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We report a spatially resolved spectroscopic study of the visible chemiluminescence emission from different premixed ammonia-air-oxygen flames stabilized on a laminar flat flame burner, with equivalence ratio ranging from 0.7 to 1.35 and an O 2 /N 2 ratio of 0.4. In the reaction zone of the observed flames, the visible emission was recognized to be the chemiluminescence of excited NH 2 * radicals, while in the post-flame zone, two types of chemiluminescence were observed: NO 2 * chemiluminescence dominated in the fuel-lean flames and NH 2 * chemiluminescence dominated in the fuel-rich flames. The high-resolution spectra of the NO 2 * and NH 2 * chemiluminescence in the visible region (400-700 nm) were recorded. The intensity of both spectra increased gradually with wavelength. However, the NO 2 *-chemiluminescence spectrum appeared to be continuous and unstructured, while the NH 2 *-chemiluminescence spectrum consisted of groups of distinct emission lines. Based on the spectral feature, the ratios of the integrated chemiluminescence intensities over the 598-603 nm wavelength range to the intensities over the 586-592 nm range and 447-453 nm range were used to sense equivalence ratio. In addition, slightly different colors of the fuel-lean and fuel-rich flames were observed, due to the fact that NO 2 * chemiluminescence had a relatively stronger signal in the blue region than NH 2 * chemiluminescence. The difference was used to infer flame equivalence ratio using the flame images recorded by a RGB digital camera, where the ratio of the signal from the red channel to the signal from the blue channel was calculated. [ABSTRACT FROM AUTHOR]

To clarify the role of differential diffusion in highly turbulent premixed flames, a series of turbulent premixed ammonia/hydrogen/air flames were investigated using the NH-PLIF diagnostics. The investigated flames have almost the same laminar burning velocity, S L , but are characterized by different Lewis number, Le , from 0.56 to 1.77. The Karlovitz number, Ka , of these flames ranges from 11 to 1052, and the turbulence intensity, u' / S L , covers from 10 to 156. It is observed that the global consumption speed, S T,GC / S L , of sub-unity Le flames is much larger than that of super-unity Le flames at high Ka , indicating that the differential diffusion plays a significant role in highly turbulent flames. The flame surface density and the area ratio of turbulent flames with different Le are, however, similar under wide turbulent conditions. The stretch factor of sub-unity Le flames is estimated to be significantly larger than that of super-unity Le cases. The enhanced S T,GC of sub-unity Le flames is suggested to be attributed to the promotion of local burning rates by the couple effect of differential diffusion and turbulent flame stretch within the flame brush, rather than the enlargement of flame surface area at high Ka. Furthermore, three correlations for the S T,GC were developed based on Damkohler's second hypothesis with consideration of the Le effect. The correlation of S T,GC / S L ∼ (Re T · Le -2)0.5 is further validated by using small-scale methane/air and large-scale ammonia/air flames at high Ka , where Re T is turbulent Reynolds number. It suggests that the S T,GC is roughly inversely proportional to the Le , and the differential diffusion effect should be included in the theoretical analysis and numerical simulation of highly turbulent flames. [ABSTRACT FROM AUTHOR]

Two-dimensional laser-induced photofragmentation fluorescence (LIPF) was employed to quantitatively visualize the potassium hydroxide (KOH) and potassium chloride (KCl) vapor in the plume above burning wood and straw pellets. In the LIPF measurement, two excitation lasers at 266 and 193 nm were adopted to discriminate KOH and KCl. Meanwhile, tunable diode laser absorption spectroscopy (TDLAS), laser-induced breakdown spectroscopy (LIBS) and two-color pyrometry were used to measure the atomic potassium concentration, total elemental potassium concentration and surface temperature of the burning pellets, respectively. The combustion environment had a temperature of 1550 K and an oxygen concentration of 4.6 vol.%. Two peaks were observed from the temporal potassium release profile of the burning wood, corresponding to the devolatilization and char oxidation stage, while only a single release peak was observed from the burning straw attributed to its high ash content. During the char oxidation and ash cooking stages, KOH was observed to be the dominant potassium species released from the wood, while only KCl was observed for the straw which had a high content of chlorine. About 45% of the total potassium in the wood samples and about 10% in the straw samples were measured to be released during the combustion process. The high content of silicon in the straw retained a considerable amount of potassium in the ash. The wood had the potassium release mainly in the char oxidation stage (∼53% of the total release), while the straw had the main release during the ash cooking stage (∼49% of the total release). During the char oxidation and ash cooking stages, about 32% of Cl was released from the straw pellets in KCl, while the other part of Cl was considered to be released during the devolatilization stage in other Cl species form, such as HCl. [ABSTRACT FROM AUTHOR]

Turbulent premixed flames subjected to extreme levels of turbulence exhibit reaction zone broadening and thinning. Instantaneous flame structures visualized using advanced laser diagnostic methods have shown significantly different behaviours of flame/turbulence interaction in jet flames stabilized on burners of different sizes. This paper aims to reconcile the controversy about flame broadening on burners of different sizes. Premixed ammonia/air flames are investigated owing to the importance of ammonia in the future carbon-free energy system. Due to the low laminar flame speed of ammonia/air mixture, the intensity of turbulence (u ′ / S L) can be 5 times higher than that of methane/air flames at the same jet velocities. Planar laser-induced fluorescence (PLIF) imaging and large eddy simulation based on detailed chemical kinetics are carried out to systematically study the structures of ammonia/air premixed jet flames under extreme levels of turbulence conditions, with u ′ / S L up to 240 and Karlovitz number up to 3019. Consistent with the observation in methane/air jet flames reported in the literature, the reaction zones of premixed ammonia/air jet flames show significantly different behaviour on burners of different sizes under similar Karlovitz number conditions. It is found that reaction zone broadening in jet flames is a spatially evolving process, not only depending on the local eddy/flame interaction but also on the upstream history. The onset of distributed reaction zones is found to take place at positions located several jet orifice diameters above the burner and thereafter eddies are self-produced in the reaction layer due to the continuous vortex stretch interacting in the layer and broadening the reaction zone. The flames on a large burner have turbulence eddies of larger integral length scales that do not fit directly inside the reaction zones, and as such the eddies could not broaden the reaction zones. [ABSTRACT FROM AUTHOR]

The sulfation of gas-phase KOH and KCl was investigated in both oxidizing and reducing atmospheres at temperatures of 1120 K, 1260 K, 1390 K, and 1550 K. Well-defined gas environments were generated in a laminar flame burner fuelled with CH 4 /air/O 2 /N 2. Atomized K 2 CO 3 and KCl water solution fog and SO 2 were introduced into the hot gas as sources of potassium, chlorine, and sulfur, respectively. The in situ concentrations of KOH, KCl, and OH radicals were measured using broadband UV absorption spectroscopy, and the concentration of K atom was measured using TDLAS at 769.9 nm and 404.4 nm. The nucleated and condensed K 2 SO 4 aerosols were visualized as illuminated by a green laser sheet. With SO 2 addition, reduced concentrations of KOH, KCl, and K atom were measured in the hot gas. The sulfation was more significant for the low temperature cases. KOH was sulfated more rapidly than KCl. K 2 SO 4 aerosols, formed by homogeneous nucleation, were observed at temperatures below 1260 K. At 1390 K, no aerosols were formed, indicating that the consumed KOH was transformed into gaseous KHSO 4 or K 2 SO 4. K atoms formed in the hot flue gas with KOH addition enhanced the consumption of OH radicals except at the high-temperature case at 1550 K. At 1120 K and 1260 K, the sulfation of KOH with SO 2 seeding reduced the concentration of K atom, resulting in less OH radical consumption. Studies were also conducted in a hot reducing environment at 1140 K, with the flame at an equivalence ratio of 1.31. Similar to the observation in the oxidizing atmosphere, the concentrations of KOH and K atom decreased dramatically with SO 2 seeding. An unknown absorption spectrum observed was attributed to UV absorption by KOSO. The experimental results were used to evaluate a detailed K-Cl-S reaction mechanism, and a reasonable agreement was obtained. [ABSTRACT FROM AUTHOR]

This work investigated the combustion characteristics of single pulverized biomass-derived char particles. The char particles, in the size range 224–250 µm, were prepared in a drop tube furnace at pyrolysis temperatures of 1273 or 1473 K from four types of biomass particles – wheat straw, grape pomace, kiwi branches and rice husk. Subsequently, the char particles were injected upward into a confined region of hot combustion products produced by flat flames stabilized on a McKenna burner, with mean temperatures of 1460, 1580 and 1670 K and mean O 2 concentrations of 4.5, 6.5 and 8.5 vol%. The data reported include particle temperature, obtained using a two-color pyrometry technique, and potassium release rate, measured using a laser-induced photofragmentation fluorescence imaging technique. In addition, particle ignition delay time and burning time, obtained from the temporal evolution of the thermal radiation intensity of the burning char particles, are also reported. The results indicated that ignition of the char particles occurs simultaneously with the starting of the potassium release, then the particle burning intensity increases rapidly until it reaches a maximum, after which both the particle temperature and the potassium release rate remain approximately constant until the end of the char oxidation process. The char ignition process is temperature controlled, and the char oxidation process is oxygen diffusion controlled, with the total potassium release being independent of the oxygen concentration and the temperature of the combustion products. The combustion behavior of the chars studied is more affected by the char type than by the conditions used to prepare them. [ABSTRACT FROM AUTHOR]

We report accurate in-situ optical measurements of surface temperature, volatile gas temperature, and polycyclic aromatic hydrocarbon (PAH) emission over the whole burning history of individual biomass pellets in various combustion atmospheres. Two biomass fuels, wood and straw, were prepared in cylindrical pellets of ∼300 mg. The pellets were burned in a well-controlled combustion atmosphere provided by a laminar flame burner with temperature ranging from 1390 K to 1840 K, and oxygen concentration from zero to 4.5%. The surface temperature of burning biomass pellets was accurately measured, for the first time, using phosphor thermometry, and the volatile gas temperature was measured using two-line atomic fluorescence thermometry. PAH emission was monitored using two-dimensional laser-induced fluorescence. During the devolatilization stage, a relatively low surface temperature, ∼700 K, was observed on the burning pellets. The volatile gas temperature was ∼1100 K and ∼1500 K 5 mm above the top of the pellets in a gas environment of ∼1800 K with 0.5% and 4.5% oxygen, respectively. PAH mainly released when the temperature of the pellet exceeded ∼600 K with the highest concentration close to the surface and being consumed downstream. The weight of the released PAH molecules shifted towards lighter with a reduction of gas environment temperature. The wood and straw pellets had almost the same surface and volatile gas temperature but different compositions in the released volatile gases. The temperature information provided in the present work aids in revealing the reactions in the burning biomass fuels regarding species release, such as various hydrocarbons, nitrogen compounds, and potassium species, and is valuable for further development of biomass thermal conversion models. [ABSTRACT FROM AUTHOR]

The propagation of laminar and turbulent expanding flames subjected to Darrieus–Landau (DL), hydrodynamic instability was experimentally studied by employing stoichiometric H 2 /O 2 /N 2 flames under quiescent and turbulent conditions performed in a newly developed medium-scale, fan-stirred combustion chamber. In quiescent environment, DL unstable laminar flame exhibits three-stage propagation, i.e. smooth expansion, transition acceleration, and self-similar acceleration. The self-similar acceleration is characterized by a power-law growth of acceleration exponent, α , with normalized Peclet number, which is different from the usually suggested self-similar propagation with a constant α. The imposed turbulence advances the onset of both transition acceleration and self-similar acceleration stages and promotes the strength of flame acceleration as additional wrinkles are invoked by turbulence eddies. A DL–turbulent interaction regime is confirmed to be the classical corrugated flamelets regime. Furthermore, the DL instability significantly facilitates the propagation of expanding flames in medium and even intense turbulence. The development of DL cells is not suppressed by turbulence eddies, and it needs to be considered in turbulent combustion. [ABSTRACT FROM AUTHOR]

Laser diagnostics has been one of the most powerful tools in advancing state-of-the-art combustion research over the last five decades. Prof. Marcus Aldén, one of the most well-known pioneers in this field, has contributed many influential original publications over more than 40 years of his research career. In this paper, we will review some selected contributions, with emphasis on optical techniques developed and applied by Marcus and the group in Lund under his leadership, which are expected to play important roles in facing the challenge of the transition to a carbon-neutral energy system. A brief biography of Marcus is presented in the introduction section, including a summary of his academic services and achievements. The following sections are organized into chapters on different well-recognized techniques, i.e. , laser-induced fluorescence, Raman and Rayleigh scattering, thermographic phosphors, coherent anti-Stokes Raman spectroscopy, resonant four-wave mixing, and additional novel technical developments and approaches. The description often follows different categories, including technique development, activities using relatively mature techniques for studies of combustion phenomena, and applications of the most mature techniques in practical devices. The last part of the paper includes a short interview with Marcus for his comments, suggestions, and thoughts on the future challenges and opportunities in this field. This article, on the one hand, is an acknowledgment of the outstanding contribution of Marcus to the field of combustion research and, on the other hand, intends to provide a valuable review of laser diagnostics in combustion research. [ABSTRACT FROM AUTHOR]

In this work we report the impact of using mid-infrared laser-induced thermal grating spectroscopy (IR-LITGS) for temperature measurements in flames by probing hot water lines. The measurements have been performed in the product zone of laminar atmospheric CH 4 /H 2 /air flat flames with equivalence ratio ranging between 0.6 and 1.05. LITGS is a technique based on thermalization through collisions of the excited molecules for generation of a laser-induced grating, which then decays through thermal diffusion. As such, it tends to have limited application in atmospheric flames compared to flame measurements at elevated pressures due to the faster decay at low gas densities. However, by using mid-IR pump laser beams, it enables the generation of laser-induced gratings with large grating spacing, resulting in strong signal intensities and long signal durations. Single-shot IR-LITGS signals were recorded in the different CH 4 /H 2 /air flames that covered a temperature range between 1500 and 1800 K. To test the accuracy, the IR-LITGS flame temperature measurements were compared with laser Rayleigh scattering measurements and the result were in good agreement with each other. The IR-LITGS flame temperature measurements show a repetitive single-shot temperature precision better than 1% and an accuracy of 2.5% of the flame temperature. An IR-LITGS excitation scan of water in the flame shows that some ro-vibrational transitions exhibit no IR-LITGS signal, probably due to less efficient collisional energy transfer mechanism. This is important when deciding the wavelength to use for IR-LITGS flame temperature measurements using water absorption. [ABSTRACT FROM AUTHOR]

The mechanism of stabilizing a turbulent premixed methane-air flame using warm filamentary plasma is investigated by using laser diagnostics. First, stabilization of a turbulent jet flame is demonstrated in a setup using a pin-to-pin plasma discharge. The coupled plasma-flame structures were visualized utilizing planar laser-induced fluorescence (PLIF) of formaldehyde (CH 2 O) and methylidyne radicals (CH), as well as laser Rayleigh scattering thermometry imaging. The results show that the plasma channel and the flame front are spatially separated by a layer of hot burning products attributed to the flame propagation from the plasma core. Because of this spatial separation, the impacts of plasma on combustion are primarily thermal since the energetic radical species (such as O, H), produced by the discharge, have short equilibration time and cannot spread far away from the discharge channel before reaching the equilibrium state. From this point of view, turbulence would be beneficial for promoting the transport of plasma-produced radicals and thus bridge the gap between the plasma and the flame front. The plasma is still able to stabilize the flame. Based upon the experimental results, a frequent ignition-flame propagation (FIFP) model is proposed to explain the flame stabilization process. For the contracted plasma filament, the local power density is high enough to initialize the flame kernel that propagates away from the plasma channel until extinction. The propagation process is, however, strongly affected by turbulence. Local extinction is highly probable and thus the flame front has to be close to the ignition source at strong turbulence. At such conditions, the stabilized flame can be regarded as a large number of flame pockets, repeating the three phases of ignition, propagation and extinction, which can be summarized as the FIFP model. It infers that the flame propagation phase is important for sustaining the flame to complete combustion. Hence, this phase should be extended, which is more probable to achieve if the plasma ignition pilot is located in a section of limited turbulence. [ABSTRACT FROM AUTHOR]

Abstract A burner design with integrated electrodes was used to couple a gliding arc (GA) discharge to a high-power and large-scale turbulent flame for flame stabilization. Simultaneous OH and CH 2 O planar laser-induced fluorescence (PLIF) and CH PLIF measurements were conducted to visualize instantaneous structures of the GA-assisted flame. Six different regions of the GA-assisted flame were resolved by the multi-species PLIF measurements, including the plasma core, the discharge-induced OH region, the post-flame OH region, the flame front, the preheat CH 2 O region and the fresh gas mixture. Specifically, the OH profile was observed to be ring-shaped around the gliding arc discharge channel. The formaldehyde (CH 2 O) was found to be widely distributed in the entire measurement volume even at a low equivalence ratio of 0.4, which suggest that long-lived species from the gliding arc discharge have induced low-temperature oxidations of CH 4. The CH layer coincides with the interface of the OH and CH 2 O regions and indicates that the flame front and the discharge channel are spatially separated by a distance of 3–5 mm. These results reveal that the discharge column acts as a movable pilot flame, providing active radicals and thermal energy to sustain the flame. High-speed video photography was also employed to record the dynamics of the GA-assisted flame. This temporally resolved data was used to study the ignition and propagation behaviors of the flame in response to a temporally modulated burst-mode discharge. The results indicate that turbulent flame can be sustained by matching temporal parameters of the high-voltage bursts to the extinction time of flame. [ABSTRACT FROM AUTHOR]

This study investigated polycyclic aromatic hydrocarbons (PAHs) and soot formation characteristics in laminar jet flames fuelled by primary reference fuels (PRFs) at elevated pressures. Qualitative PAHs and quantitative soot profiles were acquired by using laser-induced fluorescence and laser-induced incandescence, respectively. The backpressure of flames ranged from 1 bar to 5 bar. Proper flames with the volume fraction of iso-octane in PRFs varying between 0% and 100% and flame equivalent ratio varying between 3.0 and 11.4 were stabilised in a pressurised chamber. The effects of backpressure, equivalent ratio and iso-octane ratio on PAHs and soot formation were evaluated. PAHs and soot formation can be promoted by increasing iso-octane ratio, equivalent ratio and backpressure. The data suggest that PAHs with large molecular size are more sensitive to the increase of backpressure compared with those with small molecular size. Backpressure played a positive role in the growth of PAHs size. The averaged soot volume fraction showed an approximate power-law relation with pressure. The measured averaged soot volume fraction was proportional to pn. Pressure exponent n was 1.34–2.17, 1.41–2.12 and 1.56–2.20 at equivalent ratios of 6.2, 8.5 and 11.4, respectively. [ABSTRACT FROM AUTHOR]

Abstract This work examines the combustion behavior of single pulverized biomass particles from ignition to early stages of char oxidation. The biomass residues investigated were pine wood, wheat straw, rice husk and grape pomace. The biomass particles, in the size range 224–250 µm, were injected upward into a confined region with hot combustion products, produced by a flat flame McKenna burner, with a mean temperature of 1610 K and a mean O 2 concentration of 6.5 vol%. Temporally and spectrally resolved images of the single burning particles were recorded with an intensified charge-coupled device camera equipped with different band-pass spectral filters. Data are reported for CH*, C 2 *, Na* and K* chemiluminescence, and thermal radiation from soot and char burning particles. The data on CH* and C 2 * chemiluminescence and soot thermal radiation permits to identify important differences between the ignition delay time, volatiles combustion time and soot formation propensity of the four biomass residues, which are mainly affected by their volatile matter content. The Na* and K* emission signals follow the same trends of the CH* and C 2 * emission signals until the end of the volatiles combustion stage, beyond which, unlike the CH* and C 2 * emission signals, they persist owing to their release from the char burning particles. Moreover, during the volatiles combustion stage, the Na*/CH* and K*/CH* ratios present constant values for each biomass residue. The CH* and thermal radiation emission data suggest that all biomass char particles experienced heterogeneous oxidation at or immediately after the extinction of the homogeneous volatiles combustion. [ABSTRACT FROM AUTHOR]

Abstract A series of turbulent premixed methane/air jet flames are studied using simultaneous planar laser diagnostic imaging of OH/CH/temperature and CH/OH/CH 2 O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH 2 O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH 2 O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor. [ABSTRACT FROM AUTHOR]

Abstract For the first time, to the best of the authors' knowledge, nonintrusive quantitative measurement of hydrogen cyanide (HCN) released during the devolatilization phase of straw pellets gasification is demonstrated with high spatial and temporal resolution. Mid-infrared degenerate four-wave mixing (IR-DFWM) measurements of HCN were performed by probing the interference-free P(20) line in the v 1 vibrational band at around 3 µm and the IR-DFWM signal was detected with an upconversion-based detector, providing discrimination of thermal noise and increased sensitivity. A novel single-pellet setup consisting of a multi-jet burner was used to provide hot flue gas environments with an even and well-defined temperature distribution, for single straw pellet gasification at atmospheric pressure. The environments had temperatures of 1380 K, 1540 K and 1630 K with a constant oxygen concentration of 0.5 vol%. In order to quantify the amount of HCN released during the devolatilization of straw pellets, calibration measurements were performed in well-defined HCN gas flows. Selected hot water lines were probed with IR-DFWM in the interrogated volume to obtain the instantaneous temperature, which were used to correct the temperature effect. HCN concentrations up to 1500 ppm were detected during the devolatilization stage, and the results indicate a strong temperature dependence of the HCN release. [ABSTRACT FROM AUTHOR]

Detailed numerical simulations are conducted in comparison with experimental results to study the flame structure and burning rate of a steadily burning aluminum droplet in hot steam-dominated environments. The droplet surface temperature, flame temperature, and flame stabilization position are measured along with the droplet burning rate estimated from the droplet size evolution. A numerical model accounting for detailed transport properties and chemical kinetics is presented and applied to unveil the flame structure, species and temperature distributions, and heat/mass transfer between the droplet and the surrounding gas. The numerical results of the temperature, velocity, and species distribution profiles demonstrate that the aluminum vapor flame is of classical diffusion flame structure, where near the droplet, there is a non-negligible amount of AlOAl apart from the main product Al 2 O 3 (l). This supports the deposition and formation of an alumina cap on the surface proposed in the literature. The simulation correctly captured the flame temperature and flame stabilization distance for a range of droplet sizes. Net heat flux analysis shows that conduction heat from the flame front accounts for less than 30% of the heat needed in aluminum evaporation, which warrants further quantification on other heat sources. The experimental and numerical results enrich the knowledge of the heat/mass transfer and chemical reactions near the droplet, which helps deepen the understanding of aluminum droplet burning. [ABSTRACT FROM AUTHOR]

Micron-sized aluminum (Al) particles have recently been proposed as promising carbon-free energy carriers. To facilitate the application of micron-sized Al particles as fuel in practical energy generation system, this study experimentally investigates the underlying mechanisms of the ignition, stabilization and particle-particle collision in lifted Al particle cloud flames. High-resolution shadowgraphy and luminosity measurements at a frame rate of up to 50 kHz are implemented to reveal the transient dynamics involved in these processes. It is shown that small particles, e.g. , with diameters less than 10 µm, ignite and combust further upstream than larger ones. This leads to the formation of massive and wide spreading hot Al 2 O 3 smokes, which contributes to the ignition of particles with larger diameters (e.g. , > 30 µm) downstream. The combustion of these large particles, in turn, promotes the ignition of adjacent particles through the deposition of hot Al 2 O 3 product particles on the particle surfaces. This process then sustains the group combustion of Al particle cloud. Additionally, the critical interparticle distance that triggers the ignition of the cloud flame is estimated to be around 6.5 times the mean diameter of the fresh particles. This again suggests that the individually burning Al particles can have a much broader influence on the surrounding non-burning particles due to the wide spreading hot Al 2 O 3 smokes. Moreover, the collision and the consecutive coalescence of two burning micron-sized Al particles are firstly studied. Interesting features, e.g. , the contact of the two flame envelopes, the collision and coalescence of the Al droplet cores, the variations in the droplet velocity and flame envelope radius, are analyzed and discussed. A schematic model accounting for this process is also proposed. [ABSTRACT FROM AUTHOR]

The concentrations of gas-phase potassium hydroxide (KOH), potassium chloride (KCl) and atomic potassium (K(g)) were quantitatively measured from the near-surface to downstream area of burning pinewood pellets by a newly developed photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS) technique to reveal the original form of the released potassium. The novel arrangement of the PF-TDLAS system enabled a spatial resolution of ∼1 mm3, which made it possible to obtain temporal release profiles of K(g)/KOH/KCl at different heights above the burning pellet surface. Surface temperature and mass loss of the wood pellet as well as the gas temperature at measurement points were measured simultaneously. During the devolatilization stage, the release of all three potassium species was observed, with each of them accounting for ∼1/3; while in char oxidation stage, the release of KOH was dominant, but the release intensity was strongly influenced by the local oxygen content. The results from different measurement heights showed there was a notable difference in potassium release profiles of different potassium species over the near-surface area, where the detected potassium forms were the best representative of the originally released forms of potassium. For a period of time during the devolatilization stage, only K(g) was detected in the near-surface area, and the concentration was significantly lower than the downstream area where KOH and KCl coexisted. This suggested that a large amount of potassium might leave the pellet as organic-K, which cannot be detected by the PF-TDLAS method. During char oxidation stage, the total potassium concentration at the near-surface area was also lower than the downstream area, but it was due to the lack of oxygen at the measurement position. A potassium release mechanism during the devolatilization stage of biomass combustion was proposed based on the experimental measurements, and the results also indicated the importance of spatially resolved measurement. [ABSTRACT FROM AUTHOR]

The elucidation of premixed ethylene/air combustion mechanisms holds theoretical significance for optimizing engines such as pulse detonation engines and rotating detonation engines. The flame front structure constitutes a vital factor of premixed ethylene/air combustion mechanisms, while the prevailing planar laser-induced fluorescence techniques currently in use fall short of directly visualizing the reaction zone of ethylene flames. Here, we, for the first time, employ the planar laser-induced photofragmentation fluorescence technique to achieve direct visualization of the reaction zone in premixed ethylene/air jet flames over a broad range of equivalence ratios (φ = 0.4–1.8). The application of a 212.8 nm laser for the photofragmentation of ethylene combustion intermediates results in the generation of abundant C 2 Swan bands fluorescence. Through a comparative analysis of experimental and simulation outcomes, it is established that this fluorescence primarily originates from C 2 * produced after the three-photon photofragmentation of C 2 H 2 , which is present in relatively high concentrations in ethylene/air flames, enabling direct visualization of the flame front structure. To enhance the signal-to-noise ratio (SNR), we further adopt a pump-probe approach by introducing a 516.5 nm probe laser to excite the low-energy state C 2 produced from the 212.8 nm laser-induced photofragmentation of C 2 H 2. This methodology results in a doubling of the overall imaging SNR. [ABSTRACT FROM AUTHOR]

In-situ measurements of the volume fraction of nanoparticles generated during the combustion of single iron particles were conducted using a high-speed light extinction method. The process of nanoparticles generation from iron particles burning under conditions of 21% oxygen concentration and ambient temperatures of 1400 K and 1850 K can be divided into three stages: acceleration of productivity, stable productivity, and continuous growth of volume fraction after the cessation of nanoparticles generation on the iron particle's surface. The productivity of nanoparticles in stage 2, which is proportional to the particle diameter, suggests that the nanoparticles generation process in this stage may be influenced by the outward diffusion of gaseous Fe and FeO. Approximately 2% to 4% of iron is converted into nanoparticles after combustion, and the conversion rate decreases with increasing particle diameter. Accurate values of the refractive index of iron and its oxides at high temperatures and in the liquid phase will contribute to improving the accuracy of the light extinction measurement since those values significantly affect the measured results of the volume fraction of nanoparticles. [ABSTRACT FROM AUTHOR]

The refined inertia of a square real matrix B , denoted ri ( B ) , is the ordered 4-tuple ( n + ( B ) , n − ( B ) , n z ( B ) , 2 n p ( B ) ) , where n + ( B ) (resp., n − ( B ) ) is the number of eigenvalues of B with positive (resp., negative) real part, n z ( B ) is the number of zero eigenvalues of B , and 2 n p ( B ) is the number of pure imaginary eigenvalues of B . For n ≥ 3 , the set of refined inertias H n = { ( 0 , n , 0 , 0 ) , ( 0 , n − 2 , 0 , 2 ) , ( 2 , n − 2 , 0 , 0 ) } is important for the onset of Hopf bifurcation in dynamical systems. An n × n sign pattern A is said to require H n if H n = { ri ( B ) | B ∈ Q ( A ) } . In this paper, we show that no path sign pattern of order n ≥ 5 requires H n . [ABSTRACT FROM AUTHOR]

Coal has been the main fuel in the generation of heat and power for many years and will continue to be so for years to come. Global warming awareness has led to the Kyoto Protocol, which targets the abatement of six greenhouse gases, including carbon dioxide (CO 2 ). For the abatement of CO 2 emissions, co-combustion of coal and biomass is an attractive solution. Other mitigation solutions include oxy-fuel combustion followed by Carbon Capture and Storage (CCS). However, co-combustion with coal and biomass in oxy-fuel atmosphere has been found to be conducive to chlorine-induced corrosion, slagging and fouling due to the use of complex fuel mixtures rich in alkali metals and chlorine. In order to address problems such as high-temperature corrosion, slagging and fouling caused by the use of these types of fuel mixtures, an alkali-measuring device is needed. The In-situ Alkali Chloride Monitor (IACM) utilizes UV light and Differential Optical Absorption Spectroscopy to measure gas-phase potassium chloride (KCl). This device has been successfully employed during biomass (wood chips and demolition wood) combustion and waste incineration. To investigate its performance in oxy-fuel combustion, the instrument was mounted on a 100 kW oxy-fuel combustion test unit equipped with a propane burner and a system for seeding of aqueous potassium chloride solution. The unit was operated in air- and oxy-fuel mode and the experiments substantiated the feasibility of online monitoring of gas-phase KCl in the flue gas during oxy-fuel combustion. The employment of an online alkali monitoring device during oxy-fuel combustion has the potential to increase the understanding of the release and capture of alkali chlorides, which in turn opens up for mitigation methods to reduce high temperature corrosion, slagging and fouling. This is a cost-efficient, sustainable solution because it extends the operational time of the boiler, reducing related maintenance costs. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of robust stability for quaternion-valued neural networks (QVNNs) with leakage delay, discrete delay and parameter uncertainties. Based on Homeomorphic mapping theorem and Lyapunov theorem, via modulus inequality technique of quaternions, some sufficient conditions on the existence, uniqueness, and global robust stability of the equilibrium point are derived for the delayed QVNNs with parameter uncertainties. Furthermore, as direct applications of these results, several sufficient conditions are obtained for checking the global robust stability of QVNNs without leakage delay as well as complex-valued neural networks (CVNNs) with both leakage and discrete delays. Finally, two numerical examples are provided to substantiate the effectiveness of the proposed results. [ABSTRACT FROM AUTHOR]

For the first time, laser-induced thermal grating spectroscopy (LITGS) in the spectral range around 3 µm is demonstrated as a versatile diagnostic tool. This spectral region is of particular interest in combustion diagnostics as many relevant species such as hydrocarbons and water exhibit fundamental vibrational modes and hence can be probed with high sensitivity. Another benefit of the IR-LITGS is that it allows performing spectroscopy in the infrared combined with signal detection in the visible. Hence, the strong thermal radiation inherent in flames does not represent an interference. As the first step, we present the application of IR-LITGS to cold gas flows, where traces of ethylene and water vapor are detected. The time-resolved LITGS signals, which can be acquired in a single laser shot, are rich in information and allow deriving temperature and to some extend chemical composition. In the second step, the IR-LITGS technique is applied to ethylene/air flames stabilized on a flat flame burner. A proof-of-concept study is carried out, in which the temperature is determined in the burned region of flames with systematically varied equivalence ratio (0.72 < Φ < 2.57). Moreover, in a highly sooty flame, LITGS signals were recorded as a function of height above the burner and allowed the determination of the temperature profile. The proposed IR-LITGS method has the potential for enabling single-shot measurements of several parameters at a time. Its applicability to sooty flame environments opens up new opportunities to study the complex formation of carbonaceous particles in flames. [ABSTRACT FROM AUTHOR]

Flame emission spectra and OH * imaging are carried out together with planar laser-induced fluorescence (PLIF) of OH and CH 2 O to characterize the reaction zone structures of three combustion modes, i.e. a flame mode, a transition mode and a flameless mode, established in a laboratory combustor. Spectroscopic measurements indicate that the invisibility of the flameless mode can be attributed to a moderate suppression of the CH * and C 2 * emissions as well as a significant increase of the continuous background from the CO 2 * emission. PLIF measurements of OH and CH 2 O show that all studied cases experience a premixing of unburnt reactants with the recirculated oxygen-containing hot burnt gases in the near-inlet region. It is found that the interplay between turbulence intensity and the availability of oxygen from both the inlet air jet and recirculation is important in establishing flameless combustion in the present combustor. Consistent with a previous chemical kinetic study, the present experimental results suggested that the flameless condition is in favor of converting a highly diluted CH 4 /air mixture into CO and H 2 , which will be further oxidized. For the flameless case, CH 2 O is detected slightly before the OH * region, where a relatively high level of minimal OH is observed. The reaction zone region marked by the OH * emission for the flameless case is characterized by a uniform instantaneous OH distribution with small standard deviation, suggesting a distributed oxidization of intermediate species such as CO and H 2 for this combustion mode. [ABSTRACT FROM AUTHOR]

Single-shot imaging of methyl radical (CH 3 ) in premixed methane/air flames is demonstrated using photofragmentation laser-induced fluorescence (PF-LIF) technique. A pump-probe strategy was adopted with the pump laser at 212.8 nm photolyzing CH 3 , and with the probe laser at 426.8 nm detecting the photolyzed CH ( X 2 Π) fragments. Spatially resolved spectrograph of the PF-LIF signal from a stable laminar flame was recorded across the reaction zone to investigate potential interferences. The results indicate that the single-photon channel, CH 3 + 212.8 nm → CH ( X 2 Π) + H 2 , dominates the photofragmentation process. The CH 2 radical was excluded from being an interfering precursor of the CH ( X 2 Π) fragments owing to its relatively low concentration and small absorption cross section. Naturally present CH in the flame was identified as the main interference, but was conservatively estimated to account for only less than 4% of the total PF-LIF signal. Signal-to-noise ratio of around 10 was realized for single-shot imaging of natural CH 3 in turbulent jet flames. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) direct numerical simulation (DNS) of an experimental turbulent premixed jet flame at high Karlovitz number was studied. The DNS resolution adequately resolves both the flame and turbulence structures. A reduced chemical mechanism for premixed CH 4 /air flames with NO x based on GRI-Mech3.0 was used, including 268 elementary reactions, and 28 transported species. Consistent post-processing methods were applied to both the DNS and experimental data to evaluate turbulent burning velocity-related statistics, namely the flame surface density (FSD), and the flame curvature. Good agreement was achieved for the 2D comparisons. The DNS data were further analysed and provide 3D statistics unattainable from the experiment. The ratio of the 3D and 2D flame surface densities was estimated. The results are comparable with other values reported for various experimental flames. The 3D and 2D flame curvatures were also compared and their distributions are shown to be quite different owing to the round on-average geometry. Instantaneous images of the heat release surrogate, [CH 2 O][OH], between the DNS and experiment agreed qualitatively. Various other experimentally obtainable surrogates for heat release rate including [CH 2 O][H], [CH 2 O][O], [HCO], and [CH] are also evaluated and compared using the DNS. The inner structure of the flame was compared between the DNS and experiment in terms of the joint PDFs of OH concentration and temperature. Generally good agreement was obtained; discrepancies may be due to the inconsistency of assumed equilibrium levels of OH concentration in the co-flow. [ABSTRACT FROM AUTHOR]

Flame stabilized by a bluff-body is a common scene in many engineering applications due to the enhanced mixing characteristics, improved flame stability, and ease of combustion control. We recently designed a burner which has a conical bluff body with a central air injector. In the current work, effects of the central air jet on the heat load of the bluff body, the flame structures and the flame blowoff limits were investigated. It was found that the central air jet can significantly reduce the heat load to the bluff body. It is a considerable solution to the problem caused by the high heat load in practical applications. The flame structures and blowout limits were altered with the addition of central air jet as well. Different blowout behaviors caused by the air jet were observed and reported. The bluff-body could be cooled down by the center air injection but then it seems not to stabilize the flame any more. [ABSTRACT FROM AUTHOR]

Laminar methane/air premixed Bunsen flames were studied using detailed numerical simulations and laser diagnostics. In the numerical simulations one-dimensional and two-dimensional configurations were considered with detailed transport properties and chemical kinetic mechanism. In the measurements OH PLIF was employed. The flame structures vary with varying equivalence ratio and pressure. For stoichiometric mixture at atmospheric pressure the flame exhibits a single reaction zone structure, while at high-pressures the flame exhibits a two-reaction zone structure: an inner premixed flame and an outer diffusion flame. The predicted two-zone structure is confirmed in the OH PLIF measurements. Using the numerical and the experimental data the methods of flame-cone-angle and flame-area have been used to extract the laminar flame speed for different equivalence ratios and pressures. It is found that although the flame cone angle method is widely used, it yields a lower accuracy than that of the flame surface area method. The inlet velocity of the burner is shown to affect the accuracy of extracted laminar flame speed. It is suggested that the most suitable inlet velocity of methane-air mixture is about 6 times the laminar flame speed. [ABSTRACT FROM AUTHOR]

The refined inertia of a square real matrix B , denoted ri ( B ) , is the ordered 4-tuple ( n + ( B ) , n − ( B ) , n z ( B ) , 2 n p ( B ) ) , where n + ( B ) (resp., n − ( B ) ) is the number of eigenvalues of B with positive (resp., negative) real part, n z ( B ) is the number of zero eigenvalues of B , and 2 n p ( B ) is the number of pure imaginary eigenvalues of B . For n ≥ 3 , the set of refined inertias H n = { ( 0 , n , 0 , 0 ) , ( 0 , n − 2 , 0 , 2 ) , ( 2 , n − 2 , 0 , 0 ) } is important for the onset of Hopf bifurcation in dynamical systems. An n × n sign pattern A is said to require H n if H n = { ri ( B ) | B ∈ Q ( A ) } . The star sign patterns of order n ≥ 5 that require H n are characterized. More specifically, it is shown that for each n ≥ 5 , a star sign pattern requires H n if and only if it is equivalent to one of the five sign patterns identified in the paper. [ABSTRACT FROM AUTHOR]

A sign pattern matrix is a matrix whose entries are from the set { + , − , 0 }. For a sign pattern matrix A , the qualitative class of A , denoted Q (A) , is the set of all real matrices whose entries have signs given by the corresponding entries of A. An n × n sign pattern matrix A requires all distinct eigenvalues if every real matrix in Q (A) has n distinct eigenvalues. Li and Harris (2002) [13] characterized the 2 × 2 and 3 × 3 irreducible sign pattern matrices that require all distinct eigenvalues, and established some useful general results on n × n sign patterns that require all distinct eigenvalues. In this paper, we characterize 4 × 4 irreducible sign patterns that require four distinct eigenvalues. This is done by characterizing 4 × 4 irreducible sign patterns that require four distinct real eigenvalues, that require four distinct nonreal real eigenvalues, or that require two distinct real eigenvalues and a pair of conjugate nonreal eigenvalues. The last case turns out to be much more involved. Some interesting open problems are presented. Three important tools that are used in the paper are the following: the discriminant of a polynomial; the fact that if a square sign pattern matrix A requires all distinct eigenvalues then A requires a fixed number of real eigenvalues; and the known result that if A is a " k -cycle" sign pattern then for each B ∈ Q (A) , the k nonzero eigenvalues of B are evenly distributed on a circle in the complex plane centered at the origin. [ABSTRACT FROM AUTHOR]

The refined inertia of a square real matrix A is the ordered 4-tuple ( n + , n − , n z , 2 n p ) , where n + (resp., n − ) is the number of eigenvalues of A with positive (resp., negative) real part, n z is the number of zero eigenvalues of A , and 2 n p is the number of nonzero pure imaginary eigenvalues of A . For n ≥ 3 , the set of refined inertias H n = { ( 0 , n , 0 , 0 ) , ( 0 , n − 2 , 0 , 2 ) , ( 2 , n − 2 , 0 , 0 ) } is important for the onset of Hopf bifurcation in dynamical systems. We say that an n × n sign pattern A requires H n if H n = { ri ( B ) | B ∈ Q ( A ) } . Bodine et al. conjectured that no n × n irreducible sign pattern that requires H n exists for n sufficiently large, possibly n ≥ 8 . However, for each n ≥ 4 , we identify three n × n irreducible sign patterns that require H n , which resolves this conjecture. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is a promising carbon-free energy carrier. Co-firing of ammonia in solid fuel-fired facilities is a feasible solution to reduce carbon dioxide (CO 2) emissions. Solid fuels, such as coal and biomass, contain various trace elements, such as alkali metals and sulfur, which are released to the gas phase during combustion. Experimental characterization and modeling are used to study the participation of alkali and sulfur species in ammonia conversion in a post-flame environment, focusing on the characteristics of NO emissions and NH 3 slip. The combustion environment was provided by a laminar flame burner with a temperature decreasing from about 2000 K in reaction zone to 1500 or 1100 K in flue gas zone and an equivalence ratio of around 0.65 or 1.3. Known amounts of ammonia (up to 20,000 ppm), potassium hydroxide (KOH, representative of alkaline substances, up to 25 ppm), and sulfur dioxide (SO 2 , up to 1500 ppm) were uniformly introduced into the burner for high-temperature thermochemical research. The concentrations of NH 3 , nitric oxide (NO), KOH, SO 2 , and hydroxyl radicals (OH) downstream of the burner were measured quantitatively in situ using broadband UV (ultraviolet) absorption spectroscopy. In the oxidizing reaction environments, the influence of SO 2 on the NO formation was negligible, while KOH significantly reduced the concentration of NO, and even led to residual ammonia in the low temperature case. Under reducing conditions, both SO 2 and KOH significantly inhibited the decomposition of ammonia, especially at relatively low temperature. Meanwhile, consumption of KOH/K was observed after the mixing with ammonia, possibly due to a direct reaction of KOH/K with ammonia. One dimensional modeling using a detailed mechanism containing N/S/K chemistry qualitatively predicted the impact of S/K on ammonia oxidation and decomposition. The effect was mainly contributed to the enhanced radical consumption by SO 2 and KOH. However, the model could not describe the observed consumption KOH/K by ammonia. Potassium amide (KNH 2) can be generated through KOH + NH 3 = KNH 2 + H 2 O. However, according to quantum chemistry calculations for KNH 2 , this reaction is endothermic by 80 kJ mol−1, shifting the equilibrium strongly towards KOH + NH 3 , and more work is required to clarify the mechanism of removal of potassium by NH 3. [ABSTRACT FROM AUTHOR]