Staged Combustion Research Articles

A comparative study on the premixed ammonia/hydrogen/air combustion with spark ignition and turbulent jet ignition

Turbulent jet ignition (TJI) is an advanced ignition mode that can enhance ignition and combustion performance, and it is a potential ignition strategy for ammonia/hydrogen internal combustion engines. This study aims to investigate the ignition and combustion characteristics of lean ammonia/hydrogen/air ignited by TJI, and the different ignition modes were compared. The results indicate that active TJI with auxiliary hydrogen injection in the pre-chamber improves the ignition and combustion performance of ammonia/hydrogen/air, and the appropriate shift of the pre-chamber equivalence ratio towards the rich side is considered optimal. As the ammonia fraction increases, the ignition mechanism in the main chamber changes from flame ignition to jet ignition. The advantage of TJI is mainly shown in high ammonia fraction mixtures, which is reflected in significantly lower ignition delay and combustion duration. In addition, TJI reduces the sensitivity of combustion to ammonia fraction compared to SI, due to the high ignition energy, initial flame area and turbulence provided by the hot jet. In TJI mode, the increase in jet velocity seems to be detrimental to the radial development of flames near the orifice, which may result in an increase in the duration of the final stage of combustion.

Linear adiabatic analysis for general-relativistic instability in primordial accreting supermassive stars

Accreting supermassive stars of $ will eventually collapse directly to a black hole via the general-relativistic (GR) instability. Such direct collapses of supermassive stars are thought to be a possible formation channel for supermassive black holes at $z > 6$. In this work, we investigate the final mass of accreting Population III stars with constant accretion rates between $0.01$ and 1000\ We determined the final mass by solving the differential equation for GR linear adiabatic radial pulsations. We find that models with accretion rates $ experience the GR instability at masses depending on the accretion rates. The critical masses are larger for higher accretion rates, ranging from $8 for $0.05\ to $ for $1000\ The $0.05\ model reaches the GR instability at the end of the core hydrogen burning. The higher-mass models with higher accretion rates reach the GR instability during the hydrogen burning stage.

Biofuel–Electric Hybrid Aircraft Application—A Way to Reduce Carbon Emissions in Aviation

As global warming intensifies, the world is increasingly concerned about carbon emissions. As an important industry that affects carbon emissions, the air transportation industry takes on the important task of energy saving and emission reduction. For this reason, major airlines have designed or will design different kinds of new-energy aircraft; however, each aircraft has a different scope of application according to its energy source. Biofuels have an obvious carbon emission reduction effect in the whole life cycle, which can offset the drawback of the high pollutant emission of traditional fossil fuels in the preparation and combustion stages. At the same time, a battery has zero emissions in the operating condition, while the low energy density also makes it more applicable to short-range navigation in small aircraft. In this paper, the development direction of a biofuel–electric hybrid aircraft is proposed based on the current development of green aviation, combining the characteristics of biofuel and electric aircraft.

Mechanistic study on the modification of Al NPs by PVDF with different interfacial binding methods to suppress agglomeration and promote combustion

The modification of aluminum nanoparticles (Al NPs) by polyvinylidene fluoride (PVDF) can significantly enhance their reaction characteristics and reduce combustion agglomeration. However, the impact and underlying mechanisms of changes in interfacial binding methods between them on the reaction process remain unclear. In this study, the oxidation and combustion processes, as well as the agglomeration behavior of Al NPs under three different interfacial binding methods (mixing, with oxygenation layer coating, and without oxygenation layer coating) between Al NPs and PVDF were investigated by utilizing ReaxFF molecular dynamics simulations, density functional theory calculations, and ignition experiments. The results indicate that changes in the interfacial binding method significantly affect the oxidation behavior of Al NPs, and PVDF molecules exhibit different decomposition pathways on the surfaces of Al and Al2O3. During the combustion stage, different interfacial binding methods determine distinct reaction modes between Al NPs and PVDF. Based on these findings, a reaction mechanism for Al NPs and PVDF is proposed. This study addresses the gap in the field of the impact of interfacial binding methods on the reaction processes between Al NPs and PVDF, laying a theoretical foundation for future research and providing guidance for the practical application of Al-based modified fuels.

Thermal decomposition kinetics and spectral analysis of mixed ester propellants

Mixed ester propellants are extensively used in artillery weapons due to their superior mechanical properties. This study investigates the decomposition kinetics and mechanism of mixed ester propellants using Differential Scanning Calorimetry (DSC) and Thermogravimetry-Fourier Transform Infrared Spectroscopy-Mass Spectrometry (TG-FTIR-MS). Kinetic parameters were determined via a model-free approach, while laser diagnostic technology captured radicals and molecular fragments during the decomposition-combustion process. Results indicate a single exothermic peak during decomposition, with apparent activation energy (Ea) increasing initially (0.01 ≤ α ≤ 0.4) and stabilizing around 278.43 kJ/mol later. The thermal decomposition behavior of mixed ester propellants follows the Sestak-Berggren equation, consistent with an autocatalytic reaction. NO2 generated during decomposition acts as a catalyst, accelerating the decomposition process. CN* radicals were detected throughout the combustion stages, indicating the resilience of the CN bond against breaking. This study can provide a theoretical basis for predicting fire development and for the selection or design of fire sensors.

Characterisation of wood combustion and emission under varying moisture contents using multiple imaging techniques

In this study, the complex combustion process of natural wood, with different moisture contents, has been characterised using a pioneering approach that integrated Mid-Wavelength Infrared (MWIR) Hyperspectral Imaging (HSI), Schlieren imaging, Long-Wavelength Infrared (LWIR) thermal imaging and visual imaging. The experiment involved the combustion of oak wood samples with moisture contents varying from dry (defined as 0%) to 30%. This setup enabled a detailed investigation into the combustion process, examining aspects such as weight loss rates, thermal behaviours, spectral radiance of gas emissions and flow structures. Higher wood moisture levels were associated with decreased fuel consumption and sustainability of the combustion. The radiance ratios of CH4/CO2 and CO/CO2 were higher in samples with increased moisture during the initial combustion stages, indicating reduced efficiency in gas-phase combustion. The different combustion stages: flaming and smouldering, were characterised by the thermal profiles. Results demonstrated the moisture level significantly affected the initiation of both flaming and smouldering combustion, as moisture can suppress the thermal pyrolysis of flammable gases during ignition and the evaporation of the residual water can cool the surface locally after ignition. Additionally, the samples with high moisture content transitioned more quickly and directly to smouldering combustion. This transition is further evidenced by lower emissions of flammable gases and a significant decrease in the intensity gradient of flow structures in high moisture conditions. This comprehensive study highlights the potential of multi-wavelength techniques in combustion research. The findings have significant implications for optimising biomass combustion processes in various applications, contributing to the broader field of energy and combustion science.

DETERMINATION OF THE FEATURES OF THERMAL DECOMPOSITION OF SUNFLOWER HUSK IN A FLUIDIZED BED

Sunflower husk (SH) is a plant waste fuel. The carbon content in different samples of SH ranges from 40.5% to 54.5% in an operating state, with ash content ranging from 1.5% to 8.5%, moisture content ranging from 6.9% to 9.5%, chlorine content ranging from 0.05% to 0.3%, and lower heating value ranging from 14.5 MJ/kg to 20.5 MJ/kg. These characteristics make it a suitable substitute for fossil fuels in power boilers. Waste-to-energy (WTE) technologies are rapidly developing worldwide, offering the potential for generating renewable energy from waste, including agricultural and food industry waste such as SH. According to our estimates, Ukraine has an annual energy potential of approximately 3.4 million tons of SH or about two million tons of fuel equivalent. Approximately half of this volume is currently being burned in oil extraction plants' boilers; however, up to one million tons of SH end up in landfills annually, resulting in significant energy losses. To develop new and improve existing WTE technologies that utilize SH as a fuel source, it's essential to understand the thermal processing characteristics of SH under conditions similar to those found in different zones within real power boilers - specifically the heating of fuel particles at rates up to 500 °C/s over a temperature range of 500–1000 °C. In this study, we aimed to investigate the thermal processing characteristics by subjecting SH particles within a laboratory fluidized bed reactor to high-speed heating within the aforementioned temperature range. During rapid heating between 500–1000 °C temperatures range, SH particles undergo conversion into volatile compounds and solid carbon residue. Two distinct stages can be observed on the dynamic yield curves for volatiles. The release and burnout of volatiles occurs during the first stage while the second stage involves coke ash residue burnout. We obtained empirical temperature-dependents for total heat treatment time and carbon residue burnout time under fast heating conditions in the investigated temperature range. The stage of carbon residue combustion is the most enduring and determines the overall duration of thermal treatment. This stage determines the degree of fuel transformation, especially in cases where low-reactivity carbon residue enters the low-temperature combustion chamber area of the boiler. The obtained regularities have practical significance in designing combustion chambers for thermal processing of sunflower husk.

Evolution of rotating massive stars adopting a newer, self-consistent wind prescription at Small Magellanic Cloud metallicity

Aims. We aim to measure the impact of our mass-loss recipe in the evolution of massive stars at the metallicity of the Small Magellanic Cloud (SMC). Methods. We used the Geneva-evolution code (GENEC) to run evolutionary tracks for stellar masses ranging from 20 to 85 M⊙ at SMC metallicity (ZSMC = 0.002). We upgraded the recipe for stellar winds by replacing Vink’s formula with our self-consistent m-CAK prescription, which reduces the value of the mass-loss rate, Ṁ, by a factor of between two and six depending on the mass range. Results. The impact of our new [weaker] winds is wide, and it can be divided between direct and indirect impact. For the most massive models (60 and 85 M⊙) with Ṁ ≳ 2 × 10−7 M⊙ yr−1, the impact is direct because lower mass loss make stars remove less envelope, and therefore they remain more massive and less chemically enriched at their surface at the end of their main sequence (MS) phase. For the less massive models (20 and 25 M⊙) with Ṁ ≲ 2 × 10−8 M⊙ yr−1, the impact is indirect because lower mass loss means the stars keep high rotational velocities for a longer period of time, thus extending the H-core burning lifetime and subsequently reaching the end of the MS with higher surface enrichment. In either case, given that the conditions at the end of the H-core burning change, the stars will lose more mass during their He-core burning stages anyway. For the case of Mzams = 20–40 M⊙, our models predict stars will evolve through the Hertzsprung gap, from O-type supergiants to blue supergiants (BSGs), and finally red supergiants (RSGs), with larger mass fractions of helium compared to old evolution models. New models also sets the minimal initial mass required for a single star to become a Wolf-Rayet (WR) at metallicity Z = 0.002 at Mzams = 85 M⊙. Conclusions. These results reinforce the importance of upgrading mass-loss prescriptions in evolution models, in particular for the earlier stages of stellar lifetime, even for Z ≪ Z⊙. New values for Ṁ need to be complemented with upgrades in additional features such as convective-core overshooting and distribution of rotational velocities, besides more detailed spectroscopical observations from projects such as XShootU, in order to provide a robust framework for the study of massive stars at low-metallicity environments.

Evaluating potential of increasing flow intensity and reducing crevice volume to improve thermal efficiency and hydrocarbon emission in spark-ignition natural gas engines

Low thermal efficiency and high hydrocarbon emissions caused by slow combustion rates and high methane emissions hinder the development of natural gas (NG) engines. Engine structure modifications have always been an important means of improving the combustion effects of engines. This study investigates the effects of combustion chamber improvements on the thermal efficiency and hydrocarbon emissions of stoichiometric spark-ignition NG engines through numerical simulations. The improvements aimed to enhance squish flow, organize high tumble flow, and reduce crevice volume. The results indicate that strengthening the squish intensity by increasing the squish area of the piston can accelerate flame propagation and reduce exhaust loss. However, this improvement is hindered by a significant rise in heat transfer loss, which limits further gains in thermal efficiency. Compared to the traditional high swirl combustion chamber, a high tumble combustion chamber (dual straight intake port combined with a hemispherical piston) can promote flame propagation in the early stages of combustion and increase thermal efficiency by 1.43%. However, the accelerated flame propagation rate resulting from increased flow intensity has little potential to reduce hydrocarbon emissions due to the small contribution of flame quenching to unburned hydrocarbon production. The primary source of hydrocarbon emissions from NG engines is unburned methane in the piston crevice. Therefore, when addressing hydrocarbon emissions from NG engines, the first consideration should be to minimize the volume of the piston crevice, which will significantly reduce methane emissions and incomplete combustion loss.

Study on auto-ignition characteristics of N-heptane/methanol/ammonia mixed fuel

An investigation was carried out on the auto-ignition properties of n-heptane/ammonia/methanol fuel mixtures, characterized by a small amount of n-heptane concentration (2 %). The experiment was conducted using a rapid compression machine (RCM) within a temperature bracket of 790–1112 K, under pressures of 1.5 and 3.05 MPa. The fuel incorporates a methanol content of 0 to 98 %, with mixture equivalence ratios set at 0.5, 1.0, and 2.0. Observations indicate that the fuel component and the equivalence ratio influence the ignition delay time (IDT). It was found that the reactivity of the mixtures increases upon the induction of n-heptane. For the 1 % CH3OH mixture, the IDT is the shortest with stoichiometry. When the methanol content increases, the IDT decreases with increased equivalence ratios. In addition, a detailed mechanism has been modified to improve the predictive performance of the experimental mixture. The chemical reaction kinetic analysis shows that n-heptane promotes the combustion reaction. The consumption of fuels in the combustion process follows a sequential pattern, with n-heptane being consumed first, followed by methanol and ammonia. Both n-heptane and methanol experience nearly complete consumption before the combustion. In contrast, the consumption of ammonia proceeds at a significantly slower rate and is primarily consumed in substantial quantities at the combustion stage. The dehydrogenation reactions compete for key radicals, which inhibit the oxidation process of the system. The extraction of H from methanol by HO2 to produce H2O2 is a sensitive reaction to promote combustion.

NO emission during the co-combustion of biomass and coal at high temperature: An experimental and numerical study

Biomass/coal co-combustion presents a promising avenue for reducing carbon dioxide emissions. However, the mechanism of NO generation during co-combustion in pulverized coal-fired furnaces remains unclear. Herein, this study investigates NO emission characteristics at elevated temperatures, emphasizing mechanistic understanding. The experiments and simulations were conducted across temperatures ranging from 1000 °C to 1600 °C. In addition to exploring how fundamental conditions (temperature, oxygen concentration, and biomass blending ratio (BBR)) affect NO generation, the study also investigates the interactions between biomass and coal, as well as between fuel NO and thermal NO. The results indicate that the change in NO production with increasing temperature is non-monotonic, attributable to the combined contribution of fuel NO and thermal NO. Moreover, the interaction between biomass and coal manifests predominantly during the char combustion stage, owing to the disparate combustion characteristics and reactivity of biomass char and coal char. Numerical simulations were highly consistent with the experimental findings, underscoring a reduction in the rate of production (ROP) for major elementary reactions leading to NO formation as the BBR increased, consequently mitigating NO emissions. By integrating experimental insights with reaction kinetics calculations, this study proposes pathways for fuel NO and thermal NO generation during co-combustion at high temperatures, involving precursors such as NCO, HNO, and NH. These findings offer a deeper understanding of the mechanism of fuel NO and thermal NO emission during co-combustion at high temperatures.

Visualization study on the ignition and combustion characteristics of methane/hydrogen ignited by diesel

Nowadays, internal combustion engine fuels need to shift from diesel and gasoline to clean fuels. As a low-carbon fuel, methane has attracted widespread attention. Diesel ignition of premixed methane is one of the measures to achieve effective application. However, limited by the equipment and the active reactivity of methane, there are few basic visualization studies on that, resulting in a lack of deep understanding of ignition and flame propagation processes. Therefore, this paper studies the diesel ignition and combustion characteristics of diesel-ignited methane and the promotion of 10 and 20% (volume fraction) hydrogen on combustion performance based on the rapid compression and expansion machine. The experimental results indicate that methane has a suppressive effect on the ignition of diesel. The combustion process can be mainly divided into the diesel ignition stage, the premixed combustion stage, and the fuel burnout stage. Blending 10% and 20% hydrogen has little effect on the diesel ignition stage, but has a relatively great impact on the premixed combustion stage. The flame propagation can be improved with hydrogen blending, but that is limited due to the effects of ignition and wall. The combustion duration can be shortened with hydrogen blending, mainly for the CA50-90, especially at a lower equivalence ratio. The combustion efficiency can be improved with hydrogen blending. Overall, the strategy of blending hydrogen is considered more suitable for large bore engines and the results can deepen the understanding of the combustion process of diesel-ignited methane/hydrogen/air mixtures and have value for the relevant engine development so that the energy shortage and environment pollution issues can be alleviated.

Research on early identification of burning status in a fire area in Xinjiang based on data-driven

The early combustion stage in coalfield fire areas (CFA) is the initial phase of coal fire development and spread, making the effective identification of the combustion state crucial. To effectively identify the early combustion state of CFA, this study investigates the CFA in Xinjiang, China. Based on the abnormal characteristics of shallow soil and surface space, a data-driven evaluation method for identifying early combustion states of CFA is proposed. Construct a soil-gas dual-layer early combustion state assessment model of CFA. The evaluation model was applied and validated in the coal fire areas of Xinjiang, China.The results show that there are obvious differences in the spatial distribution of auto-ignition gas, temperature, humidity, surface vegetation coverage and other indicators in the shallow soil and surface space of the coal fire area. The early combustion states of coal fires in areas A, B, and C in the survey area are all in the combustion stage, the combustion state in area D is in the oxidation stage. The accuracy of the evaluation method is bidirectionally verified by the radon measurement method and the infrared thermal imaging detection method. It provides a theoretical basis for controlling the development and spread of coalfield fire areas.

Enhancing Combustion Efficiency: Utilizing Graphene Oxide Nanofluids as Fuel Additives with Tomato Oil Methyl Ester in CI Engines

The research aims to conduct a thorough examination of the combustion, injection, performance, and emission characteristics of a diesel engine below different engine loads, as well as the synthesis of graphene oxide (GO) nano fuels and their utilization in combination with Tomato Oil methyl ester (TOME) and diesel fuel blend. The (GO), plays a vital role in the pre-mixed combustion phase of diesel engines. Addition of GO nano fuels enhances the high-pressure combustion stage, resulting in increased maximum pressure and heat release rate (HRR). TOME (B20GO75) and TOME (B20GO100) exhibit comparable HRR to diesel due to improved fuel characteristics and quicker ignition delay duration. While TOME (B20) shows a slight decrease in (BTE) compared to diesel, the addition of GO improves BTE, with TOME (B20GO50) displaying the highest BTE at full load, indicating enhanced combustion efficiency. Moreover, GO addition leads to a reduction in carbon monoxide (CO) and hydrocarbon (HC) emissions, with emissions decreasing as the dosage of GO increases. However, NOx emissions initially decrease with TOME (B20) compared to diesel but increase with higher GO dosages. Smoke emissions increase with TOME (B20) but decrease with higher GO dosages. Overall, the incorporation of GO nano fuels into TOME biodiesel fuel blends demonstrates potential for improving engine performance and reducing emissions.

Impact of injection pressure on a dual-fuel engine using acetylene gas and microalgae blends of chlorella protothecoides

The amount of fossil fuel usage in compression ignition (CI) engines is greatly reduced when biodiesel is used. The primary disadvantage of using biodiesel is that, due to its high viscosity, which causes fuels to remain unburned during the premixed combustion stage, leads to lower brake thermal efficiency (BTE). Gaseous fuels predominantly reducing emissions in CI engines due to its complete burning without leaving any carbon traces. Fuel injection pressure (FIP) is one of the factors which is influencing the combustion phase because they are used to optimize fuel particle atomization. The current study examines engine parameters for a dual fuel engine that runs on biodiesel blends made from 20 % methyl ester of chlorella protothecoides micro algae (B20MEOA) and acetylene gas under variable Fuel injection pressure (FIP) ranging from 200 bar to 240 bar with 10-bar steps. According to the experimental results, when 3 LPM of acetylene gas is supplied along with the intake air and B20MEOA supplied at a FIP of 240 bar, the emissions such as smoke opacity, hydrocarbon (HC), and carbon monoxide (CO) are reduced by 16.9 %, 8.3 %, and 15.4 %, respectively, whereas the oxides of nitrogen (NOx) increases by approximately 7 % when compared to B20MEOA alone operation.

Combustion and particulate I/SVOC characteristics of an aero-engine combustor with dual-stage under operational power and injection pressure

The combustion and particulate intermediate-volatile/semi-volatile organic compound (I/SVOC) emission characteristics of a technology of low emission stirred swirl aero-engine combustor, which was operated under dual combustion stages of diffusion and premixed-dominant combustion have been investigated. The operational conditions of the combustor were combustor power (7 %–100 %) and injection pressure (IP, 1.5–6.0 MPa). Initially, the ignition and extinction boundary regions were identified, and within the ignition region, the emitted I/SVOCs were comprehensively resolved via a two-dimensional gas chromatography-time of flight mass spectrometer (GC × GC-ToFMS). It is found that I/SVOCs were remarkable under diffusion combustion, and reduced significantly by 61 % under premixed-dominant combustion (at 3.0 MPa IP). Besides, I/SVOCs were suppressed prominently by 49 % as IP increased from 1.5 MPa to 6.0 MPa (at 30 % combustor power). Based on the Response Surface Method (RSM), a model has been established to simulate the particulate I/SVOCs. The simulation model reveals that high Power-IP (100 % power, 6 MPa IP) could reduce I/SVOCs maximally by 84 %, 78 % and 56 % for n-alkanes, cycloalkanes and PAHs, respectively in comparison with low Power-IP (30 % power, 1.5 MPa IP). The essence of the findings reveals that domination of premixed combustion with promotional atomization inhibits aviation-emitted organic particles effectively.

Effects of inorganic sodium on the soot generation of coal particle: Insights with PLIF and DFT calculation

Inorganic sodium in low rank coal exerts significant influence on the soot generation in the early stage of combustion. The effect of inorganic sodium on the PAHs, soot formation and OH generation of coal particle was investigated on a concentrating light heating platform using Planar Laser Induced Fluorescence (PLIF) of OH radicals and PAHs. The generation characteristics of PAHs during coal particles pyrolysis with the temperature of 500 °C and 800 °C were also investigated. Raw coal, acid-washed (AW) coal and AW impregnated with different mass ratio of NaCl (1 %, 3 % and 5 % NaCl-loaded) were studied. Theoretical calculations using density functional theory was performed to explore the effect of sodium on the conversion of small molecule to macromolecular substances during early coal combustion. The results showed that the NaCl addition inhibited the OH radical and soot generation in the early stage of coal combustion, as demonstrated by the reduced OH intensity, and decreased soot intensity by observing the flame luminosity for the NaCl-loaded coals, compared with AW coal. Simultaneously, the PAHs generation was significantly reduced for NaCl-loaded coals, both at the early stage of coal combustion and under pyrolysis conditions, compared with AW coal. The quantum chemistry calculation revealed that the formed C-Na structure enhanced coal structural stability, making the chemical bonds less prone to cleavage, thereby inhibiting the volatile and PAHs release during the early stage of coal combustion. On the other hand, the released Na+ can catalyze the addition reactions of small ring PAHs (naphthalene, naphthalene derivatives) to larger ring PAHs (phenanthrene, pyrene), which would promote the PAHs and soot generation in homogeneous reactions, explaining the highest PAHs with 3-to-5 rings for 5 % NaCl-loaded coal among NaCl-loaded coals. The above two kings of reaction pathways were related to soot formation, but considering the results of reduced soot yield after NaCl addition, the former dominated. Furthermore, the results from the two pathways also illustrated that the Na inhibit the OH formation, which was corresponding to the decreased OH intensity for NaCl-loaded coals, as detected by OH-PLIF, and thus was detrimental to soot consumption. Therefore, the presence of Na in the early stage of combustion inhibited the release of volatiles and PAHs, which was the main reason for the decreased soot generation amount of NaCl-loaded coal.

Fire zone forefront residual tar combustion characteristics and functional group transformation mechanism: Appling the based on thermogravimetric and in-situ high-temperature FTIR

When preventing spontaneous combustion disasters in extinguished coal fire zones, the influence of the tar re-ignition phenomenon is often overlooked. Therefore, in this study, using thermogravimetric analysis and high-temperature (630 °C) in-situ infrared technology, the combustion reactivity and real-time evolution mechanism of key functional groups for tar were deeply discussed under oxygen-lean conditions. Additionally, a novel two-step baseline drift correction method for spectral graphs at high temperatures is proposed. The results showed that tar combustion could be divided into two stages: pyrolysis-volatilization mass loss and crosslinked structure oxidation combustion. Reduced oxygen concentration limits the violent and stable rapid combustion of tar-crosslinked structures and reduces the re-ignition tendency of tar. Among, with a decrease in oxygen concentration, H (burnout performance), Cb (the reaction ability in the early stage of ignition) and S (overall combustion performance) decreased, while Ti (mass loss endpoint temperature), Th(burnout temperature), and HF (combustion stability) increased. This provides information for the selection of a reasonable process to prevent tar re-ignition. The contents of methylidene and aldehyde carbonyl in tar increased by approximately 2 and 3.7 times, respectively, compared to those of raw coal. In the first thermal decomposition stage of tar, the aromatic hydrocarbons were relatively stable under the action of the structural framework, whereas aliphatic hydrocarbons continued to decrease. Before burnout, a decrease in oxygen concentration increased the crosslinking ability of tar and delayed the tar burnout process. The contents of aromatic hydrocarbons and aldehyde groups are highly sensitive to the evolution of the oxygen concentration. Specifically, the Kubelka–Munk value of the aromatic hydrocarbons increased from 0.015 to 0.10, while that of aldehyde carbonyl groups increased from 0 to 0.23. Additionally, in the early stage of tar combustion, the volatile mass loss was basically not affected by the time-scale effect, and mass loss endpoint residual mass was approximately 48.11 %. In the later stage of tar combustion, owing to the cumulative of time-scale effect, the tar re-ignition tendency increased, H, Cb, and S increased, and HF decreased from 1.388 to 1.098. This research is helpful guiding disaster prevention and control of tar re-ignition in a fire area, which leads to reburning in the surrounding area.

Research on the impacts of operating frequency at combustion process for opposed single-cylinder free piston generator under direct injection

The opposed single-cylinder free piston generator（OSFPG）has the characteristics of compact structure, variable compression ratio and low emission characteristics of common free piston generator, etc. More, it also has unique features such as high unit power, low vibration noise, and good motion stability, etc. So, in this research, a physical prototype platform based on OSFPG is built. For the key factors, both experiment and simulation have been combined to investigate the changes in OSFPG's work performance under different operating frequencies. It was found that operating frequency affected OSFPG's working characteristics through the entire combustion period. More, the combustion period of each combustion stage increased with the increase of the operating frequency, and the increase amplitude was gradually reduced with the advancing of the ignition advance angle, but the operating frequency affected the after burning period most. When the operating frequency changed from 15 Hz to 35 Hz, the after burning period was extended by at least 100 %. The important thing was that the indicated thermal efficiency of the OSFPG increased first and then decreased with the increase of operating frequency. When the operating frequency was 25 Hz, the OSFPG's indicated thermal efficiency could reach 40.4 %.

Numerical Investigation of In-Cylinder Combustion Behaviors in a Medium-Speed Diesel Engine

Abstract This study aims to advance understanding of in-cylinder combustion processes in medium-speed diesel engines, which are extensively employed in heavy-duty applications where electrification proves inefficient yet remains insufficiently examined in the literature. By modeling a four-stroke engine with dimensions of 210 mm bore and 310 mm stroke, operating at 900 rpm under full load, this research identifies distinct combustion characteristics that differentiate medium-speed engines from their high-speed counterparts. Key findings illustrate that super turbocharging in medium-speed engines enhances the combustion of the fuel–air mixture under elevated temperatures and pressures. Moreover, an increased stroke length promotes gas velocity and turbulence, facilitating fuel atomization and mixing. Notably, rapid fuel ignition occurs near the nozzle due to the high temperature of compressed air, reducing the ignition delay. As a result, the premixed combustion stage nearly disappears, with diffusion combustion dominating, especially pronounced with long-duration injection, a characteristic of medium-speed engines. The study also reveals a more uniform but elevated distribution of nitrogen oxide emissions in medium-speed engines, attributed to prolonged high-temperature conditions that both facilitate their formation. Early stages of diffusion combustion show high concentrations of incomplete combustion products. However, as the combustion process progresses, the conditions favor the complete oxidation of these products at high temperatures, resulting in decreased carbon-based pollutions. In addition, the larger combustion chamber and enhanced turbulence characteristic of medium-speed engines support efficient fuel and air mixing without necessitating the swirl effect required by high-speed engines, diminishing the dependence on wall impingement dynamics for air utilization. Consequently, efficiency optimization strategies for medium-speed engines, emphasizing adjustable injection parameters, encounter fewer constraints than those inherent to the spatial limitations of high-speed engines.