排序方式: 共有83条查询结果,搜索用时 15 毫秒
41.
The onset of convection in a horizontal layer of a porous medium saturated by a nanofluid is studied analytically. The model
used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. For the porous medium, the Brinkman
model is employed. Three cases of free–free, rigid–rigid, and rigid–free boundaries are considered. The analysis reveals that
for a typical nanofluid (with large Lewis number), the prime effect of the nanofluids is via a buoyancy effect coupled with
the conservation of nanoparticles, whereas the contribution of nanoparticles to the thermal energy equation is a second-order
effect. It is found that the critical thermal Rayleigh number can be reduced or increased by a substantial amount, depending
on whether the basic nanoparticle distribution is top-heavy or bottom-heavy, by the presence of the nanoparticles. Oscillatory
instability is possible in the case of a bottom-heavy nanoparticle distribution. 相似文献
42.
This paper presents a linear stability analysis for the onset of natural convection in a horizontal nanofluid layer. The employed model incorporates the effects of Brownian motion and thermophoresis. Both monotonic and oscillatory convection for free–free, rigid–rigid, and rigid–free boundaries are investigated. The oscillatory instability is possible when nanoparticles concentrate near the bottom of the layer, so that the density gradient caused by such a bottom-heavy nanoparticle distribution competes with the density variation caused by heating from the bottom. It is established that the instability is almost purely a phenomenon due to buoyancy coupled with the conservation of nanoparticles. It is independent of the contributions of Brownian motion and thermophoresis to the thermal energy equation. Rather, the Brownian motion and thermophoresis enter to produce their effects directly into the equation expressing the conservation of nanoparticles so that the temperature and the particle density are coupled in a particular way, and that results in the thermal and concentration buoyancy effects being coupled in the same way. 相似文献
43.
P. Auer D. Bridges T. Droege D. Hill R. Giese R. Miller K. Nield P. Rynes B. Sandler A. Yokosawa G. Hicks D. Miller C. Wilson 《Nuclear Physics B》1976,113(2):279-284
Polarization in π?p elastic scattering, with emphasis over the backward region, has been measured at 2.93 and 3.25 GeV/c. We observe large changes in polarization compared with existing data above and below these energies. Our data may be useful in determining the properties of resonances and in understanding baryon exchanges. 相似文献
44.
D. A. Nield 《Transport in Porous Media》2006,64(2):185-188
The problem of fully developed forced convection in a parallel-plates channel filled with a saturated porous medium (involving
a Brinkman model for the momentum equation), with the effect of viscous dissipation (involving a Brinkman number), is discussed.
Some general matters relating to the possibility of fully developed convection are also discussed. 相似文献
45.
Effects of Viscous Dissipation and Flow Work on Forced Convection in a Channel Filled by a Saturated Porous Medium 总被引:1,自引:3,他引:1
Fully developed forced convection in a parallel plate channel filled by a saturated porous medium, with walls held either at uniform temperature or at uniform heat flux, with the effects of viscous dissipation and flow work included, is treated analytically. The Brinkman model is employed. The analysis leads to expressions for the Nusselt number, as a function of the Darcy number and Brinkman number. 相似文献
46.
D. A. Nield 《Transport in Porous Media》1987,2(2):177-185
An analysis is made of convective heat transport, produced by uniform heating from below, in a horizontal layer of a porous medium consisting of vertical slabs or columns of different permeabilities. Estimates of the heat flux are made on the assumption that flow in one column does not interact with flow in adjacent columns. The results are compared with those for a homogeneous layer, for which previous work is reviewed. It is found that an inhomogeneous layer transports less heat than a homogeneous layer for which the mean Rayleigh number is the same, if the Rayleigh number is supercritical throughout the layer. If the Rayleigh number is subcritical in part of the layer, the inhomogeneous layer may transport more heat than the equivalent homogeneous layer. 相似文献
47.
Comments are made about the model employed by Chen and Vafai for forced convection in a porous medium channel, with a surface tension effect at the moving interface between two fluids, when one fluid is displaced by the other. A simple situation is analysed, and the circumstances under which surface tension effects are important in this case are clarified. 相似文献
48.
A perturbation approach is used to obtain analytical expressions for the velocity, temperature distribution, and transient
Nusselt number for the problem of forced convection, in a parallel-plates channel or a circular tube occupied by a saturated
porous medium modeled by the Brinkman equation, produced by an applied pressure gradient that fluctuates with small amplitude
harmonically in time about a non-zero mean. It is shown that the fluctuating part of this Nusselt number alters in magnitude
and phase as the dimensionless frequency increases. The magnitude increases from zero, goes through a peak, and then decreases
to zero. The height of the peak decreases as the modified Prandtl number increases. The phase (relative to that of the steady
component) decreases from π/2 to − π/2. The height of the peak at first increases, goes through a maximum, and then decreases
as the Darcy number decreases. 相似文献
49.
The effects of hydrodynamic and thermal heterogeneity, for the case of variation in both the horizontal and vertical directions,
on the onset of convection in a horizontal layer of a saturated porous medium uniformly heated from below, are studied analytically
using linear stability theory for the case of weak heterogeneity. Attention is focused on the case of constant flux upper
and lower boundaries, a case for which the critical horizontal wavenumber is zero, and attention is also concentrated on the
case of a shallow layer. It is found that the effect of such heterogeneity on the critical value of the Rayleigh number Ra
based on mean properties is of second order if the properties vary in a piecewise constant or linear fashion. The effects
of horizontal heterogeneity and vertical heterogeneity are then comparable once the aspect ratio is taken into account, and
to a first approximation are independent. The combination of permeability heterogeneity and conductivity heterogeneity can
be either stabilizing or destabilizing for the present case. 相似文献
50.
An analytical solution is obtained for forced convection in a circular tube occupied by a core–sheath-layered saturated porous
medium with counterflow produced by pulsating pressure gradients. The case of the constant heat-flux boundary conditions is
considered, and the Brinkman model is employed for the porous medium. A perturbation approach is used to obtain analytical
expressions for the velocity, temperature distribution, and transient Nusselt number for convection produced by an applied
pressure gradient that fluctuates with small amplitude harmonically in time about a non-zero mean. It is shown that the fluctuating
part of the Nusselt number alters in magnitude and phase as the dimensionless frequency increases. The magnitude increases
from zero, goes through a peak, and then decreases to zero. The height of the peak depends on the values of various parameters.
The phase (relative to that of the steady component) decreases from π/2 to − π/2 as the frequency increases. 相似文献