共查询到20条相似文献,搜索用时 31 毫秒
1.
Bert Vreman Bernard J. Geurts N. G. Deen J. A. M. Kuipers J. G. M. Kuerten 《Flow, Turbulence and Combustion》2009,82(1):47-71
Large-eddy simulations (LES) of a vertical turbulent channel flow laden with a very large number of solid particles are performed.
The motivation for this research is to get insight into fundamental aspects of co-current turbulent gas-particle flows, as
encountered in riser reactors. The particle volume fraction equals about 1.3%, which is relatively high in the context of
modern LES of two-phase flows. The channel flow simulations are based on large-eddy approximations of the compressible Navier–Stokes
equations in a porous medium. The Euler–Lagrangian method is adopted, which means that for each individual particle an equation
of motion is solved. The method incorporates four-way coupling, i.e., both the particle-fluid and particle–particle interactions
are taken into account. The results are compared to single-phase channel flow in order to investigate the effect of the particles
on turbulent statistics. The present results show that due to particle–fluid interactions the mean fluid profile is flattened
and the boundary layer is thinner. Compared to single-phase turbulent flow, the streamwise turbulence intensity of the gas
phase is increased, while the normal and spanwise turbulence intensities are reduced. This finding is generally consistent
with existing experimental data. The four-way coupled simulations are also compared with two-way coupled simulations, in which
the inelastic collisions between particles are neglected. The latter comparison clearly demonstrates that the collisions have
a large influence on the main statistics of both phases. In addition, the four-way coupled simulations contain stronger coherent
particle structures. It is thus essential to include the particle–particle interactions in numerical simulations of two-phase
flow with volume fractions around one percent. 相似文献
2.
An experimental study was conducted of incompressible, moderate Reynolds number flow of air over heated rectangular blocks
in a two-dimensional, horizontal channel. Holographic interferometry combined with high-speed cinematography was used to visualize
the unsteady temperature fields in self- sustained oscillatory flow. Experiments were conducted in the laminar, transitional
and turbulent flow regimes for Reynolds numbers in the range from Re = 520 to Re = 6600. Interferometric measurements were
obtained in the thermally and fluiddynamically periodically fully developed flow region on the ninth heated block. Flow oscillations
were first observed between Re = 1054 and Re = 1318. The period of oscillations, wavelength and propagation speed of the Tollmien–Schlichting
waves in the main channel were measured at two characteristic flow velocities, Re = 1580 and Re = 2370. For these Reynolds
numbers it was observed that two to three waves span one geometric periodicity length. At Re = 1580 the dominant oscillation
frequency was found to be around 26 Hz and at Re = 2370 the frequency distribution formed a band around 125 Hz. Results regarding
heat transfer and pressure drop are presented as a function of the Reynolds number, in terms of the block-average Nusselt
number and the local Nusselt number as well as the friction factor. Measurements of the local Nusselt number together with
visual observations indicate that the lateral mixing caused by flow instabilities is most pronounced along the upstream vertical
wall of the heated block in the groove region, and it is accompanied by high heat transfer coefficients. At Reynolds numbers
beyond the onset of oscillations the heat transfer in the grooved channel exceeds the performance of the reference geometry,
the asymmetrically heated parallel plate channel.
Received on 26 April 2000 相似文献
3.
Comparative evaluation of three heat transfer enhancement strategies in a grooved channel 总被引:1,自引:0,他引:1
Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel
plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported.
Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic
grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block.
In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed
cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations
of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified
by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining
insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities
in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding
to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic
grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At
Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated
grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat
transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5,
respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties
also increased significantly.
Received on 5 April 2001 相似文献
4.
The effects of the magnetothermal force on the flows of heat and fluid through a pipe are investigated numerically when the pipe wall is either heated or cooled at constant heat flux. The flow is laminar and a paramagnetic fluid is presumed as the working fluid. Because the magnetic susceptibility of a paramagnetic fluid depends on the inverse of its temperature, the magnetothermal force is induced by coupling of the temperature field and magnetic induction. First, the effects are discussed using the case of a magnetic field induced by a single-turn concentrically placed electric coil. It is found that the effects of the magnetothermal force differ according to whether the pipe is cooled or heated. When cooled, the heat and fluid flows are affected behind the coil; the flow is repelled from the wall to the center and the thermal boundary layer thickens. By decomposing the force into the radial and axial directions in the heated and cooled cases, it is clarified that the axial force changes from positive to negative depending on the coil location in the heated case. Therefore, it can be concluded that the effects are not simply oppositional in the heated and cooled cases. In relation to the heat transfer, only when the coil is placed at the threshold of the heating/cooling zone do the effects on the local heat transfer become the opposite of each other. At other coil locations, the suppression of heat transfer is dominant ahead of the coil in the heated case, as indicated in previous work by our group. However, in the cooled case, this effect occurs behind the coil. For a more practical case, a solenoid coil is employed in the simulation. It is then found that the effect on the heat transfer becomes remarkable at the solenoid edges, especially for the heat-transfer suppression in both the heated and cooled cases. 相似文献
5.
A numerical study is performed to analyze steady laminar forced convection in a channel in which discrete heat sources covered
with porous material are placed on the bottom wall. Hydrodynamic and heat transfer results are reported. The flow in the porous
medium is modeled using the Darcy–Brinkman–Forchheimer model. A computer program based on control volume method with appropriate
averaging for diffusion coefficient is developed to solve the coupling between solid, fluid, and porous region. The effects
of parameters such as Reynolds number, Prandtl number, inertia coefficient, and thermal conductivity ratio are considered.
The results reveal that the porous cover with high thermal conductivity enhances the heat transfer from the solid blocks significantly
and decreases the maximum temperature on the heated solid blocks. The mean Nusselt number increases with increase of Reynolds
number and Prandtl number, and decrease of inertia coefficient. The pressure drop along the channel increases rapidly with
the increase of Reynolds number. 相似文献
6.
In high-velocity open channel flows, the measurements of air–water flow properties are complicated by the strong interactions
between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single-
and dual-tip conductivity probe signals is developed to provide further details on the air–water turbulent level, time and
length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility
with flow Reynolds numbers ranging from 3.8E+5 to 7.1E+5. The air water flow properties presented some basic characteristics
that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were
observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble
count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions
at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic
results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of
the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the
characteristic air–water depth Y
90. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95–0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences
with the rest of the flow. 相似文献
7.
E. N. Vasil'ev D. A. Nesterov 《Journal of Applied Mechanics and Technical Physics》2005,46(6):773-779
A two-dimensional computational model is proposed to calculate radiative-convective heat transfer in gas flows with large
gradients of physical properties. The model is based on the numerical solution of the unsteady dynamic equations for a compressible
inviscid gas and the radiative transfer equations. Flow calculations for the magnetogasdynamic channel of a rail accelerator
show that the dynamics of the process is substantially affected by the flow in the discharge region and hydrodynamic instability,
resulting in the nonstationarity and nonuniformity of the flow and discharge structure. During the process, the discharge
can exist both in the form of several current-carrying channels and in the form of a unified plasma formation. Results of
the numerical calculations agree qualitative with experimental data.
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Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 6, pp. 5–13, November–December, 2005. 相似文献
8.
V. Yu. Liapidevskii 《Journal of Applied Mechanics and Technical Physics》1998,39(3):393-398
Steady-state density flows in a horizontal channel are studied based on a two-layer shallow water model, developed by the
author, with allowance for the mixing between the layers. The structure of a gravity flow and the intensity of mixing in the
flow head are shown to depend significantly on the channel depth. Conditions behind the flow front, which determine the basic
characteristics of a gravity flow, are found.
Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 3, pp. 79–85, May–June, 1998. 相似文献
9.
B. N. Elemesova 《Journal of Applied Mechanics and Technical Physics》1999,40(1):28-35
The plane-parallel unsteady-state shear gas flow in a narrow channel of constant cross section is considered. The existence
theorem of solutions in the form of simple waves of a set of equations of motion is proved for a class of isentropic flows
with a monotone velocity profile over the channel depth. The exact solution described by incomplete beta-functions is found
for a polytropic equation of state in a class of isentropic flows.
Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from
Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 36–43, January–February, 1999. 相似文献
10.
The stability of a laminar boundary layer flow under natural convection on a vertical isothermally heated wall is studied
analytically. The analysis is performed by using two different two-dimensional linear models: (1) The non-parallel flow model
in which the steady mean flow as well as the disturbance amplitude functions can change in the streamwise direction; (2) The
parallel flow model in which the effects of the mean flow and disturbance changes in the streamwise direction are neglected.
The linear non-parallel stability analysis is based on the so-called parabolised stability equations (PSEs) which have been
successfully applied to the stability analysis of forced convection boundary layers. In this study the PSE equations are applied
to natural convection boundary layers in order to show the difference between parallel and non-parallel stability analysis.
A second part of this study deals with the effects of variable properties, which are always present in natural convection
flows. They are analysed by an extended version of the Orr–Sommerfeld equation (EOSE).
Received on 31 May 2000 相似文献
11.
Fluid flow at the interface of a porous medium and an open channel is the governing phenomenon in a number of processes of
industrial importance. Traditionally, this has been modeled by applying the Brinkman’s modification of Darcy’s law to obtain
the velocity profile in terms of an additional parameter known as the “apparent viscosity” or the “slip coefficient”. To test
this ad hoc approach, a detailed experimental investigation of the flow was conducted using Laser Doppler Anemometry (LDA) in the close
vicinity of the permeable boundary of a porous medium. The porous medium used in the experiments consisted of a network of
continuous glass strands woven together in a random fashion.
A Hele–Shaw cell was partially filled with a fibrous preform such that an open channel flow is coupled with the Darcy flow
inside the preform through the permeable interface of the preform. The open channel portion of the Hele–Shaw cell also acts
as an ideal porous medium of known in-plane permeability which is much higher than the permeability of the fibrous porous
medium. A viscous fluid is injected at a constant flow rate through the above arrangement and a saturated and steady flow
is established through the cell. Using LDA, steady state velocity profiles are accurately measured by traversing across the
cell in the direction perpendicular to the flow. A series of experiments were conducted in which fluid viscosity, flow rate,
solid volume fraction of the porous medium and depth of the Hele–Shaw cell were varied. For each and every case in which the
conditions for Hele–Shaw approximation were valid, the depth of the boundary layer zone or the screening length inside the
fibrous preform was found to be of the order of the channel depth. This is much larger as compared to the Brinkman’s prediction
of the screening length which is of the order of √K, where K is the permeability of the fibrous porous medium. Based on this finding, we modified the boundary condition in the Brinkman’s
solution and found that the velocity profile results compared well with the experimental data for the planar geometry and
the fibrous preforms for volume fractions of 7%, 14% and 21% for Hele–Shaw cell depths of 1.6 and 3.175 mm. For a cell depth
of 4.8 cm, in which the Hele–Shaw approximation was not valid, the boundary layer thickness or the screening length was found
to be less than the mold or channel depth but was still much larger than the Brinkman’s prediction.
Received: 10 May 1996 / Accepted: 26 August 1996 相似文献
12.
David Harris 《Meccanica》2006,41(3):351-362
Some properties of a new continuum model for the bulk flow of a dense granular material in which neighbouring grains are in
contact for a finite duration of time and in which the contact force is non-impulsive – the so called slow flow regime – are
presented. The model generalises both the plastic potential and double-shearing models and contains an additional kinematic
quantity – the intrinsic spin. The stress tensor is, in general, non-symmetric and separate yield conditions govern translational
and rotational yield. We consider homogeneous, quasi-static loadings for the symmetric part of the stress and dynamic loading
for the anti-symmetric part of the stress. A solution for the stress state in terms of a single parameter, namely the major
principal direction of the symmetric part of the stress, is presented. This direction itself is determined by a consideration
of the flow equations in the context both dilatant and isochoric simple shear flows. These simple flows are used to complete
the characterisation of the relationship between the anti-symmetric part of the stress and the intrinsic spin. 相似文献
13.
Through an improved ε transport equation, a major quality enhancement of the cubic k–ε model, earlier developed in[13], is obtained. The ε-equation of [13],yielding good results for wall-bounded and rotating
flows, is combined with the one derived by Shih et al. [20], which produces good results for free shear flows (e.g. the plane
jet–round jet anomaly is resolved).Results are presented for the following flows: fully developed stationary and rotating
channel and pipe, backward-facing step, sudden pipe expansion, smooth channel expansion and contraction, plane and round jet.
Heat transfer predictions in turbulent impinging jets are also discussed. Accurate results are obtained for the mean flow
quantities for all test cases, without case dependent model tuning.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
The steady boundary-layer flow near the stagnation point on an impermeable vertical surface with slip that is embedded in
a fluid-saturated porous medium is investigated. Using appropriate similarity variables, the governing system of partial differential
equations is transformed into a system of ordinary differential equations. This system is then solved numerically. The features
of the flow and the heat transfer characteristics for different values of the governing parameters, namely, the Darcy–Brinkman,
Γ, mixed convection, λ, and slip, γ, parameters, are analysed and discussed in detail for the cases of assisting and opposing
flows. It is found that dual solutions exist for assisting flows, as well as those usually reported in the literature for
opposing flows. A stability analysis of the steady flow solutions encountered for different values of the mixed convection
parameter λ is performed using a linear temporal stability analysis. This analysis reveals that for γ = 0 (slip absent)
and Γ = 1 the lower solution branch is unstable while the upper solution branch is stable. 相似文献
15.
This study reports an investigation on the characteristics of single-phase (brine) and two-phase (DNAPL–brine) flows in induced
fractures. The fracture aperture and fluid phase distributions were determined using X-ray computer tomography. In the single-phase
flow tests, the pressure gradient across the induced fractures increases linearly with increasing flow rate. However, models
based on the measured aperture do not yield a consistent match with the experimental data because the effect of pressure losses
due to aperture variation and undulation are not taken into account. In the two-phase flow tests, the measured phase distributions
reveal that the flow pattern is dominated by a dispersed or mixed flow in which either DNAPL or brine phase is discontinuous.
The channel flow pattern, in which DNAPL and brine phases are continuous in the fracture and well represented by the widely
used Romm’s relative permeability relationship was not observed in this study. In contrast, a Lockhart–Martinelli-type correlation
developed for gas–liquid flow in pipes was found to match the pressure gradient and phase saturation results obtained from
the laboratory tests. 相似文献
16.
H. M. Badr M. A. Habib R. Ben-Mansour S. A. M. Said T. F. Ayinde S. Anwar 《Heat and Mass Transfer》2011,47(11):1427-1443
This paper presents the results of experimental and numerical investigations of the problem of turbulent natural convection
in a converging-plate vertical channel. The channel has two isothermally heated inclined walls and two adiabatic vertical
side walls. The parameters involved in this study are the channel geometry represented by the channel width at exit, the inclination
of the heated walls and the temperature difference between the heated walls and the ambient. The investigation covered modified
Rayleigh numbers up to 108 in the computational study and up to 9.3 × 106 in the experimental work. The experimental measurements focused on the velocity field and were carried out using a PIV system
and included measurements of the mean velocity profiles as well as the root-mean-square velocity and shear stress profiles.
The experiments were conducted for an inclination angle of 30°, a gap width of 10 mm and two temperature differences (∆T=25.4°C
and 49.8°C). The velocity profiles in the lower part of the channel indicated the presence of two distinct layers. The first
layer is adjacent to the heated plate and driven by buoyancy forces while the second layer extends from the point of maximum
velocity to the channel center plane and driven mainly by shear forces. The velocity profile at the upper portion of the channel
has shown the merging of the two boundary layers growing over the two heated walls. The measured values of the Reynolds shear
stress and root mean square of the horizontal and vertical velocity fluctuation components have reached their maximum near
the wall while having smaller values in the core region. The computational results have shown that the average Nusselt number
increases approximately linearly with the increase of the modified Rayleigh number when plotted on log–log scale. The variation
of the local Nusselt number indicated infinite values at the channel inlet (leading edge effect) and high values at the channel
exit (trailing edge effect). For a fixed value of the top channel opening, the increase of the inclination angle tended to
reduce flow velocity at the inlet section while changing the flow structure near the heated plates in such a way to create
boundary-layer type flow. The maximum value of the average Nusselt number occurs when θ = 0 and decreases with the increase
of the inclination angle. On the other hand, the increase of the channel width at exit for the same inclination angle caused
a monotonic increase in the flow velocity at the channel inlet. 相似文献
17.
Results of a parametric study of unsteady laminar flows are analyzed. Three-dimensional unsteady equations of hydromechanics
for a compressible medium are solved. The range of the characteristic Reynolds number Re = 400–900 is considered. It is demonstrated that the laminar flow in a plane channel ceases to be steady at Re = 415. As the Reynolds number increases, the unsteady processes become more intense, disturbances penetrate inward the channel, and
separation zones lose their stability. In the vicinity of the channel exit, however, the flow tends to stabilize, though it
remains unsteady. No transition to a turbulent flow occurs in the examined range of Reynolds numbers. 相似文献
18.
Sergey A. Suslov 《Theoretical and Computational Fluid Dynamics》2007,21(4):271-290
The problem of non-Boussinesq mixed convection in a vertical channel formed by two differentially heated infinite plates is
investigated and the complete convective/absolute instability boundary is computed for a wide range of physical parameters.
A physical insight into the mechanisms causing instabilities is given. In particular, it is shown that the appearance of absolute
instability is always dictated by a flow reversal within a channel; however, existence of the flow reversal does not exclude
the possibility of convective instability. It is also shown that fluid’s non-linear transport property variations have a dramatic
effect on the structure and complexity of spatio-temporal instabilities of the co-existing buoyancy and shear modes as the
temperature difference across the channel increases. The validity of the stability results obtained using the procedure described
in Suslov (J Comp Phys 212, 188–217, 2006) is assessed using the method of steepest descent.
This work was partially supported by a computing grant from the Australian Partnership for Advanced Computing, 2000–2003. 相似文献
19.
S. V. Kulikov 《Journal of Applied Mechanics and Technical Physics》2006,47(4):505-509
A single-species gas flow into vacuum in a constant-section channel is computed by means of the Direct Simulation Monte Carlo
method. It is shown that the longitudinal, transverse, and total kinetic temperatures are significantly different in the head
part of the flow, which is a consequence of the arising translational nonequilibrium. The flow is almost self-similar in the
entire region of flow expansion (except for distributions of the transverse and total kinetic temperatures in the head part
of the gas flow), which allows one to predict flow parameters at times greater than those used in simulations.
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Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 54–59, July–August, 2006. 相似文献
20.
A two-scale model of ion transfer in a porous medium is obtained for one-dimensional horizontal flows under the action of
a pressure gradient and an external electric field by the method of homogenization. Steady equations of electroosmotic flows
in flat horizontal nano-sized slits separated by thin dielectric partitions are averaged over a small-scale variable. The
resultant macroequations include Poisson’s equation for the vertical component of the electric field and Onsager’s relations
between flows and forces. The total horizontal flow rate of the fluid is found to depend linearly on the pressure gradient
and external electric field, and the coefficients in this linear relation are calculated with the use of microequations.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 49, No. 4, pp. 162–173, July–August, 2008. 相似文献