Strong flame acceleration and detonation limit of hydrogen-oxygen mixture at cryogenic temperature

A series of experiments were carried out in a closed tube at cryogenic temperature (77 K) for hydrogen-oxygen mixtures. Flame propagation speed and overpressure were measured by optical fibers and pressure sensors, respectively. The first and second shock waves were captured in the cryogenic experiments, although the shock waves always precede the flames in all cases indicating the absence of stable detonation. However, strong flame acceleration was observed for all situations, which is consistent with the prediction by expansion ratio and Zeldovich number. Besides, the tube diameter and length are also critical for flame acceleration to supersonic. All the flames in this work accelerate drastically reaching the C-J deflagration state. But at 0.4 atm, only fast flame is formed, while at higher initial pressures, the flame further accelerates to a galloping mode manifesting a near-limit detonation, which could be indicated by the stability parameter $\chi$.     

Lewis number effects on laminar and turbulent expanding flames of NH3/H2/air mixtures at elevated pressures

This study clarifies the effects of Lewis number (Le) on laminar and turbulent expanding flames of NH₃/H₂/air mixtures. The laminar burning velocity (S_L) and turbulent burning velocity (S_T) were measured using a medium-scale, fan-stirred combustion chamber with ammonia/hydrogen molar ratio (NH₃/H₂) of 50/50 and 80/20 under the maximum pressures of 5 atm. The lean laminar flame with NH₃/H₂ = 50/50 is significantly accelerated by the diffusional–thermal instability, which dominated the trend of S_T,c=0.1 with the equivalence ratio (ϕ). The lean normalized turbulent burning velocity (S_T/S_L) increases with the decrease of hydrogen content due to the weakening effects of S_L. However, the S_T/S_L reaches peak with hydrogen volumetric content less than 20% due to effects made by diffusional–thermal instability than S_L did. The turbulent flame of NH₃/H₂/air mixtures is characterized by self-similar acceleration propagation, and propagation with Le < 1 is faster. A modified correlation considering the effects of Le was proposed, as (d<r>/dt)/σS_L = 0.118(Re_T,flameLe⁻²)^0.57, which was able to predict not only the self-similar propagation of NH₃/H₂/air but also the previous syngas/air flames. The Kobayashi correlations modified by three kinds of Le power exponents were used to clarify the effects of Le by comparing their fitting parameters and predictive powers on experimental data and literature data. Similar pre-factors, power exponents and the goodness of fit (R²) were obtained with Le ranging from 0.58 to 1.62, which suggested that the determination of Le power exponent had no significant effect on the prediction accuracy of the S_T/S_L trend with data of Le near unity. This might be attributed to the fact that the variation ranges of the dimensionless number that characterizes the experimental conditions is much larger than that of the Le.     

Critical temperature and reactant mass flux for radiative extinction of ethylene microgravity spherical diffusion flames at 1 bar

In microgravity combustion, where buoyancy is not present to accelerate the flow field and strain the flame, radiative extinction is of fundamental importance, and has implications for spacecraft fire safety. In this work, the critical point for radiative extinction is identified for normal and inverse ethylene spherical diffusion flames via atmospheric pressure experiments conducted aboard the International Space Station, as well as with a transient numerical model. The fuel is ethylene with nitrogen diluent, and the oxidizer is an oxygen/nitrogen mixture. The burner is a porous stainless-steel sphere. All experiments are conducted at constant reactant flow rate. For normal flames, the ambient oxygen mole fraction was varied from 0.2 to 0.38, burner supply fuel mole fraction from 0.13 to 1, total mass flow rate, ṁ_total, from 0.6 to 12.2 mg/s, and adiabatic flame temperature, T_ad, from 2000 to 2800 K. For inverse flames, the ambient fuel mole fraction was varied from 0.08 to 0.12, burner supply oxygen mole fraction from 0.4 to 0.85, ṁ_total from 2.3 to 11.3 mg/s, and T_ad from 2080 to 2590 K. Despite this broad range of conditions, all flames extinguish at a critical extinction temperature of 1130 K, and a fuel-based mass flux of 0.2 g/m²-s for normal flames, and an oxygen-based mass flux of 0.68 g/m²-s for inverse flames. With this information, a simple equation is developed to estimate the flame size (i.e., location of peak temperature) at extinction for any atmospheric-pressure ethylene spherical diffusion flame given only the reactant mass flow rate. Flame growth, which ultimately leads to radiative extinction if the critical extinction point is reached, is attributed to the natural development of the diffusion-limited system as it approaches steady state and the reduction in the transport properties as the flame temperature drops due to increasing flame radiation with time (radiation-induced growth.)     

Propagation of Darrieus–Landau unstable laminar and turbulent expanding flames

The propagation of laminar and turbulent expanding flames subjected to Darrieus–Landau (DL), hydrodynamic instability was experimentally studied by employing stoichiometric H₂/O₂/N₂ 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.     

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.     

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.     

Investigation on spherically expanding flame temperature of n-butane/air mixtures with tunable diode laser absorption spectroscopy

A joint schlieren imaging, pressure recording and tunable diode laser absorption spectroscopy (TDLAS) thermometry technique was developed to simultaneously determine the flame radius, pressure and line-of-sight averaged temperature of spherically expanding flames of n-butane/air mixtures at initial temperature of 298 K, initial pressure of 1 atm and equivalence ratios of 0.9–1.5. To probe the flame temperature, a mid-infrared interband cascade laser at 4.2 µm was used to measure the time-resolved direct absorption spectra of CO₂ which are strongly related to flame temperature, CO₂ mole fraction, flame radius and pressure. Quantitative line-of-sight averaged temperatures of burnt gas were obtained by fitting the normalized absorbance spectra. Three typical stages, including the spark affected initial stage, quasi-steady stage and the pressure induced growing stage are determined from the evolution of measured temperature as a function of time and flame radius. The relation between flame temperature, stretch rate and burning velocity of burnt gas are analyzed. Stretch rate is found to have minor effect on the measured temperature in the quasi-steady stage. The relative variation of temperature is much smaller than that of velocity. The flame with lower normalized temperature tends to propagate slower.     

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.     

Computational study on lean and rich combustion of flame ball,counterflow flame and planar flame: Their limits and stoichiometries

To investigate (fuel-)lean/rich limits and essential stoichiometries, i.e., the borders of lean/rich combustion, one-dimensional steady computations with detailed chemistry for flame balls, counterflow flames, and stretch-free planar flames were conducted using a CH₄/O₂/Xe mixture that has been used in microgravity experiments. As continuous converged solutions were obtained under lean/rich conditions, it was suggested that the existence of flame ball not only under lean but also under rich condition. Flame radii and temperatures of flame balls decreased and increased toward the lean/rich limits from their maximum and minimum values, respectively. The lean limits were wider in the order of the flame ball, counterflow flame, and stretch-free planar flame. Therefore, the lean flammability limit corresponded to the lean limit of the flame ball in the mixture. Conversely, the rich limits were wider in the order of the counterflow flame, stretch-free planar flame, and flame ball. Thus, the rich flammability limit corresponded to the rich limit of the counterflow flame in the mixture. Essential stoichiometry, which represents the actual stoichiometry depending on the dominant transport in near-flame front, was not uniquely determined as conventional stoichiometry (ϕ = 1); it was located between the equivalence ratio of ϕ = 1 and ϕ_c, where ϕ _c denotes the critical equivalence ratio is evaluated using the fuel and oxidizer Lewis number of a target mixture. The results indicated that the essential stoichiometry of the stretch-free planar flame corresponded to ϕ = 1, that of the flame ball corresponded to ϕ = ϕ _c, and that of the stretched flame was located between ϕ = 1 and ϕ _c depending on the stretch rate.     

Studies of the dynamics of autoignition assisted outwardly propagating spherical cool and double flames under shock-tube conditions

The initiation, propagation, and transition of the autoignition assisted spherical cool flame and double flame are studied numerically and experimentally using n-heptane/air/He mixtures under shock-tube experimental conditions over a wide range of temperatures. The primary goal of the current study is to understand the effects of the ignition Damkohler number, ignition energy, flame curvature, and autoignition-induced flow compression on the propagation of spherical flames to ensure the proper interpretation of shock-tube flame speed measurements at engine-relevant conditions. The results show that at high ignition Damkohler number, there are three different flame regimes, cool flame, double flame, and hot flame. The cool flame speed accelerates dramatically with the increase of ignition Damkohler number. In addition, it is found that the change of flame regime, low-temperature autoignition, flame stretch, and autoignition-induced flow compression result in a complicated non-linear dependence of flame speed on stretch. The results also reveal that the spherical cool flame has much lower Markstein length compared to the hot flame at T > 600 K. Moreover, it is found that both the autoignition assisted cool flame and the trailing hot flame front in the double flame can propagate much faster that the hot flame alone at the same mixture conditions, leading to a nonlinear dependence of flame speed on the mixture initial temperature. The simulated flame trajectories and the flame speed dependence on temperature agree qualitatively well with the shock-tube experiments. A quantitative criterion to ensure the accurate speed measurement of the cool and hot flame is proposed. The present study provides important physical insight and guidance for the flame speed measurement using a shock-tube at engine relevant conditions.     

Modification of the CVD-graphene resistivity by post-processing sample annealing

In this paper, we present the results of an additional annealing effect on the temperature dependences of the resistivity for CVD-graphene samples of a large area. We found that an annealing in a Ar/H₂ mixture at different temperatures modifies both the value of the resistivity and the slope of its temperature dependence. The annealing effect on the resultant sample quality depends on the type of the ρ(T) dependence for the initial sample. For samples with a metallic-like ρ(T) dependence, a low-temperature annealing (at T = 250 °C) results in a slight decrease in the resistivity value and an increase of the ρ(T) curve slope. Increasing the annealing temperature up to T = 400 °C leads to a stronger increase in the ρ(T) curve slope but to an increase in the resistivity value. For samples with a semiconductor-like ρ(T) dependence, increasing the annealing temperature up to T = 750 °C results in a gradual suppression of the activation character of the resistivity behavior at low temperatures. The additional annealing is concluded to be accompanied by two processes: a cleaning of the graphene surface from adsorbed contaminations and an additional defect formation in the graphene structure. A relative role of these processes in dependence on the annealing temperature and the type of the ρ(T) dependence for the initial sample is discussed.     

Spatiotemporal measurements of flame stretch and propagation rates for lean and rich CH4/air premixed flames interacting with a turbulent-wake

A 1.5 m long turbulent-wake combustion vessel with a 0.15 m × 0.15 m cross-sectional area is proposed for spatiotemporal measurements of curvature, strain, dilatation and burning rates along a freely downward-propagating premixed flame interacting with a parallel row of staggered vortex pairs having both compression (negative) and extension (positive) strains simultaneously. The wanted wake is generated by rapidly withdrawing an electrically-controlled, horizontally-oriented sliding plate of 5 mm thickness for flame–wake interactions. Both rich and lean CH₄/air flames at the equivalence ratios  = 1.4 and  = 0.7 with nearly the same laminar burning velocity are studied, where flame–wake interactions and their time-dependent velocity fields are obtained by high-speed, high-resolution DPIV and laser-tomography. Correlations among curvature, strain, stretch, and dilatation rates along wrinkled flame fronts at different times are measured and thus their influences on front propagation rates can be analyzed. It is found that strain-related effects have significant influence on front propagation rates of rich CH₄/air (diffusionally stable) flames even when the curvature weights more in the total stretch than the strain rate does. The local propagation rates along the wrinkled flame front are more intense at negative strain rates corresponding to positive peak dilatation rates but the global propagation rate averaged along the rich flame front remains constant during all period of flame–wake interaction. For lean CH₄/air (diffusionally unstable) flames, the curvature becomes a dominant parameter influencing the structure and propagation of the wrinkled flame front, where both local and global propagation rates increase significantly with time, showing unsteady flame propagation. These experimental results suggest that the theory of laminar flame stretch can be applicable to a more complex flame–wake interaction involving unsteadiness and multitudinous interactions between vortices.     

Recombination processes in lanthanum beryllate single crystals doped with Ce3+ and Pr3+ ions

The paper presents experimental results on recombination processes in lanthanum beryllate (BLO) single crystals (undoped BLO, and doped with 0.5 at % by Ce³⁺ and Pr³⁺ ions) obtained using XRL-spectroscopy at T = 8, 80 and 290 K and thermoluminescence technique in the temperature range of 8–650 K. The paper discusses spectra of the steady-state XRL-luminescence recorded in the energy range from 1.5 to 6.2 eV at different temperatures between 8 and 290 K; temperature dependences of XRL intensity recorded in the temperature range from 8 to 650 K; thermoluminescence glow curves recorded in spectral-integrated regime after X-ray exposure at T₀ = 8 or 290 K.     

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.     

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.     

Flame fingers and interactions of hydrodynamic and thermodiffusive instabilities in laminar lean hydrogen flames

Lean hydrogen/air flames are prone to hydrodynamic and thermodiffusive instabilities. In this work, the contribution of each instability mechanism is quantified separately by performing detailed simulations of laminar planar lean hydrogen/air flames with different diffusivity models and equations of state to selectively suppress the hydrodynamic or thermodiffusive instability mechanism.From the analysis of the initial phase of the simulations, the thermodiffusive instability is shown to dominate the flame dynamics. If differential diffusion and, hence, the thermodiffusive instability is suppressed, the flame features a strong reduction of the instability growth rates, whereas if present, a wide range of unstable wave numbers is observed due to the strong destabilizing nature of differential diffusion. When instabilities are fully developed, lean hydrogen/air flames feature the formation of small-scale cellular structures and large-scale flame fingers. While the size of the former is known to be close to the most unstable wave length of a linear stability analysis, this work shows that flame fingers also originate from the thermodiffusive instability and most noteworthy, are not linked to an interaction of the two instability mechanisms. They are stable with respect to external perturbations and feature an enhanced flame propagation as the formation of a central cusp at their tip enables the co-existence of two strongly curved leading edges with high reactivity. The thermodiffusive instability is shown to significantly affect the flames’ consumption speed, while the consumption speed enhancement caused by the hydrodynamic instability is significantly smaller. Further, the surface area increase due to wrinkling is strongly diminished if one of the two instability mechanisms is missing. This is linked to a synergistic interaction between the two mechanisms, as the propagation of flame fingers is enhanced by the presence of the hydrodynamic instability due to a widening of the streamlines ahead of the flame fingers.     

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).     

Magnetic properties of RPdIn (R=Gd–Er) compounds

AC susceptibility and DC magnetization measurements were performed for the RPdIn (R=Gd–Er) compounds both in the paramagnetic and in the ordered state. In opposite to GdPdIn, which is a ferromagnet (T_c=102 K), the other samples show a complex ferrimagnetic behavior with the additional transition at T_t<T_c. In the high-temperature phase (for T_t<T<T_c), a ferromagnetic interaction dominates, while in the low-temperature phase (for T⩽T_t) antiferromagnetic interactions with the magnetocrystalline anisotropy, especially strong for TbPdIn, come into play. The ordering temperatures are T_c=70, 34, 25 and 12.3 K for Tb-, Dy-, Ho- and ErPdIn respectively, while transition temperatures are T_t=6, 14 and 6 K for Tb-, Dy- and HoPdIn respectively. TbPdIn reveals an additional transition at 27 K connected with the intermediate ferrimagnetic phase. The critical fields for the magnetization process of the low-temperature phase are high (52 and 150 kOe for TbPdIn and 32 kOe for DyPdIn at T=4.2 K) yet these values decrease remarkably with increasing temperature. Results of the study are compared with magnetic and neutron diffraction data hitherto available. We state that irreversibility of the zero-field cooled–field cooled magnetization is not connected with the spin-glass phase claimed elsewhere.     

Photoluminescence of cubic and monoclinic Gd2O3:Eu phosphors prepared by flame spray pyrolysis

In this paper, europium-doped gadolinium phosphor, which is a potentially bifunctional material with both fluorescent and magnetic properties, has been prepared in a one-step procedure via flame spray pyrolysis, and its crystal structure, morphology, and PL intensity were investigated. All the prepared phosphors were submicron-sized with spherical shapes and either a pure cubic or pure monoclinic phase. In order to observe the effects of temperature on the crystal phases of the prepared phosphors, we applied a H₂ vs. N₂/O₂ diffusion flame, with the maximum flame temperature ranging from T_max=1375 to 2050 K. The temperature profiles under various flame conditions are also reported herein to further elucidate the rapid synthesis process. The PL intensity in the cubic phase improved linearly with increasing flame temperature until the transition to a monoclinic phase. The peak of the photoluminescence(PL) spectrum from the phosphors prepared at T_max=1733 K in the cubic phase was narrower and twice as strong as the peak of the PL spectrum from the phosphors prepared at T_max=2050 K in the monoclinic phase. This paper provides important data showing the relationship between the synthesis temperature and the phase transition in Gd₂O₃:Eu in the continuous one-step use of flame spray pyrolysis.