The burner stabilized stagnation flame technique coupled with micro-orifice probe sampling and mobility sizing has evolved into a useful tool for examining the evolution of the particle size distribution of nascent soot in laminar premixed flames. Several key aspects of this technique are examined through a multi-university collaborative study that involves both experimental measurement and computational modeling. Key issues examined include (a) data reproducibility and facility effects using four burners of different sizes and makers over three different facilities, (b) the mobility diameter and particle mass relationship, and (c) the degree to which the finite orifice flowrate affects the validity of the boundary condition in a pseudo one dimensional stagnation flow flame formulation. The results indicate that different burners across facilities yield nearly identical results after special attention is paid to a range of experimental details, including a proper selection of the sample dilution ratio and quantification of the experimental flame boundary conditions. The mobility size and mass relationship probed by tandem mass and mobility measurement shows that nascent soot with mobility diameter as small as 15 nm can deviate drastically from the spherical shape. Various non-spherical morphology models using a mass density value of 1.5g/cm3 can reconcile this discrepancy in nascent soot mass. Lastly, two-dimensional axisymmetric simulations of the experimental flame with and without the sample orifice flow reveal several problems of the pseudo one-dimensional stagnation flow flame approximation. The impact of the orifice flow on the flame and soot sampled, although small, is not negligible. Specific suggestions are provided as to how to treat the non-ideality of the experimental setup in experiment and model comparisons
This study focuses on promoting the low temperature performance of vanadium-based catalysts; for this, the SHS method was applied to synthesize a series of Ti0.9V
x
M0.1–x
O2−δ catalysts. The performances of the catalysts (Ti0.9V0.1O2−δ, Ti0.9Mn0.05V0.05O2−δ, and Ti0.9Ce0.05V0.05O2−δ) were fully investigated with the temperature-programmed-reaction, which proved that these SHS catalysts with nanometer size had high activity over a broad temperature window of 150–400 °C. Compared with Ti0.9V0.1O2−δ, the substituted catalysts, Ti0.9Mn0.05V0.05O2−δ and Ti0.9Ce0.05V0.05O2−δ, showed higher N2 selectivity at high temperatures. The Ce substituted catalyst exhibited good resistance to H2O and SO2 poisoning at low temperatures. The structural and physical-chemical properties of catalysts were characterized comprehensively by BET, XRD, FTIR, TEM, EDX, XPS, and TPD. The XRD results indicated that the active components of V, Mn, and Ce were highly dispersed over the catalysts. The Mn substitution could enhance the Brønsted acid sites on the catalyst surface and accelerate the SCR reaction at low temperatures. The XPS shows that the Ce substitution led to high concentration of chemisorbed oxygen, which diminished the unselective oxidation of NH3 by O2 to N2O, NO, or NO2 and resulted in superior N2 selectivity. The active components of the catalysts, such as V, Mn, and Ce, mostly existed in the form of mixed-valence which was beneficial for the oxidation of NO to NO2. Furthermore, the SCR reaction mechanism over Ti0.9Ce0.05V0.05O2−δ catalyst was also examined using in situ DRIFTS. The results revealed that high active monodentate nitrate and bridging nitrate species as well as abundant ionic NH4
+ (Brønsted acid sites) were the key intermediates in the SCR reaction since the ad-monodentate nitrate and bridging nitrate species disappeared quickly in the presence of NH3.
Three-point bending experiments were performed on as-cast and annealed samples of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 (Vit105) bulk metallic glasses over a wide range of temperatures varying from room temperature (293 K) to liquid nitrogen temperature (77 K). The results demonstrated that the free volume decrease due to annealing and/or cryogenic temperature can reduce the propensity for the formation of multiple shear bands and hence deteriorate plastic deformation ability. We clearly observed a sharp ductile-to-brittle transition (DBT), across which microscopic fracture feature transfers from micro-scale vein patterns to nano-scale periodic corrugations. Macroscopically, the corresponding fracture mode changes from ductile shear fracture to brittle tensile fracture. The shear transformation zone volume, taking into account free volume, temperature and strain rate, is proposed to quantitatively characterize the DBT behavior in fracture of metallic glasses.
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