Peak OH Research Articles

Ozone production sensitivity analysis for the Chengdu Plain Urban Agglomeration based on a multi-site and two-episode observation

Chengdu Plain Urban Agglomeration (CPUA) is one of the most serious areas suffering from ozone pollution in China. A comprehensive field observation focused on the ozone production rate and its sensitivity was conducted at CPUA in the summer of 2019. Six sampling sites were set and two ozone pollution episodes were recognized. The daily maximum 8-h average (MDA8) O3 concentration reached 137.9 ppbv in the urban sites during the ozone pollution episode. Peak concentration of O3 was closely related to intense solar radiation, high temperatures, and precursor emissions. The OH-HO2-RO2 radical chemistry and ozone production rate (P(O3)) were calculated using an observation-based model (OBM). The daily peak OH concentration varied in the range of 3-13 × 106 molecules cm−3, and peak HO2 and RO2 were in the range of 2–14 × 108 molecules cm−3 during ozone pollution episodes. During the ozone pollution episode, the average maximum of P(O3) in suburban sites (about 30 ppbv h-1.) was compared with urban sites, and the maximum of P(O3) was 18 ppbv h-1 in rural sites. The relative incremental reactivity (RIR) results demonstrate that it was a VOCs-limited regime in the central urban area of Chengdu, with NOx suppression effect in some regions. In the southern neighboring suburb of Chengdu, it was VOCs-limited as well. However, the northern suburban area was a transition region. In the remote rural areas of the southern CPUA, it was highly NOx-limited. Local ozone production driven by the photochemical process is crucial to the ozone pollution formation in CPUA. The geographically differentiated recognition of the ozone regime found by this study can help to tailor control strategies for local conditions and avoid the negative effects of a one-size-fits-all approach.

Combustion characteristics and flame development of ammonia in an optical spark-ignition engine

To meet the ever-increasing serious challenges of global energy shortages and environmental pollution, it is urgent to apply alternative carbon-free fuels in the transportation field to reduce global carbon dioxide emissions. Ammonia is considered a prospective carbon-free fuel for engines due to its advantages in production, storage and transportation. The combustion and flame development characteristics of ammonia at different excess air ratios (λ) were researched in an optical spark ignition (SI) engine employing high-speed natural flame luminosity (NFL) imaging, OH* chemiluminescence imaging and flame spectra measurement. The combustion performance and flame development of ammonia were compared with methane. Results indicate that ammonia has the optimal combustion performance at λ = 0.9, with the most concentrated heat release, the highest power output and the best combustion stability. The peak natural flame luminosity intensity is highest at λ = 0.9, and the flame speed of 13.7 m/s is the fastest among testing cases, which results in the shortest ignition delay and combustion duration. OH* and NH2* spectra intensity of the ammonia flame were quantified and analyzed under different λ. The peak OH* intensity is highest at λ = 0.9, while the largest intensity of NH2* is obtained at the fuel-rich case (λ = 0.8). The maximum overall OH* chemiluminescence intensity at the same crank angles is obtained at λ = 0.9. Finally, despite the delayed spark timing, the combustion performance of methane presents higher indicated mean effective pressure (IMEP) and better combustion stability as well as faster flame propagation speed than ammonia at the same λ. In summary, ammonia shows the best combustion and flame development characteristics at λ = 0.9. The combustion performance of ammonia in SI mode is significantly worse than that of methane.

Exploring the promoting effect of nitrilotriacetic acid on hydroxyl radical and humification during magnetite-amended composting of sewage sludge

The OH production by adding magnetite (MGT) alone has been reported in composting. However, the potential of nitrilotriacetic acid (NTA) addition for magnetite-amended sludge composting remained unclear. Three treatments with different addition [control check (CK); T1: 5 % MGT; T2: 5 % MGT + 5 % NTA] were investigated to characterize hydroxyl radical, humification and bacterial community response. The NTA addition manifested the best performance, with the peak OH content increase by 52 % through facilitating the cycle of Fe(Ⅱ)/Fe(Ⅲ). It led to the highest organic matters degradation (22.3 %) and humic acids content (36.1 g/kg). Furthermore, NTA addition altered bacterial community response, promoting relative abundances of iron-redox related genera, and amino acid metabolism but decreasing carbohydrate metabolism. Structural equation model indicated that temperature and Streptomyces were the primary factors affecting OH content. The study suggests that utilizing chelators is a promising strategy to strengthen humification in sewage sludge composting with adding iron-containing minerals.

Comparative study of the effects of ignition location on the flame propagation characteristics and spectral properties of a methane–air premixed gas in a vertical pipeline

Methane is an explosion hazard during its pipeline transportation and distribution. This study investigated the characteristics of methane explosions in a vertical pipeline for various ignition locations and methane concentrations. Flame propagation properties were more sensitive to methane concentration changes in central ignition than in bottom ignition; in lean fuel-air mixtures, they were more sensitive to methane concentration changes than in rich fuel-air mixtures. Because the central ignition flame burned over a larger area, it shows more explosive intensity than the bottom ignition. At methane concentration of 10 vol%, the maximum pressure (Pmax) and maximum pressure increase ((dP/dt)max) of the central ignition increased by 6.2 % and 44.2 %, respectively, compared with that of the bottom ignition, and the time to reach the peak explosion pressure (tP) was advanced by 14.6 %. Bottom ignition showed faster maximum flame velocity (vmax) and flame pattern change, higher maximum flame temperature (Tmax), and stronger OH• peak intensity than central ignition. When the flame velocities were all upward, vmax was most significantly reduced by 64.6 % when the methane concentration was 8 vol% compared with the bottom ignition. At a methane concentration of 10 vol%, Tmax, OH• peak signal intensity and OH• signal duration increased by 2.2 %, 1.3 %, and 39.5 %, respectively, for the bottom ignition compared to the central ignition. The experimental results of this study can be used as a reference to design targeted measures for preventing methane explosions in pipelines and to develop new methane explosion suppressants.

Computational study of added H2 impacts on mixture formation, OH radical and unregulated emissions of spark-ignition methanol engine under various boundary parameters

Numerical simulations of hydrogen (H2) impacts on mixture formation, OH radical generation, and formaldehyde (HCHO) and unburned methanol (CH3OH) emissions of spark-ignition (SI) methanol engine based methanol late-injection strategy under different methanol injection timings (MIT), ignition timings (IT), excess air ratios (λ) and added H2 ratios (RH2) boundary parameters were conducted. The simulation results show that the ideal methanol concentration distribution is obtained at the MIT = 85°CA BTDC. The OH radical plays a dominant role in methanol oxidation. The OHPMF (peak OH radical mass fraction) of RH2 = 9 % ≫ OHPMF of RH2 = 6 % > OHPMF of RH2 = 3 % > OHPMF of RH2 = 0 % at MIT = 85°CA BTDC. Compared with IT and λ, the RH2 is more decisive for the generation of OH radical. The HCHO is a major intermediate and an important unregulated emission of methanol engine. In the initial stage of methanol reaction, added H2 can accelerate the generation of OH radical, further promote the oxidation of methanol, and greatly raise the generation of HCHO. The peak HCHO mass fraction increases with the increase of RH2. At MIT = 85°CA BTDC and exhaust valve opening, the HCHO emission of RH2 = 3 %, 6 %, and 9 % is approximately 51 %, 76.5 % and 89.4 % lower than that of RH2 = 0 %, respectively. The generation and post-oxidation of HCHO mainly depended on average in-cylinder temperature. There is a lowest CH3OH emission at MIT = 85°CA BTDC, IT = 28°CA BTDC, λ = 1.4 and pure methanol. At MIT = 85°CA BTDC, λ = 1.4 and exhaust valve opening, the CH3OH emission of RH2 = 3 %, 6 %, and 9 % is approximately 52.8 %, 75.5 % and 92.5 % lower than that of RH2 = 0 %, respectively.

Nitrous acid budgets in the coastal atmosphere: potential daytime marine sources

Abstract. Nitrous acid (HONO), a vital precursor of atmospheric hydroxyl radicals (OH), has been extensively investigated to understand its characteristics and formation mechanisms. However, discerning fundamental mechanisms across diverse environments remains challenging. This study utilizes measurements from Mount Lao, a coastal mountain in eastern China, and an observation-based chemical box model (OBM) to examine HONO budgets and their subsequent impacts on atmospheric oxidizing capacity. The model incorporates additional HONO sources, including direct emissions, heterogeneous conversions of NO2 on aerosol and ground surfaces, and particulate nitrate photolysis. The observed mean HONO concentration was 0.46 ± 0.37 ppbv. The updated model reproduced daytime HONO concentrations well during dust and photochemical pollution events. During dust events, daytime HONO formation was dominated by photo-enhanced heterogeneous reactions of NO2 on aerosol surfaces (> 50 %), whereas particulate nitrate photolysis (34 %) prevailed during photochemical pollution events. Nevertheless, the model uncovers a significant unidentified marine HONO source in a “sea case”, with its HONO production rate reaching up to 0.70 ppbv h−1 at noon. Without considering this unidentified source, an extraordinarily high photolysis coefficient of nitrate and/or a heterogeneous uptake coefficient of NO2 would be required to match observed HONO concentrations. This missing marine HONO source affected the peak O3 production rate and OH radical concentration by 36 % and 28 %, respectively, at the observation site. Given the limited HONO observation data in coastal and marine settings, the unidentified HONO source may cause an underestimation of the atmosphere's oxidizing capacity. This study highlights the necessity for further investigation of the role of HONO in atmospheric chemistry in coastal and marine environments.

Nutritional composition, antioxidant activity and characterization of bioactive compounds from Thymus serpyllum L.: An underexploited wild aromatic plant

Thymus serpyllum is an aromatic wild plant with a wide range of usage in food, medicinal and therapeutics due to the presence of health promoting compounds. Considering the increasing demand for natural sources of bioactive compounds in the food and pharmaceutical industry, the present study is aimed to characterize the nutrients, bioactive compounds and antioxidant property of T. serpyllum which is used as food and medicine in Ladakh region of India since ancient times. The plant has rich protein content (21.39 g/100 g) and is an excellent source of micronutrients such as vitamins and mineral elements (Ca, K, Si, Fe, Mn) as analyzed by WDXRF spectrometry. Spectrophotometric analysis was conducted to estimate the phenol, flavonoid and phytosterol content. The DPPH scavenging activity of the methanol extract showed a good percentage inhibition with IC50 = 48.58 μg/mL. Six polyphenols gallic acid, caffeic acid, 4-hydroxybenzoic acid, rutin, catechol and naringin were quantified using RP-HPLC analysis. Various functional groups with high intense peak OH, C = C and CO was identified through FTIR analysis. The main compounds identified through GCMS includes thymol (55.98%) followed by Benzene, 1,1′-sulfonylbis- (5.63%) and Stigmast-5-en-3-ol, (3.beta.,24 s)- (4.05%). Thymol is a distinctive compound present in all Thyme species which have anti-inflammatory, antimicrobial, antioxidant property and is also used as food and preservatives. Compared to other thyme species, the thymol content was higher in T. serpyllum. Presence of health beneficial nutrients and a rich profile of bioactive compounds mainly gallic acid and thymol makes this plant a promising source of natural additives for the food industry and development of novel drugs.

Optical study of knocking phenomenon in a spark-ignition engine by using high-speed OH* chemiluminescence imaging: A multiple ignition sites approach

Knock in a spark-ignition (SI) engine is a complicated combustion phenomenon that stimulates high-pressure oscillations inside the combustion chamber and restricts engine performance. This study presents a high-speed OH* chemiluminescence imaging technique to investigate the knock mechanism resulting from firing multiple spark plugs. The experiment was performed using a customized liner having three spark plugs installed at equal spacing, and to compare the results with conventional SI conditions, in which one spark plug was mounted at the center of the cylinder head. In addition, multiple pressure transducers were used at various locations to record the frequencies induced by the pressure oscillations inside the cylinder during knocking events. The results showed that firing a single central spark plug generated mild knock with late combustion phasing and lower power output. However, adding more spark plugs could advance the initiation of autoignition and produce higher knock intensity along with lower combustion duration for the same operating conditions. Additionally, a weak OH* chemiluminescence intensity oscillation was monitored before the autoignition of the unburned charge occurred. The crank angle location of peak OH* intensity and peak HRR showed a good linear curve fit with a positive slope. Furthermore, the larger amount of unburned mass fraction produced stronger pressure waves due to multiple autoignition sites, and the unburned mass fraction exhibited a good linear relationship with unburned temperature and end-gas area at the knock onset point. Moreover, the frequency spectrum recorded by the multiple pressure sensors illustrated that in the case of a single central spark plug only circumferential acoustic waves were formed with low power intensity. However, multiple ignition sites promoted both circumferential and radial pressure waves inside the combustion chamber because of multiple autoignitions occurring both near the center and cylinder wall.

Analytical Approximations of the Characteristics of Nighttime Hydroxyl on Mars and Intra-Annual Variations

Observations of vibrationally excited hydroxyl (OH*) emissions are widely used to obtain information about the dynamics and composition of the atmosphere. We present some analytical approximations for the characteristics of the hydroxyl layer in the Martian atmosphere such as OH* concentration at the maximum and height of the maximum, as well as relations for estimating the influence of various factors on the OH* layer in night conditions. These characteristics depend on the temperature of the environment, concentration of atomic oxygen, and their vertical gradients. The relations are applied to the results of numerical modeling using the global atmospheric circulation model for prediction of seasonal behavior of the hydroxyl layer on Mars. Annual and intra-annual variations in the concentration of excited hydroxyl and layer height from the modeling data have both some similarities with those of the Earth and significant differences. The concentration and height maximum in the equatorial, northern and southern midlatitudes vary depending on the season; the maximum concentration and the minimum height fall on the first half of the year. Model calculations confirmed the presence of the peak OH* concentration at polar latitudes in winter at an altitude of approximately 50 km with the volume emission densities of 2.1, 1.4, and 0.6 × 104 photons cm–3 s–1 for vibrational level transitions 1–0, 2–1, and 2–0, respectively. The relations obtained may be used for the analysis of measurements and interpretation of their variations.

Temperature, soot, and OH*/CH* chemiluminescence measurements of partially-premixed CO2-diluted propane flames on a linear Hencken burner

Recent studies highlight the need for further examination of partially-premixed diffusion flames with applications in modern burner configurations that increasingly rely on technologies such as flue gas recirculation and oxy-fuel combustion, which increase the CO 2 content in the combustion environment. The partially-premixed nature of the linear Hencken burner (LHB) offers a unique platform with many advantages over comparable slot-type or radially-symmetric burner configurations. In this work, partially-premixed laminar propane flames were examined under increasing CO 2 percentages (0 to 50% v ). Prosumer grade digital CMOS cameras were calibrated and used to determine two-dimensional, quantitative measurements of soot, temperature, and OH* and CH* chemiluminescence. Soot values were additionally supported via laser extinction measurements using a 532 nm continuous wavelength laser. The peak soot volume fraction was found to decrease from 318 ppbv for the pure propane case down to 37 ppbv for the 50% v CO 2 case, while the corresponding temperature decreased from 1876 K down to 1742 K. The data also suggested that soot maturity decreased in conjunction with overall concentration. The regions of peak OH* and CH* concentrations maintained their transverse positions as well as their relative positions with respect to one another for all flame conditions, though moved axial further away from the flame base with increasing CO 2 . Additionally, the peak OH* region remained closer to the central flame axis across all conditions, a result that is counter to other studies that examined CO 2 dilution in purely non-premixed flames. • 2D quantitative soot and temperature measurements of CO2 diluted partially-premixed propane flames. • Laser extinction measurements bolster 2D soot values. • Fuel dilution over 40%v CO2 starts to significantly lower soot maturity. • 2D quantitative chemiluminescence measurements of OH* and CH* in CO2 diluted partially-premixed propane flame. • Movement of peak chemiluminescent regions show significant differences versus premixed and dilution flames.

REINFORCED PECTIN-DES BIOPLASTIC WITH DIFFERENT CONCENTRATION OF CITRIC ACID

The constant demand for synthetic plastics has led to the accumulation of plastic waste in nature due to its non-biodegradable attributes. In this study, the properties of pectin as biodegradable bioplastic were enhanced using Deep Eutectic Solvent (DES) as a plasticiser with concentrations of citric acid as a crosslinker that was ranged from 1% to 4%. The physical characteristic and mechanical properties of bioplastics were determined using the thickness test and the tensile test. The sample with 3% citric acid showed the highest thickness value (0.25±0.01 mm). The results showed that the increase in the concentration of citric acid increased the film’s flexibility, thereby influencing the properties of the bioplastic film obtained. The highest citric acid concentration at 4% showed the highest flexibility with 29.02±2.99 MPa, 2.73±0.38% and 10.14±0.88 MPa for tensile strain, tensile stress and Young’s Modulus values, respectively. The presence of peak OH, C=O and C-O from Fourier Transform Infrared (FT-IR) spectroscopy outlined the formation and showed the interactions between the pectin, DES and citric acid in the bioplastic that was produced. In conclusion, the properties of bioplastic/pectin/DES were altered with the different concentrations of citric acid either as a crosslinker or as a plasticiser to become a new green alternative to synthetic plastics.

The transition from a nitrogen oxides-limited regime to a volatile organic compounds-limited regime in the petrochemical industrialized Lanzhou City, China

Heavy ozone (O3) pollution is often observed in the petrochemical industrial city of Lanzhou, located in the semi-arid and mountainous northwest China. The mountain-valley topography, special meteorological conditions, and high precursor emissions from the petrochemical industry across the city caused significantly increasing surface ozone concentrations in recent years. We investigated temporal distributions of O3 and its precursors and explored volatile organic compounds (VOCs) ‘composition and its contribution to O3 formation. The Ozone Isopleth Plotting Research (OZIPR) model was used to illustrate the ozone isoconcentration curves and assess the responses and sensitivity of O3 concentrations to its precursors. The results show that, in summer 2018, the VOCs/ nitrogen oxides (NOx) ratio defined in the ridgeline of the Empirical Kinetic Modeling Approach (EKMA) curve was about 13.8:1, indicating that ozone was formed in the NOx-limited regime, whereas in summer 2019, this ratio was 15:1, indicating that ozone was formed in the VOCs-limited regime, featuring a transition from a NOx-limited regime in summer 2018 to a VOCs-limited regime in summer 2019. We explored potential causes of this transition and attributed it to a large-scale instrument overhaul that resulted in significantly lower OH radical concentrations in summer 2019 and lower VOC emission after the overhaul. The modeling results using the Regional Atmospheric Chemical Mechanism version 2 (RACM2) photochemical box model show that the peak OH radical concentration in the summer of 2018 was much higher than that of 2019, reaching 12.38 × 106 molecules·cm−3. The result was confirmed by the Community Multiscale Air Quality (CMAQ) modeling system.

Investigation of the OH∗ chemiluminescence characteristics in CH4/O2 lifted flames

Premixed and non-premixed flames of CH4/O2 are widely used in industrial fields, including the non-catalytic partial oxidation (NC-POX) of natural gas to produce hydrogen-rich syngas. As a major spontaneous emission present in flames, OH∗ chemiluminescence is often applied in combustion diagnostics to indicate flame structure, reaction mechanism and equivalence ratio. In this study, the flame imaging technique was applied in order to investigate the flame structure, lift-off height, peak OH∗ intensity and core reaction zone for a CH4/O2 lifted flame impinging a plate. The results demonstrated that both the increase in the global equivalence ratio (λ) as well as the reduction in the plate-to-nozzle distance (L/De) can contribute the generation of the lifted flame. In regard to the fixed L/De, the lifted flame exhibited a narrower flame front and lower chemical reaction intensity in the case of a small λ. As L/De decreased, the position of the flame core reaction zone shifted to the flame base. Meanwhile, the core reaction zone of the flame and peak OH∗ intensity was found to increase progressively. In addition, with the increase in λ, the flame lift-off height (H) initially rose gradually, after which it then declined sharply and thereafter increased. As indicated by the flame lift-off characteristic, as λ rose, a circulation zone developed at the flame base, which was located between the flame inner shear layer and the jet of oxidizer. Accordingly, the circulation zone can promote the generation of a lifted flame due to the enhanced mixing of the reactants.

Numerical Study of the Influence of Turbulence–Chemistry Interaction on URANS Simulations of Diesel Spray Flame Structures under Marine Engine-like Conditions

The present work performs unsteady Reynolds-averaged Navier-Stokes simulations to study the effect of turbulence-chemistry interaction (TCI) on diesel spray flames. Three nozzle diameters (d0) of 100, 180, and 363 μm are considered in the present study. The Eulerian stochastic fields (ESF) method (with the TCI effect) and well-stirred reactor (WSR) model (without the TCI effect) are considered in the present work. The model evaluation is carried out for ambient gas densities (ρam) of 30.0 and 58.5 kg/m3. The ESF method is demonstrated to be able to reproduce the ignition delay time (IDT) and lift-off length (LOL) with an improved accuracy than that from the WSR method. Furthermore, TCI has relatively more influence on LOL than on IDT. A normalized LOL (LOL*) is introduced, which considers the effect of d0, and its subsequent effect on the fuel-richness in the rich premixed core region is analyzed. The RO2 distribution is less influenced by the TCI effect as ambient density increases. The ESF model generally predicts a longer and wider CH2O distribution. The difference in the spatial distribution of CH2O between the ESF and WSR model diminishes as d0 increases. At ρam = 30.0 kg/m3, the ESF method results in a broader region of OH with lower peak OH values than in the WSR case. However, at ρam = 58.5 kg/m3, the variation of the peak OH value is less susceptible to the increase in d0 and the presence of the TCI model. Furthermore, the influence of TCI on the total OH mass decreases as d0 increases. The total NOx mass qualitatively follows the same trend as the total OH mass. This present work clearly shows that the influence of TCI on the global spray and combustion characteristics becomes less prominent when d0 increases. (Less)

Modeling of observations of the OH nightglow in the venusian mesosphere

Venus airglow emissions have been unambiguously detected in the wavelength ranges of 1.40–1.49 and 2.6–3.14 μm in limb observations by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) onboard the Venus Express (VEx) spacecraft and are attributed to the OH(2–0) and OH(1–0) Meinel band transitions. The integrated (limb slant path) emission rates for these bands were measured by Piccioni et al. (2008).Photochemical (Caltech/JPL KINETICS) and global circulation (Venus Thermospheric General Circulation Model - VTGCM) model calculations suggest the observed OH emission is produced primarily via the Bates-Nicolet mechanism, as on the Earth, although Venus' background atmosphere is different than that of the Earth, but the modeled contribution of the HO2 + O → OH(v) + O2 reaction increases in the lower portion of the OH airglow layer. An overall difference of ~2 km in the peak heights of the OH(1–0) and OH(2–0) layers is seen in both the KINETICS and VTGCM simulations as a result of this change in the relative importance of H + O3 → OH(v) + O2 versus HO2 + O → OH(v) + O2 reactions with altitude.First time 3-D simulations of the OH Δv = 1 nightglow limb slant emission calculate a peak intensity of ~0.6 ± 0.3 MegaRayleighs at ~102 km altitude, an intensity that is consistent with Venus Express VIRTIS observations (Gérard et al. 2010; Soret et al. 2010, 2012) and KINETICS results. Soret et al. (2010) reported the intensity of the peak OH airglow increased from 0.30 to 0.40 MR from dusk to dawn but noted the observations used are not uniformly distributed and the observed emission is extremely variable, so a more detailed assessment of the observations was not possible. Our simulations show a decrease in the average OH(1–0) emission is symmetric about the midnight meridian, but the simulations find an asymmetric decrease from the equator to the poles. Consideration of transport and chemical lifetimes suggests modeling of OH above ~96 km requires explicit description of transport and vibrational-state-dependent chemistry.

Auto-Ignition of Gas Turbine Lubricating Oils in a Shock Tube Using Spray Injection

Abstract In choosing the lubricating oil for a gas turbine system, properties such as viscosity, viscosity index, corrosion prevention, and thermal stability are chosen to optimize turbine longevity and efficiency. Another property that needs to be considered is the lubricant's reactivity, as the lubricant's ability to resist combustion during turbine operation is highly desirable. In evaluating a method to define reactivity, the extremely low vapor pressure of these lubricants makes conventional vaporization by heating impractical. To this end, a new experiment was designed and tested to evaluate the reactivity of lubricating oils using an existing shock-tube facility at Texas A&M University equipped with an automotive fuel injector. This experiment disperses a premeasured amount of lubricant into a region of high-temperature air to study auto-ignition. To ensure proper dispersal, a laser extinction diagnostic was used to detect the lubricant particles behind the reflected shock as they are dispersed and vaporized. An OH* chemiluminescence diagnostic was used to determine ignition delay time. Using this method, various 32-, 36-, and 46-weight lubricants identified as widely used in the gas turbine industry were tested. Experiments were conducted in postreflected shock conditions around 1370 K (2006 °F) and 1.2 atm, where ignition delay time, peak OH* emission, and time-to-peak values were recorded and compared. Ignition was observed for all but one of the lubricants at these conditions, and mild to strong ignition was observed for the other lubricants with varying ignition delay times.

Experimental and Numerical Study of the Effect of Fuel/Air Mixing Modes on NOx and CO Emissions of MILD Combustion in a Boiler Burner

The Moderate or Intense Low-oxygen Dilution (MILD) combustion is characterized by low emissions, stable combustion and low noise for various kinds of fuel, which has great potential in the industry. The aim of this study is to investigate the effect of fuel/air mixing modes on NOx and CO emissions of MILD combustion in a boiler burner by experiments and numerical simulations. Three types of fuel/air mixing modes (premixing mode, diffusion mode and hybrid mode) have been considered in this study. The realizable k-e turbulent model and the Eddy Dissipation Concept (EDC) combustion model were used in numerical simulations. In addition to the temperature near the burner head, the calculation results match very well with the axial temperature distribution at the furnace center. The flow pattern under different mixing modes is similar, while the hybrid mode has a higher OH concentration near the diffusive fuel nozzle than the premixing mode, and the corresponding position of the peak OH concentration is closer to the rear half of the furnace. The distribution of temperature is extremely uniform for the premixing mode in the main reactive zone, which is typical for MILD combustion. There is a distinct area where the reaction temperature is higher than 1600 K for the hybrid mode. Moreover, in the main reactive zone, the gas recirculation ratio is high enough to ensure flue gas recirculation, which is beneficial to achieve MILD combustion at local areas. At the location where the axial distance is greater than 0.2 m, the gas recirculation ratio of the premixing mode is larger than that of the hybrid mode, which strengthens the entrainment of hot flue gas into the recirculated gas. The experimental results show that when the thermal intensity is less than 1.67 MWm−3, the NOx emissions are less than 15×10−6@3.5%O2 in near stoichiometric ratio in the premixing mode, and the CO emissions are less than 10×10−6@3.5%O2 under the same conditions. In the diffusion mode, the NOx emissions are less than 30×10−6@3.5%O2. In order to keep NOx and CO emissions low, the hybrid modes with optimized fuel distribution ratio are found under different thermal intensities.

Experimental and numerical study of the effect of elevated pressure on laminar burning velocity of lean H2/CO/O2/diluents flames

Laminar burning velocities (SL) of H2/CO/O2/diluents (He or N2) were reported at lean conditions (ϕ = 0.5–1.0), elevated pressures (1–11 bar) and a wide range of H2-CO volumetric ratios. A new high-pressure rig based on specially designed heat flux burner was applied to obtain planar, adiabatic and stretch-less flames used for laminar burning velocity measurements. Five popular syngas oxidation mechanisms were tested against the obtained SL data and showed satisfactory agreement even at high pressures up to 11 bar. SL decreases as pressure increases while the negative pressure dependence of mass burning rate is experimentally and numerically found for syngas with high diluent content in the oxidizer or high hydrogen content in the fuel, which indicated the negative dependence of the overall reaction order on adiabatic flame temperature. Sensitivity and rate of production analyses indicated that the sensitivity coefficients and HO2 mole fraction increase to the maximum at the pressure of the minimum overall reaction order and then decreases, which leads to larger uncertainty on predicting SL at higher pressures. The flame front height represented by the peak OH* chemiluminescence location also shows a non-monotonic variation trend as pressure increases and can serve as a target for mechanism validation.

A numerical investigation in buoyancy effects on micro jet diffusion flame

The buoyancy effect on micro hydrogen jet flames in still air was numerially studied. The results show that when the jet velocity is relatively large (V≥0.2 m/s), the flame height, width and temperature decrease, whereas the peak OH mass fraction increases significantly under normal gravity (g=9.8 m/s2). For a very low jet velocity (e.g., V= 0.1 m/s), both the peak OH mass fraction and flame temperature under g=9.8 m/s2 are lower than the counterparts under g=0 m/s2. Analysis reveals that when V≥0.2 m/s, fuel/air mixing will be promoted and combustion will be intensified due to radial flow caused by the buoyancy effect. However, the flame temperature will be slightly decreased owing to the large amount of entrainment of cold air into the reaction zone. For V=0.1 m/s, since the heat release rate is very low, the entrainment of cold air and fuel leakage from the rim of tube exit lead to a significant drop of flame temperature. Meanwhile, the heat loss rate from fuel to inner tube wall is larger under g=9.8 m/s2 compared to that under g=0 m/s2. Therefore, the buoyancy effect is overall negative at very low jet velocities.

Investigation of flame retarded polypropylene by high-speed planar laser-induced fluorescence of OH radicals combined with a thermal decomposition analysis

The combustion of micrometer-sized polypropylene (PP) particles is analyzed in situ using a combination of high-speed planar laser-induced fluorescence of the OH radical (OH-PLIF) and a thermal decomposition analysis. The gas phase is investigated by multiple analytical techniques to gain comprehensive knowledge on the decomposition products of flame retardants and their effect on the combustion process. Neat PP is compared with a formulation consisting of 10 wt% of a phosphorus-containing flame retardant (pentaerythritol spirobis(methylphosphonate), PSMP) which is known to provide gas phase activity. The decomposition of the neat flame retardant, PP and the flame retardant formulation is investigated using a simultaneous analysis (STA) consisting of a thermal gravimetric analysis and a differential thermal analysis device which is coupled to Fourier-transform infrared spectroscopy and mass spectrometry devices. By this, the release of decomposition products of the flame retardant additive can be determined. The excitation of OH radicals is used to temporally track the diffusion flame surrounding the particles during combustion in a laminar flow reactor. The radial distance to the peak reactivity zone of flame retardant containing particles increased by about 70 µm compared with neat PP particles. Tracking the peak OH signal in the diffusion flame, during ignition and the early phase of combustion, a decrease in the peak intensity is observed for flame retardant polymer particles. Additionally, cone calorimeter tests are used to evaluate the combustion behavior as a standard test in flame retardancy.