The effects of water dilution on hydrogen,syngas, and ethylene flames at elevated pressure

This work investigates experimentally and numerically the kinetic effects of water vapor addition on the burning rates of H₂, H₂/CO mixtures, and C₂H₄ from 1 atm to 10 atm at flame temperatures between 1600 K and 1800 K. Burning rates were measured using outwardly propagating spherical flames in a nearly constant pressure chamber. Results show good agreement with newly updated kinetic models for H₂ flames. However, there is considerable disagreement between simulations and measurements for H₂/CO and C₂H₄ flames at high pressure and high water vapor dilution. Both experiments and simulations show that water vapor addition causes a monotonic decrease in mass burning rate and the inhibitory effect increases with pressure. For hydrogen flames, water vapor addition reduces the critical pressure above which a negative pressure dependence of the burning rate is observed. However, for C₂H₄ flames, the burning rate always increases with pressure. The results also show that water vapor addition has the same effect as a pressure increase for H₂ and H₂/CO flames, shifting the reaction zone into a narrower window at higher temperatures. For all fuels, water vapor addition increases OH formation via H₂O + O while reducing the overall active radical pool for hydrogen flames. For C₂H₄, the additional HO₂ production pathway through HCO results in a dramatic difference in pressure dependence of the burning rate from that observed for hydrogen. The present work provides important additions to the experimental database for syngas and C₀–C₂ high pressure kinetic model validations.     

The structure and extinction of nonpremixed methane/nitrous oxide and ethane/nitrous oxide flames

Knowledge of combustion of hydrocarbon fuels with nitrogen-containing oxidizers is a first step in understanding key aspects of combustion of hypergolic and gun propellants. Here an experimental and kinetic-modeling study is carried out to elucidate aspects of nonpremixed combustion of methane (CH₄) and nitrous oxide (N₂O), and ethane (C₂H₆) and N₂O. Experiments are conducted, at a pressure of 1 atm, on flames stabilized between two opposing streams. One stream is a mixture of oxygen (O₂), nitrogen (N₂), and N₂O, and the other a mixture of CH₄ and N₂ or C₂H₆ and N₂. Critical conditions for extinction are measured. Kinetic-modeling studies are performed with the San Diego Mechanism. Experimental data and results of kinetic-modeling show that N₂O inhibits the flame by promoting extinction. Analysis of the flame structure shows that H radicals are produced in the overall chain-branching step 3H₂ + O₂ ? 2H₂O + 2H, in which molecular hydrogen is consumed. Hydrogen is also consumed in the overall step N₂O + H₂ ? N₂ + H₂O where stable products are formed. Inhibition of the flames by N₂O is attributed to competition between these two overall steps.     

Extinguishment of propane/air co-flowing diffusion flames by fine water droplets

In the present study, extinguishment of propane/air co-flowing diffusion flame by fine water droplets was investigated experimentally. Water droplets are generated by piezoelectric atomizers with the maximum droplets flow rate of 1500 ml/h. When the fuel injection velocity U_f is low, an attached laminar diffusion flame with a premixed flame at the base is stabilized. At some distance from the burner rim, a transition from laminar to turbulent diffusion flame occurs, and a turbulent diffusion flame is formed in the downstream region. When the fuel injector rim is thin (δ = 0.5 mm), the flame stability deteriorates with increase of the co-flowing air stream velocity U_a and the water droplets flow rate Q_m. The stability mechanism can be explained by the balance of the gas velocity and the burning velocity of premixed flame formed at the base. However, when the injector rim is thick (δ = 5 mm), a recirculation zone is produced downstream of the injector rim. The dependence of the quenching distance H_q on U_f and Q_m is relatively weak, and the stability diagram shows curious features. It was shown that U_a is crucially important since it determines flow residence time; if U_a < 0.4 m/s, water droplets can evaporate when they go by the recirculation zone, and the water vapor can diffuse into the recirculation zone. However, if U_a > 0.4 m/s, the water droplets should pass by the recirculation zone without sufficiently evaporated and are not so effective to extinguish the flame. The supply velocity of droplet-laden air should be low enough so that water droplets can evaporate and water vapor can diffuse into the premixed region at the base to obtain sufficient effectiveness of water droplets for fire suppression.     

Ignition of non-premixed cyclohexane and mono-alkylated cyclohexane flames

Ignition temperatures of non-premixed cyclohexane, methylcyclohexane, ethylcyclohexane, n-propylcyclohexane, and n-butylcyclohexane flames were measured in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 373 K, a local strain rate of 120 s^?1, and fuel mole fractions ranging from 1% to 10%. Using the recently developed JetSurf 2.0 kinetic model, satisfactory predictions were found for cyclohexane, methyl-, ethyl-, and n-propyl-cyclohexane flames, but the n-butylcyclohexane data were overpredicted by 20 K. The results showed that cyclohexane flames exhibit the highest ignition propensity among all mono-alkylated cyclohexanes and n-hexane due to its higher reactivity and larger diffusivity. The size of mono-alkyl group chain was determined to have no measurable effect on ignition, which is a result of competition between fuel reactivity and diffusivity. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to fuel diffusion and also to H₂/CO and C₁–C₃ hydrocarbon kinetics.     

On the rate constants of OH + HO2 and HO2 + HO2: A comprehensive study of H2O2 thermal decomposition using multi-species laser absorption

Hydrogen peroxide (H₂O₂) and hydroperoxy (HO₂) reactions present in the H₂O₂ thermal decomposition system are important in combustion kinetics. H₂O₂ thermal decomposition has been studied behind reflected shock waves using H₂O and OH diagnostics in previous studies (Hong et al. (2009) [9] and Hong et al. (2010) [6,8]) to determine the rate constants of two major reactions: H₂O₂ + M → 2OH + M (k₁) and OH + H₂O₂ → H₂O + HO₂ (k₂). With the addition of a third diagnostic for HO₂ at 227 nm, the H₂O₂ thermal decomposition system can be comprehensively characterized for the first time. Specifically, the rate constants of two remaining major reactions in the system, OH + HO₂ → H₂O + O₂ (k₃) and HO₂ + HO₂ → H₂O₂ + O₂ (k₄) can be determined with high-fidelity.No strong temperature dependency was found between 1072 and 1283 K for the rate constant of OH + HO₂ → H₂O + O₂, which can be expressed by the combination of two Arrhenius forms: k₃ = 7.0 × 10¹² exp(550/T) + 4.5 × 10¹⁴ exp(?5500/T) [cm³ mol^?1 s^?1]. The rate constants of reaction HO₂ + HO₂ → H₂O₂ + O₂ determined agree very well with those reported by Kappel et al. (2002) [5]; the recommendation therefore remains unchanged: k₄ = 1.0 × 10¹⁴ exp(?5556/T) + 1.9 × 10¹¹⁺exp(709/T) [cm³ mol^?1 s^?1]. All the tests were performed near 1.7 atm.     

Cup-burner flame structure and extinguishment by CF3Br and C2HF5 in microgravity

The effects of fire-extinguishing agents CF₃Br and C₂HF₅ on the structure and extinguishing processes of microgravity cup-burner flames have been studied numerically. Propane and a propane–ethanol–water fuel mixture, prescribed for a Federal Aviation Administration (FAA) aerosol can explosion simulator test, were used as the fuel. The time-dependent, two-dimensional numerical code, which includes a detailed kinetic model (177 species and 2986 reactions), diffusive transport, and a gray-gas radiation model, revealed unique flame structure and predicted the minimum extinguishing concentration of agent when added to the air stream. The peak reactivity spot (i.e., reaction kernel) at the flame base stabilized a trailing flame. The calculated flame temperature along the trailing flame decreased downstream due to radiative cooling, causing local extinction at <1250 K and flame tip opening. As the mole fraction of agent in the coflow (X_a) was increased gradually: (1) the premixed-like reaction kernel weakened (i.e., lower heat release rate) (but nonetheless formed at higher temperature); (2) the flame base stabilized increasingly higher above the burner rim, parallel to the axis, until finally blowoff-type extinguishment occurred; (3) the calculated maximum flame temperature remained at nearly constant (≈1700 K) or mildly increased; and (4) the total heat release of the entire flame decreased (inhibited) for CF₃Br but increased (enhanced) for C₂HF₅. In the lifted flame base with added C₂HF₅, H₂O (formed from hydrocarbon-O₂ combustion) was converted further to HF and CF₂O through exothermic reactions, thus enhancing the heat-release rate peak. In the trailing flame, “two-zone” flame structure developed: CO₂ and CF₂O were formed primarily in the inner and outer zones, respectively, while HF was formed in both zones. As a result, the unusual (non-chain branching) reactions and the combustion enhancement (increased total heat release) due to the C₂HF₅ addition occurred primarily in the trailing diffusion flame.     

A structural study of premixed hydrogen-air cellular tubular flames

Two dimensionally spatially resolved structural measurements are reported for cellular phenomena in lean laminar premixed hydrogen-air tubular flames. Laser-induced Raman scattering and chemiluminescence imaging are combined to investigate low Lewis number lean hydrogen-air flames. The strong effect of thermal-diffusive imbalance is observed in radial profiles interpolated through the centers of reaction and extinction zones. In the flame cell, the equivalence ratio is ～80% higher than the inlet mixture, resulting in a peak flame temperature of 1600 K that is 550 K above the adiabatic flame temperature of the inlet mixture (1055 K). In the adjacent extinction zone, the temperatures are ～900 K lower than the peak flame temperature and the equivalence ratio is similar to the inlet mixture. Despite doubling the global stretch rate from 200 s^?1 to 400 s^?1, the enhancement of local equivalence ratio and peak temperature in the flame cell remain similar. This enhancement seems dependent on the local cellular flame curvature, that is similar between both cases. With strong preferential diffusion effects, cellular flames offer unique validation data to improve the accuracy of current molecular transport modeling techniques.     

An experimental study on the formation of polycyclic aromatic hydrocarbons in laminar coflow non-premixed methane/air flames doped with four isomeric butanols

Experimental measurements were conducted for temperatures and mole fractions of C1–C16 combustion intermediates in laminar coflow non-premixed methane/air flames doped with 3.9% (in volume) 1-butanol, 2-butanol, iso-butanol and tert-butanol, respectively. Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique was utilized in the measurements of species mole fractions. The results show that the variant molecular structures of butyl alcohols have led to different efficiencies in the formation of polycyclic aromatic hydrocarbons (PAHs) that may cause the variations in sooting tendency. Detailed species information suggests that the presence of allene and propyne promotes benzene formation through the C₃H₃ + C₃H₄ reactions and consequently PAH formation through the additions of C2 and C3 species to benzyl or phenyl radicals. As a matter of fact, PAHs formed from the 1-butanol doped flame are the lowest among the four investigated flames, because 1-butanol mainly decomposes to ethylene and oxygenates rather than C3 hydrocarbon species. Meanwhile, the tert-butanol doped flame generates the largest quantities of allene and propyne among the four flames and therefore is the sootiest one.     

Stabilized flames in hybrid aluminum-methane-air mixtures

A premixed methane–air bunsen-type flame is seeded with micron-sized (d₃₂ = 5.6 μm) atomized aluminum powder over a wide range of solid fuel concentrations. The burning velocities of the resulting two-phase hybrid flame are determined using the total surface area of the inner flame cone and the known volumetric flow rate, and spatially resolved flame spectra are obtained with a spectral scanning system. Flame temperatures are derived through polychromatic fitting of Planck’s law to the continuous part of the spectrum. It is found that an increase in the solid fuel concentration changes the aluminum combustion regime from low temperature oxidation to full-fledged flame front propagation. For stoichiometric methane–air mixtures, the transition occurs in the aluminum concentration range of 140–220 g/m³ and is manifested by the appearance of AlO sub-oxide bands and an increase in the flame temperature to 2500 K. The flame burning velocity is found to decrease only slightly with an increase in aluminum concentration, in contrast to the rapid decrease in flame speed, followed by quenching, that is observed for flames seeded with inert SiC particles. The observed behavior of the burning velocity and flame temperature leads to the conclusion that intense aluminum combustion in a hybrid flame only occurs when the flame front propagating through the aluminum suspension is coupled to the methane–air flame.     

Laminar flame propagation of acetone and 2-butanone at normal to high pressures: Insight into fuel molecular structure effects of ketones

This work reports an experimental and kinetic modeling investigation on the laminar flame propagation of acetone and 2-butanone at normal to high pressures. The experiments were performed in a high-pressure constant-volume cylindrical combustion vessel at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5. A kinetic model of acetone and 2-butanone combustion was developed from our recent pentanone model [Li et al., Proc. Combust. Inst. 38 (2021) 2135–2142] and validated against experimental data in this work and in literature. Together with our recently reported data of 3-pentanone, remarkable fuel molecular structure effects were observed in the laminar flame propagation of the three C₃C₅ ketones. The laminar burning velocity increases in the order of acetone, 2-butanone and 3-pentanone, while the pressure effects in laminar burning velocity reduces in the same order. Modeling analysis was performed to provide insight into the key pathways in flames of acetone and 2-butanone. The differences in radical pools are concluded to be responsible for the observed fuel molecular structure effects on laminar burning velocity. The favored formation of methyl in acetone flames inhibits its reactivity and leads to the slowest laminar flame propagation, while the easiest formation of ethyl in 3-pentanone flames results in the highest reactivity and fastest laminar flame propagation. Furthermore, the LBVs of acetone and 3-pentanone exhibit the strongest and weakest pressure effects respectively, which can be attributed to the influence of fuel molecular structures through two crucial pressure-dependent reactions CH₃ + H (+M) = CH₄ (+M) and C₂H₄ + H (+M) = C₂H₅ (+M).     

The diffusion flame structure of an ammonium perchlorate based composite propellant at elevated pressures

Examination of the surface behavior and flame structure of a bimodal ammonium perchlorate (AP) composite propellant at elevated pressure was performed using high speed (5 kHz) planar laser-induced fluorescence (PLIF) from 1 to 12 atm and visible surface imaging spanning 1–20 atm. The dynamics of the combustion of single, coarse AP crystals were resolved using these techniques. It was found that the ignition delay time for individual AP crystals contributed significant to the particle lifetime only at pressures below about 6 atm. In situ AP crystal burning rates were found to be higher than rates reported for pure AP deflagration studies. The flame structure was studied by exciting OH molecules in the gas phase. Two types of diffusion flames were observed above the composite propellant: jet-like flames and v-shaped, inverted, overventilated, flames (IOF) lifted off the surface. While jet-like diffusion flames have been imaged at low pressures and simulated by models, the lifted IOFs have not been previously reported or predicted. The causes for the observed flame structures are explained by drawing on an understanding of the surface topography and disparities in the burning rates of the fuel and oxidizer.     

Ignition of non-premixed counterflow flames of octane and decane isomers

Ignition temperatures of non-premixed flames of octane and decane isomers were determined in the counterflow configuration at atmospheric pressure, a free-stream fuel/N₂ mixture temperature of 401 K, a local strain rate of 130 s^?1, and fuel mole fractions ranging from 1% to 6%. The experiments were modeled using detailed chemical kinetic mechanisms for all isomers that were combined with established H₂, CO, and n-alkane models, and close agreements were found for all flames considered. The results confirmed that increasing the degree of branching lowers the ignition propensity. On the other hand, increasing the straight chain length by two carbons was found to have no measurable effect on flame ignition for symmetric branched fuel structures. Detailed sensitivity analyses showed that flame ignition is sensitive primarily to the H₂/CO and C₁–C₃ hydrocarbon kinetics for low degrees of branching, and to fuel-related reactions for the more branched molecules.     

Experimental and numerical study of product gas and N2O emission characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow

In order to achieve carbon neutrality, the use of ammonia as a fuel for power generation is highly anticipated. The utilization of a binary fuel consisting of ammonia and hydrogen can address the weak flame characteristics of ammonia. In this study, the product gas characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow were experimentally and numerically investigated for various equivalence ratios for the first time. A trade-off relationship between NO and unburnt ammonia was observed at slightly rich conditions. At lean conditions, NO reached a maximum value of 8,700 ppm, which was larger than that of pure ammonia/air flames. The mole fraction of nitrous oxide (N₂O) which has large global warming potential rapidly increased around the equivalence ratio of 0.6, which was attributed to the effect of a decrease in flame temperature downstream of the reaction zone owing to heat loss to the stagnation wall. To understand this effect further, numerical simulations of ammonia/hydrogen/air flames were conducted using the stagnation flame model for various equivalence ratios and stagnation wall temperatures. The results show that the important reactions for N₂O production and reductions are NH +NO = N₂O + H, N₂O + H = N₂ + OH, and N₂O (+M) = N₂ + O (+M). A decrease in flame temperature in the post flame region inhibited N₂O reduction through N₂O (+M) = N₂ + O (+M) because this reaction has a large temperature dependence, and thus N₂O was detected as a product gas. N₂O is reduced through N₂O (+M) = N₂ + O (+M) in the post flame region if the stagnation wall temperature is sufficiently high. On the other hand, it was clarified that an increase in equivalence ratio enhances H radical production and promotes N₂O reduction by H radical through the reaction of N₂O + H = N₂ + OH.     

Fundamental burning velocities of meso-scale propagating spherical flames with H2, CH4 and C3H8 mixtures

This study is performed to experimentally examine the fundamental burning velocity characteristics of meso-scale outwardly propagating spherical laminar flames in the range of flame radius r_f approximately from 1 to 5 mm for hydrogen, methane and propane mixtures, in order to make clear a method for improving combustion of micro–meso scale flames. Macro-scale laminar flames with r_f > 7 mm are also examined for comparison. The mixtures have nearly the same laminar burning velocity (S_L0 = 25 cm/s) for unstretched flames and different equivalence ratios ?. The radius r_f and the burning velocity S_Ll of meso-scale flames are estimated by using sequential schlieren images recorded under appropriate ignition conditions. It is found that S_Ll of hydrogen and methane premixed meso-scale flames at the same r_f or the Karlovitz number Ka shows a tendency to increase with decreasing ?, whereas S_Ll of propane flames increases with ?. However, S_Ll tends to decrease with the Lewis number Le and the Markstein number Ma, irrespective of the type of fuel and ?. It also becomes clear that the optimum flame size and Ka to improve the burning velocity exist for some mixtures depending on Le and fuel types.     

Prompt NO formation in flames: The influence of NCN thermochemistry

The influence of the route via the NCN radical on NO formation in flames was examined from a thermochemistry and reaction kinetics perspective. A detailed analysis of available experimental and theoretical thermochemical data combined with an Active Thermochemical Tables analysis suggests a heat of formation of 457.8 ± 2.0 kJ/mol for NCN, consistent with carefully executed theoretical work of Harding et al. (2008) [5]. This value is significantly different from other previously reported experimental and theoretical values. A combination of an extensively validated comprehensive hydrocarbon oxidation model extended by the GDFkin3.0_NCN-NO_x sub-mechanism reproduced NCN and NO mole fraction profiles in a recently characterized fuel-rich methane flame only when heat of formation values in the range of 445–453 kJ/mol are applied. Sensitivity analysis revealed that the sensitivities of contributing steps to NO and NCN formation are strongly dependent on the absolute value of the heat of formation of NCN being used. In all flames under study the applied NCN thermochemistry highly influences simulated NO and NCN mole fractions. The results of this work illustrate the thermochemistry constraints in the context of NCN chemistry which have to be taken into account for improving model predictions of NO concentrations in flames.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Kinetic analysis of the pathways to naphthalene formation from phenyl + 1,3-Butadiyne reaction

Having a better understanding of polycyclic aromatic hydrocarbon (PAH) formation under flame conditions contributes to optimizing the fuel reforming process, where soot poisons the downstream catalyst. In this work, the phenyl + 1,3-Butadiyne reaction is systematically investigated to examine its contribution to naphthalene formation. The reaction potential energy surfaces were calculated using DFT/M06–2X/cc-pvtz and G4 methods. The temperature- and pressure-dependent reaction rate constants were calculated using RRKM theory with solving master equation. The results revealed that 2-naphthyl could be directly formed by phenyl + 1,3-Butadiyne reaction. With H assistance, naphthalene could be formed by the pathway of phenyl + 1,3-Butadiyne → C₆H₅CHCCCH (+H) → C₆H₅CHCHCCH (+H) →naphthalene +H. The proposed pathway is kinetically favorable, and featured by relatively low energy barrier. The importance of the proposed pathway reaction was confirmed in a premixed and a diffusion C₂H₄/O₂/Ar flame simulations, where the enhancement of naphthalene by the investigated reactions is notable. The mole fraction of A₂ is promoted by a factor of 10% in premix C₂H₄/O₂/Ar flame and 30% in C₂H₄/O₂/Ar counterflow flame, bringing the prediction results closer to the experimental results. The relative contribution of different reaction route to A₂ formation is evaluated for HACA, cyclopentadienyl radical-cyclopentadienyl radical, phenyl-vinylacetylene[1], benzyl radical-propargyl radical, indene-CH₂ and phenyl-1,3-Butadiyne routes in premixed and diffusion C₂H₄/O₂/Ar flames. This work suggests that the PAH growth by 1,3-Butadiyne addition reaction is an effective pathway for A₂ formation, which should be considered in future PAH mechanism.     

Response of non-premixed flames to bulk flow perturbations

This paper describes the dynamics of non-premixed flames responding to bulk velocity fluctuations, and compares the dynamics of the flame sheet position and spatially integrated heat release to that of a premixed flame. The space–time dynamics of the non-premixed flame sheet in the fast chemistry limit is described by the stoichiometric mixture fraction surface, extracted from the solution of the

-equation. This procedure has some analogies to premixed flames, where the premixed flame sheet location is extracted from the G = 0 surface of the solution of the G-equation. A key difference between the premixed and non-premixed flame dynamics, however, is the fact that the non-premixed flame sheet dynamics are a function of the disturbance field everywhere, and not just at the reaction sheet, as in the premixed flame problem. A second key difference is that the non-premixed flame does not propagate and so flame wrinkles are convected downstream at the axial flow velocity, while wrinkles in premixed flames convect downstream at a vector sum of the flame speed and axial velocity. With the exception of the flame wrinkle propagation speed, however, we show that that the solutions for the space–time dynamics of the premixed and non-premixed reaction sheets in high velocity axial flows are quite similar. In contrast, there are important differences in their spatially integrated unsteady heat release dynamics. Premixed flame heat release fluctuations are dominated by area fluctuations, while non-premixed flames are dominated by mass burning rate fluctuations. At low Strouhal numbers, the resultant sensitivity of both flames to flow disturbances is the same, but the non-premixed flame response rolls off slower with frequency. Hence, this analysis suggests that non-premixed flames are more sensitive to flow perturbations than premixed flames at O(1) Strouhal numbers.     

Influence of radiation losses on the stability of premixed flames on a porous-plug burner

Analysis of the planar premixed flames on a porous plug was performed numerically for finite activation energy within the diffusive-thermal model. The paper is focused on the influence of radiation heat loses on the flame standoff distance and its linear stability. We show that the presence of volumetric heat losses limits the range of the mass flow range as well as it can promote the flame instabilities of different kinds, both oscillatory and cellular. The oscillatory instability, which for freely propagating flames can be usually observed for the Lewis number larger than one, in the porous-plug case occurs also for flames with unity and lower than unity Lewis number. For flames with Le < 1 both cellular and oscillatory instabilities can be observed simultaneously in a certain range of the mass flow rate.     

Upconversion luminescence in Tm3+/Yb3+co-doped lead tungstate tellurite glasses

This work reports the upconversion luminescence properties of Tm³⁺/Yb³⁺ ions in lead tungstate tellurite (LTT) glasses. Judd–Oflet intensity parameters have been obtained from the absorption band intensities of Tm³⁺ singly-doped and Tm³⁺/Yb³⁺ co-doped LTT glasses. The spontaneous emission probabilities, radiative lifetimes and branching ratios for ¹G₄ and ³H₄ emission levels of Tm³⁺ have been determined. Upconversion luminescence has been observed by exciting the samples at 980 nm (Yb³⁺:²F_7/2→²F_5/2) at room temperature. Four upconversion emission bands corresponding to the ¹G₄→³H₆ (477 nm), ¹G₄→³F₄ (651 nm), ¹G₄→³H₅ (702 nm) and ³H₄→³H₆ (810 nm) transitions have been identified. The relative variation in the intensities of upconversion bands, the different channels responsible for upconversion spectra and the effect of Yb³⁺ ions concentration on the upconversion luminescence of Tm³⁺ ions have also been discussed.