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101.
102.
Large eddy simulation of gas-particle turbulent channel flow with momentum exchange between the phases 总被引:1,自引:0,他引:1
This paper presents results of a large eddy simulation (LES) combined with Lagrangian particle tracking and a point-force approximation for the feedback effect of particles on the downward turbulent gaseous flow in a vertical channel. The LES predictions are compared with the results obtained by direct numerical simulation (DNS) of a finer computational mesh. A parametric study is conducted for particles with two response times in simulations with and without streamwise gravitational settling and elastic, binary interparticle collisions. It is shown that the classical and the dynamic Smagorinsky turbulence models adequately predict the particle-induced changes in the mean streamwise velocity and the Reynolds stresses of the carrier phase for the range of parameters studied. However, the largest discrepancies between the LES and DNS results are found in the cases of particle-laden flows. Conditional sampling of the instantaneous resolved flow fields indicates that the mechanisms by which particles directly oppose the production of momentum and vorticity of the organized fluid motions are also observed in the LES results. However, the geometric features of the near-wall quasistreamwise vortices are overestimated by the use of both turbulence models compared to the DNS predictions. 相似文献
103.
Wall-mounted roughness features, such as ribs, are often placed along the walls of a channel to increase the convective surface
area and to augment heat transfer and mixing by increasing turbulence. Depending on the relative roughness size and orientation,
the ribs also have varying degrees of increased pressure losses. Designs that use ribs to promote heat transfer encompass
the full range of having only a few streamwise ribs, which do not allow fully developed flow conditions, to multiple streamwise
ribs, which do allow the flow to become fully developed. The majority of previous studies have focused on perturbing the geometry
of the rib with little attention to the spatially and temporally varying flow characteristics and their dependence on the
Reynolds number. A staggered rib-roughened channel study was performed using time-resolved digital particle image velocimetry
(TRDPIV). Both the developing (entry region) and a fully developed region were interrogated for three Reynolds numbers of
2,500, 10,000, and 20,000. The results indicate that the flow was more sensitive to Reynolds number at the inlet than within
the fully developed region. Despite having a similar mean-averaged flowfield structure over the full Reynolds number range
investigated, the population and distribution of coherent structures and turbulent dissipation within the fully developed
region were also found to be Reynolds number dependent. Exploring the time-accurate flow characteristics revealed that in
addition to vortices shed from the rib shear layer, the region of the rib wake was governed by a periodic process of bursting
of the wake vortices resulting in the intermittent ejection of the inter-rib recirculation region into the core flow. This
periodic process was the driving mechanism resulting in mixing and heat transfer augmentation. A quadrant-splitting burst
analysis was also performed to determine the characteristic frequency and duration of inter-rib bursting as well as the wake
shedding frequency, both of which were determined to be Reynolds number dependent. 相似文献
104.
We use the conversion of protonated glycerol to acrolein for a case study of the mechanism of acid-catalyzed dehydration of polyols in aqueous environments. We employ hybrid Quamtum Mechanics/Molecular Mechanics Molecular Dynamics (QM/MM MD) simulations with biased sampling and perform free energy calculations for the elementary steps of the reaction. We investigate the effects of solvent dynamics and in particular the role of quantum mechanical water in the dehydration mechanism. We present results supporting a mechanism that proceeds via water-mediated proton transfers and thus through an enol intermediate. We find that the first dehydration may take place by two, low-energy pathways requiring, respectively, 20.9 and 18.8 kcal/mol of activation free energy. The second dehydration requires 19.9 kcal/mol of activation free energy while for the overall reaction we compute a free energy change of -8 kcal/mol. 相似文献
105.
Matheus Dorneles de Mello Gaurav Kumar Tarnuma Tabassum Dr. Sheetal K. Jain Tso-Hsuan Chen Dr. Stavros Caratzoulas Xinyu Li Prof. Dionisios G. Vlachos Prof. Songi-I Han Prof. Susannah L. Scott Prof. Paul Dauenhauer Prof. Michael Tsapatsis 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(32):13362-13368
Phosphorus-modified all-silica zeolites exhibit activity and selectivity in certain Brønsted acid catalyzed reactions for biomass conversion. In an effort to achieve similar performance with catalysts having well-defined sites, we report the incorporation of Brønsted acidity to metal–organic frameworks with the UiO-66 topology, achieved by attaching phosphonic acid to the 1,4-benzenedicarboxylate ligand and using it to form UiO-66-PO3H2 by post-synthesis modification. Characterization reveals that UiO-66-PO3H2 retains stability similar to UiO-66, and exhibits weak Brønsted acidity, as demonstrated by titrations, alcohol dehydration, and dehydra-decyclization of 2-methyltetrahydrofuran (2-MTHF). For the later reaction, the reported catalyst exhibits site-time yields and selectivity approaching that of phosphoric acid on all-silica zeolites. Using solid-state NMR and deprotonation energy calculations, the chemical environments of P and the corresponding acidities are determined. 相似文献
106.
Vlachos K. Koonen Ton Theophilopoulos G. Avramopoulos H. 《Optical and Quantum Electronics》2003,35(9):865-872
Simultaneous oscillation of 23 wavelengths, spaced at 100 GHz, is demonstrated from a single source using a semiconductor optical amplifier linear cavity. The wavelength comb is generated from an intra cavity, fiber implemented Lyot filter. Each oscillating wavelength has a linewidth of 12.5 GHz and the maximum power variation between the 23 wavelengths was less than 3 dB. 相似文献
107.
We propose a new method for constructing kinetic response surfaces used in the development and optimization of gas‐phase and surface reaction kinetic models. The method, termed as the sensitivity analysis based (SAB) method, is based on a multivariate Taylor expansion of model response with respect to model parameters, neglecting terms higher than the second order. The expansion coefficients are obtained by a first‐order local sensitivity analysis. Tests are made for gas‐phase combustion reaction models. The results show that the response surface obtained with the SAB method is as accurate as the factorial design method traditionally used in reaction model optimization. The SAB method, however, presents significant computational savings compared to factorial design. The effect of including the partial and full third order terms was also examined and discussed. The SAB method is applied to optimization of a relatively complex surface reaction mechanism where large uncertainty in rate parameters exists. The example chosen is laser‐induced fluorescence signal of OH desorption from a platinum foil in the water/oxygen reaction at low pressures. We introduce an iterative solution mapping and optimization approach for improved accuracy. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 94–106, 2004 相似文献
108.
While recently derived continuum mesoscopic equations successfully bridge the gap between microscopic and macroscopic physics, so far they have been derived only for simple lattice models. In this paper, general deterministic continuum mesoscopic equations are derived rigorously via nonequilibrium statistical mechanics to account for multiple interacting surface species and multiple processes on multiple site types and/or different crystallographic planes. Adsorption, desorption, reaction, and surface diffusion are modeled. It is demonstrated that contrary to conventional phenomenological continuum models, microscopic physics, such as the interaction potential, determines the final form of the mesoscopic equation. Models of single component diffusion and binary diffusion of interacting particles on single-type site lattice and of single component diffusion on complex microporous materials' lattices consisting of two types of sites are derived, as illustrations of the mesoscopic framework. Simplification of the diffusion mesoscopic model illustrates the relation to phenomenological models, such as the Fickian and Maxwell-Stefan transport models. It is demonstrated that the mesoscopic equations are in good agreement with lattice kinetic Monte Carlo simulations for several prototype examples studied. 相似文献
109.
This paper presents laser measurements of particle velocities in a horizontal turbulent two-phase pipe flow. A phase Doppler particle analyzer, (PDPA), was used to obtain particle size, velocity, and rms values of velocity fluctuations. The particulate phase consisted of glass spheres 50 m in diameter with the volume fraction of the suspension in the range
p=10-4 to
p=10-3. The results show that the turbulence increases with particle loading.List of symbols
a
particle diameter
-
C
va
velocity diameter cross-correlation
-
d
pipe diameter
-
Fr
2
Froude number
-
g
gravitational constant
- p(a)
Probability density of the particle diameter
- Re
pipe Reynolds number based on the friction velocity
-
T
characteristic time scale of the energy containing eddies
-
T
L
integral scale of the turbulence sampled along the particle path
-
u, U, u
characteristic fluid velocities: fluctuating, mean and friction
-
v
characteristic velocity of the paricle fluctuations
- f
expected value of any random variable f
- f¦g
expected value of f given a value of the random variable g
-
p
particle volume fraction
-
p
particle response time
-
absolute fluid viscosity
-
v
kinematic fluid viscosity
-
p, f
densities, particle and fluid
-
a
2
particle diameter variance
-
va
2
velocity variance due to the particle diameter variance
-
vT
2
total particle velocity variance
-
vt
2
particle velocity variance due to the response to the turbulent field 相似文献
110.
Accuracy of out-of-plane vorticity estimation from in-plane experimental velocity measurements is investigated with particular
application to digital particle image velocimetry (DPIV). Simulations of known flow fields are used to quantify errors associated
with amplification of the velocity measurement noise and method bias error due to spatial sampling resolution. A novel, adaptable,
hybrid estimation scheme combining implicit compact finite difference and Richardson extrapolation schemes is proposed for
improved vorticity estimation. The scheme delivers higher-order truncation error with less noise amplification than an explicit
second order finite difference scheme. Finally, a complete framework for predicting, a priori, the random, bias, and total
error of the vorticity estimation on the basis of the error of the resolved velocities and the choice of differentiation scheme
is developed and presented.
A portion of this work was presented at ASME IMECE 2003 conference
An erratum to this article is available at . 相似文献