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41.
Bradley A. Ochs Dan Fries Devesh Ranjan Suresh Menon 《Flow, Turbulence and Combustion》2018,101(3):927-951
It is unclear whether turbulent flame speed scalings established in low speed regimes are applicable to supersonic flames. To investigate this question, the canonical flame kernel is investigated in a scramjet-like channel having a one degree wall divergence. The growth, shape and internal kernel dynamics are investigated. Results are presented for three Mach numbers, four equivalence ratios, and three turbulence generators. Schlieren photography provides flame images for growth rate statistics and particle image velocimetry (PIV) provides turbulence statistics and investigation of internal kernel dynamics. Supersonic flame kernels are self-propagating and respond to the equivalence ratio in a fashion that is similar to low speed flames. However, supersonic flame kernels have features that are not present in subsonic flame kernels. Baroclinicity, resulting from pressure-density misalignment, creates a reacting vortex ring structure. Further, the mean kernel shape has a Mach number dependence and the vortex ring enhances the turbulent flame speed through entrainment of reactants and augmented flame surface growth. Hence, the previously established (low speed) flame speed scalings are inappropriate for supersonic flame kernels. Drawing motivation from vortex ring literature, the ring propagation velocity is used as the characteristic velocity and a new flame speed scaling is proposed. 相似文献
42.
Large eddy simulation (LES) models for flamelet combustion are analyzed by simulating premixed flames in turbulent stagnation
zones. ALES approach based on subgrid implementation of the linear eddy model(LEM) is compared with a more conventional approach
based on the estimation of the turbulent burning rate. The effects of subgrid turbulence are modeled within the subgrid domain
in the LEM-LES approach and the advection (transport between LES cells) of scalars is modeled using a volume-of-fluid (VOF)
Lagrangian front tracking scheme. The ability of the VOF scheme to track the flame as a thin front on the LES grid is demonstrated.
The combined LEM-LES methodology is shown to be well suited for modeling premixed flamelet combustion. The geometric characteristics
of the flame surfaces, their effects on resolved fluid motion and flame-turbulence interactions are well predicted by the
LEM-LES approach. It is established here that local laminar propagation of the flamelets needs to be resolved in addition
to the accurate estimation of the turbulent reaction rate. Some key differences between LEM-LES and the conventional approach(es)
are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
43.
Thin films of, N-N′diphenyl 1-4phenylene-diamineane are prepared using vacuum sublimation technique. The electrical conductivity from room temperature down to 127 K is studied. It is found that the conduction of charge carriers obeys T−1/2 dependence on temperature. The average hopping distance, hopping energy, density of states and their variation due to post-deposition heat treatment are studied. Schottky diodes are fabricated with gold as ohmic contact and aluminium as Schottky contact. From the observed current voltage characteristics the saturation current density, diode ideality factor and the barrier height are determined. Their variation with air annealing is also investigated. 相似文献
44.
45.
We study fluctuations in the force at the boundary of a 2D granular flow. The forces are mainly impulsive at all flow rates. The probability distribution of impulses decays exponentially at large impulses, as do the forces in a static granular medium. At small impulses, the distribution evolves continuously with flow rate with no indication of the transition from collisional flow to intermittently jamming flows. However, the distribution of the time interval between collisions tends to a power law, P(tau) - tau(-3/2), showing a clear dynamical signature of the approach to jamming. 相似文献
46.
K. R.?RajeshEmail author C. S.?Menon 《The European Physical Journal B - Condensed Matter and Complex Systems》2005,47(2):171-176
Sandwich and planar structures were fabricated using
manganese phthalocyanine (MnPc) as active layer and gold (Au) as electrodes
by thermal evaporation method. The permittivity ε of MnPc was determined from
the dependence of capacitance on film thickness. J-V
characteristics of Au/MnPc/Au structure at room temperature were performed.
Thermally generated hole concentration p0, hole
mobility μp, total trap concentration
Nt and depth of the trap level were estimated. The
activation energies of MnPc films were determined from the Arrhenius plots
of ln σ versus 1000/T. The absorption and reflectance spectra of MnPc thin film deposited at room
temperature were recorded in the spectral range 300–900 nm. The optical band
gap of MnPc thin film was determined from the α2 versus hν graph. The optical constants n and k were found. The real and imaginary parts of the optical dielectric constant
ε1
and ε2 were calculated. 相似文献
47.
A. C.?VargheseEmail author C. S.?Menon 《The European Physical Journal B - Condensed Matter and Complex Systems》2005,47(4):485-489
The electrical properties of mixed copper phthalocyanine (CuPc) and nickel
phthalocyanine (NiPc) thin films devices with gold and lead electrodes have
been investigated. The two phthalocyanines were co-evaporated and deposited
on to a pre-cleaned glass substrate by thermal evaporation technique at room
temperature. By the co-sublimation technique an integrated phthalocyanine
sample was prepared. At low voltages the film showed an ohmic conduction
whereas at higher voltage levels the conduction mechanisms were dominated by
space charged limited conduction (SCLC). The value of mobility of holes were
calculated as μ= 3.1×10-8 m2 V-1 s-1
which is lower for the mixed phthalocyanines whereas the trap concentrations
Nt(e) = 8.06 ×1025 m-3 have increased in the SCLC
region. Further the reverse conduction mechanisms have also been
investigated. From the current limitations in the reverse condition a strong
rectifying behaviour was evident. 相似文献
48.
[reaction: see text] Pyridine catalyzes the reaction of 1,2-diaryl diones with dimethyl butynedioate to afford diaroyl maleates. This unprecedented rearrangement involves a unique benzoyl migration and proceeds with complete stereoselectivity. 相似文献
49.
We study the statistics of the power flux into a collection of inelastic beads maintained in a fluidized steady state by external mechanical driving. The power shows large fluctuations, including frequent large negative fluctuations, about its average value. The relative probabilities of positive and negative fluctuations in the power flux are in close accord with the fluctuation theorem of Gallavotti and Cohen, even at time scales shorter than those required by the theorem. We also compare an effective temperature that emerges from this analysis to the kinetic granular temperature. 相似文献
50.
Results based on a symmetry- and spin-unrestricted tight-binding molecular-dynamics study are presented for the ground-state geometries of intermediate Ni(n), n in [39,49], clusters. A structural phase change is found to take place around n=43 during which a structural transition from fcc/hcp structure to icosahedral one is observed. This is in good agreement with recent experimental findings. This structural transition is found to be associated with a degradation of the inter-atomic bond energy which indicates that the inter-atomic bond does not only depend on the coordination number of each atom but also on its point group symmetry. 相似文献