共查询到20条相似文献,搜索用时 78 毫秒
1.
This paper reports the use of the technique of combining asymptotics with computational fluid dynamics (CFD), known as asymptotic
computational fluid dynamics (ACFD), to handle the problem of combined laminar mixed convection and surface radiation from
a two dimensional, differentially heated lid driven cavity. The fluid under consideration is air, which is radiatively transparent,
and all the walls are assumed to be gray and diffuse and having the same hemispherical, total emissivity (ɛ). The computations
have been performed on FLUENT 6.2. The full radiation problem (i.e. all the walls are radiatively black corresponding to ɛ
= 1) is first taken up and the method of “perturbing and blending” is used wherein, first, limiting solutions of natural and
forced convection are perturbed, to obtain correlations for the weighted average convective Nusselt numbers for the full radiation case. These correlations are then blended suitably in order to obtain a composite
correlation for the weighted average convective Nusselt number that is valid for the entire mixed convection range, i.e., 0 ≤ Ri ≤ ∞. This correlation is then expanded in terms of ɛ to obtain an expression for the average convective Nusselt number that is valid for any ɛ in the range 0 ≤ ɛ ≤ 1. In so far as radiation heat transfer is concerned, using asymptotic
arguments, a new weighted average radiation Nusselt number is defined such that this quantity can be expanded just in terms of ɛ. Hence, by the use of ACFD, the number
of solutions required to obtain reasonably accurate correlations for both the convective and radiative heat transfer rates
and hence the total heat transfer rate (Nu
total = Nu
C + Nu
R), is substantially reduced. More importantly, the correlations for convection and radiation are asymptotically correct at
their ends. The effect of secondary variables like aspect ratio and the case of unequal wall emissivities can also be included
without significant additional effort. 相似文献
2.
Javad Rostami 《Heat and Mass Transfer》2008,44(9):1079-1087
In this paper, unsteady heat transfer and fluid flow characteristics in an enclosure are investigated. The enclosure consists
of two vertical wavy and two horizontal straight walls. The top and the bottom walls are considered adiabatic. Two wavy walls
are kept isothermal and their boundaries are approximated by a cosine function. Governing equations including continuity,
momentum and energy were discretized using the finite-volume method and solved by SIMPLE method in curvilinear coordinate.
Simulation was carried out for a range of Grashof number Gr = 103–106, Prandtl number Pr = 0.5–4.0, wave ratio A (defined by amplitude/wavelength) 0.0–0.35 and aspect ratio W (defined by average width/wavelength) 0.5–1.0. Streamlines and isothermal lines are presented to corresponding flow and thermal
fields. Local and average Nusselt number distributions are presented. The obtained results are in good agreement with available
numerical and experimental data. 相似文献
3.
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
4.
H. H. Liakos E. P. Keramida M. A. Founti N. C. Markatos 《Heat and Mass Transfer》2002,38(4-5):425-432
Impinging jet combusting flows on granite plates are studied. A mathematical model for calculating heat release in turbulent
impinging premixed flames is developed. The combustion including radiative heat transfer and local extinction effects, and
flow characteristics are modeled using a finite volume computational approach. Two different eddy viscosity turbulence models,
namely the standard k–ɛ and the RNG k–ɛ model with and without radiation (discrete transfer model) are assessed. The heat released predictions are compared with
experimental data and the agreement is satisfactory only when both radiative heat transfer and local extinction modeling are
taken into account. The results indicate that the main effect of radiation is the decrease of temperature values near the
jet stagnation point and along the plate surface. Radiation increases temperature gradients and affects predicted turbulence
levels independently of the closure model used. Also, the RNG k–ɛ predicts higher temperatures close the solid plate, with and without radiative heat transfer.
Received on 13 November 2000 / Published online: 29 November 2001 相似文献
5.
Closed-form solutions are derived for the steady magnetohydrodynamic (MHD) viscous flow in a parallel plate channel system
with perfectly conducting walls in a rotating frame of reference, in the presence of Hall currents, heat transfer and a transverse
uniform magnetic field. A mathematical analysis is described to evaluate the velocity, induced magnetic field and mass flow
rate distributions, for a wide range of the governing parameters. Asymptotic behavior of the solution is analyzed for large
M
2 (Hartmann number squared) and K
2 (rotation parameter). The heat transfer aspect is considered also with Joule and viscous heating effects present. Boundary
layers arise close to the channel walls for large K
2, i.e. strong rotation of the channel. For slowly rotating systems (small K
2), Hall current parameter (m) reduces primary mass flow rate (Q
x
/R
ρ
v). Heat transfer rate at the upper plate (d
θ/d
η)
η=1 decreases, while at the lower plate (d
θ/d
η)
η=−1 increases, with increase in either K
2 or m. For constant values of the rotation parameter, K
2, heat transfer rate at both plates exhibits an oscillatory pattern with an increase in Hall current parameter, m. The response of the primary and secondary velocity components and also the primary and secondary induced magnetic field
components to the control parameters is also studied graphically. Applications of the study arise in rotating MHD induction
machine energy generators, planetary and solar plasma fluid dynamics systems, magnetic field control of materials processing
systems, hybrid magnetic propulsion systems for space travel etc. 相似文献
6.
The mixing length theory is employed to simulate the fully developed turbulent heat transfer in annular-sector ducts with
five apex angles (θ0=18,20,24,30,40∘) and four radius ratios (R
o/R
i=2,3,4,5). The Reynolds number range is 104105. The numerical results agree well with an available correlation which was obtained in following parameter range: θ0=18,20,24,30,40∘, R
o/R
i=4 and Re=1045×104. The present work demonstrates that the application range of the correlation can be much extended. Apart from the mixing
length theory, the kɛ model with wall function and the Reynolds stress model are also employed. None of the friction factor results predicted
by the three models agrees well with the test data. For the heat transfer prediction the mixing length theory seems the best
for the cases studied.
Received on 17 July 2000 / Published online: 29 November 2001 相似文献
7.
The main objective of this article is to study the effect of discrete heating on free convection heat transfer in a rectangular
porous enclosure containing a heat-generating substance. The left wall of the enclosure has two discrete heat sources and
the right wall is isothermally cooled at a lower temperature. The top and bottom walls, and the unheated portions of the left
wall are adiabatic. The vorticity–stream function formulation of the governing equations is numerically solved using an implicit
finite difference method. The effects of aspect ratio, Darcy number, heat source length, and modified Rayleigh number on the
flow and heat transfer are analyzed. The numerical results reveal that the rate of heat transfer increases as the modified
Rayleigh number and the Darcy number increases, but decreases on increasing the aspect ratio. The average heat transfer rate
is found to be higher at the bottom heater than at the top heater in almost all considered parameter cases except for ε = 0.5. Also, the maximum temperature takes place generally at the top heater except for the case ε = 0.5, where the maximum temperature is found at the bottom heater. Further, the numerical results reveal that the maximum
temperature decreases with the modified Rayleigh number and increases with the aspect ratio. 相似文献
8.
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
9.
In this paper a new model and computer code is presented by considering singular and discontinuous heat flow divergence. A hybrid model including Smith’s WSGG model and Coppale and Vervish’s model is used for calculating gas radiative properties. Energy equation is solved simultaneously to reach temperature field which specify gas radiative properties. S 8 order of discrete ordinate method is used to solve RTE. It is assumed that walls of enclosure are gray, diffuse and opaque with specified temperature. Boundary conditions are corrected in each iteration that change temperature field. 相似文献
10.
This work is focused on the numerical modeling of steady laminar mixed convection flow in a lid-driven inclined square enclosure filled with water–Al2O3 nanofluid. The left and right walls of the enclosure are kept insulated while the bottom and top walls are maintained at constant temperatures with the top surface being the hot wall and moving at a constant speed. The developed equations are given in terms of the stream function–vorticity formulation and are non-dimensionalized and then solved numerically subject to appropriate boundary conditions by a second-order accurate finite-volume method. Comparisons with previously published work are performed and found to be in good agreement. A parametric study is conducted and a set of graphical results is presented and discussed to illustrate the effects of the presence of nanoparticles and enclosure inclination angle on the flow and heat transfer characteristics. It is found that significant heat transfer enhancement can be obtained due to the presence of nanoparticles and that this is accentuated by inclination of the enclosure at moderate and large Richardson numbers. 相似文献
11.
We present the effect of a magnetic field on three-dimensional fluid flow and heat transfer during solidification from a melt in a cubic enclosure. The walls of the enclosure are considered perfectly electrically conducting and the magnetic field is applied separately in three directions. The finite-volume method with enthalpy formulation is used to solve the mathematical model in the solid and liquid phases. The results obtained by our computer code are compared with the numerical and experimental data found in the literature. For Gr = 5 × 105 and Ha = 0, 25, 50, 75, and 100 (where Gr and Ha are the Grashof and Hartmann numbers, respectively), the effects of magnetic field on flow and thermal fields, and on solid/liquid interface shape are presented and discussed. The interface is localized with and without magnetic field. The results show a strong dependence between the interface shape and the intensity and orientation of magnetic field. When the magnetic field is applied along the X-direction, the magnetic stability diagrams (Vmax–Ha) and (Nuavg–Ha) show the strongest stabilization of the flow field and heat transfer. 相似文献
12.
Sherifull-Din Jamalud-Din D. Andrew S. Rees B. V. K. Reddy Arunn Narasimhan 《Heat and Mass Transfer》2010,46(3):333-343
Steady state natural convection of a fluid with Pr ≈ 1 within a square enclosure containing uniformly distributed, conducting square solid blocks is investigated. The side
walls are subjected to differential heating, while the top and bottom ones are kept adiabatic. The natural convection flow
is predicted employing the nondimensional volumetric flow rate (Qmax* Q_{\max }^{*} ) by using a network model and also using numerical simulations. For identical solid and fluid thermal conductivities (i.e.
k
s
= k
f
), a parametric study of the effect of number of blocks (N
2), gap size (δ) and enclosure Rayleigh number (Ra) on Qmax* Q_{\max }^{*} is performed using the two approaches. Network model predictions are observed to agree well with that from the simulations
until Raδ3 ~ 12. Considering the enclosure with blocks as a porous medium, for a fixed enclosure Ra number, increasing the number of blocks for a fixed volumetric porosity leads to a decrease in enclosure permeability, which
in turn reduces the flow rate. When the number of blocks is fixed, and for a given Ra number, the flow rate increases as the porosity increases by widening the gap between the blocks. 相似文献
13.
The effect of uniform suction on the steady two-dimensional laminar forced flow of a viscous incompressible fluid of temperature
dependent viscosity past a wedge with uniform surface heat flux is considered. The governing equations for the flow are obtained
by using suitable transformations and are solved by using an implicit finite difference method. Perturbation solutions are
also obtained near the leading edge and in the downstream regime. The results are obtained in terms of the local skin friction
coefficient and the rate of heat transfer for various values of the pertinent parameters, such as the Prandtl number, Pr,
the velocity gradient parameter, m, the local suction parameter, ξ, and the viscosity variation parameter, ɛ. Perturbation solutions are compared with the finite
difference solutions and are found to be in excellent agreement. The effect of ξ, m and ɛ on the dimensionless velocity profiles and viscosity distribution are also presented graphically for Pr = 0.7 and 7.0,
which are the appropriate values for gases and water respectively.
Received on 22 July 1999 相似文献
14.
Laminar steady state buoyancy induced flows in a two-dimensional, air filled partial open enclosure with a discrete flush
mounted iso-flux heat source on one of its walls is investigated numerically. The transport equations for energy and vorticity
are solved with the aid of the ADI finite difference scheme on uniform mesh. Because of the specific application of the present
study in the air cooling of electronic equipments, results are obtained only for a Prandtl number of 0.71 with an aspect ratio
of 1.0 for a range of Rayleigh numbers, Ra (≤105), heat flux parameter, Q and opening parameter, A
0 using constant properties and Boussinesq approximation by imposing approximate conditions at the opening. Results of flow
and temperature patterns, velocity and temperature profiles shows that the outgoing flow is governed by strong characteristics
of the cavity condition whereas the incoming flow influenced by outside conditions. It is observed that Rayleigh number considerably
affects the flow and thermal fields within the open enclosure when compared with intensity of heat flux and size of the opening.
Received on 22 January 2001 / Published online: 29 November 2001 相似文献
15.
Steady-state conjugate natural convection in a square cavity filled with a porous medium is studied numerically in this paper.
The enclosure consists of two horizontal conductive walls of finite thickness and two vertical walls at different uniform
temperatures. The focus is on the role of solid-fluid conductivity ratio, k, on the flow and heat transfer characteristics and the average Nusselt number, , over the vertical hot and cold walls of the cavity for a limited set of particular parameters. It was shown that the interface
temperature, θw, along the top of the solid wall decreases with the increase in the wall conductivity k. Also, the values of decreases with the increase of the values of the parameter k. Comparison with known results from the open literature when the wall thickness of the horizontal solid walls is neglected
(non-conjugate problem) is excellent.
Received on 4 April 2000 相似文献
16.
The current study addresses the mathematical modeling aspects of transport phenomena in steady, two-dimensional, laminar flow accompanied by heat transfer in a lid-driven differentially heated cavity in presence of radiatively absorbing, emitting and scattering gray medium. The walls of the enclosure are considered to be opaque, diffusive and gray. Mixed convection is the outcome of the interaction of forced convection induced by the moving vertical hot and cold wall with the natural convection induced due to the differentially heated enclosure. Two different orientations of the wall movement have been considered to simulate opposing and aiding mixed convection phenomenon and to study its interaction with radiation. Vorticity-stream function formulation of N–S equation has been employed. The discrete ordinate method has been used in modeling the radiative transport equation followed with finite volume method as discretisation technique. The effect of influencing parameters on fluid flow and heat transfer has been studied. 相似文献
17.
H. M. Badr 《Heat and Mass Transfer》1998,34(2-3):229-236
The paper deals with the problem of two-dimensional laminar forced convection heat transfer from a straight isothermal tube
of elliptic cross-section placed in a uniform stream. The study is based on numerical solutions of the conservation equations
of mass, momentum, and energy which covers the entire flow domain including the wake region. The parameters influencing the
heat transfer process are essentially the Reynolds number, Re, the tube geometry represented by its minor to major axis ratio,
Ar, and the angle of inclination, λ. The study focuses on the effect of Re, Ar, and λ on the heat transfer process in the
range of Re from 20 to 500. The study reveals that the rate of heat transfer reaches its maximum when λ=0∘ while the minimum occurs when λ=90∘. The results also show that smaller axis ratio gives higher heat transfer rate when λ=0∘. The local Nusselt number and surface vorticity distributions are plotted for a number of cases and the effect of vortex
shedding on the overall rate of heat transfer is briefly discussed.
Received on 20 September, 1997 相似文献
18.
A transient natural convection caused by abruptly heating of a vertical wall in a square enclosure which is under time-dependent gravitational acceleration field is studied numerically. During the computing process, a penalty finite element method is adopted to solve the governing equations. The factor of the time-dependent gravitational acceleration is included in Rayleigh number, therefore two variation of Rayleigh number are considered, one is from 105 via 106 to ? 105 and the other is from 104 via 105 to ? 104. Since the variations of flow field and temperature distribution of the fluid in the square enclosure are complicated and unpredictable, the isotherms, streamlines and time dependent heat transfer rate are examined for investigating the heat transfer mechanism during the transient process. The results show that the agreement of the heat transfer rate of this study with that of quasi-steady state is mainly dependent on the response of the variation rate of boundary layer along the vertical walls to that of time-dependent gravitational acceleration. 相似文献
19.
M. Ould-Rouiss L. Redjem-Saad G. Lauriat 《International Journal of Heat and Fluid Flow》2009,30(4):579-589
Fully developed turbulent flow and heat transfer in a concentric annular duct is investigated for the first time by using a direct numerical simulation (DNS) with isoflux conditions imposed at both walls. The Reynolds number based on the half-width between inner and outer walls, δ=(r2-r1)/2, and the laminar maximum velocity is Reδ=3500. A Prandtl number Pr=0.71 and a radius ratio r*=0.1 were retained. The main objective of this work is to examine the effect of the heat flux density ratio, q*=q1/q2, on different thermal statistics (mean temperature profiles, root mean square (rms) of temperature fluctuations, turbulent heat fluxes, heat transfer, etc.). To validate the present DNS calculations, predictions of the flow and thermal fields with q*=1 are compared to results recently reported in the archival literature. A good agreement with available DNS data is shown. The effect of heat flux ratio q* on turbulent thermal statistics in annular duct with arbitrarily prescribed heat flux is discussed then. This investigation highlights that heat flux ratio has a marked influence on the thermal field. When q* varies from 0 to 0.01, the rms of temperature fluctuations and the turbulent heat fluxes are more intense near the outer wall while changes in q* from 1 to 100, lead to opposite trends. 相似文献
20.
An experimental investigation was carried out to study the enhancement of the heat transfer from a heated flat plate fitted
with rectangular blocks of 1 × 2 × 2 cm3 dimensions in a channel flow as a function of Reynolds number (Reh), spacing (S
y
) of blocks in the flow direction, and the block orientation angle (α) with respect to the main flow direction. The experiments were performed in a channel of 18 cm width and 10 cm height, with
air as the working fluid. For fixed S
x
=3.81 cm, which is the space between the blocks in transverse to the flow direction, the experimental ranges of the parameters
were S
y
=3.33–4.33 cm, α=0–45°, Reh=7625–31550 based on the hydraulic diameter and the average velocity at the beginning of the test section in the channel.
Correlations for Nusselt number were developed, and the ratios of heat transfer with blocks to those with no blocks were given.
The results indicated that the heat transfer could be enhanced or reduced depending on the spacing between blocks, and the
block orientation angle. The maximum heat transfer rate was obtained at the orientation angle of 45°.
Received on 13 December 2000 / Published online: 29 November 2001 相似文献