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1.
It is considered a heat conduction in a layer made of two conductors distributed in the form of laminas with varied thicknesses.
Macroscopic (averaged) properties of the layer are continuously “transversally” graded across its thickness (TGL layer), cf.
Fig. 1. The aim of the paper is to present and apply an averaged model of the heat conduction, obtained within the tolerance
averaging technique, discussed in the book edited by Woźniak et al. (Thermomechanics of microheterogeneous solids and structures. Tolerance averaging approach, Łódź, Wydawnictwo Politechniki Łódzkiej, 2008). It is shown that the proposed model describes the microstructural effect on the heat conduction of the TGL layer. Moreover,
results obtained within this model are compared to results by the higher order theory (cf. Aboudi et al., Composites B, 30:777–832,
1999). 相似文献
2.
Adrian Postelnicu 《Meccanica》2012,47(6):1349-1357
The paper studies the problem of free convection about a vertical frustum of a cone in a micropolar fluid. It is assumed that the flow is laminar, steady and the wall is subjected to a constant heat flux and the angle of the frustum of the cone is large enough so that the transverse curvature effects are negligible. Under these assumptions, the governing boundary layer equations subject to appropriate boundary conditions are transformed into a set of equations of parabolic type, that are solved using the local non-similarity method. The space of parameters contains the Prandtl number Pr, the micropolar parameter Δ and the microrotation parameter n. Numerical solutions are obtained by varying Pr from 6.7 to 100, Δ from 0 (Newtonian fluid) to 2 and considering two values of n with physical significance (0 and 0.5). Flow and heat transfer characteristics are determined and are shown in graphs. The results are discussed and compared at some extent with those reported by the present author in a previous study (Postelnicu in Int. J. Eng. Sci. 44:672–682, 2006) on the isothermal case. 相似文献
3.
The present paper focuses on the analysis of unsteady flow and heat transfer regarding an axisymmetric impinging synthetic
jet on a constant heat flux disc. Synthetic jet is a zero net mass flux jet that provides an unsteady flow without any external
source of fluid. Present results are validated against the available experimental data showing that the SST/k − ω turbulence model is more accurate and reliable than the standard and low-Re k − ε models for predicting heat transfer from an impinging synthetic jet. It is found that the time-averaged Nusselt number
enhances as the nozzle-to-plate distance is increased. As the oscillation frequency in the range of 16–400 Hz is increased,
the heat transfer is enhanced. It is shown that the instantaneous Nu distribution along the wall is influenced mainly by the interaction of produced vortex ring and wall boundary layer. Also,
the fluctuation level of Nu decreases as the frequency is raised. 相似文献
4.
The Darcy Model with the Boussinesq approximation is used to study natural convection in a horizontal annular porous layer
filled with a binary fluid, under the influence of a centrifugal force field. Neumann boundary conditions for temperature
and concentration are applied on the inner and outer boundary of the enclosure. The governing parameters for the problem are
the Rayleigh number, Ra, the Lewis number, Le, the buoyancy ratio, j{\varphi } , the radius ratio of the cavity, R, the normalized porosity, e{\varepsilon } , and parameter a defining double-diffusive convection (a = 0) or Soret induced convection (a = 1). For convection in a thin annular layer (R → 1), analytical solutions for the stream function, temperature and concentration fields are obtained using a concentric
flow approximation and an integral form of the energy equation. The critical Rayleigh number for the onset of supercritical
convection is predicted explicitly by the present model. Also, results are obtained from the analytical model for finite amplitude
convection for which the flow and heat and mass transfer are presented in terms of the governing parameters of the problem.
Numerical solutions of the full governing equations are obtained for a wide range of the governing parameters. A good agreement
is observed between the analytical model and the numerical simulations. 相似文献
5.
The onset of buoyancy-driven convection in an initially quiescent ferrofluid saturated horizontal porous layer in the presence
of a uniform vertical magnetic field is investigated. The Brinkman-Lapwood extended Darcy equation with fluid viscosity different
from effective viscosity is used to describe the flow in the porous medium. The lower boundary of the porous layer is assumed
to be rigid-paramagnetic, while the upper paramagnetic boundary is considered to be either rigid or stress-free. The thermal
conditions include fixed heat flux at the lower boundary, and a general convective–radiative exchange at the upper boundary,
which encompasses fixed temperature and fixed heat flux as particular cases. The resulting eigenvalue problem is solved numerically
using the Galerkin technique. It is found that increase in the Biot number Bi, porous parameter σ, viscosity ratio Λ, magnetic susceptibility χ, and decrease in the magnetic number M
1 and non-linearity of magnetization M
3 is to delay the onset of ferroconvection in a porous medium. Further, increase in M
1, M
3, and decrease in χ, Λ, σ and Bi is to decrease the size of convection cells. 相似文献
6.
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
7.
We study flow and heat transfer to a cylinder in cross flow at Re = 3,900–80,000 by means of three-dimensional transient RANS (T-RANS) simulations, employing an RNG k − ε turbulence model. Both the case of a bare solid cylinder and that of a solid cylinder surrounded at some fixed distance by
a thin porous layer have been studied. The latter configuration is a standard test geometry for measuring the insulating and
protective performance of garments. In this geometry, the flow in the space between the solid cylinder and the porous layer
is laminar but periodic, whereas the outer flow is transitional and characterized by vortex shedding in the wake of the cylinder.
The results from the T-RANS simulations are validated against data from Direct Numerical Simulations and experiments. It is
found that T-RANS is very well suited for simulating this type of flow. The transient nature of the flow underneath the porous
layer is well reproduced, as well as the influence of vortex shedding on the heat transfer in the downstream stagnation zone.
T-RANS results are found to be in much better agreement with DNS and experimental data than results from steady-state RANS. 相似文献
8.
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
9.
Steady mixed convection boundary layer flow from an isothermal horizontal circular cylinder embedded in a porous medium filled
with a nanofluid has been studied for both cases of a heated and cooled cylinder. The resulting system of nonlinear partial
differential equations is solved numerically using an implicit finite-difference scheme. The solutions for the flow and heat
transfer characteristics are evaluated numerically for various values of the governing parameters, namely the nanoparticle
volume fraction φ and the mixed convection parameter λ. Three different types of nanoparticles are considered, namely Cu, Al2O3 and TiO2. It is found that for each particular nanoparticle, as the nanoparticle volume fraction φ increases, the magnitude of the skin friction coefficient decreases, and this leads to an increase in the value of the mixed
convection parameter λ which first produces no separation. On the other hand, it is also found that of all the three types
of nanoparticles considered, for any fixed values of φ and λ, the nanoparticle Cu gives the largest values of the skin friction coefficient followed by TiO2 and Al2O3. Finally, it is worth mentioning that heating the cylinder (λ > 0) delays separation of the boundary layer and if the cylinder
is hot enough (large values of λ > 0), then it is suppressed completely. On the other hand, cooling the cylinder (λ < 0) brings
the boundary layer separation point nearer to the lower stagnation point and for a sufficiently cold cylinder (large values
of λ < 0) there will not be a boundary layer on the cylinder. 相似文献
10.
This paper presents an analytical and numerical study of natural convection of nanofluids contained in a rectangular enclosure
subject to uniform heat flux along the vertical sides. Governing parameters of the problem under study are the thermal Rayleigh
number Ra, the Prandtl number Pr, the aspect ratio of the cavity A and the solid volume fraction of nanoparticles, Φ. Three types of nanoparticles are taken into consideration: Cu, Al2O3 and TiO2. Various models are used for calculating the effective viscosity and thermal conductivity of nanofluids. In the first part
of the analytical study, a scale analysis is made for the boundary layer regime situation. In the second part, an analytical
solution based on the parallel flow approximation is reported for tall enclosures (A ≫ 1). In the boundary layer regime a good agreement is obtained between the predictions of the scale analysis and those of
the analytical solution. Solutions for the flow fields, temperature distributions and Nusselt numbers are obtained explicitly
in terms of the governing parameters of the problem. A numerical study of the same phenomenon, obtained by solving the complete
system of the governing equations, is also conducted. A good agreement is found between the analytical predictions and the
numerical simulations. 相似文献
11.
M. A. Antar 《Heat and Mass Transfer》2008,45(2):139-146
Forced convection heat transfer characteristics around a microsphere subjected to uniform heat flux boundary condition is
numerically investigated in this study. Moderate to high values of Reynolds number and a wide range of Prandtl number are
considered. The analysis assumes that the continuity assumption is valid and hence the Navier–Stokes equations are solved
for the range of Knudsen number of 0.001 ≤ Kn ≤ 0.1. The appropriate boundary conditions at the surface of the microsphere; the velocity slip and temperature jump are
applied. The effect of the flow parameters: Re, Pr and Kn on the velocity and temperature distribution is presented and hence a better control on the boundary layer thickness can
be achieved in the microscale level. Furthermore, the effect of the controlling parameters on the delay of flow separation,
reduced shear stress, drag coefficient and on the Nusselt number profiles is also presented in the results. 相似文献
12.
In this paper, the problem of laminar free convection from a vertical permeable circular cone maintained with non-uniform
surface heat flux is considered. The governing boundary layer equations are reduced non-similar boundary layer equations with
surface heat flux proportional to x
n
(where x is the distance measured from the leading edge). The solutions of the reduced equations are obtained by using three distinct
solution methodologies; namely, (i) perturbation solution for small transpiration parameter, ξ, (ii) asymptotic solution for
large ξ, and (iii) the finite difference solutions for all ξ. The solutions are presented in terms of local skin-friction
and local Nusselt number for smaller values of Prandtl number and heat flux gradient and are displayed in tabular form as
well as graphically. Effects of pertinent parameters on velocity and temperature profiles are also shown graphically. Solutions
obtained by finite difference method are also compared with the perturbation solutions for small and large ξ and found to
be in excellent agreement.
Received on 1 October 1999 相似文献
13.
This paper presents a new technique for non-contact calorimetry measurement of specific heat capacity and thermal conductivity.
Based on pseudo-white noise modulation and system identification, commonly used in electronics and communication engineering,
this procedure can be used to measure the transfer function of the sample temperature variation due to heating power variation.
The heat capacity and internal heat transfer coefficient are then determined using the equivalence between the identified
transfer functions of the temperatures measured at two locations and the analytical model proposed by Fecht and Johnson (Rev
Sci Instrum 62:1299–1303, 1991) and Wunderlich and Fecht (Measur Sci Technol 16:402–416, 2005). This inverse problem is solved numerically using a Gauss–Seidel algorithm. A numerical simulation of a non-contact modulated
calorimetry experiment is used to demonstrate the relevance of this new technique for indirect measurement of the heat capacity
and heat transfer coefficients of solid samples presenting large Biot numbers (Bi > 0.4). 相似文献
14.
D. Estruch D. G. MacManus J. L. Stollery N. J. Lawson K. P. Garry 《Experiments in fluids》2010,49(3):683-699
The understanding of the behaviour of the flow around surface protuberances in hypersonic vehicles is developed and an engineering
approach to predict the location and magnitude of the highest heat transfer rates in their vicinity is presented. To this
end, an experimental investigation was performed in a hypersonic facility at freestream Mach numbers of 8.2 and 12.3 and Reynolds
numbers ranging from Re
∞/m = 3.35 × 106 to Re
∞/m = 9.35 × 106. The effects of protuberance geometry, boundary layer state, freestream Reynolds number and freestream Mach numbers were
assessed based on thin-film heat transfer measurements. Further understanding of the flowfield was obtained through oil-dot
visualizations and high-speed schlieren videos. The local interference interaction was shown to be strongly 3-D and to be
dominated by the incipient separation angle induced by the protuberance. In interactions in which the incoming boundary layer
remains unseparated upstream of the protuberance, the highest heating occurs adjacent to the device. In interactions in which
the incoming boundary layer is fully separated ahead of the protuberance, the highest heating generally occurs on the surface
just upstream of it except for low-deflection protuberances under low Reynolds freestream flow conditions in which case the
heat flux to the side is greater. 相似文献
15.
In the present case, the conjugate heat transfer involving a turbulent plane offset jet is considered. The bottom wall of
the solid block is maintained at an isothermal temperature higher than the jet inlet temperature. The parameters considered
are the offset ratio (OR), the conductivity ratio (K), the solid slab thickness (S) and the Prandtl number (Pr). The Reynolds number considered is 15,000 because the flow becomes fully turbulent and then it becomes independent of the
Reynolds number. The ranges of parameters considered are: OR = 3, 7 and 11, K = 1–1,000, S = 1–10 and Pr = 0.01–100. High Reynolds number two-equation model (k–ε) has been used for turbulence modeling. Results for the solid–fluid interface temperature, local Nusselt number, local
heat flux, average Nusselt number and average heat transfer have been presented and discussed. 相似文献
16.
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
17.
In the present work, the effect of MHD flow and heat transfer within a boundary layer flow on an upper-convected Maxwell (UCM)
fluid over a stretching sheet is examined. The governing boundary layer equations of motion and heat transfer are non-dimensionalized
using suitable similarity variables and the resulting transformed, ordinary differential equations are then solved numerically
by shooting technique with fourth order Runge–Kutta method. For a UCM fluid, a thinning of the boundary layer and a drop in
wall skin friction coefficient is predicted to occur for higher the elastic number. The objective of the present work is to
investigate the effect of Maxwell parameter β, magnetic parameter Mn and Prandtl number Pr on the temperature field above the sheet. 相似文献
18.
Absorber is an important component in absorption machines and its characteristics have significant effects on the overall
efficiency of absorption machines. This article reports a model of simultaneous heat and mass transfer process in absorption
of refrigerant vapor into a lithium bromide solution of water––cooled vertical plate absorber in the Reynolds number range
of 5 < Re < 150. The boundary layer assumptions were used for the transport of mass, momentum and energy equations and the fully implicit
finite difference method was employed to solve the governing equations in the film flow. Dependence of lithium bromide aqueous
properties to the temperature and concentration and film thickness to vapor absorption was employed. This model can predict
temperature, concentration and properties of aqueous profiles as well as the absorption heat and mass fluxes, heat and mass
transfer coefficients, Nusslet and Sherwood number of absorber. An analysis for linear distribution of wall temperature condition
carries out to investigation the reliability of the present numerical method through comparing with previous investigation. 相似文献
19.
This work is related to the flow of an electro-conducting Newtonian fluid presenting thermoelectric properties in the presence of magnetic field. The flow is considered to be governed an incompressible viscous fluid. The electro-conducting thermofluid equation heat transfer with one relaxation time is derived. The state space formulation developed in Ezzat (Can. J. Phys. Rev. 86:1242–1450, 2008) or one-dimensional problems is introduced. The Laplace transform technique is used. The resulting formulation is applied to a thermal shock problem; that is, a problem of a layer media and a problem for the infinite space in the presence of heat sources. A numerical method is employed for the inversion of the Laplace transforms. Numerical results are given and illustrated graphically for each problem. The effects of thermoelastic properties on the thermofluid flow are studied. 相似文献
20.
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