Flame Characteristics Research Articles

Investigation on Flame and Spray Characteristics of Butanol and Lemon Peel Oil Blends With Gasoline Using Optical Engine

Abstract The current research investigates the spray behavior of lemon peel oil (LPO) and butanol in a controlled environment under various engine-like conditions. The liquid spray morphology of both fuel blends is captured using a standard Mie scattering technique, and the liquid spray penetration length is compared to a baseline fuel isooctane. In order to simulate and create engine-like conditions, these experiments are carried out in a constant volume chamber under various pressure and temperature conditions. Furthermore, the combustion quality of binary and ternary blends is studied using an optical gasoline direct injection engine at three different injection timings. According to the constant volume spray study, isooctane has the shortest penetration. Because of its higher boiling point, LPO has a longer liquid spray penetration length. Despite its lower boiling point, butanol penetrates better than isooctane. The temperature was also discovered to influence liquid spray tip penetration length more than pressure significantly. In-cylinder combustion imaging results also revealed that injection timing significantly impacts combustion. Although butanol improves combustion, LPO-dominant blends demonstrated more diffusion burning due to poor evaporation characteristics. The blends prepared for the study were similar to gasoline in combustion conditions. It was discovered that these blends ran optimally without requiring any modifications to existing engines, even though late injection is recommended to improve combustion quality and peak performance.

Numerical investigations on hydrothermal flame characteristics of water-cooled hydrothermal burner

Abstract Supercritical hydrothermal combustion, as a quick homogeneous oxidizing process, offers a promising treatment option for industrial wastewater. This paper established a computational fluid dynamics model of a water-cooled hydrothermal combustion burner to investigate the thermal flame characteristics. The effects of the fuel mass flow rate, fuel concentration, initial reactor temperature, reaction pressure, and oxidant temperature on the thermal combustion ignition were revealed. The results indicate that the fuel concentration (from 10 wt% to 60 wt%) and initial reactor temperature (from 623 to 773 K) had less effect on the ignition temperature. In contrast, the ignition temperature increases by 398 K with increasing fuel mass flow rate (from 24 kg h−1 to 1080 kg h−1). As the oxygen temperature increases (from 273 to 673 K), the ignition temperature gradually decreases to 573 K and then increases. An increase in reaction pressure can facilitate a decrease in ignition temperature to a certain extent, and the optimal reaction pressure is 25 MPa. This study provides a vital reference for a hydrothermal burner’s scale-up design and ignition operation.

Fundamental Study of Premixed Methane Air Combustion in Extreme Turbulent Conditions Using PIV and C-X CH PLIF

This paper presents the flow and flame characteristics of a highly turbulent reactive flow over a backward-facing step inside a windowed combustor. Flow and combustion experiments were performed at Re = 15,000 and Re = 30,000 using high-resolution 10 kHz PIV and 10 kHz PLIF diagnostic techniques. Grid turbulators (Grid) with two different hole diameters (HD of 1.5 mm and 3 mm) and blockage ratios (BR of 46%, 48%, 62%, and 63%) were considered for the turbulence study. Grids introduced different turbulent length scales (LT) in the flow, causing the small eddies and turbulence intensity to increase downstream. The backward-facing step increased the turbulence level in the recirculation zone. This helped to anchor the flame in that zone. The small HD grids (Grids 1 and 3) produced continuous fluid structures (small-scale), whereas the larger HD grids (Grids 2 and 4) produced large-scale fluid structures. Consequently, the velocity fluctuation was lower (~25.6 m/s) under small HD grids and higher (~27.7 m/s) under large HD grids. The flame study was performed at ɸ = 0.8, 1.0, and 1.2 using C-X CH PLIF. An Adaptive MATLAB-based flame imaging scheme has been developed for turbulent reacting flows. Grids 1 and 3 induced more wrinkles in the flame due to higher thermal instabilities, pressure fluctuation, and diffusion under those grids. The flamelet breakdown and burnout events were higher under Grids 2 and 4 due to higher thermal diffusivity and a slower diffusion rate. It was observed that the flame wrinkling and flame stretching are higher at Re = 30,000 compared to Re = 15,000. The Borghi–Peters diagram showed that the flames were within the thin reaction zone except for Grid 1 at Re =15,000, where flames fell in the corrugated zone. It was observed from PIV and PLIF analyses that Re and LT mostly controlled the flame and flow characteristics.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

Numerical investigation of ammonia/coal co-combustion in a low NOx swirl burner

Co-firing of ammonia (NH3) and coal in boilers is a promising technology to reduce CO2 emissions from power plants. However, NH3/coal co-firing in swirl burners can impact the original flame structures and may lead to serious NOx emission issues due to the high concentration of nitrogen in NH3. In this study, a numerical simulation of a low-NOx swirl burner was carried out to investigate the changes in flame structure, carbon emission, and characteristics of NOx generation. The results show that, when the NH3 co-firing ratio is 10 cal%, the flame retains its original swirl structure, and a significant increase in NO formation is observed in situations with different injection positions. When the co-firing ratio increases to 20 cal% and 30 cal% and NH3 is injected through the central air pipe, the flame changes from a swirl flame to an elongated flame. The injected NH3 is wrapped by the low-temperature primary air, separating the NH3 from the high-temperature zone, and pyrolysis becomes the primary reaction pathway for NH3 consumption rather than oxidation. The NO concentrations measured at the chamber outlet can be reduced to 11.2mg/Nm3, which is lower than that from coal combustion. As the co-firing ratio further increases to 50 cal%, NH3 and air are well mixed and burned, forming a “candle-type” flame. This results in a significant rise in NO emissions.

Role of hydrogen on aviation sector: A review on hydrogen storage, fuel flexibility, flame stability, and emissions reduction on gas turbines engines

Hydrogen is being recognized as a versatile energy carrier that can aid in the transition towards a decarbonized energy future due to its high specific energy density. Gas turbine engines are primary source of energy for the modern aircraft. To make the air transportation economical it is essential to achieve higher thermal efficiencies and the capability to operate on carbon–neutral fuels such as hydrogen. The current review article examines the effect of hydrogen in the gas turbine engines. Furthermore, the green routes for the hydrogen production and storage were discussed in detail. Additionally, the implementation of the conceptual design for the onboard hydrogen utilization in the long-range endurance aircrafts was considered. The utilization of hydrogen as a fuel in gas turbines is subject to the influence of different factors, including the functioning of the turbine components, ambient conditions, combustion stability, flame characteristics and operating parameters. This study explores the use of hydrogen as a fuel in aircraft gas turbine engines, which has the potential to reduce NOx emissions, improve fuel efficiency, and increase range while significantly decreasing pollutants such as carbon monoxide. However, modifications to the engine system are necessary, and careful consideration of factors such as fuel storage, combustion, and emissions is essential. While there are significant challenges to storing hydrogen for use as a fuel in aircraft, finding an optimized solution is crucial for reducing carbon emissions and moving towards a more sustainable future. Overall, hydrogen has the potential to be a viable alternative to conventional fuels in aircraft engines.

Influence of sidewall restriction on flame characteristics of double fires in a channel

In this paper, the influences of sidewall restriction on flame characteristics of double fires in a channel were studied. Two typical fire scenarios (center fire and wall fire), different heat release rates and burner separation distances were considered. The results show that ΔT = 300 K can be used as the boundary temperature of the flame. Further, the merging phenomenon is more likely to occur for wall fires, and the critical flame merging separation distances (Sc) of center and wall fires are around 0.13∼0.16 and 0.21, respectively. The flame tilt angle decreases with the increasing fire separation distance, and it is not sensitive to HRR for both center and wall fires. The flame tilt angle of wall fire is slightly larger than that of center fire under a given fire separation distance. Finally, two prediction models are proposed to estimate the normalized effective flame lengths of impinging and non-impinging flames for the center and wall fires in a channel. This study will provide a basis for exploring the interaction mechanisms of double fires under restricted boundary conditions.

Early smoke and flame detection based on transformer

Fire-detection technology plays a critical role in ensuring public safety and facilitating the development of smart cities. Early fire detection is imperative to mitigate potential hazards and minimize associated losses. However, existing vision-based fire-detection methods exhibit limited generalizability and fail to adequately consider the effect of fire object size on detection accuracy. To address this issue, in this study a decoder-free fully transformer-based (DFFT) detector is used to achieve early smoke and flame detection, improving the detection performance for fires of different sizes. This method effectively captures multi-level and multi-scale fire features with rich semantic information while using two powerful encoders to maintain the accuracy of the single-feature map prediction. First, data augmentation is performed to enhance the generalizability of the model. Second, the detection-oriented transformer (DOT) backbone network is treated as a single-layer fire-feature extractor to obtain fire-related features on four scales, which are then fed into an encoder-only single-layer dense prediction module. Finally, the prediction module aggregates the multi-scale fire features into a single feature map using a scale-aggregated encoder (SAE). The prediction module then aligns the classification and regression features using a task-aligned encoder (TAE) to ensure the semantic interaction of the classification and regression predictions. Experimental results on one private dataset and one public dataset demonstrate that the adopted DFFT possesses high detection accuracy and a strong generalizability for fires of different sizes, particularly early small fires. The DFFT achieved mean average precision (mAP) values of 87.40% and 81.12% for the two datasets, outperforming other baseline models. It exhibits a better detection performance on flame objects than on smoke objects because of the prominence of flame features.

OH-PLIF study on the mechanism regulating flame-wall interaction with catalytically active CeO2-ZrO2 coatings

Technically, using catalytically active coatings regulate flame-wall interactions to enhance flame stability in small-scale burners, such as the common ZrO2-based ceramics with doping CeO2 which benefit to improve the high-temperature mobility and reactivity of lattice oxygen as an oxygen source to promote gas phase combustion. Probing by OH-PLIF laser diagnosis, the aim of this study was to investigate the dopant content effect (0–20 wt.% CeO2) on the flame characteristics of methane-air mixtures of varying equivalence ratio (0.9, 1.0, and 1.2) in an adjustable-gap narrow channel (2–12 mm) at a wall temperature range of 373 K to 973 K, including the dynamic flame morphology, flame quenching performance, and near-wall spatial distribution of OH radicals. Results show that the flame-wall distances are strongly correlated with wall temperature, equivalence ratio, and channel scale. After doping CeO2, the measured quenching distances can significantly drop, but do not show a linear downward trend with increasing CeO2 content. The case of 5%CeO2-ZrO2 coating demonstrates a small critical quenching Peclet number at high wall temperatures. The maximum OH fluorescence intensity in flame core areas increases with CeO2 doping into CeO2-ZrO2 at 973 K when the channel clearance is less than 4 mm. However, the absolute concentration of near-wall surviving OH radicals in the presence of 20%CeO2-ZrO2 coating is substantially lower than in the 5%CeO2-ZrO2 case due to differences in the redox properties. In situ diffuse reflectance infrared Fourier transformations spectroscopy tests further indicate that the adsorption quantities of C2H4 and C2H6 species on the coating surface also show a non-monotonous CeO2 dependence, as they are important intermediates in the low-temperature C2 chain reaction of methane.

Waste heat recovery optimization in ammonia-based gas turbine applications

E-fuels are promising alternatives to fossil fuels in the transition towards zero-emission energy system. In this study, a novel chemical-recuperated and humidified gas turbine concept aiming at the application of e-fuel ammonia is proposed to overcome the technical hitches and exploit the waste heat to enhance the cycle performance. The thermodynamic analysis shows that the highest net electrical efficiency (56.7%) under the design conditions exceeds that of the ammonia-fueled Brayton cycle by 20.6%-points. The chemical recuperation of fuel contributes to the efficiency improvement by 7%-points under dry condition, while steam injection provides 8%-points to 12%-points efficiency increase corresponding to the ammonia decomposition ratio of 5%–96%. A non-monotonic relation between the net electrical efficiency and steam-to-air ratio is found to be the result of the competition between the enthalpy change from to the varied steam and air mass flow rates. Analyzing the flame characteristics in the combustor under the design conditions, we found that conditions with high decomposition ratio (>88%) and high steam-to-air ratio (>0.3) exhibit similar flame speed with that of methane fueled combustor and thus existing designs can be reused. The prediction shows the NOx emission can be restricted when the steam-to-air ratio exceeds 0.4.

On the soot formation characteristics of an ammonia-ethylene counterflow diffusion flame based on the orthogonal decoupling method

Ammonia is regarded to be one of the most competitive fuels for achieving carbon neutral due to its carbon-free properties. In this work, the effects of adding ammonia (NH3) to the fuel stream of an ethylene counterflow diffusion flame on the soot formation characteristics were numerically investigated. The orthogonal decoupling method was used to completely separate the dilution, chemical and thermal effects of the added NH3 by introducing two different virtual NH3. The influence of these effects and their interactions were explored by the range analysis, respectively. The results show that the soot volume fraction (SVF) of the flame decreases under the dilution, chemical and thermal effects of NH3, with the dominated effect being the chemical effect, followed by the dilution effect, and the least is the thermal effect. Besides, the chemical-thermal interaction also has an important impact on the SVF. The increase of SVF is dominated by the HACA surface growth, while the decrease of that is dominated by the OH oxidation. The HACA surface growth reaction is inhibited under the three effects of NH3 addition mainly due to a lower concentrations of C2H2 and H radical. The number density of soot increases under the chemical effect, while decreases under the dilution effect. The mole fractions of polycyclic aromatic hydrocarbons (PAHs) reduces under these three effects, leading to a lower inception rate, which in turn has a significant effect on the number density of soot.

Experimental and numerical assessment of the effects of hydrogen admixtures on premixed methane-oxygen flames

Thermal utilization of hydrogen is becoming increasingly important as the climate change progresses. Fossil fuels such as natural gas are replaced by green hydrogen in the natural gas grid to reduce CO2 emissions. However, the switch to hydrogen causes significant changes in combustion properties, such as higher flame temperatures or higher burning velocities. As a result, this change in flame properties impacts application processes. In order to prevent a reduction in quality, the processes must be optimally adapted to the fuel switch. Therefore, the combustion properties of the gas mixture have to be investigated in more detail.In this work a premixed oxy-fuel combustion of hydrogen-methane mixtures under atmospheric conditions has been investigated. Therefore, two scenarios are of further interest, low hydrogen admixture (0 vol-%, 10 vol-%, 20 vol-%, and 35 vol-%) and high hydrogen admixture (50 vol-%, 75 vol-%, 95 vol-%/100 vol-%). In this study, a premixed burner is utilized at three different burner powers with a maximum power of 1.2 kW. The flame characteristics has been analyzed by optical (chemilumicence and OH-PLIF), spectroscopic, and thermal measurements. This study aims to determine the change in flame shape, the temperature profiles at different flame heights, and the species formed in the combustion zone. For this purpose, various compositions of hydrogen-methane-oxygen mixtures at different equivalence ratios ϕ=0.8–1.2 are experimentally investigated at atmospheric conditions. The effect of hydrogen addition to the methane-oxygen flame has been numerically considered utilizing three different mechanisms (GRI 3.0, Glarborg, and Aramco). The adiabatic flame temperature, laminar burning velocity and radical formation have been studied numerically for different gas compositions.

Consequence analysis of heptane multiple pool fire in a dike

Multiple Pool Fire (MPF) reasonably enhances the flame height, the rate of fuel combustion and irradiation due to cascading effect and flame merging of multiple pools. The distribution of temperature and radiative heat flux from the source is the key parameter in predicting safety distance. The current study’s major objective is to develop a Computational Fluid Dynamics (CFD) model to calculate the safety distance of MPF and validate the predicted results (flame temperature, radiative heat flux, CO2 and O2 mass fraction) with the experimental data. The flame structure and its thermal properties have been evaluated using the Reynolds Averaged Navier Stokes (RANS) and the Large Eddy Simulation (LES) model. The CFD results are found to be in close agreement with the experimental findings. The LES turbulence model predicts more accurately the flame structures and the fluctuations with an error of less than 3%, while the Re-Normalization Group k-ε model predicts the average characteristics. This authenticates the accuracy of computational methodology and robustness of the LES turbulence model to predict MPF flame characteristics. Furthermore, this computational methodology can be utilised by the industries for quantitative risk assessment and the worst-case scenario of various pool fires to save the human and material of the workplace beforehand.

Dynamic response characteristics of flame during condition transition in a cavity-based scramjet combustor

Experimental study is performed to investigate the dynamic response characteristics of the flame when global equivalence ratio increases suddenly during the condition transition in a cavity-based scramjet combustor. High-enthalpy supersonic flow enters the combustor on conditions that Mach number, stagnation temperature, and total pressure are 2.52, 1486 K, and 1.6 MPa, respectively. In this paper, more attention is paid to the characteristics of flame evolution after sudden increase of fuel injection pressure and combustion characteristic when the injection pressure gets stabilized again. Three flame modes after condition transition have been observed and analyzed according to the transient chemiluminescence image captured by high-speed photography. There are two kinds of stable flame modes after transition mission at a low injection pressure difference, which are mainly reflected in the distribution of flame and intensity of visible light. Whereas, once the injection pressure difference exceeds a critical value, intense periodic flame flashback phenomenon begins to appear in the combustor. It can clearly capture the back and forth motion of the flame in the combustor, along with the local blowout and reignition phenomenon. Furthermore, the dominant frequency of flame flashback decreases and contrast of combustion intensity becomes more obvious as the injection pressure difference increases.

Optical diagnosis study of fuel volatility on combustion characteristics of spray flame and wall-impinging flame

Finding suitable alternative fuels and reducing carbon emissions are important development directions for future engines. Many carbon-neutral fuels, such as alcohol fuels are widely used, and they are more volatile than traditional diesel fuel. Therefore, exploring the development characteristics of spray flames and near-wall flames of different volatile fuels has important theoretical significance and application value for the optimization of fuel characteristics and the development of efficient and clean combustion technology. In this paper, the flame structure, flame development characteristics and soot evolution of three fuels, including n-hexane, 74%v n-heptane mixed 26%v isooctane (PRF26) and 53%v n-dodecane mixed 47%v isooctane (D53) were tested, that the cetane number of three fuels is essentially same. High speed self-luminous imaging and RGB two-color method were used to investigated the effect of fuel volatility on combustion. Results can be seen as following. Firstly, during the early stages of n-hexane combustion, a blue premixed flame can be found, and there is a pale-yellow flame in the later stage. As the boiling point increases, the area of the blue flame in the first stage gradually decreases, the area of the yellow flame in the later stage gradually increases, and the spatially integrated natural luminosity (SINL) of flame gradually increases. In addition, as the boiling point increases, the initial spontaneous combustion ignition point is away from the nozzle, the ignition delay time reduces and flame area becomes larger. Furthermore, the flame of high boiling fuels result in higher soot emissions. Secondly, in the event of flame wall-impingement, the near-wall flame of the three fuels has higher flame SINL and more soot formation in comparison to the free spray flame. The flame development of the three fuels is similar. However, there are some differences in the flame brightness distribution. The bright flame areas in n-hexane and PRF26 are concentrated near the wall, while the bright flame area in the D53 flame is concentrated on the wall and the flame head area rolled up with a larger distribution area. The flame near the wall is impacted by the wall temperature that the lower wall temperature leads to prolonged fuel evaporation and mixing time, which increases the ignition delay time. In conclusion, fuel volatility has less influence on flame structure and development, but a significant impact on ignition delay time and emissions. Regardless of whether wall impingement occurs, higher volatile fuels can extend the ignition delay time, increase the flame area and decrease soot emissions from combustion.

Visual investigation on optimizing combustion characteristic of porous media burner combined with bluff body

ABSTRACT Coalbed methane as an unconventional natural gas is suitable for the utilization in thermal photovoltaic systems through optimized burners. In order to improve the radiation and utilization efficiency of coalbed methane, a combination of bluff body and porous media was proposed. Moreover, the bluff body spacing and porous media structure were changed to analyze the influence on flame characteristics. The results showed that the maximum temperature and radiation efficiency increased with decreasing of bluff body spacing. When the bluff body spacing was 30 mm, the maximum temperature of 933 K was obtained, and the radiation efficiency increased by 68%. The increase of the equivalence ratio also improved the radiation efficiency, and the maximum radiation efficiency was 14.5% at φ = 0.90. In addition, the increase of inlet velocity and bluff body spacing was beneficial to downstream flame propagation, but had a negative effect on upstream propagation. At v in = 40 cm/s, the maximum downstream propagation velocity was 0.267 mm/s. Due to the difference in thermal conductivity, flame fragmentation occurred in the 8 mm pellets, but the flame propagation was gradually stable with the increase of bluff body temperature.

Effect of oxygen concentration on the split injection combustion process of diesel spray injected into 2D piston cavity

The exhaust gas recirculation (EGR) technique effectively reduces NOx emissions, and the split injection can lower engine emissions. There is little research on the formation and oxidation of soot during fuel combustion by combining EGR and split injection. Therefore, the effect of oxygen concentration on the split injection strategies' flame characteristics was investigated by analyzing the integrated KL factor, the mean flame temperature, and the soot evolution. The experiment combines various conditions of single main injection, double main injection, high injection pressure, and low injection pressure, with pre-injection and without pre-injection, to discuss the influence of oxygen concentration on combustion characteristics. The 2D piston cavity has been used to form the impingement spray flame, and the two-color method has been used to analyze the combustion characteristics. Results reveal that there will not be fuel accumulation along the cavity groove between the first and the second main injection under low oxygen concentration with pre-injection conditions. Pre-injection reduces the soot oxidation rate under double main injection with low oxygen concentration. The ignition timing of the first main injection under double main injection with pre-injection conditions occurred in advance, even under low oxygen concentration conditions. The effect of oxygen concentration on the total flame area is not as significant as that of pre-injection or without pre-injection conditions. Considering all conditions, split injection (D160_50:50) without pre-injection has higher soot oxidation rate under 15% O2. The injection strategy (S160) without pre-injection can minimize soot and NOx emissions at a concentration of 15% O2.

Characteristics of flame acceleration and deflagration-to-detonation transition enhanced by SF6 jet-in-cross-flow/flame interaction

Jet-in-crossflow (JICF), as a turbulence generator, is proposed to promote the flame acceleration and deflagration-to-detonation transition (DDT) process. Previous work has suggested that JICF using CO2 or Ar was in favor of flame acceleration under optimal conditions. The competition mechanism between the positive combustion-enhancement effect caused by jet-induced turbulence and the negative combustion-inhibition caused by non-reactive gas has been demonstrated. It is worth noting that the difference in molar weight between the medium of JICF and the combustible mixture may significantly impact deflagration propagation prior to DDT. Thus, in this work, to investigate the flame propagation perturbed by the enhancement on the difference in molar weight between the JICF medium and combustible mixture, an experimental study of the flame propagation perturbed by SF6 JICF is carried out. By adjusting the injection pressure (Pj) of the SF6 JICF, the characteristics of flame propagation and DDT are analyzed. The results showed that the flame can accelerate to the onset of detonation within a short run-up distance in the cases with SF6 JICF. Schlieren images of flame evolution show that the flame disturbed by SF6 JICF rapidly transitions to a wrinkled flame. It is suggested that the SF6 JICF-enhanced turbulence promotes flame acceleration and DDT. The effect of SF6 JICF on the flame characteristics can be concluded in two aspects: one is that the inhomogeneously distributed vortices that are induced by JICF cause a nonuniform diffusion of SF6 in the tube, resulting in a nonuniform dilution of the combustible mixture near the jet; another one is that the vortices induced by JICF stretch the flame surface to increase the flame area. Increasing the injection pressure could promote the formation of complex wrinkles in the flame, however, it shows few further improvements in promoting DDT. Different jet mediums are also tested, and the results show that SF6 JICF can shorten the run-up distance significantly compared with helium and nitrogen JICF. The small and densely distributed wrinkles significantly increase the flame area to enhance heat release. The flame disturbed by SF6 JICF could accelerate to the local sound speed of the unburnt mixture, which results in the facilitation of the precursor shock wave formation. After the flame passes through the SF6 JICF, it remains a larger acceleration rate and then transitions to quasi-detonation rapidly. The conclusion of this work could help to qualitatively understand the positive effect of JICF-induced turbulence on flame acceleration and DDT. It also provides a relatively efficient and safe technology to promote detonation initiation in the design of detonation engines.

Optimizing RCCI combustion for improved engine performance under low load conditions: Impact of low-reactivity fuel and direct injection timing

The objective of this study is to explore the challenges associated with the RCCI (Reactivity Controlled Compression Ignition) concept under low loads, where there may be insufficient combustion or misfires caused by a lean fuel mixture in the combustion chamber. To address this issue, optical diagnostics and post-processing techniques are utilized to investigate the effects of low-reactivity fuel and direct injection timing on combustion and flame characteristics in a dual-fuel engine under such conditions. The study tested three different low-reactivity fuels and five different direct injection (DI) timings while maintaining hydrogenated catalytic biodiesel (HCB) as a high-reactivity fuel. The research findings reveal that an increase in ethanol content results in longer ignition delay and a displacement of the ignition kernels towards the downstream of the fuel stream. Consequently, a more homogeneous combustion process and slower flame propagation occur, ultimately reducing the combustion intensity, cylinder pressure, and peak apparent heat release rate. Premixing E10 allows for a higher indicated mean efficient pressure (IMEP) of 0.20 MPa, exceeding E0 by 0.02 MPa and E100 by 0.05 MPa. The research also highlights that delaying the DI timing of HCB reduces ignition delay time and creates a higher concentration gradient of the combustible mixture, resulting in a higher apparent heat release rate and shorter combustion duration. Additionally, natural combustion luminosity images indicate that delaying the DI timing substantially impacts the characteristics of flame kernels generation and propagation. The ignition kernels formed shifted gradually from the near-wall region to upstream region of the spray, as the blue flame area and front propagation speed showed a significant increase. Retarded DI timing can achieve more favorable flame propagation and a higher IMEP under low loads conditions with a higher premixed energy ratio, while effectively reducing the impact of spray-wall impingement and pool burning phenomena. These findings provide insights into optimizing RCCI combustion for improved engine performance under low load conditions.

Analysis of explosion and laminar combustion characteristics of premixed ammonia-air/oxygen mixtures

Ammonia is suggested to be a potential carbon-free energy source because of its high volumetric energy density and has attracted attention recently. However, systematic investigations into the explosion characteristics and flame instability of ammonia are still lacking, particularly for low-pressure ammonia-oxygen mixture. The use of low-pressure ammonia-oxygen mixtures could not only benefit for increasing engine's efficiency but also enhance its operational safety. In this study, explosion experiments of premixed ammonia-air/oxygen mixtures are carried out in a 20-L spherical chamber. By adjusting the initial pressure (P0) and equivalence ratio (ϕ), the maximum explosion pressure (Pmax) and maximum rate of pressure rise ((dP/dt)max) are measured. The laminar flame characteristics are analysed simultaneously. The results show that for the ammonia-air mixture, Pmax increases as P0 increases and reaches the peak at ϕ = 1.1; the unstretched laminar burning velocity (SL), Markstein length (Lb), and laminar flame thickness (δ) decrease as the initial pressure increases and reach the peak at ϕ = 1.1. For the ammonia-oxygen mixture, the explosion parameters linearly increase as P0 increases, while the maximum values of the parameters are obtained at ϕ = 1.0; the unstretched laminar burning velocity increases first and then gradually decreases as P0 increases; the maximum SL is 1.14 m/s at P0 = 0.1 MPa and ϕ = 1.0, which is 18 times that of the premixed ammonia-air mixture under the same condition. The schlieren images demonstrate that the buoyancy instability has significant impacts on the premixed ammonia-air flame propagation, while it has negligible impacts on the premixed ammonia-oxygen flame propagation. The effect of hydrodynamic instability on flame propagation could be observed in both the premixed ammonia-air flame at ϕ > 1.0 and the premixed ammonia-oxygen flame at P0 > 0.1 MPa and ϕ < 1.0. In addition, it is proposed that for the premixed ammonia-air flame, the buoyancy instability would remarkably influence the flame propagation when the unstretched laminar burning velocity is < 5 cm/s. The fundamental research on the combustion characteristics of ammonia-air and ammonia-oxygen in this study can provide an experimental reference for the later development of chemical reaction mechanisms, especially for the mechanism suitable for ammonia combustion in oxygen-rich environments. Meanwhile, the combustion characteristics of the ammonia-oxygen mixture at ultra-low pressure also provide a data basis for the possibility of using ammonia fuel as a propellant for hypersonic vehicles in the future.