共查询到20条相似文献,搜索用时 31 毫秒
1.
A. Benderradji A. Haddad R. Taher M. Médale C. Abid F. Papini 《Heat and Mass Transfer》2008,44(12):1465-1476
Two mechanisms of roll initiation are highlighted in a horizontal channel flow, uniformly heated from below, at constant heat
flux (Γ = 10, Pr = 7, 50 ≤ Re ≤ 100, 0 ≤ Ra ≤ 106). The first mechanism is the classical one, it occurs for low Rayleigh numbers and is initiated by the lateral wall effect.
The second occurs for higher Rayleigh numbers and combines the previous effect with a supercritical vertical temperature gradient
in the lower boundary layer, which simultaneously triggers pairs of rolls in the whole zone in between the two lateral rolls.
We have found that in the present configuration, the transition between the two roll initiation mechanisms occurs for Ra/Re
2 ≈ 18. Consequently, the heat transfer is significantly enhanced compared to the pure forced convection case owing to the
flow pattern responsible of the continuous flooding the heated wall with cold fluid. 相似文献
2.
S. A. Nada 《Heat and Mass Transfer》2009,45(8):1083-1097
The present article reports on heat transfer characteristics associated with multiple laminar impinging air jet cooling a
hot flat plat at different orientations. The work aims to study the interactions of the effects of cross flow, buoyancy induced
flow, orientation of the hot surface with respect to gravity, Reynolds numbers and Rayleigh numbers on heat transfer characteristics.
Experiments have been carried out for different values of jet Reynolds number, Rayleigh number and cross flow strength and
at different orientations of the air jet with respect to the target hot plate. In general, the effective cooling of the plate
has been observed to be increased with increasing Reynolds number and Rayleigh number. The results concluded that the hot
surface orientation is important for optimum performance in practical applications. It was found that for Re ≥ 400 and Ra ≥ 10,000 (these ranges give 0.0142 ≤ Ri ≤ 1.59 the Nusselt number is independent on the hot surface orientation. However, for Re ≤ 300 and Ra ≥ 100,000 (these ranges give 1.59 ≤ Ri ≤ 42.85): (i) the Nusselt number for horizontal orientation with hot surface facing down is less that that of vertical orientation
and that of horizontal orientation with hot surface facing up, and (ii) the Nusselt number of vertical orientation is approximately
the same as that of horizontal orientation with hot surface facing up. For all surfaces orientations and for the entire ranges
of Re and Ra, it was found that increasing the cross flow strength decreases the effective cooling of the surface. 相似文献
3.
In this paper, a non-isobaric Marangoni boundary layer flow that can be formed along the interface of immiscible nanofluids
in surface driven flows due to an imposed temperature gradient, is considered. The solution is determined using a similarity
solution for both the momentum and energy equations and assuming developing boundary layer flow along the interface of the
immiscible nanofluids. The resulting system of nonlinear ordinary differential equations is solved numerically using the shooting
method along with the Runge-Kutta-Fehlberg method. Numerical results are obtained for the interface velocity, the surface
temperature gradient as well as the velocity and temperature profiles for some values of the governing parameters, namely
the nanoparticle volume fraction φ (0≤φ≤0.2) and the constant exponent β. Three different types of nanoparticles, namely Cu, Al2O3 and TiO2 are considered by using water-based fluid with Prandtl number Pr =6.2. It was found that nanoparticles with low thermal conductivity,
TiO2, have better enhancement on heat transfer compared to Al2O3 and Cu. The results also indicate that dual solutions exist when β<0.5. The paper complements also the work by Golia and Viviani (Meccanica 21:200–204, 1986) concerning the dual solutions in the case of adverse pressure gradient. 相似文献
4.
The two-dimensional forced convection stagnation-point flow and heat transfer of a viscoelastic second grade fluid obliquely impinging on an infinite plane wall is considered as an exact solution of the full partial differential equations. This oblique flow consists of an orthogonal stagnation-point flow to which a shear flow whose vorticity is fixed at infinity is added. The relative importance of these flows is measured by a parameter γ. The viscoelastic problem is reduced to two ordinary differential equations governed by the Weissenberg number We, two parameters α and β, the later being a free parameter β, introduced by Tooke and Blyth [A note on oblique stagnation-point flow, Physics of Fluids 20 (2008) 033101-1–3], and the Prandtl number Pr. The two cases when α=β and α≠β are, respectively, considered. Physically the free parameter may be viewed as altering the structure of the shear flow component by varying the magnitude of the pressure gradient. It is found that the location of the separation point xs of the boundary layer moves continuously from the left to the right of the origin of the axes (xs<0). 相似文献
5.
The present study is devoted to investigate the influences of mass transfer on buoyancy induced flow over vertical flat plate
embedded in a non-Newtonian fluid saturated porous medium. The Ostwald–de Waele power-law model is used to characterize the
non-Newtonian fluid behavior. Similarity solution for the transformed governing equations is obtained with prescribed variable
surface heat flux. Numerical results for the details of the velocity, temperature and concentration profiles are shown on
graphs. Excess surface temperature as well as concentration gradient at the wall associated with heat flux distributions,
which are entered in tables, have been presented for different values of the power-law index n, buoyancy ration B and the exponent λ as well as Lewis number Le.
Received on 26 April 2000 相似文献
6.
Rafael Cortell 《Meccanica》2012,47(3):769-781
An analysis is presented for the steady non-linear viscous flow of an incompressible viscous fluid over a horizontal surface
of variable temperature with a power-law velocity under the influences of suction/blowing, viscous dissipation and thermal
radiation. Numerical results are illustrated by means of tables and graphs. The governing partial differential equations are
converted into nonlinear ordinary differential equations by a similarity transformation. The effects of the stretching parameter
n, suction/blowing parameter b, Prandtl number σ, Eckert number Ec(Ec * )E_{c}(E_{c}^{ *} ) and radiation parameter N
R
are discussed. Two cases are studied, namely, (i) Prescribed surface temperature (PST case) and, (ii) Prescribed heat flux
at the sheet (PHF case). 相似文献
7.
Natural convection heat transfer in an inclined fin attached square enclosure is studied both experimentally and numerically.
Bottom wall of enclosure has higher temperature than that of top wall while vertical walls are adiabatic. Inclined fin has
also adiabatic boundary conditions. Numerical solutions have been done by writing a computer code in Fortran platform and
results are compared with Fluent commercial code and experimental method. Governing parameters are Rayleigh numbers (8.105 ≤ Ra ≤ 4 × 106) and inclination angle (30° ≤ and ≤ 120°). The temperature measurements are done by using thermocouples distributed uniformly
at the wall of the enclosure. Remarkably good agreement is obtained between the predicted results and experimental data. A
correlation is also developed including all effective parameters on heat transfer and fluid flow. It was observed that heat
transfer can be controlled by attaching an inclined fin onto wall. 相似文献
8.
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
9.
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
10.
Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric,
turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface,
is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric
fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D
c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re
d
≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R
th increased with the confining wall diameter D
c and also with the nozzle diameter d. A minimum value of the thermal resistance of R
th,min=0.4 cm2 K/W was attained for a combination of d=4.42 mm, D
c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of
the thermal resistance and the R
th-predictions. The total fin effectiveness ɛf was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More
than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the
heated round disk.
Received on 30 August 2000 / Published online: 29 November 2001 相似文献
11.
This paper presents the numerical study of internal free convection of Al2O3 water nanofluid in vertical annuli. Vertical walls are maintained at constant temperatures and horizontal walls are adiabatic.
Results are validated by experimental data. Effect of nanofluids on natural convection is investigated as a function of geometrical
and physical parameters and particle fractions for aspect ratio of 1 ≤ H/L ≤ 5, Grashof number of 103 ≤ Gr ≤ 105 and concentration of 0 ≤ ϕ ≤ 0.06. More than 330 different numerical cases are investigated to develop a new correlation for the Nusselt number. This
correlation is presented as a function of Nusselt number of base fluid and particle fraction which is a linear decreasing
function of particle fraction. The developed correlation for annuli is also valid for the natural convection of Al2O3 water nanofluid in a square cavity. Furthermore, the effect of the viscosity and conductivity models on the Nusselt number
of nanofluids in cylindrical cavities are discussed. 相似文献
12.
The flow developing downstream of a step change from smooth to rough surface condition is studied in the light of Townsend’s
wall similarity hypothesis. Previous studies seem to support the hypothesis for channel and pipe flows, but there are considerable
controversies about its application to boundary layers and in particular to surface roughness formed by spanwise bars. It
has been suggested that this controversy arises from insufficient separation of scales between the boundary layer thickness
and the roughness length scale. An experimental investigation has therefore been undertaken where the flow evolves from a
fully developed smooth wall boundary layer at high Reynolds numbers over a step in surface roughness (Re
θ = 13,400 at the step). The flow is mapped through the development of the internal layer until the flow is fully developed
over the rough wall. The internal layer is found to grow as δ ∼ X
0.73, and after about 15 boundary layer thicknesses at the step, the internal layer has reached the outer edge of the incoming
layer. At the last rough wall measurement station, the Reynolds number has grown to Re
θ ≈ 32,600 and the ratio of boundary layer to roughness length scales is δ/k ≈ 140. The outer layer differences between the smooth and the rough wall data were found to be sufficiently small to conclude
that for this setup the Townsend’s wall similarity hypothesis appears to hold. 相似文献
13.
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
14.
In the present study laminar transition to oscillatory convection of fluids having different Prandtl numbers in a laterally
heated vertical cylindrical enclosure for different aspect ratios (melt height to crucible radius) of 2–4 is investigated
numerically for 0.01 ≤ Pr ≤ 10. Numerical solution to two-dimensional axisymmetric transient Navier Stokes equations and energy equation were solved
by finite volume method using SIMPLE algorithm. Numerical results illustrate that there exists a critical Rayleigh number
for each Prandtl number beyond which sustained laminar oscillatory flow sets in. The oscillatory regime was characterised
by the oscillation of the average kinetic energy and average thermal energy of the melt. For a given aspect ratio, critical
Rayleigh number increases with Pr upto 1 and then flattens. It was observed that for low Prandtl number fluids, Pr < 1.0, critical Rayleigh number is found to increase with increase in aspect ratio while for high Prandtl number fluids,
Pr ≥ 1.0, it is found to decrease with increase in aspect ratio. The influence of aspect ratio on the transient behaviour of
the melt volume below and above the critical Rayleigh number was studied. 相似文献
15.
Summary A boundary layer problem of a nonnewtonian fluid flow with fluid injection on a semi-infinite flat plate whose surface moves
with a constant velocity in the opposite direction to that of the uniform mainstream is analyzed. Concluding similarity equations
are solved numerically to show the dependence of the problem to the velocity ratio λ of the plate to uniform flow and to the
injection velocity parameter C. The critical values of λ and C for each nonnewtonian power-law index n are obtained, and their significance in drag reduction is discussed.
Received 26 August 1997; accepted for publication 21 October 1998 相似文献
16.
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
17.
A numerical study of laminar forced convective flows of three different nanofluids through a horizontal circular tube with
a constant heat flux condition has been performed. The effect of Al2O3 volume concentration 0 ≤ φ ≤ 0.09 in the pure water, water-ethylene glycol mixture and pure ethylene glycol as base fluids,
and Reynolds number of 100 ≤ Re ≤ 2,000 for different power inputs in the range of 10 ≤ Q(W) ≤ 400 have been investigated. In this study, all of the nanofluid properties are temperature and nanoparticle volume concentration
dependent. The governing equations have been solved using finite volume approach with the SIMPLER algorithm. The results indicate
an increase in the averaged heat transfer coefficient with increasing the mass of ethylene glycol in the water base fluid,
solid concentration and Reynolds number. From the investigations it can be inferred that, the pressure drop and pumping power
in the nanofluids at low solid volumetric concentration (φ < 3%) is approximately the same as in the pure base fluid in the
various Reynolds numbers, but the higher solid nanoparticle volume concentration causes a penalty drop in the pressure. Moreover,
this study shows it is possible to achieve a higher heat transfer rate with lower wall shear stress with the use of proper
nanofluids. 相似文献
18.
Conjugate natural convection-conduction heat transfer in a square porous enclosure with a finite-wall thickness is studied
numerically in this article. The bottom wall is heated and the upper wall is cooled while the verticals walls are kept adiabatic.
The Darcy model is used in the mathematical formulation for the porous layer and the COMSOL Multiphysics software is applied
to solve the dimensionless governing equations. The governing parameters considered are the Rayleigh number (100 ≤ Ra ≤ 1000), the wall to porous thermal conductivity ratio (0.44 ≤ K
r ≤ 9.90) and the ratio of wall thickness to its height (0.02 ≤ D ≤ 0.4). The results are presented to show the effect of these parameters on the heat transfer and fluid flow characteristics.
It is found that the number of contrarotative cells and the strength circulation of each cell can be controlled by the thickness
of the bottom wall, the thermal conductivity ratio and the Rayleigh number. It is also observed that increasing either the
Rayleigh number or the thermal conductivity ratio or both, and decreasing the thickness of the bounded wall can increase the
average Nusselt number for the porous enclosure. 相似文献
19.
An experimental investigation of water flow in a T-shaped channel with rectangular cross section (20 × 20 mm inlet ID and
20 × 40 mm outlet ID) has been conducted for a Reynolds number Re range of 56–422, based on inlet diameter. Dynamical conditions and the T-channel geometry of the current study are applicable
to the microscale. 2-D planar particle imaging velocimetry (PIV) and laser-induced fluorescence (LIF) were used in multiple
locations of the T-channel to investigate local dynamical behaviors. Steady symmetric and asymmetric flow regimes predicted
in the literature, which is largely numerical, are experimentally verified. Unsteady flow regimes, which are numerically predicted
to occur at higher Re but have not yet been experimentally characterized, are also examined, and real-time LIF results illuminate the evolution
of unsteady structure. Experimental data of the present resolution and scope are not presently available for unsteady flow
regimes. Time scales are presented for unsteady flow regimes, which are found to exhibit periodic behavior and to occur for
Re ≥ 195. An unsteady symmetrical regime is identified for Re ≥ 350 that is detrimental to mixing. Momentum fields and dynamical behaviors of all flow regimes are characterized in detail,
such that published mixing trends may be better understood. Results of all experimental trials were used to construct a regime
map. A symmetric topology is found to be dominant for Re from 56 to 116, when flow is steady, and 350 to 422, when flow is characterized by unsteady stagnation-point oscillation
in the T-channel junction. Asymmetric flow, which is positively indicated for mixing, is dominant for Re between 142 and 298, and the fluid interface exhibits both steady (two standing vortices) and unsteady (shear-layer type
roll-up) behaviors. This result is based on multiple experiments and suggests a practical operating range of 142 ≤ Re ≤ 298 where asymmetric flow is highly likely to experimentally occur. The identification of an upper limit on Re, beyond which mixing appears negatively impacted by a more symmetrical momentum field, is practically important as pressure
drops on the microscale are significant. 相似文献
20.
Experiments are carried out in the wake of a cylinder of d
c
= 10 mm diameter placed symmetrically between two parallel walls with a blockage ratio r = 1/3 and a Reynolds number varying between 75 ≤ Re ≤ 277. Particle image velocimetry is exerted to obtain the instantaneous velocity components in the cylinder wake. A snapshot
proper orthogonal decomposition (POD) is also applied to these PIV results in order to extract the dominant modes through
the implementation of an inhomogeneous filtering of these different snapshots, apart from an interpolation to estimate the
wall shear rate at the lower wall downstream the cylinder. Mass transfer circular probes are placed at the lower wall downstream
this obstacle so as to further determine the time evolution of the wall shear rate, by bringing the inverse method to bear
on the convective-diffusion equation. Comparisons between the two synchronized techniques demonstrate that electrochemical
method can give more accurate information about the coherent structures present in the flow and about the interaction of the
von Kármán vortices with the walls of the tunnel as well. The comparison between the two measurement techniques in the flow
regions concerns the spatiotemporal evolutions of the wall shear rate obtained from PIV measurements and the wall shear rate
using mass transfer probes. Discrepancy between the PIV measurements and the electrochemical ones near the wall, where the
secondary vortices P
1′ are generated at wall, are caused by a PIV bias and a limitations of the singular mass transfer probes. 相似文献