was almost constant over the stability limit at constant equivalence ratio while this parameter decreased with increasing equivalence ratio. Also, the effect of outlet diameter of the burner on the flame stabilization suggested that the optimum outlet diameter eventuating the highest flame stability limit was double the amount of inlet diameter. The study of the effect of the length of first porous layer on the flame status showed that the optimum ratio of preheat zone to combustion zone length was 0.33 to reach the highest burner operating range. [ABSTRACT FROM AUTHOR]

The present work experimentally studies the correlation between the cellular structure of a flame front and the pressure of methane/hydrogen/air laminar premixed flame at various equivalence ratios and hydrogen fractions in a spherical combustion chamber. The effects of the equivalence ratio and the hydrogen fraction on the explosion characteristics and evolution of the cellular structure of the flame front of methane/hydrogen/air were determined, and the correlation between cellular structure and pressure was analyzed. The results show that with an increase in the equivalence ratio or the hydrogen fraction, the p max and its time increase monotonously. The mean area of all cells reaches a constant value, which tends to increase with an increase in the hydrogen fraction or the equivalence ratio. The absolute value of growth rate decreases with an increase in the equivalence ratio under 70% hydrogen fraction or with an increase in the added amount of hydrogen. The influence of pressure on the cellular structure decreases in the early stage after the flame edge reaches the quartz window under a certain equivalence ratio. When the flame develops to a certain degree, the influence of pressure on the cellular structure is limited. [ABSTRACT FROM AUTHOR]

Three dimensional DNS are performed to investigate the auto-ignition behavior of n -heptane droplets in high-temperature products of premixed H 2 /air combustion. A skeletal chemistry which contains 44 species 122 element reactions is adopted. Two-way coupling is considered for the interaction between liquid phase and gas phase by Eulerian-Lagrangian point-source approach. The initial turbulence is homogenous and isotropic. Five cases of diverse initial droplet size and global equivalence ratio are carried out to study their effects on the ignition process. The most reactive mixture fraction η MR is determined. The statistical characteristics of mixture fraction η and the scalar dissipation rate χ are examined, in terms of the probability density function (PDF) P ( η ) and conditional averaged dissipation rate 〈χ| η 〉 respectively. The temporal evolution of joint PDF of η and χ at the point ( η = η MR , χ = 0) is analyzed. It is found that P ( η MR , χ = 0 ) is a key parameter relating to the happening of ignition. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lean-fuel, stoichiometric and rich-fuel mixtures combustion were investigated. • Flame quenching occurred when the equivalence ratio was 0.5. • Copper foam promoted the explosion when the equivalence ratio was 1.5. • Mechanism of flame retardant and explosion suppression of copper foam was verified. This study investigates the effect of copper foam sheets on explosion suppression in hydrogen/air mixtures with different equivalence ratios (φ). The explosion experiment is conducted in a circular tube with a large aspect ratio (1 0 8). The phenomenon of flame quenching can be observed distinctly in the case of φ = 0.5. In the case of φ = 1.5, the copper foam sheets are destroyed by the reflected shock wave, and the overpressure rises by 65 %. In addition, a longer duration of peak overpressure and a lower attenuation rate can be observed due to the interaction between the shock wave and flame front. For the cases of φ = 1.0 and 2.0, the copper foam sheet effectively prevents flames and shock waves from reflecting. The experimental results provide an essential reference for the design of hydrogen flame arresters, the safety of hydrogen pipeline transportation, and engineering application. [ABSTRACT FROM AUTHOR]

Polyoxymethylene dimethyl ethers (PODE) are a promising alternative fuel for diesel engines. PODE have high volatility, high ignitability and high oxygen content, and is thus also an ideal additive fuel for blend and dual-fuel combustion due to its low-temperature chemistry. In this work, the characteristics of PODE homogenous charge compression ignition (HCCI) combustion are investigated for the first time. The effects of charge mass equivalence ratio ( Φ m ) and exhaust gas recirculation (EGR) on PODE HCCI are studied. The results indicate that PODE HCCI exhibits two-stage ignition with a strong low temperature heat release (LTHR) before the high temperature heat release (HTHR). HTHR switches from one-stage to two-stage with an increase of Φ m due to rapid CO oxidation. At a specific EGR rate, with an increase of Φ m , the end-of-compression charge temperature decreases, the ignition timing of LTHR delays. With an increase of Φ m , the ignition timing of the HTHR advances at the EGR lower than 42%, but it delays at the EGR of 52% in general. For a specific Φ m , with an increase of EGR, the end-of-compression charge temperature decreases, the ignition timing of both LTHR and HTHR delays, and the combustion duration of the HTHR increases. CO emissions decrease with an increase of Φ m and a decrease of EGR. Φ m and EGR have only a slight effect on HC emissions. This work also provides fundamental data of PODE combustion characteristics for the future development of PODE reaction mechanisms. [ABSTRACT FROM AUTHOR]

• Syngas can be generated from sweet sorghum stalk residue for heating or electricity. • Optimum gasification air flux and Ø for sweet sorghum stalk are 12.9 g/m2·s and 2.1. • Optimum gasification thermal efficiency of sweet sorghum stalk is 81% • The yield of particles was lowest at the optimum gasification conditions. Sweet sorghum (SS) is an agricultural crop that is produced commercially in Nigeria. The crop has a high biowaste energy in its stalk, which is an attractive source of bioenergy in rural areas where it is produced. The residue–to-produce ratio (RPR) of the crop is 1.25 kg of biowaste for 1 kg of SS produced. The solid residue that results from the crop can be subjected to gasification to produce combustible gases: carbon monoxide (CO), hydrocarbon gases (total hydrocarbons) and hydrogen. The combustible gases can be piped into a burner for heat or into a Compression Ignition (CI) engine for electricity generation. This will enhance energy security as well as energy equity in rural areas in Nigeria and sub-saharan African countries where the crop is also produced. This research was aimed at optimising the gasification of SS stalk residue to maximise the yield of combustible gases from the first stage of the process. The restricted ventilation cone calorimeter method was used to gasify SS stalks on a laboratory scale. The test was carried out at air flow rates per exposed flat surface area of 9, 11.2, 12.9, 14.3, 15.5, 16.3, and 19.2 g/s·m2 respectively, which controls the gasification rate or power output. The speciation of the gases that evolved from the gasification of the biomass samples was carried out by an FTIR that was calibrated for 60 species. Current uses of biomass residues in open fire heating generates toxic fine particulate emissions and this work aimed to show that this was not a greater problem with gasification. A dynamic electrical mobility particle spectrometer (DMS500) was used to measure the particulate size distribution and concentration, as an efficient gasifier should not be generating major yields of soot, which would be a problem for a downstream reciprocating engine. The optimum equivalence ratio (Ф) for the best energy transfer to the gaseous products was 2.1, which was similar to previous work on pine using this equipment where the optimum equivalence ratio was 2.8. The hot gases efficiency at the optimum Ф was 81%, which compares well to that of 78% for pine. [ABSTRACT FROM AUTHOR]

This paper presents the effects of equivalence ratio ( Φ ) on mixture properties, combustion, thermal efficiency and exhaust emissions of a pilot-ignited natural gas (NG) engine using diesel as an ignition source at low loads. Φ was changed in a wide range (0.4–0.9) by changing throttle opening with other combustion boundaries being unchanged with different NG percentage energy substitutions (PES) at 1335 rpm and 25% load. The experiments were arranged based on an electronically controlled heavy-duty, turbocharged, intercooler, 6-cylinder and four-stroke pilot-ignited NG engine. Thermodynamic properties of the NG–air mixture with decreasing air were analyzed theoretically using thermodynamic relations and ideal gas equations. In addition, the time-sequenced characteristic and the heat release rate (HRR)-imbalanced characteristic were found in the engine combustion process, with their quantitative indicators being defined. The results show that as Φ increases, the density, the specific heat ratio and the heat capacity of the NG–air mixture decrease, but the gas constant and the specific heat capacity increase. CA10 moves far away from top dead centre (TDC), CA50 moves near TDC first and then far away with increasing Φ . Three kinds of profiles of HRR curves with different time-sequenced and HRR-imbalanced characteristics appear as Φ increases. The engine thermal efficiency increases first and then decreases with increasing Φ . In addition, as Φ increases, total hydrocarbons (THC) emissions decrease significantly (decrease by 97% as Φ increases from 0.45 to 0.91 with 90% PES) and nitrogen oxides (NOx) emissions increase first and then decrease. [ABSTRACT FROM AUTHOR]

• Mixing lengths variation impact on flame stability is more pronounced for LPG than NG. • Increasing the disk slot diameter enhanced the NG flame stability. • Increasing the disk slot diameter reduced the LPG flame stability. • Wider slot turbulence generator disc produced homogeneous radial profiles of equivalence ratio. This study is geared toward generating highly stabilized partially premixed flames at various levels of turbulence and partially premixing. Therefore, with the help of the laser-induced breakdown spectroscopy (LIBS) technique, a new burner was constructed and employed to quantitatively estimate the mixture equivalence ratio (Φ) within the flame. Two turbulence generator disks, five degrees of partial premixing, and two fuels were used to assess the flame stability. Natural gas (NG) and liquefied petroleum gas (LPG) were used as fuels. The LIBS spectrum's most common atomic emission lines which include hydrogen, nitrogen, oxygen, and carbon, were chosen to establish the correlation between emission lines' intensity and the flame's mixture equivalence ratio. The results showed that the stability of NG flame was less sensitive to the variation of the partially premixing levels. In contrast, the LPG flames were more susceptible to the variation of the mixing degree. At a lower level of partially premixing, NG flames were more stable, and as the mixing degree increased, the stability of NG flames was reduced compared to LPG flames. In addition, the results showed that the equivalence ratio radial profiles are more homogeneous and have lower RMS fluctuation for the wider slot of the turbulence generator disc. Furthermore, the larger turbulence generator disk's higher turbulent intensity contributed in posting the mixing process and enhancing mixture homogeneity over even shorter recess distances than the smaller disk generator. [ABSTRACT FROM AUTHOR]

[Display omitted] • Two stages of combustion (LTR, HTR) under HCCI mode. • Knock under HCCI mode occurs beyond 0.55 equivalence ratio. • Indicated thermal efficiency increases till 0.55 equivalence ratio. • Ultra-Low NO x (10–15 ppm) and zero smoke emissions. • Significant reduction in CO and HC emissions. This study deals with the effects of different equivalence ratios on combustion with a knock, performance, emissions of a DME fueled automotive CI engine under HCCI mode. The conventional common rail direct injection (CRDI) CI engine was modified to run with DME fuel (manifold injection) under HCCI mode. The experiments were conducted on the engine at different equivalence ratios at a constant speed of 1400 rpm. Two stages of combustion (low-temperature reaction (LTR) region and high-temperature reaction (HTR) region) were observed under HCCI mode. The LTR region occurs at the temperature range from 700 to 750 K. In contrast, the HTR region was at a temperature beyond 1000 K. The cyclic variations with the coefficient of variation (COV) of maximum in-cylinder pressure, indicated mean effective pressure with knock peak pressure and knocking intensity (KI) were analyzed for assessment of combustion stability. Based on the knocking intensity, the optimum equivalence ratio with controlled auto-ignition (CAI) was found as 0.55, with peak in-cylinder pressure of 70.87 bar and peak in-cylinder temperature of 1815 K. The COV for all the parameters and KI found below the limit value (COV limit − 5% and knock limit − 5 MW/m2) till 0.55 equivalence ratio and beyond the equivalence ratio, the knock occurred resulting in uncontrolled auto-ignition (UAI). Ultra-Low NO x (10–15 ppm) and zero smoke emissions with a considerable reduction in CO and HC emissions were observed at a higher equivalence ratio from DME fueled HCCI mode. However, CO and HC emissions were higher at lower equivalence ratio. This study shows that the simultaneous reduction of NO x and smoke emissions could be achieved in a DME fueled HCCI engine. [ABSTRACT FROM AUTHOR]

The primitive purpose of the present paper is to address the wall heat flux modeling of n-heptane fueled direct injection (DI) diesel engine with the application of a coupled computational fluid dynamics (CFD) and artificial neural network (ANN) approach. The numerical model was established for a Ford 1.8 l DI diesel engine equipped with a prototype Lucas CAV HPCR system, and an Allied Signal VGT. The turbulent flows within the combustion chamber were simulated using the RNG k – ɛ turbulence model. The input parameters of crank angle, mass flux, liquid mass evaporated, equivalence ratio, turbulence kinetic energy, and pressure were included in the system. It was concluded that more wall heat flux was transferred with fuel injection around TDC and along with combustion initiation for 2000 rpm and the higher pressure can be achieved at the same engine speed. Furthermore, a feed-forward with back propagation learning algorithm and Levenberg–Marquardt training technique were employed for various ANN modeling implementations. At 17 neurons in the hidden layer, the MSE equal to 0.5217 was yielded and the coefficient of determination values of 0.99 and 0.99 were obtained for training and testing phases. The optimum values of the learning rate and momentum were also yielded at 0.6 and 0.7, respectively. [ABSTRACT FROM AUTHOR]

This study is dedicated to explore the effect of in-cylinder combustion parameters on the accumulated heat release at rated EGR levels using the CFD implemented code data which were coupled to the artificial neural network (ANN) approach to construct a model to predict the accumulated heat release of DI diesel engines. To this end, at two different engine speeds of 3000 and 4000 rpm, crank angle, equivalence ratio, turbulence kinetic energy and temperature varied to obtain the corresponding accumulated heat release data at three EGR levels of 0.2, 0.3 and 0.4. It was discovered that that application of higher EGR is conducive to temperature reduction while it leads to the decreased equivalence ratios. It was also concluded that the accumulated heat increases with equivalence ratio and temperature but decreases with increment of Exhaust Gas Recirculation (EGR) levels. Numerous ANN modeling implementations were carried out using different training algorithms of trainlm , trainscg , traingdx , and trainrp at diversified number of neurons in the single hidden layer. At 17 neuron numbers in the hidden layer, the trainlm method denoted MSE equal to 0.1057 which was the best performance among the various implemented models. The coefficient of determination ( R 2 ) values equal to 0.99 and 0.99 were obtained for training and testing phases. The obtained results confirm the promising ability of ANN for the prognostication of accumulated heat release of DI engines. [ABSTRACT FROM AUTHOR]

Highlights: [•] The ROP of CO of R167, R166, R79 and R23 markedly decrease with increasing Kv and . [•] The CO formation is controlled by the consumption reaction R99 in MILD combustion. [•] The reaction R99 has been a direct reaction in MILD combustion. [ABSTRACT FROM AUTHOR]

Highlights: [•] Air motion caused by ultra-high injection pressure single and twin diesel sprays is investigated. [•] CFD/empirical analysis shows more entrainment at higher injection pressure and ambient density. [•] Gas aspiration, recirculation and pushed out zones for single sprays are identified. [•] Effects of incidence angle and separation distance of twin sprays on spray parameters are studied. [•] Vortical structures are used to characterize the spray-induced air motion in twin sprays. [ABSTRACT FROM AUTHOR]

Abstract: Dimethyl ether (DME) is the simplest ether and it is used as an alternative fuel or fuel additive to reduce toxic emissions from combustion processes. The effects of DME on n-butane oxidation were investigated for two different concentrations of DME in the fuel mixture (i.e., 20% and 50%) and two different fuel-rich equivalence ratios (i.e., 2.6 and 3.0) using detailed chemical kinetic modeling. Reactor model was selected as atmospheric-pressure, adiabatic, tubular reactor, operated under laminar flow conditions. The concentration profiles of major, minor, and trace species were obtained for n-butane/DME/oxygen/argon at six different reactor inlet temperatures, and the results were compared with those attained for pure n-butane oxidation case (n-butane/oxygen/argon). Dimethyl ether addition decreased formations of various toxic species such as carbon monoxide, aromatic species, and polycyclic aromatic hydrocarbons, while it increased the formations of formaldehyde and acetaldehyde. Increasing equivalence ratio increased the formations of carbon monoxide, methane, aromatic species, and polycyclic aromatic hydrocarbons, while its effects on formaldehyde and acetaldehyde were not pronounced under the conditions studied. [Copyright &y& Elsevier]