Flame Area Research Articles

Investigation of Split Diesel Injections in Methanol/Diesel Dual-Fuel Combustion in an Optical Engine

Methanol is a promising alternative fuel due to its wide availability of raw materials, mature production processes, and low production cost. However, because of the low cetane number, methanol must include a more reactive fuel to assist with combustion when used in compression ignition (CI) engines. In this study, based on the optical CI engine platform, methanol is injected into the intake port, and diesel is directly injected into the cylinder to achieve dual-fuel combustion. The effects of the methanol energy ratios and diesel split injection strategies on combustion are investigated. The results show that the premixed blue flame was mainly concentrated in the near wall region, whereas the yellow flame produced by diesel combustion tended to concentrate in the central region as the methanol energy ratio increased. When the methanol energy ratio exceeded 50%, the ignition delay was significantly prolonged, while the flame area was greatly reduced. Meanwhile, the peak values for the cylinder pressure and heat release rate decreased significantly, indicating a significant deterioration in combustion. At the earlier diesel pre-injection timing at −58°, the overall dual-fuel combustion at each main injection timing exhibited low-temperature premixed combustion characteristics, with a lower peak exothermic rate and flame brightness. At the later pre-injection timing at −33°, the spray flame at all main injection timings could be observed, with higher peak heat release rates and indications of thermal efficiency. Combustion at later main injection timings was characterized by diffusion combustion, and the main injection timing could effectively regulate the combustion process through phase adjustment.

Simplified identification of fire spread risk in building clusters based on digital image processing technology

ABSTRACT Due to factors such as smoke, the non fire identification rate and fire identification accuracy in building fire identification are low, resulting in poor identification results. Therefore, a simplified identification method for building fire spread risk based on digital image processing technology is proposed. A fire spread model is constructed by considering both thermal radiation and thermal plumes, and based on this model, changes in smoke, temperature, and other factors during the occurrence of a fire are obtained. Using K-means to improve the dark channel defogging algorithm, the image after defogging is obtained, making the colour of the image more prominent and the feature information more abundant. Using the area threshold method to further extract the flame target contour in the defogged image, and determine whether the suspected flame area is a flame. Finally, four aspects of colour, sharp corner characteristics, area growth characteristics, and automatic estimation of propagation direction were identified to obtain simplified identification results for fire propagation risk. The experimental results show that the average non fire recognition rate of the proposed method reaches 95.8%, with good fire recognition accuracy and recognition effect, which verifies its feasibility.

Research on species categorical authentication of accelerants based on flame characteristics analysis

Species categorical authentication of accelerants has traditionally relied on fire debris analysis. To explore a novel method for identifying the accelerants species, four commonly used accelerants for arson were loaded onto different substrates and ignited at different locations. The entire combustion process was recorded and flame characteristics were analyzed. The results showed that the probability density function (PDF) of flame apex angle counts within a certain period after ignition can be used to distinguish accelerant species, and this method is not affected by accelerant loading amount, ignition location, and substrate, demonstrating strong stability and universality, while the temporal variation of flame area and the value obtained by dividing half of the flame width by the flame height (tangent of flame cone angle) can effectively differentiate gasoline and diesel. The utilization of flame characteristics for identifying accelerants species holds significant implications for arson investigation.

Direct numerical simulations of pure and partially cracked ammonia/air turbulent premixed jet flames

Ammonia has been identified as a promising fuel to diminish greenhouse gas emission. However, ammonia combustion presents certain challenges including low reactivity and high NO emission. In the present study, three-dimensional direct numerical simulations (DNS) of ammonia/air premixed slot jet flames with varying Karlovitz numbers (Ka) and cracking ratios were performed. Three cases were considered, including two pure ammonia/air flames with different turbulence intensities and one partially cracked ammonia/air flame with high turbulence intensity. The effects of turbulence intensity and partial ammonia cracking on turbulence–flame interactions and NO emission characteristics of the flames were investigated. It was shown that the turbulent flame speed is higher for the flames with high turbulence intensity. In general, the flame displacement speed is negatively correlated with curvature in negative curvature regions, while the correlation is weak in the positive curvature regions for highly turbulent flames. Most flame area is consumed in negatively curved regions and produced in positively curved regions. It was found that the NO mass fraction is higher in the flame with partial ammonia cracking compared to the pure ammonia/air flames. The NO pathway analysis shows that the NH → NO pathway is enhanced, while the NO consumption pathway is suppressed in the partially cracked ammonia/air flame. The NO mass fraction is higher in regions of negative curvature than positive curvature. Interestingly, the NO mass fraction is found to be negatively correlated with the local equivalence ratio, which is consistent in both the DNS and the corresponding laminar premixed flames.

Research on the thermal runaway mechanism and eruption flame dynamics of NCM811 lithium-ion battery stimulated by the coupling abuse of heat and penetration

Thermal abuse and mechanical abuse are the main triggers of thermal runaway (TR) in lithium-ion batteries (LIBs). With the emergence of stimulus superposition application scenarios makes the TR mechanism of LIBs under a single stimulus no longer applicable, and the TR characteristics of LIBs induced by heat-penetration coupled stimulation has become one of the key bottlenecks constraining the intrinsic safety design and control of LIBs. In this paper, an experimental platform for heat-penetration coupled stimulation on LIBs was built, and an experimental research on the TR mechanism and eruption dynamics of LIBs under penetration with different initial thermal load was carried out. The results show that the maximum temperature of cell induced by penetration increases from 537.4 °C to 746 °C, an increase of 38.8 %, as the initial thermal load of the cell increases from 10 °C to 130 °C. When the initial thermal load is further increased to 160 °C, the penetration stimulation no longer induces TR, which is closely related to the “separator melt protection channel”. Based on this, a “kebab” micro-short circuit model is proposed. At 10 °C, 40 °C, 70 °C, and 100 °C, the cell safety valve is destroyed and a “column fire” is formed above the positive electrode. And at 100 °C, the flame eruption height is the largest, up to 185.10 cm, and the jet flame area is the largest, up to 5687.62 cm2.And with the initial thermal load increases, the eruption particles are smaller in size. Increasing content of O in eruption particles, which characterizes the strength of the reaction within the cell, and F elements characterizing toxicity are detected in the eruption particles. The results enrich the theory of TR induced by thermal–mechanical abuse, which is of great guiding significance for the intrinsic safety design of LIBs and the prevention and control of TR risk.

The Spatial Development Characteristics of High-Pressure Methane Jet Impinging on Methane Lean-Burn Premixed Flame

This work investigated the effect of high-pressure (10 MPa) methane gas jet impinging on the methane lean-burn premixed flame (equivalent ratio of 0.7) based on a three-dimensional numerical simulation by CONVERGE software. The results show that laminar premixed flame is accelerated to develop into a stable turbulent flame under the action of the methane jet, the whole process of flame front development is divided into three stages: laminar (Reynolds number maintains stable, 1–1.3 ms), transition (Reynolds number shows an increasing trend, 1.4–8 ms), and turbulent (Reynolds number tends to stabilize at a high value, 1.9–3 ms). The effect of high-pressure jet on flame development along jet direction (Z axis) is greater than that on vertical direction (Y axis). During turbulence stage, the momentum and kinetic energy of Z axis are 2.7 and 6.3 times greater than that of Y axis, respectively. The high-pressure methane jet causes a change in heat distribution, resulting in local flameout. The rate of change in the local flameout area is greater than that in the flame area, causing a temperature drop in Z axis. This temperature drop increases with the increase in equivalence ratio and with the decrease in distance between cross-section position and ignition center.

Optical study on the effect of ozone addition on a diesel/ammonia dual fuel engine with pilot injection

Influenced by the concept of carbon neutrality, ammonia is gaining widespread attention in the transport sector as a zero-carbon fuel. Diesel/ammonia dual fuel engines can achieve stable combustion of ammonia and reduce carbon emissions. To further enhance the low load combustion performance of a diesel/ammonia dual fuel engine, the effect of ozone addition on in-cylinder combustion and flame development was investigated based on an optical engine. Beyond that, ozone addition on pure diesel was also tested to understand the effect of ozone addition on diesel combustion enhancement in diesel/ammonia reactivity controlled compression ignition (RCCI) mode. The results show that the addition of ozone significantly advances the ignition phase, increases the flame area and brightness, and results in a growth in flame luminescence intensity along the radial length and closer to the center of the view field for diesel/ammonia RCCI combustion. In addition, chemical kinetic analysis shows that diesel/ammonia combustion requires higher ozone concentrations to overcome the inhibitory effect of ammonia to reach the upper limit of the promotional effect on the ignition phase and indicated mean effective pressure (IMEP), due to the large amount of OH consumed by ammonia compared to pure diesel. The promotional effect of added ozone on IMEP is similar for different pilot injection ratios (PIRs). At a low PIR and high ozone concentration, the diesel/ammonia RCCI engine achieves the highest IMEP, indicating that ozone addition combined with optimization of diesel injection parameters contributes to improve the performance of the diesel/ammonia RCCI engine at low loads.

Experimental research on dynamic response characteristics of lean premixed flame excited by acoustic standing wave

The compact combustor holds great potential in the aerospace field. However, it faces challenges in terms of interaction between acoustic waves and flames, which affects combustion efficiency and intensity. In this study, the dynamic response of lean premixed flames under the excitation of a transverse standing wave acoustic field with different frequencies and amplitudes. The results indicate that the flame's response frequency is consistent with the excitation frequency. Decreasing the sound wave frequency and increasing the sound amplitude both lead to an increase in flame area oscillation, thereby enhancing the heat release rate. However, excessively high sound pressure values can enhance fuel diffusion, resulting in a reduction in flame area or even local flame extinction, which significantly decreases flame intensity. This research provides valuable insights into the behavior of lean premixed flames under acoustic excitation, contributing to the optimization of combustion processes and improvement of efficiency and performance in compact combustors.

Optical engine investigation of ammonia combustion enhancement with polyoxymethylene dimethyl ethers (PODE₃) as pilot fuel

Ammonia, gaining attention as a carbon-free fuel amid declining fossil fuel reserves, faces challenges like combustion instability and misfiring in its application in internal combustion engines. In this study, polyoxymethylene dimethyl ethers (PODE3), a clean, renewable, and electrically synthesized fuel with high cetane number and oxygen content, is evaluated as a pilot fuel for ammonia in dual-fuel engines. The study, performed in a dual-fuel optical engine, investigates the use of PODE₃ compared to diesel as direct injection fuels for ammonia combustion, focusing on the impact of PODE3 injection timing and dwell (the interval between dual-stage injections) on the ignition characteristics of ammonia. The results demonstrate that, compared to diesel, using PODE3 enhances ammonia combustion in dual-fuel engines. This is evident from increased in-cylinder pressure, higher apparent heat release rate (AHRR), advanced combustion phases, and improved indicated mean effective pressure (IMEP), enhancing overall stability. PODE3 also effectively minimizes soot generation, evidenced by a 71.5 % reduction in spatially integrated natural luminosity (SINL), accompanied by a 16.9 % increase in peak flame area. Adjusting PODE3 injection timing is crucial; initial retardation improves combustion performance, marked by higher in-cylinder pressure and AHRR, as well as expanded flame area. However, over-delaying injection timing reduces these gains. The optimal performance is achieved under the SOI-25 condition, balancing combustion and flame development, though some areas remain unburnt. In a dual-stage injection strategy using PODE3, where the second injection timing remains fixed at −25 °CA ATDC, the critical adjustment of advancing the first injection timing to −35 °CA ATDC markedly enhances combustion performance compared to a single injection condition. This setting reduces ignition delays, increases peak in-cylinder pressures, and improves stability, which collectively contribute to a significant 20.4 % increase in the peak flame area, particularly between adjacent fuel jets. Expanding upon this, advancing the first injection timing further to −45 °CA ATDC leads to a decrease in peak in-cylinder pressure and AHRR, accompanied by a notable 40.3 % reduction in the peak flame area. This research affirms the advantages of PODE3 over diesel in ammonia-powered dual-fuel engines, markedly enhancing combustion performance and environmental sustainability, where the injection strategy of PODE3 emerges as a crucial element.

Investigation of the dynamic interaction mechanism of split injections during the ignition process under diesel engine-like cold- and hot-start conditions

In a range of operating conditions with variable injection parameters, interactions among split sprays present uncertainties across both temporal and spatial dimensions. Understanding these dynamic interactions is crucial for optimizing injection strategies to improve both the performance and environmental adaptability of diesel engines. In this study, optical diagnostics and numerical simulations were conducted to explore the interaction mechanisms between split injections and their impact on the ignition process. Engine-like cold and hot-start operating conditions were configured, and four injection strategies, including a single injection, were examined. The findings reveal that ambient temperature influences split spray interactions by altering their ignition characteristics, while injection dwell time modifies these interactions by changing the combustion spatial and temporal phase between split sprays. Compared to a single injection, split injection promotes the ignition and combustion of the main spray such as shortening ignition delay, reducing ignition location, and increasing flame area, but this promoting effect weakens under hot-start conditions. When the main spray chases the pilot spray before ignition, it increases the local concentration and reduces the temperature upstream of the pilot spray plume. However, since the effect front fails to reach the expected ignition region before the pilot ignition occurs, the ignition of the pilot fuel pocket remains unaffected. Under cold-start conditions, the main ignition delay shows a non-monotonic trend, reaching its minimum at an injection dwell of 900 μs, where the pilot ignition provides the main spray with elevated local temperatures and an increased hydroxyl mass fraction. Under hot-start conditions, the temporal overlap of ignition in split sprays is enhanced, yet spatial interaction weakens. Despite varying degrees of pilot flame decay before the main spray ignites with increasing injection dwells, the thermal radiation emanating from their combustion elevates the local temperature in the interaction zone beyond 1200 K. Consequently, upon catching up with the pilot injection, the main spray mixture rapidly ignites through the high-temperature reaction pathway, rendering the ignition delay insensitive with injection dwells.

Comprehending flame development and misfire at advanced engine conditions: Detailed experimental characterizations and machine learning-assisted kinetic analyses

Through comprehensive experimental and modeling efforts, this work unravels the underlying mechanisms governing flame development and misfire at advanced engine conditions that are representative of low-load and lean blow-out operations. Toward this, preliminary heat release, autoignition, and flame developing patterns are characterized, via a case study of n-heptane, at ultra-lean conditions in a well-controlled optical engine under various combustion modes including homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and reactivity-controlled compression ignition (RCCI). Changes in preliminary heat release and flame developing patterns at three overall equivalence ratios (0.12, 0.18, and 0.24) are first characterized under the PPC mode. Flame development characteristics including flame areas and number of initial flame kernels at close-to-misfire conditions are further extracted and compared across the HCCI, RCCI, and three PPC modes, with two distinctive and one transition regimes identified. Further analyses indicate that sustainable flame development and misfire are largely controlled by the spatial distribution of local equivalence ratio (phi) and local temperature in the mixture, which dictate the initial flame kernel generation and the subsequent flame propagation through localized preliminary heat release and autoignition. Chemical kinetic modeling is also undertaken, using a recently updated gasoline chemistry model, in conjunction with a backpropagation neural network, where the predicted ignition delay map well captures the different regions of flame development. Further kinetic analysis and heat rate of production per reaction analysis corroborate the CH2O planar laser-induced fluorescence experiments and highlight the important chemical kinetics that govern the initial flame development patterns.

Modeling effects of swirl burners on the heating quality of slabs and flame morphology in a walking-beam reheating furnace

This paper developed a model of zone segment swirl burners to realize individual separation of swirl burners within a CFD model of an industrial-scale walking-beam reheating furnace. Numerical results are validated against the experimental measurements, demonstrating good agreement and credibility. Leveraging this model, a comprehensive investigation on the effect of swirl angle on the flame morphology and the slab heating quality has been conducted. Comparing various swirl angles (0°, 25°, 45°, and 65°) provides valuable insights into their impact on flame morphology, and slab heating quality. A swirl angle of 25° has been identified as optimal, yielding the maximum flame area within the furnace, measuring 326.09 m2. Furthermore, it reduces the maximum absolute temperature deviation of the discharged slab’s top surface centerline to 7.64 K, marking a substantial 25.83 % reduction compared to the 0° angle. These results highlight the critical role swirl burners play in the reheating furnace and identify the optimal swirl angle for effectively enhancing both the economic benefits and product quality in industrial processes.

Influence of primary jets on the combustion performance of a rich quench lean combustor: A numerical and experimental investigation

The effect of primary hole jet on the Rich-burn/Quick-mix/Lean-burn (RQL) combustor was studied here by both experimental and numerical analysis approaches, where with a standard k-ε model was applied. To understand the influence of primary hole on the performance of RQL combustor, three primary hole jets configurations with different dimensions of layout were tested. Results from both experimental and numerical studies demonstrate that an increased number of primary holes results in a moderated reduction in the height of the central recirculation zone and the emergence of vortical structures in the quenched zone. Additionally, in the rich fuel region, the flame area decreases with additional primary holes when the equivalence ratio is below 1.0, while it expands when the equivalence ratio exceeds 1.0. The interaction between local recirculation zones is enhanced by additional primary holes, facilitating rapid fuel-air mixing and reducing circumferential ignition time. Notably, a symmetric jet flow scheme with fewer primary holes displays a favorable outlet temperature distribution, characterized by a weighted standard deviation ranging from 0.02 to 0.06, indicating efficient mixing. These comprehensive insights highlight the intricate relationship between primary hole jets and combustor performance.

Effects of nano-metal oxide additives on ignition and combustion properties of MICs-boron rich fuels

Boron has been considered a promising powdered metal fuel for enhancing composite propellants' energy output due to its high energy density. However, the high ignition temperature and low combustion efficiency limit the application of boron powder due to the high boiling point of the boron oxide layer. Much research is ongoing to overcome these shortcomings, and one potential approach is to introduce a small quantity of metal oxide additives to promote the reaction of boron. This study prepared boron-rich fuels with 10 wt% of eight nano-metal oxide additives by mechanical ball milling. The effect of metal oxides on the thermo-oxidation, ignition, and combustion properties of boron powder was comprehensively studied by the thermogravimetric analysis (TG), the electrically heated filament setup (T-jump), and the laser-induced combustion experiments. TG experiments at 5 K/min found that Bi2O3, MoO3, TiO2, Fe2O3, and CuO can promote thermo-oxidation of boron. Compared to pure boron, Tonset can be reduced from 569 °C to a minimum of 449 °C (B/Bi2O3). Infrared temperature measurement in T-jump tests showed that when heated by an electric heating wire at rates from 1000 K/s to 25000 K/s, the ignition temperatures of B/Bi2O3 are the lowest, even lower than the melting point of boron oxide. Ignition images and SEM for the products further showed that the high heating rate is beneficial to the rapid reaction of boron powder in the single-particle combustion state. Fuels (B/Bi2O3, B/MoO3, and B/CuO) were mixed with the oxidant AP and ignited by laser to study the combustion performance. The results showed that B/CuO/AP has the largest flame area, the highest BO2 characteristic spectral intensity, and the largest burn rate for powder lines. To combine the advantages of CuO and Bi2O3, binary metal oxide (CBO, mass ratio of 3:1) was prepared and the test results showed that CBO can very well improve both ignition and combustion properties of boron. Especially B/CBO/AP has the highest burn rate compared with all fuels containing other additives. It was found that multi-component metal-oxide additive can more synergistically improve the reaction characteristics of boron powder than unary additive. These findings contribute to the development of boron-rich fuels and their application in propellants.

Research on explosion venting characteristics of CH4/H2/Air mixture in square explosion vessels

Hydrogenation of methane (CH4) is an important means of achieving energy savings and large-scale hydrogen (H2) transportation. Explosion venting is considered to be an effective method to reduce the risk of gas explosions. To investigate the parameters of the explosion characteristics of CH4/H2/Air premixed fuel under venting conditions, the explosion characteristics and the dynamic propagation law of venting flame in CH4/Air mixtures with 5% concentration were studied in 20 L and 40 L square explosion venting vessels. The development process of flames inside vessels, the evolution characteristics of explosive venting flames, and the changes in explosion pressure were tested using a high-speed camera and pressure sensors. The maximum explosion pressure, the maximum explosion pressure rise rate, and the maximum flame velocity increased first and then decreased with the addition of H2. However, the maximum flame length increased continuously with the increase of H2. With the increase of hydrogenation, the maximum flame area showed an exponential increase. At equivalence ratio less than 1, the effect of hydrogenation on the flame area was much attenuated than that at equivalence ratio was greater than 1.

Numerical study on the shock wave and vortex and their coupling effect on the behavior of the flame induced by the spontaneous ignition of high-pressure hydrogen release

High-pressure hydrogen storage is currently the mainstream way of hydrogen storage. However, the risk of spontaneous ignition and subsequent jet flame are serious obstacles to the wide application of hydrogen energy. To further reveal the interaction of shock waves, vortices and flame induced by the spontaneous ignition of high-pressure hydrogen release under the effect of release process and burst pressure, a numerical study is conducted by employing LES, RNG, EDC models and detailed hydrogen/air combustion mechanism. The qualitative and quantitative comparison of flow field and flame splitting process outside the tube between this numerical study and previous experimental results validate the numerical results. It is found that an initial vortex is formed at the edge of the jet. Then, several vortices are generated at the edge of the jet and ahead of the Mach disk. The number of vortices in the outer side of the barrel shock is larger when the opening time is shorter. After moving out of the tube, the flame front becomes hemisphere and the lateral edge has a hemispherical bulge which is induced by the initial vortex. The temperature is low and the hydrogen concentration is high in the high-speed region which is induced by the reflected shock wave, leading to the splitting of the flame into two parts: the first and the second half of the flame. However, the flame cannot be formed at the initial stage when the opening time increases to 150 μs and the flame area is larger than with smaller opening times. After splitting, under the affection of the vortices generated at the lateral side, the first half of the flame rotates backward and gradually merges with the second half of the flame, finally forming a complete hydrogen jet flame.

Optical diagnostics in impact of ammonia energy ratio and diesel split ratio on combustion process and flame propagation in an Ammonia-Diesel Dual-Fuel engine

In the context of ammonia-diesel dual-fuel (ADDF) engines, challenges such as ignition difficulties and misfires emerge prominently at elevated ammonia energy ratios. To alleviate these challenges, this study aims to elucidate the effects of ammonia energy ratios (AER) and diesel split ratios (DSR) on the combustion process and flame propagation in an optical ADDF engine configuration. Utilizing optical diagnostics, including flame luminescence and high-speed photography, the study provided insights into combustion boundary conditions specific to ADDF engine operation. The results highlight that an increase in AER to 60 % results in a marked decrease in peak cylinder pressure, peak apparent heat release rate (AHRR), and the in-cylinder pressure rise rate. This is accompanied by a reduction in IMEP from 4.16 bar to 3.24 bar, indicating a decline in combustion efficiency upon greater ammonia addition. Flame luminosity exhibits a shift from a diesel-like blue hue to an ammonia-tinted pale yellow. This change coincides with a significant reduction in soot formation and restrictions in flame area and propagation speed, largely due to ammonia's intrinsically slower burning velocity and its chemical interplay with diesel. Implementing a dual injection approach further accentuates the peak in-cylinder pressure and AHRR, with the second injection playing a crucial role in ignition. Optimizing the DSR to 40 % improves combustion efficiency, attributed to enhanced fuel-oxygen interactions. Conversely, DSR values of 60 % and 80 % pose challenges. A reduction in diesel content in the subsequent injection leads to a leaner blend, subsequently affecting combustion efficiency, as indicated by reduced heat release rate and diminished IMEP. This study emphasizes understanding the ammonia-diesel interaction and the importance of choosing the appropriate DSR to exploit the advantages of ADDF engines.

Quantitative 2D reconstruction of temperature and OH concentration in axisymmetric flames based on ultraviolet broadband absorption tomography

Absorption tomography plays a crucial role in quantitatively imaging temperature and absolute concentration in combustion. However, there is scarce research on the tomography of OH radicals, which are significant for combustion diagnostics. This study presented a new tomography method to image OH radicals, termed ultraviolet broadband absorption tomography (UV-BAT). Three strategies were proposed to solve the nonlinear problem in UV-BAT tomography, namely all-spectra fitting, weak-absorption approximation, and multi-temperature approximation. The proposed UV-BAT method was applied for OH imaging in methane/air axisymmetric laminar Bunsen flames with different equivalence ratios (ϕ = 0.9, 1.0, and 1.1). The reconstructed OH distributions showed distinctive structures of Bunsen flames including premixed and diffusion flame areas. The accuracy of the UV-BAT method was validated by comparing tomography results with computational fluid dynamics (CFD) simulations using the USC-Mech II mechanism, demonstrating good agreement in two-dimensional (2D) temperature and OH concentration distributions. This novel method offers new opportunities for tomographic reconstruction of free radicals such as OH and temperature in small-scale flames.Novelty and significanceThis study demonstrated a novel tomography imaging method using ultraviolet broadband absorption spectroscopy to reconstruct temperature and radical concentration in combustion. This method possesses high sensitivity to free radicals and employs compact and inexpensive equipment, making it one of the most viable methods for realizing free radical absorption tomography. The 2D imaging of OH radicals using this method based on axisymmetric absorption tomography was accomplished for the first time. In conclusion, this study enriched the diagnostic methods of flame radicals based on absorption spectroscopy and provided new ideas for small-scale temperature tomography.

Combustion enhancement in a model scramjet by a simple pin-to-pin AC arc plasma

A simple pin-to-pin alternative-current (AC) arc plasma located upstream of the cavity was developed to achieve significant combustion enhancement in a Mach 2.52 C2H4-fueled and cavity-based model scramjet. The arc plasma-enhanced combustion process was recorded by high-speed CH⁎ chemiluminescence imaging, optical emission spectroscopy, static pressure measurements, and discharge waveform measurements. In the absence of the arc plasma, the flame heat release intensity was low, and the heat release region was located at the shear layer of the cavity; the flame stayed in a relatively weak combustion state and even appeared in a lower heat release intensity. Once the arc plasma was turned on, the static pressure of the model scramjet at the typical positions was increased by up to 9 %, and the flame area and the flame intensity were significantly enhanced. In the presence of the arc plasma, the position of the flame heat release region moved upstream and even caused the long-scale displacement for the flame with a higher heat release intensity several times. The arc plasma provided a large number of reactive species and Joule heat at the practical influence region of the arc plasma, which contributed to igniting an initial flame upstream of the cavity. The propagation and development of the continuous initial flame generated by the arc plasma can enable the flame to be more frequently transferred into the cavity-assisted jet-wake stabilized combustion mode, leading to significant combustion enhancement.

Optical investigation of low temperature combustion and soot emission characteristics of biodiesel/n-pentanol engine

Biodiesel/n-pentanol blend fuels have been regarded as the attractive alternatives for the utilization of diesel engines. However, the fundamental studies of low temperature combustion and soot formation characteristics of biodiesel/n-pentanol blend fuels in diesel engines are still scarce. The low temperature combustion and soot emission characteristics of pure waste cooking oil (WCO) biodiesel (B100) and 70% WCO biodiesel/30% n-pentanol blend (B70P30) were experimentally studied in an optical engine in the present study. Results reveal that B70P30 has longer ignition delays than B100 at low exhaust gas recirculation (EGR) rate, but the ignition delays of B70P30 become similar or even shorter when the EGR rate is over 12%. Adding n-pentanol into biodiesel increases the in-cylinder combustion pressure peak and maximum pressure rise rate. In addition, the delay in the appearance of ignition kernels and two-color images are observed for B70P30 fuel. In the initial stage of fuel combustion, B70P30 has less ignition kernels and lower soot KL factor distribution area. In the middle and late stages of combustion, flame area of B70P30 is small and flame brightness is weaker. Also, at the end of combustion, the two-color images of B70P30 show that the soot KL factor distribution around the periphery of the chamber is decreasing at a higher rate compare to B100.