共查询到20条相似文献,搜索用时 140 毫秒
1.
Mathematical modeling is performed to simulate forced convection flow of 47 nm- Al2O3/water nanofluids in a microchannel using the lattice Boltzmann method (LBM). Single channel flow and conjugate heat transfer
problem are taken into consideration and the heat transfer rate using a nanofluid is examined. Simulations are conducted at
low Reynolds numbers (2 ≤ Re ≤ 16). The computed average Nusselt number, which is associated with the thermal conductivity of nanofluid, is in the range
of 0.6 £ [`(Nu)] £ 13 0.6 \le \overline{Nu} \le 13 . Results indicate that the average Nusselt number increases with the increase of Reynolds number and particle volume concentration.
The fluid temperature distribution is more uniform with the use of nanofluid than that of pure water. Furthermore, great deviations
of computed Nusselt numbers using different models associated with the physical properties of a nanofluid are revealed. The
results of LBM agree well with the classical CFD method for predictions of flow and heat transfer in a single channel and
a microchannel heat sink concerning the conjugate heat transfer problem, and consequently LBM is robust and promising for
practical applications. 相似文献
2.
Natural convection in a fluid saturated porous medium has been numerically investigated using a generalized non-Darcy approach.
The governing equations are solved by using Finite Volume approach. First order upwind scheme is employed for convective formulation
and SIMPLE algorithm for pressure velocity coupling. Numerical results are presented to study the influence of parameters
such as Rayleigh number (106 ≤Ra ≤108), Darcy number (10−5 ≤ Da ≤ 10−2), porosity (0.4 ≤ ɛ ≤ 0.9) and Prandtl number (0.01 ≤ Pr ≤ 10) on the flow behavior and heat transfer. By combining the method of matched asymptotic expansions with computational
fluid dynamics (CFD), so called asymptotic computational fluid dynamics (ACFD) technique has been employed to generate correlation
for average Nusselt number. The technique is found to be an attractive option for generating correlation and also in the analysis
of natural convection in porous medium over a fairly wide range of parameters with fewer simulations for numerical solutions. 相似文献
3.
The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity
components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Red of 7,200 based on free stream velocity (U
∞) and cylinder diameter (d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream
locations, i.e., x/d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further
to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases
by about 70%. Similar results are found at x/d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing
α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise
spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence
slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to [`(v2)] \overline{{v^{2}}} is remarkable at x/d = 10 and 20 and significantly larger than that to [`(u2)] \overline{{u^{2}}} and [`(w2)]. \overline{{w^{2}}}. This contribution to all three Reynolds normal stresses becomes negligibly small at x/d = 40. The coherent contribution to [`(u2)] \overline{{u^{2}}} and [`(v2)] \overline{{v^{2}}} decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for
a larger α. 相似文献
4.
A. Klaczak 《Heat and Mass Transfer》2001,37(4-5):443-448
This article presents the results of laboratory research on heat exchange while heating water in horizontal and vertical
tubes with twisted-tape inserts.
The scope of the research:
70 ≤ Re ≤ 4000
3.6 ≤ Pr ≤ 5.9
8.6 ≤ Gz ≤ 540
The research was held for three cases:
– horizontal experimental tube
– vertical experimental tube, the direction of flow according to the free convection vector
– vertical experimental tube, the direction of flow not in accordance with the free convection vector
For such cases the correlation equation was defined NuT=f(Gz; y), Nu = f(Gz) and the proportion NuT/Nu was analysed.
Received on 30 March 2000 相似文献
5.
A test rig incorporating the injection from a single cylindrical hole with an inclination of 30° to a thermally uniform mainstream
flow was used for determining variations in flow structures due to injectant pulsation. The average blowing ratios ([`(M)] \overline{M} ) were 0.65, 1, and 1.25. The periodic variations in injectant flow were rendered by a loudspeaker-based pulsation system
to nondimensionalized excitation frequency (St St ) of 0, 0.2, 0.3, and 0.5. Pulsation resulting in a close-wall orientation of injectant fluid compared with steady blowing
bearing outward orientation was only observed in few cases. At [`(M)] \overline{M} = 0.65, jet fluid remains aligned and covers a significant part of the wall under steady blowing. At higher blowing ratios,
pulsation induces large spatial variations in the jet trajectory, collapsing of the jet body, and the shedding of wake structures
due to the periodic variation of injection flow rate. It was found that the pulsation improves wall coverage of the injectant
fluid under low frequency excitation as the separation of the jet from the wall becomes evident ([`(M)] \overline{M} = 1 and 1.25). 相似文献
6.
Non-Darcy mixed convection in a porous medium from horizontal surfaces with variable surface heat flux of the power-law distribution
is analyzed. The entire mixed convection regime is divided into two regions. The first region covers the forced convection
dominated regime where the dimensionless parameter ζ
f
=Ra*
x
/Pe2
x
is found to characterize the effect of buoyancy forces on the forced convection with K
′
U
∞/ν characterizing the effect of inertia resistance. The second region covers the natural convection dominated regime where
the dimensionless parameter ζ
n
=Pe
x
/Ra*1/2
x
is found to characterize the effect of the forced flow on the natural convection, with (K
′
U
∞/ν)Ra*1/2
x
/Pe
x
characterizing the effect of inertia resistance. To obtain the solution that covers the entire mixed convection regime the
solution of the first regime is carried out for ζ
f
=0, the pure forced convection limit, to ζ
f
=1 and the solution of the second is carried out for ζ
n
=0, the pure natural convection limit, to ζ
n
=1. The two solutions meet and match at ζ
f
=ζ
n
=1, and R
*
h
=G
*
h
.
Also a non-Darcy model was used to analyze mixed convection in a porous medium from horizontal surfaces with variable wall
temperature of the power-law form. The entire mixed convection regime is divided into two regions. The first region covers
the forced convection dominated regime where the dimensionless parameter ξ
f
=Ra
x
/Pe
x
3/2 is found to measure the buoyancy effects on mixed convection with Da
x
Pe
x
/ɛ as the wall effects. The second region covers the natural convection dominated region where ξ
n
=Pe
x
/Ra
x
2/3 is found to measure the force effects on mixed convection with Da
x
Ra
x
2/3/ɛ as the wall effects. Numerical results for different inertia, wall, variable surface heat flux and variable wall temperature
exponents are presented.
Received on 8 July 1996 相似文献
7.
Nonsimilarity solutions for non-Darcy mixed convection from a vertical impermeable surface embedded in a saturated porous
medium are presented for variable surface heat flux (VHF) of the power-law form. The entire mixed convection region is divided
into two regimes. One region covers the forced convection dominated regime and the other one covers the natural convection
dominated regime. The governing equations are first transformed into a dimensionless form by the nonsimilar transformation
and then solved by a finite-difference scheme. Computations are based on Keller Box method and a tolerance of iteration of
10−5 as a criterion for convergence.
Three physical aspects are introduced. One measures the strength of mixed convection where the dimensionless parameter Ra*
x
/Pe3/2
x
characterizes the effect of buoyancy forces on the forced convection; while the parameter Pe
x
/Ra*2/3
x
characterizes the effect of forced flow on the natural convection. The second aspect represents the effect of the inertial
resistance where the parameter K′U
∞/ν is found to characterize the effect of inertial force in the forced convection dominated regime, while the parameter (K′U
∞/ν)(Ra*2/3
x
/Pe
x
) characterizes the effect of inertial force in the natural convection dominated regime. The third aspect is the effect of
the heating condition at the wall on the mixed convection, which is presented by m, the power index of the power-law form heating condition.
Numerical results for both heating conditions are carried out. Distributions of dimensionless temperature and velocity profiles
for both Darcy and non-Darcy models are presented.
Received on 26 May 1997 相似文献
8.
G-equations are well-known front propagation models in turbulent combustion which describe the front motion law in the form
of local normal velocity equal to a constant (laminar speed) plus the normal projection of fluid velocity. In level set formulation,
G-equations are Hamilton–Jacobi equations with convex (L
1 type) but non-coercive Hamiltonians. Viscous G-equations arise from either numerical approximations or regularizations by
small diffusion. The nonlinear eigenvalue [`(H)]{\bar H} from the cell problem of the viscous G-equation can be viewed as an approximation of the inviscid turbulent flame speed s
T. An important problem in turbulent combustion theory is to study properties of s
T, in particular how s
T depends on the flow amplitude A. In this paper, we study the behavior of [`(H)]=[`(H)](A,d){\bar H=\bar H(A,d)} as A → + ∞ at any fixed diffusion constant d > 0. For cellular flow, we show that
$\bar H(A,d)\leqq C(d) \quad \text{for all}\ d >0 ,$\bar H(A,d)\leqq C(d) \quad \text{for all}\ d >0 , 相似文献
9.
Mixed convection flow in a two-sided lid-driven cavity filled with heat-generating porous medium is numerically investigated.
The top and bottom walls are moving in opposite directions at different temperatures, while the side vertical walls are considered
adiabatic. The governing equations are solved using the finite-volume method with the SIMPLE algorithm. The numerical procedure
adopted in this study yields a consistent performance over a wide range of parameters that were 10−4 ≤ Da ≤ 10−1 and 0 ≤ Ra
I
≤ 104. The effects of the parameters involved on the heat transfer characteristics are studied in detail. It is found that the
variation of the average Nusselt number is non-linear for increasing values of the Darcy number with uniform or non-uniform
heating condition. 相似文献
10.
We classify new classes of centers and of isochronous centers for polynomial differential systems in
\mathbb R2{\mathbb R^2} of arbitrary odd degree d ≥ 7 that in complex notation z = x + i
y can be written as
|
设为首页 | 免责声明 | 关于勤云 | 加入收藏 |
Copyright©北京勤云科技发展有限公司 京ICP备09084417号 |