48 research outputs found
Recommended from our members
Regulation of Wages and Hours Prior to 1938
Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure
Luminescent properties of lithium-phosphate-borate glasses doped with Tb{3+}/Eu{3+} ions
The luminescence of Li[2]O-B[2]O[3]-P[2]O[5]-CaF[2] scintillation glass doped Tb{3+}, Eu{3+} under different types of excitation sources are investigated. Changing the europium concentration of 0.5 to 1 wt% leads changes in luminescence intensity of Tb{3+} ions. The luminescence spectrum of the Tb3+ ions are depend on the concentration of Eu3+. It was found, that the luminescence decay kinetics of terbium ion in the band 543 nm depending on the concentration of europium and from type of excitation. The difference in the nature of the luminescence decay kinetics of glasses under pulsed photo- and electronic excitation discussed
Influence of gas compression on flame acceleration in the early stage of burning in tubes
The mechanism of finger flame acceleration at the early stage of burning in
tubes was studied experimentally by Clanet and Searby [Combust. Flame 105: 225
(1996)] for slow propane-air flames, and elucidated analytically and
computationally by Bychkov et al. [Combust. Flame 150: 263 (2007)] in the limit
of incompressible flow. We have now analytically, experimentally and
computationally studied the finger flame acceleration for fast burning flames,
when the gas compressibility assumes an important role. Specifically, we have
first developed a theory through small Mach number expansion up to the
first-order terms, demonstrating that gas compression reduces the acceleration
rate and the maximum flame tip velocity, and thereby moderates the finger flame
acceleration noticeably. This is an important quantitative correction to
previous theoretical analysis. We have also conducted experiments for
hydrogen-oxygen mixtures with considerable initial values of the Mach number,
showing finger flame acceleration with the acceleration rate much smaller than
those obtained previously for hydrocarbon flames. Furthermore, we have
performed numerical simulations for a wide range of initial laminar flame
velocities, with the results substantiating the experiments. It is shown that
the theory is in good quantitative agreement with numerical simulations for
small gas compression (small initial flame velocities). Similar to previous
works, the numerical simulation shows that finger flame acceleration is
followed by the formation of the "tulip" flame, which indicates termination of
the early acceleration process.Comment: 19 pages, 20 figure
Effect of surface friction on ultrafast flame acceleration in obstructed cylindrical pipes
The Bychkov model of ultrafast flame acceleration in obstructed tubes [Valiev et al., “Flame Acceleration in Channels with Obstacles in the Deflagration-to-Detonation Transition,” Combust. Flame 157, 1012 (2010)] employed a number of simplifying assumptions, including those of free-slip and adiabatic surfaces of the obstacles and of the tube wall. In the present work, the influence of free-slip/non-slip surface conditions on the flame dynamics in a cylindrical tube of radius R, involving an array of parallel, tightly-spaced obstacles of size αR, is scrutinized by means of the computational simulations of the axisymmetric fully-compressible gasdynamics and combustion equations with an Arrhenius chemical kinetics. Specifically, non-slip and free-slip surfaces are compared for the blockage ratio, α, and the spacing between the obstacles, ΔZ, in the ranges 1/3 ≤ α ≤ 2/3 and 0.25 ≤ ΔZ/R ≤ 2.0, respectively. For these parameters, an impact of surface friction on flame acceleration is shown to be minor, only 1∼4%, slightly facilitating acceleration in a tube with ΔZ/R = 0.5 and moderating acceleration in the case of ΔZ/R = 0.25. Given the fact that the physical boundary conditions are non-slip as far as the continuum assumption is valid, the present work thereby justifies the Bychkov model, employing the free-slip conditions, and makes its wider applicable to the practical reality. While this result can be anticipated and explained by a fact that flame propagation is mainly driven by its spreading in the unobstructed portion of an obstructed tube (i.e. far from the tube wall), the situation is, however, qualitatively different from that in the unobstructed tubes, where surface friction modifies the flame dynamics conceptually
Emission properties of YAG: Ce ceramics with barium fluoride flux
In this work, we investigated the luminescent properties of YAG, Ce ceramics with an addition of barium fluoride flux. The ceramic samples were sintered from obtained YAG: Ce, BaF2 phosphor powder. Morphological, luminescent and decay time characteristics of YAG: Ce ceramics were investigated. The luminescence decay kinetics analysis of the ceramics showed that the decay time in phosphors and ceramics is different in the visual spectral range
Luminescent properties of Li[2]O-K[2]O-Al[2]O[3]-B[2]O[3] glass-ceramics doped with Cr{3+} ions
Li[2]O-K[2]O-Al[2]O[3-B[2]O[3 glass-ceramics doped with trivalent chromium ions was fabricated by melt-quenching technique. The glass-ceramics preparation process included 5 stages. The structure, optical and luminescent properties of glass-ceramics were investigated. The effect of Sb[2]O[3] with different concentration on structure and luminescent properties was carried out. It was demonstrated the introduction of antimony oxide leads to a shift of the maxima of exothermic transformations towards higher temperatures. The luminescence intensity of Cr{3+} ions increased with increasing of Sb[2]O[3]. The maximum quantum yield was 44%, which confirms the possibility of using of glass-ceramics as active media in laser and fiber technologies, as well as for creating phosphor materials
Compositional dependence of thermal, optical and mechanical properties of oxyfluoride glass
Tungsten oxyfluoride glasses are characterized by low phonon energy. This is due to the presence of fluoride ions that have low phonon energy and formation of low phonon energy WO6 units. Oxyfluoride glasses based on WO3–BaF2–RF, where RF=LiF, NaF or mixed (LiF–NaF) have been prepared by melt quenching technique. The density and molar volume of the prepared glasses show a decrease with the increase of RF instead of WO3 content. The glass transition temperature Tg is found to decrease with increasing RF content. The refractive index increases with the addition of heavy polarizable fluorides. The decrease of the elastic moduli and microhardness of these glasses may be due to the decrease in density and the depolymrization effect. The Poisson's ratio increases with increasing RF content due to the structural changes and formation of (NBOs) and (NBF) units. The aim of this work is to prepare a glass host with low phonon energy to be an efficient host with good luminescence properties, when doped with rare earth ions, and to study its structural, thermal, optical and mechanical properties
Nanocomposition of PEDOT:PSS with metal phthalocyanines as promising hole transport layers for organic photovoltaics
PEDOT:PSS is one of the most widely used materials as a hole selective layer in organic photovoltaics due to its
easy processing and high reproducibility. Unfortunately, the material is limited when testing new donor:acceptor
systems due to its intrinsic frontier energy levels which typically leads to energy losses due to inadequate energy
level alignment and presence of resistive losses. In this work, PEDOT:PSS:metal phthalocyanines nanocomposite
thin films are formulated and used as hole transport layer for organic solar cells (OSCs). PEDOT:PSS is formulated
with H2Pc, CuPc, CoPc and ZnPc metal phthalocyanines (MPc) with nanobelt morphology which confers the
compatibility with the active layer. Atomic force microscopy (AFM) and x-ray diffraction (XRD) were used to
study the morphology and structure of nanocomposite films, respectively. OSCs based on PEDOT:PSS:MPc
nanocomposite films were fabricated and the effect of hybrid hole transport layer with various phthalocyanines
on photovoltaics properties was studied. Overall, nanocomposition of PEDOT:PSS with metal phthalocyanines
improves the final power conversion efficiency of solar cells by 20% by a reduction of the resistive losses due to
inadequate energy level alignment. The addition of metal phthalocyanines to PEDOT:PSS is a promising method
for tailor-made hole transport materials for new donor:acceptor systems to improve their efficiencies.Funding for open access charge: CRUE-Universitat Jaume