Xiao Qin

Flame Synthesis of Y 2 O 3 :Eu Nanophosphors Using Ethanol as Precursor Solvents

Y 2 O 3 :Eu nanophosphors were prepared by flame synthesis using ethanol or water as precursor solutions. The effects of precursor solvents and flame temperature on particle size, morphology, and photoluminescence intensity were investigated. The results showed that flame synthesis using ethanol solution could produce nanoparticles with better homogeneity, smoother surface structure, and stronger photoluminescence intensity than using water. It was found that the concentration quenching limit of the as-prepared nanophosphors from both ethanol and water solution was 18 mol% Eu, which is higher than the reported limit at similar particle size. The x-ray diffraction (XRD) spectra showed that the ethanol precursor solvent produced monoclinic phase Y 2 O 3 :Eu nanoparticles at a lower flame temperature than previously reported. It was also shown that the particle size could be controlled by varying the precursor concentration and flame temperature.

Measurements of Hydrocarbon Flame Speed Enhancement in High-Q Microwave Cavity

In this work we demonstrate that a small amount of microwave power below its breakdown threshold can be locally absorbed into a flame combustion zone. The absorbed microwave power can significantly change the flame speed of both laminar and turbulent flames. PIV technique was employed to measure the laminar flame speed. It was found that microwave assisted flame speed enhancement was greatly dependent on Q of the microwave cavity. Due to the unsteady nature of interaction, microwave assisted flame speed measurements were difficult to make, however, preliminary observations of the flame luminosity indicated that there was energy addition occurring without microwave breakdown and the flame speed was increased.

Enhancement of Combustion and Flame Stabilization Using Stabilized Non-Equilibrium Plasma

Abstract : The effect of non-equilibrium plasma on both partially premixed and non-premixed flames was investigated through the development of a newly integrated magnetic gliding arc (MGA) system. The lifted jet diffusion flame experiments showed a significant enhancement of the flame stabilization with plasma discharge in the air co-flow. The counterflow experiments also demonstrated that the extinction limits were extended dramatically. Laser diagnostics of flame temperature and OH distribution using planar Rayleigh scattering and planar laser-induced fluorescence revealed that the plasma-flame interaction at low air temperature was dominated by thermal effects due to rapid radical quenching. Counterflow ignition experiments for CH4-air and H2-air non-premixed flames demonstrated clearly that the MGA significantly decreased the ignition temperatures via kinetic enhancement by the NOx, catalytic effect. Numerical modeling showed that there were two ignition regimes for plasma enhanced ignition, kinetic at low strain rates and thermal at high strain rates. Comparison between experiment and simulation were in good agreement and also suggested the possibility of enhancement by ions, excited species or other mechanisms. Theoretical analysis of minimum ignition energy in a quiescent mixture showed that the production of small hydrocarbon fuel fragments by plasma discharge also led to a significant decrease of ignition energy due to radiation and transport coupling.

Experimental and Numerical Studies of Spectral Radiation Reabsorption on CO 2 Diluted Flames

*† ‡ The effects of spectral radiation absorption on the flame speed at normal and elevated pressures were experimentally and numerically investigated using the CO2 diluted outward propagating CH4-O2-He flames. Experimentally, the laminar burning velocities of CH4-O2He-CO2 mixtures at both normal and elevated pressures (up to 5 atm) were measured by using a pressure-release type spherical bomb. The results showed that radiation absorption with CO2 addition increases the flame speed and extends the flammability limit. In addition, it was also shown that the increase of pressure augments the effect of radiation absorption. Computationally, a fitted statistical narrow-band correlated-k (FSNB-CK) model was developed and validated for accurate radiation prediction in spherical geometry. This new radiation scheme was integrated to the HLLC Riemann solver for radiation prediction in a compressible reactive flow. The comparison between experiment and computation showed a very good agreement. The results showed that the flame geometry had a significant impact on radiation absorption and that the one-dimensional planar radiation model was not valid for the computation of the flame speed of a spherical flame. It was clearly demonstrated that the effect of radiation absorption increases with the pressure and flame size. An effective Boltzmann number was extracted from numerical simulation. Furthermore, the FSNB-CK model was compared with the grey band SNB model. It was shown that the grey band SNB model over-predicts the radiation absorption. It is concluded that any quantitative prediction of flame speed and flammability limit of CO2 diluted flame requires accurate spectral dependent radiation model.

Experimental and Numerical Study of Spectral Dependent Radiation Reabsorption on Flame Propagation

Premixed flames in CO2 diluted CH4-O2-He mixture s were studied both experimentally and numerically to investigate the effects of spectral dependent radiation reabsorption on the flame propagation and flammability limit. Laminar burning velocities of CH4-O2-He -CO2 mixtures at both normal and elevated pressures up to 5 atm were measured by using a pressure-release type spherical bomb. The measured data were compared with the results from computations performed with and without spectral radiation absorption. It was found that the spectral radiation reabsorption results in higher burning velocities of CH 4-O2-HeCO2 mixtures and wider flammability limits than predictions using the optically thin model. Furthermore, the enhancement becomes more significant at higher pressures. The results demonstrated that reabsorption effects must be considered to correctly predict flame speeds and extinction limits in flammability limit and burning properties of mixtures with CO2 addition.

Study of Liftoff Mechanism of Nonpremixed Jet Flame near Unity Schmidt Number

Nonpremixed jet flames of dimethyl ether (DME) were studied both experimentally and theoretically to investigate flame liftoff near unity Schmidt number. It was found experimentally that although the DME nonpremixed flames have a Schmidt number larger than unity it cannot be lifted directly by increasing the flow rate. Lifted flames can only be established by igniting the mixture in a narrow region downstream of the jet at low flow rates. The results also show that the liftoff flow rate is less than that of the blowout limit of the attached flame. Theoretically, the self-similar Landau-Squire solution for a round jet is revisited and the combined effects of stretch and flame curvature on triple flame propagation speed were considered. It was found that the critical Schmidt number for liftoff shifts around unity. The critical Schmidt number is less than unity for fuel Le numbers larger than 0.5 and larger than unity for Le numbers less than 0.5.