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1.
The development of the thermo-viscous fingering instability of miscible displacements in homogeneous porous media is examined. In this first part of the study dealing with stability analysis, the basic equations and the parameters governing the problem in a rectilinear geometry are developed. An exponential dependence of viscosity on temperature and concentration is represented by two parameters, thermal mobility ratio β T and a solutal mobility ratio β C , respectively. Other parameters involved are the Lewis number Le and a thermal-lag coefficient λ. The governing equations are linearized and solved to obtain instability characteristics using either a quasi-steady-state approximation (QSSA) or initial value calculations (IVC). Exact analytical solutions are also obtained for very weakly diffusing systems. Using the QSSA approach, it was found that an increase in thermal mobility ratio β T is seen to enhance the instability for fixed β C , Le and λ. For fixed β C and β T , a decrease in the thermal-lag coefficient and/or an increase in the Lewis number always decrease the instability. Moreover, strong thermal diffusion at large Le as well as enhanced redistribution of heat between the solid and fluid phases at small λ is seen to alleviate the destabilizing effects of positive β T . Consequently, the instability gets strictly dominated by the solutal front. The linear stability analysis using IVC approach leads to conclusions similar to the QSSA approach except for the case of large Le and unity λ flow where the instability is seen to get even less pronounced than in the case of a reference isothermal flow of the same β C , but β T = 0. At practically, small value of λ, however, the instability ultimately approaches that due to β C only. 相似文献
2.
This paper reports a numerical study of double diffusive natural convection in a vertical porous enclosure with localized
heating and salting from one side. The physical model for the momentum conservation equation makes use of the Darcy equation,
and the set of coupled equations is solved using the finite-volume methodology together with the deferred central difference
scheme. An extensive series of numerical simulations is conducted in the range of −10 ⩽ N ⩽ + 10, 0 ⩽ R
t
⩽ 200, 10−2 ⩽ Le ⩽ 200, and 0.125 ⩽ L ⩽ 0.875, where N, R
t
, Le, and L are the buoyancy ratio, Darcy-modified thermal Rayleigh number, Lewis number, and the segment location. Streamlines, heatlines,
masslines, isotherms, and iso-concentrations are produced for several segment locations to illustrate the flow structure transition
from solutal-dominated opposing to thermal dominated and solutal-dominated aiding flows, respectively. The segment location
combining with thermal Rayleigh number and Lewis number is found to influence the buoyancy ratio at which flow transition
and flow reversal occurs. The computed average Nusselt and Sherwood numbers provide guidance for locating the heating and
salting segment. 相似文献
3.
The boundary layer problem of a power-law fluid flow with fluid injection on a wedge whose surface is moving with a constant
velocity in the opposite direction to that of the uniform mainstream is analyzed. The free stream velocity, the injection
velocity at the surface, moving velocity of the wedge surface, the wedge angle and the power law index of non-Newtonian fluid
are assumed variables. The fourth order Runge–Kutta method modified by Gill is used to solve the non-dimensional boundary
layer equations for non-Newtonian flow field. Without fluid injection, for every angle of wedge β, a limiting value for velocity ratio λ
cr
(velocity of the wedge surface/velocity of the uniform flow) is found for each power-law index n. The value of λ
cr
increases with the increasing wedge angle β. The value of wedge angle also restricts the physical characteristics of the fluid to be used. The effects of the different
parameters on velocity profile and on skin friction are studied and the drag reduction is discussed. In case of C = 2.5 and velocity ratio λ = 0.2 for wedge angle β = 0.5 with the fluid with power law-index n = 0.5, 48.8% drag reduction is obtained. 相似文献
4.
N. K. Aluri P. K. G. Pantangi S. P. R. Muppala F. Dinkelacker 《Flow, Turbulence and Combustion》2005,75(1-4):149-172
This numerical investigation carried out on turbulent lean premixed flames accounts for two algebraic – the Lindstedt–Vaos
(LV) and the classic Bray–Moss–Libby (BML) – reaction rate models. Computed data from these two models is compared with the
experimental data of Kobayashi et al. on 40 different methane, ethylene and propane Bunsen flames at 1 bar, where the mean
flame cone angle is used for comparison. Both models gave reasonable qualitative trend for the whole set of data, in overall.
In order to characterize quantitatively, firstly, corrections are made by tuning the model parameters fitting to the experimental
methane–air (of Le = 1.0) flame data. In case of the LV model, results obtained by adjusting the pre-constant, i.e., reaction rate parameter,
CR, from its original value 2.6 to 4.0, has proven to be in good agreement with the experiments. Similarly, for the BML model,
with the tuning of the exponent n, in the wrinkling length scale, Ly = Cl⋅ lx(sL/u′)n from value unity to 1.2, the outcome is in accordance with the measured data. The deviation between the measured and calculated
data sharply rises from methane to propane, i.e., with increasing Lewis number. It is deduced from the trends that the effect
of Lewis number (for ethylene–air mixtures of Le = 1.2 and propane–air mixtures of Le = 1.62) is missing in both the models. The Lewis number of the fuel–air mixture is related to the laminar flame instabilities.
Second, in order to quantify for its influence, the Lewis number effect is induced into both the models. It is found that
by setting global reaction rate inversely proportional to the Lewis number in both the cases leads to a much better numerical
prediction to this set of experimental flame data. Thus, by imparting an important phenomenon (the Lewis number effect) into
the reaction rates, the generality of the two models is enhanced. However, functionality of the two models differs in predicting
flame brush thickness, giving scope for further analysis. 相似文献
5.
Dilip K. Maiti 《Heat and Mass Transfer》2011,47(3):245-257
This study provides physical insights into a lid-driven square cavity filled with a mixture of a solvent vapor and non-condensable
gas, subjected to the vertically parallel thermal and solutal gradients. The top lid is maintained at constant speed while
bottom lid and the other two walls are kept fixed. Zero heat and mass fluxes are imposed on the vertical side walls. The transport
equations are solved numerically through a pressure-correction-based iterative algorithm (SIMPLE) with the QUICK scheme for
convective terms. The diffusivities of heat and salt are assumed to be equal throughout this investigation. The essential
details of flow, temperature and concentration fields are presented for the opposing buoyancy forces ratio (B < 0) with special attention being given for the values of parameters for which either the flow inside the cavity is operated
by the mechanically induced convection; or the flow structure inside the cavity is akin to a single-diffusive thermal or solutal
convection. The variations of average rates of heat and mass transfer are uniform with the Reynolds number, while the variations
of these quantities against the solutal Richardson number (Ri
C
) and thermal Richardson number (Ri
T
) point to the existence of the local minimum/maximum. Finally, two linear relations between Ri
C
and Ri
T
at a constant sliding speed are proposed to identify the above points of high and low transport phenomena and justified with
the exhibition of the flow structures inside the cavity. 相似文献
6.
The optimal dimensions of convective-radiating circular fins with variable profile, heat-transfer coefficient and thermal
conductivity, as well as internal heat generation are obtained. A profile of the form y=(w/2) [1+(r
o/r)
n
] is studied, while variation of thermal conductivity is of the form k=k
o[1+ɛ((T−T
∞)/ (T
b−T
∞))
m
]. The heat-transfer coefficient is assumed to vary according to a power law with distance from the bore, expressed as h=K[(r−r
o)/(r
e−r
o)]λ. The results for λ=0 to λ=1.9, and −0.4≤ɛ≤0.4, have been expressed by suitable dimensionless parameters. A correlation for
the optimal dimensions of a constant and variable profile fins is presented in terms of reduced heat-transfer rate. It is
found that a (quadratic) hyperbolic circular fin with n=2 gives an optimum performance. The effect of radiation on the fin performance is found to be considerable for fins operating
at higher base temperatures, whereas the effect of variable thermal conductivity on the optimal dimensions is negligible for
the variable profile fin. It is also observed, in general, that the optimal fin length and the optimal fin base thickness
are greater when compared to constant fin thickness.
Received on 22 February 1999 相似文献
7.
M. K. Partha 《Heat and Mass Transfer》2008,44(8):969-977
Thermophoresis particle deposition in free convection on a vertical plate embedded in a fluid saturated non-Darcy porous medium
is studied using similarity solution technique. The effect of Soret and Dufour parameters on concentration distribution, wall
thermophoretic deposition velocity, heat transfer and mass transfer is discussed in detail for different values of dispersion
parameters (Ra
γ, Ra
ξ) inertial parameter F and Lewis number Le. The result indicates that the Soret effect is more influential in increasing the concentration distribution in both aiding
as well as opposing buoyancies. Also, the non-dimensional heat transfer coefficient and non-dimensional mass transfer coefficient
changes according to different values of thermophoretic coefficient k. 相似文献
8.
Chien-Hsin Chen 《Heat and Mass Transfer》2006,42(9):853-860
Forced convection flow in a microchannel with constant wall temperature is studied, including viscous dissipation effect. The slip-flow regime is considered by incorporating both the velocity-slip and the temperature-jump conditions at the surface. The energy equation is solved for the developing temperature field using finite integral transform. To increase βv
Kn is to increase the slip velocity at the wall surface, and hence to decrease the friction factor. Effects of the parameters βv
Kn, β, and Br on the heat transfer results are illustrated and discussed in detail. For a fixed Br, the Nusselt number may be either higher or lower than those of the continuum regime, depending on the competition between the effects of βv
Kn and β. At a given βv
Kn the variation of local Nusselt number becomes more even when β becomes larger, accompanied by a shorter thermal entrance length. The fully developed Nusselt number decreases with increasing β irrelevant to βv
Kn. The increase in Nusselt number due to viscous heating is found to be more pronounced at small βv
Kn. 相似文献
9.
We study the onset of time dependent Marangoni-Bénard convection in binary mixtures subject to Soret effect by numerical computation
of linear instability thresholds in infinite fluid layers and two-dimensional boxes. The calculations are done for positive
Marangoni numbers (Ma > 0) and negative Marangoni Soret parameters S
M = –(D
S
γ
c
)/(Dγ
T
) where D
S
and D are the Soret and mass diffusion coefficients, respectively, and γ
T
, γ
c
are the first derivatives of the surface tension with respect to temperature and concentration. Our purpose is to understand
why for particular choices of Prandtl and Schmidt numbers, the increase of the stabilizing solutal contribution leads to a
decrease of the critical temperature difference, a phenomenon already reported by Chen & Chen [5] and Skarda et al. [12] For
various choices of Prandtl and Schmidt numbers we analyze the evolution of the critical Marangoni number Ma
c
, critical wavenumber k
c
and angular frequency ω
c
with S
M
and compute the corresponding eigenvectors. We next propose a physical mechanism which explains how the stabilizing solutal
contribution acts as a catalyst for overstability. Finally, we extend our results to two dimensional boxes of small aspect
ratio. 相似文献
10.
The problem of the self-similar boundary flow of a “Darcy-Boussinesq fluid” on a vertical plate with temperature distribution
T
w(x) = T
∞+A·x
λ and lateral mass flux v
w(x) = a·x
(λ−1)/2, embedded in a saturated porous medium is revisited. For the parameter values λ = 1,−1/3 and −1/2 exact analytic solutions
are written down and the characteristics of the corresponding boundary layers are discussed as functions of the suction/ injection
parameter in detail. The results are compared with the numerical findings of previous authors.
Received on 8 March 1999 相似文献
11.
A linear stability analysis is used to study the conditions marking the onset of secondary flow in the form of longitudinal
vortices for plane Poiseuille flow of water in the thermal entrance region of a horizontal parallel-plate channel by a numerical
method. The water temperature range under consideration is 0∼30°C and the maximum density effect at 4°C is of primary interest.
The basic flow solution for temperature includes axial heat conduction effect and the entrance temperature is taken to be
uniform at far upstream location jackie=−∞ to allow for the upstream heat penetration through thermal entrance jackie=0. Numerical results for critical Rayleigh number are obtained for Peclet numbers 1, 10, 50 and thermal condition parameters
(λ
1, λ
2) in the range of −2.0≤λ
1≤−0.5 and −1.0≤λ
2≤1.4. The analysis is motivated by a desire to determine the free convection effect on freezing or thawing in channel flow
of water. 相似文献
12.
A model for predicting the refractive index of sodium iodide (NaI) aqueous solution n
NaI as a function of temperature T, NaI concentration c and wavelength λ was determined for moderate parameter variations. The equation accurately predicted the salt concentration
required to match n
NaI to the refractive index of Pyrex n
P.
Received: 30 July 1998/Accepted: 14 December 1998 相似文献
13.
Robert McKibbin 《Transport in Porous Media》1986,1(3):271-292
The theory describing the onset of convection in a homogeneous porous layer bounded above and below by isothermal surfaces
is extended to consider an upper boundary which is partly permeable. The general boundary condition p + λ ∂p/∂n = constant is applied at the top surface and the flow is investigated for various λ in the range 0 ⩽ λ < ∞. Estimates of the magnitude and horizontal distribution of the vertical mass and heat fluxes at the surface, the horizontally-averaged
heat flux (Nusselt number) and the fraction of the fluid which recirculates within the layer are found for slightly supercritical
conditions. Comparisons are made with the two limiting cases λ → ∞, where the surface is completely impermeable, and λ = 0, where the surface is at constant pressure. Also studied are the effects of anisotropy in permeability, ξ = K
H
/K
V
, and anisotropy is thermal conductivity, η = k
H
/k
V
, both parameters being ratios of horizontal to vertical quantities. Quantitative results are given for a wide variety of
the parameters λ, ξ and η. In the limit ξ/η → 0 there is no recirculation, all fluid being converted out of the top surface, while in the limit ξ/η → ∞ there is full recirculation. 相似文献
14.
Low-order moments of the increments δu andδv where u and v are the axial and radial velocity fluctuations respectively, have been obtained using single and X-hot wires mainly on the
axis of a fully developed pipe flow for different values of the Taylor microscale Reynolds numberR
λ. The mean energy dissipation rate〉ε〈 was inferred from the uspectrum after the latter was corrected for the spatial resolution of the hot-wire probes. The corrected Kolmogorov-normalized
second-order structure functions show a continuous evolution withR
λ. In particular, the scaling exponentζ
v
, corresponding to the v structure function, continues to increase with R
λ in contrast to the nearly unchanged value of ζ
u
. The Kolmogorov constant for δu shows a smaller rate of increase with R
λ than that forδv. The level of agreement with local isotropy is examined in the context of the competing influences ofR
λ and the mean shear. There is close but not perfect agreement between the present results on the pipe axis and those on the
centreline of a fully developed channel flow.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
The Darcy Model with the Boussinesq approximation is used to study natural convection in a shallow porous layer, with variable
permeability, filled with a binary fluid. The permeability of the medium is assumed to vary exponentially with the depth of
the layer. The two horizontal walls of the cavity are subject to constant fluxes of heat and solute while the two vertical
ones are impermeable and adiabatic. The governing parameters for the problem are the thermal Rayleigh number, R
T, the Lewis number, Le, the buoyancy ratio, φ, the aspect ratio of the cavity, A, the normalized porosity, ε, the variable permeability constant, c, and parameter a defining double-diffusive convection (a = 0) or Soret induced convection (a = 1). For convection in an infinite layer, an analytical solution of the steady form of the governing equations is obtained
on the basis of the parallel flow approximation. The onset of supercritical convection, or subcritical, convection are predicted by the present theory. A linear stability analysis of the parallel flow model is conducted and the
critical Rayleigh number for the onset of Hopf’s bifurcation is predicted numerically. Numerical solutions of the full governing
equations are found to be in excellent agreement with the analytical predictions. 相似文献
16.
Recently, in Diaz and Brevdo (J Fluid Mech 681: 567–596, 2011), further in the text referred to as D&B, we found an absolute/convective instability dichotomy at the onset of convection
in a flow in a saturated porous layer with either horizontal or vertical solutal and inclined thermal gradients, and horizontal
throughflow. The control parameter in D&B triggering the destabilization is the vertical thermal Rayleigh number, R
v. In this article, we treat the parameter cases considered in D&B in which the onset of convection has the character of convective
instability and occurs through longitudinal modes. By increasing the vertical thermal Rayleigh number starting from its critical
value, R
vc, we determine the value R
vt of R
v at which the transition from convective to absolute instability takes place and compute the physical characteristics of the
emerging absolutely unstable wave packet. In some cases, the value of the transitional vertical thermal Rayleigh number, R
vt, is only slightly greater than the critical value, R
vc, meaning that at the onset of convection the base convectively unstable state can be viewed as marginally absolutely unstable.
However, in several cases considered, the value of R
vt is significantly greater than the critical value, R
vc, implying that the base state is not marginally but essentially absolutely stable at the point of destabilization. 相似文献
17.
We consider reaction diffusion equations of the prototype form u
t
= u
xx
+ λ u + |u|
p-1
u on the interval 0 < x < π, with p > 1 and λ > m
2. We study the global blow-up dynamics in the m-dimensional fast unstable manifold of the trivial equilibrium u ≡ 0. In particular, sign-changing solutions are included. Specifically, we find initial conditions such that the blow-up
profile u(t, x) at blow-up time t = T possesses m + 1 intervals of strict monotonicity with prescribed extremal values u
1, . . . ,u
m
. Since u
k
= ± ∞ at blow-up time t = T, for some k, this exhausts the dimensional possibilities of trajectories in the m-dimensional fast unstable manifold. Alternatively, we can prescribe the locations x = x
1, . . . ,x
m
of the extrema, at blow-up time, up to a one-dimensional constraint. The proofs are based on an elementary Brouwer degree
argument for maps which encode the shapes of solution profiles via their extremal values and extremal locations, respectively.
Even in the linear case, such an “interpolation of shape” was not known to us. Our blow-up result generalizes earlier work
by Chen and Matano (1989), J. Diff. Eq. 78, 160–190, and Merle (1992), Commun. Pure Appl. Math. 45(3), 263–300 on multi-point blow-up for positive solutions, which were not constrained to possess global extensions for all
negative times. Our results are based on continuity of the blow-up time, as proved by Merle (1992), Commun. Pure Appl. Math. 45(3), 263–300, and Quittner (2003), Houston J. Math. 29(3), 757–799, and on a refined variant of Merle’s continuity of the blow-up profile, as addressed in the companion paper Matano
and Fiedler (2007) (in preparation).
Dedicated to Palo Brunovsky on the occasion of his birthday. 相似文献
18.
The existence and linear stability problem for the Stokes periodic wavetrain on fluids of finite depth is formulated in terms
of the spatial and temporal Hamiltonian structure of the water-wave problem. A proof, within the Hamiltonian framework, of
instability of the Stokes periodic wavetrain is presented. A Hamiltonian center-manifold analysis reduces the linear stability
problem to an ordinary differential eigenvalue problem on ℝ4. A projection of the reduced stability problem onto the tangent space of the 2-manifold of periodic Stokes waves is used
to prove the existence of a dispersion relation Λ(λ,σ, I
1, I
2)=0 where λ ε ℂ is the stability exponent for the Stokes wave with amplitude I
1 and mass flux I
2 and σ is the “sideband’ or spatial exponent. A rigorous analysis of the dispersion relation proves the result, first discovered
in the 1960's, that the Stokes gravity wavetrain of sufficiently small amplitude is unstable for F ε (0,F0) where F
0 ≈ 0.8 and F is the Froude number. 相似文献
19.
In this communication a generalized threedimensional steady flow of a viscous fluid between two infinite parallel plates is
considered. The flow is generated due to uniform stretching of the lower plate in x- and y-directions. It is assumed that the upper plate is uniformly porous and is subjected to constant injection. The governing
system is fully coupled and nonlinear in nature. A complete analytic solution which is uniformly valid for all values of the
dimensionless parameters β, Re and λ is obtained by using a purely analytic technique, namely the homotopy analysis method. Also the effects of the parameters
β, Re and λ on the velocity field are discussed through graphs. 相似文献
20.
Elliptical flow is common in the near vertical fracture area and in anisotropic reservoirs. However, the classical radial
flow models cannot provide a complete analysis for elliptical flow. This article presents a new mathematic model for gas elliptical
flow in tri-porosity gas reservoirs. The differential equation of the new model is written in Mathieu equation, so that the
solution can also be expressed by Mathieu functions. The numerical solution of the corresponding Mathieu functions ce
2n
(ξ, −q), Ke
2n
(ξ, −q) and their derivatives are obtained to derive the dimensionless pseudo pressure drop in Laplace space. The sensitivities
of tri-porosity systems, including the parameters related to anisotropies C
De2S
and ξ
w, the storativity ratios ω
f and ω
m, and the interporosity flow coefficients λvf and λmf, are studied using Laplace numerical inversion. The new solution includes not only the factors considered in classic solutions
in previous articles, but also incorporates the effect of reservoir anisotropy. 相似文献