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21.
D. A. Nield 《Transport in Porous Media》2008,75(2):269-270
This note is a comment on the reply by Aydin and Kaya (Transp Porous Media (to appear), 2008b) to comments by Rees and Magyari
(Transp Porous Media (to appear), 2008) on an article by Aydin and Kaya (Transp Porous Media (to appear), 2008a) concerning
the combined effects of viscous dissipation and surface mass flux on the forced convection boundary-layer flow in a saturated
porous medium modeled by the Brinkman equation. It is argued that the statements by Rees and Magyari are in fact appropriate.
The thermal boundary condition imposed at the edge of the boundary-layer by Aydin and Kaya is incompatible with the energy
equation, and thus the results of their paper are inconsistent with the physics of the situation. The attempts of Aydin and
Kaya to justify their paper are flawed by an inappropriate assumption and calculations with an insufficiently large parameter. 相似文献
22.
The problem of the onset of convective roll instabilities in a horizontal porous layer with isothermal boundaries at unequal
temperatures, well known as the Horton–Rogers–Lapwood problem, is revisited including the effect of pressure work and viscous
dissipation in the local energy balance. A linear stability analysis of rolls disturbances is performed. The analysis shows
that, while the contribution of viscous dissipation is ineffective, the contribution of the pressure work may be important.
The condition of marginal stability is investigated by adopting two solution procedures: method of weighted residuals and
explicit Runge–Kutta method. The pressure work term in the energy balance yields an increase of the value of the Darcy–Rayleigh
number at marginal stability. In other words, the effect of pressure work is a stabilizing one. Furthermore, while the critical
value of the Darcy– Rayleigh number may be considerably affected by the pressure work contribution, the critical value of
the wave number is affected only in rather extreme cases, i.e. for very high values of the Gebhart number. A nonlinear stability
analysis is also performed pointing out that the joint effects of viscous dissipation and pressure work result in a reduction
of the excess Nusselt number due to convection, when the Darcy–Rayleigh number is replaced by the superadiabatic Darcy–Rayleigh
number. 相似文献
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25.
The effect of vertical throughflow on the onset of convection in a rectangular box occupied by a saturated porous medium uniformly
heated from below, is studied using linear stability theory. It is found that, for small values of the throughflow, the stabilizing
effect of the throughflow and the stabilizing effect of the confining lateral walls of the box are approximately independent
of each other. 相似文献
26.
D. A. Nield 《Transport in Porous Media》2011,88(2):187-191
The singular behavior of the Horton–Rogers–Lapwood problem when a Newtonian fluid is replaced by a standard power-law fluid
is investigated further. Using weakly nonlinear stability theory, an estimate is made of the amplitude of convection at which
the convection is initiated (in the case of a fluid with index n > 1) or levels off (in the case of a fluid with n < 1). 相似文献
27.
D. A. Nield 《Transport in Porous Media》2011,87(1):121-123
The published literature on convection patterns arising in natural convective flow in an inclined porous layer is surveyed,
and the situation with respect to the occurrence of polyhedral cells is clarified. These are observed in experiments but are
not predicted by the simple classical theory nor are they observed in simulations based on that simple theory. 相似文献
28.
The effect of vertical heterogeneity of permeability, on the onset of convection in a horizontal layer of a saturated porous
medium, uniformly heated from below but with a non-uniform basic temperature gradient resulting from transient heating or
otherwise, is studied analytically using linear stability theory. Two particular situations, corresponding to instantaneous
bottom heating and constant-rate bottom heating, are studied. Estimates of the timescale for the development of convection
instability are obtained. 相似文献
29.
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
30.
The onset of convection in a horizontal layer of a porous medium saturated by a nanofluid is analytically studied. The model
used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. For the porous medium, the Darcy model
is employed. The effect of local thermal non-equilibrium among the particle, fluid, and solid-matrix phases is investigated
using a three-temperature model. The analysis reveals that in some circumstances the effect of LTNE can be significant, but
for a typical dilute nanofluid (with large Lewis number and with small particle-to-fluid heat capacity ratio) the effect is
small. 相似文献