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1.
A numerical study of the steady forced convection heat transfer from an unconfined circular cylinder
Forced convection heat transfer from an unconfined circular cylinder in the steady cross-flow regime has been studied using
a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400. The numerical results are used to develop simple correlations for Nusselt number as a function of the pertinent dimensionless
variables. In addition to average Nusselt number, the effects of Re, Pr and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt number distributions
have also been presented to provide further physical insights into the nature of the flow. The rate of heat transfer increases
with an increase in the Reynolds and/or Prandtl numbers. The uniform heat flux condition always shows higher value of heat
transfer coefficient than the constant wall temperature at the surface of the cylinder for the same Reynolds and Prandtl numbers.
The maximum difference between the two values is around 15–20%. 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.
The present investigation deals with the numerical analysis of steady-state laminar buoyancy-driven convection in an inclined
triangular enclosure filled with fluid saturated porous media using the Darcy law equation. One wall of the enclosure is isothermally
heated and the other is cooled, while the remaining wall is adiabatic. The effect of inclination angle on natural convection
is investigated by varying the angle of inclination (φ) between 0° and 360°. The governing transformed equations are solved numerically using a finite-difference method. Obtained
results are shown in the form of streamlines, isotherms, mean Nusselt numbers and dimensionless stream function for different
values of the Rayleigh number Ra in the range 100 ≤ Ra ≤ 1,000. It is found that the values of the maximum and minimum mean Nusselt number are reached for φ = 330° and φ = 210° , respectively. However, the lowest flow strength is formed at φ = 240° for all values of Ra. 相似文献
4.
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
5.
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
6.
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
7.
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
8.
The flow and heat transfer in an inclined and horizontal rectangular duct with a heated plate longitudinally mounted in the
middle of cross section was experimentally investigated. The heated plate and rectangular duct were both made of highly conductive
materials, and the heated plate was subjected to a uniform heat flux. The heat transfer processes through the test section
were under various operating conditions: Pr ≈ 0.7, inclination angle ϕ = −60° to +60°, Reynolds number Re = 334–1,911, Grashof number Gr = 5.26 × 102–5.78 × 106. The experimental results showed that the average Nusselt number in the entrance region was 1.6–2 times as large as that
in the fully developed region. The average Nusselt numbers and pressure drops increased with the Reynolds number. The average
Nusselt numbers and pressure drops decreased with an increase in the inclination angle from −60° to +60° when the Reynolds
number was less than 1,500. But when the Reynolds number increased to over about 1,800, the heat transfer coefficients and
pressure drops were independent of inclination angles. 相似文献
9.
A detailed numerical study of laminar forced convection in a porous channel which contains a fibrous medium saturated with
a power-law fluid was performed. Hydrodynamic and heat transfer results are presented for a configuration that has uniform
heat flux or uniform temperature heating at the walls. The flow in the porous medium was modeled using the modified Brinkman-Forchheimer-extended
Darcy model for power law fluids in which the non-Darcy effects of inertia and boundary were considered. Parametric studies
were conducted to examine the effects of Darcy number, power law index, inertia parameter and Prandtl number. The results
indicate that when the power law index is decreased, the velocity gradient near the walls increases but these effects are
reduced gradually as the Darcy number decreases until the Darcy regime (Da≤10−6) is reached in which case the effects of power law index become negligible. As the power law index is decreased, the thermal
boundary layer thickness decreases significantly only in the non-Darcy regime. Consequently, as the power law index decreases,
the fully developed Nusselt number increases considerably in the non-Darcy regime whereas in the Darcy regime the change in
Nusselt number is very small. As the Prandtl number increases, the local Nusselt number increases and this effect is more
significant for shear thinning fluids (n<1.0).
Received on 2 March 1998 相似文献
10.
MHD boundary-layer flow of a non-Newtonian nanofluid past a stretching sheet with a heat source/sink
The goal of the present paper is to examine the magnetohydrodynamic effects on the boundary layer flow of the Jeffrey fluid model for a non-Newtonian nanofluid past a stretching sheet with considering the effects of a heat source/sink. The governing partial differential equations are reduced to a set of coupled nonlinear ordinary differential equations by using suitable similarity transformations. These equations are then solved by the variational finite element method. The profiles of the velocity, temperature, and nanoparticle volume fraction are presented graphically, and the values of the Nusselt and Sherwood numbers are tabulated. The present results are compared with previously published works and are found to be in good agreement with them. 相似文献
11.
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
12.
Adrian Postelnicu 《Heat and Mass Transfer》2007,43(6):595-602
The heat and mass transfer characteristics of natural convection about a vertical surface embedded in a saturated porous medium
subjected to a chemical reaction is numerically analyzed, by taking into account the diffusion-thermo (Dufour) and thermal-diffusion
(Soret) effects. The transformed governing equations are solved by a very efficient numerical method, namely, a modified version
of the Keller-box method for ordinary differential equations. The parameters of the problem are Lewis, Dufour and Soret numbers,
sustentation parameter, the order of the chemical reaction n and the chemical reaction parameter γ. Local Nusselt number and local Sherwood number variations and dimensionless concentration
profiles in the boundary layer are presented graphically and in tables for various values of problem parameters and it is
concluded that γ and n play a crucial role in the solution. 相似文献
13.
A prolate spheroid submerged in water can be heated for decreasing the viscous drag because of a decrease in viscosity with
increasing temperature. The heated boundary layer experiment, based on this principle, was carried out in a circulating water
channel of NCKU, and the viscous drag of a spheroid, with a five-to-one ratio of length to mid-diameter, was also measured
by means of wake surveys. The difference of total-head between wake and undisturbed region was measured by two total-head
tubes, and the water speed in the wake area was measured by an electromagnetic flowmeter.
The results indicate that the viscous drag of the model decreases with surface heating, and the decrease in viscous drag of
the model is 13% when the surface temperature is 17 °C above the ambient water temperature. The velocity gradient in the wake
region and the total-heating differential readings between undisturbed and wake region also decrease with surface heating
due to the delayed laminar-turbulent transition in the boundary layer. The values of the coefficient of viscous drag obtained
without surface overheating are found to be in agreement, for the range of Froude numbers investigated, with the results obtained
from previous experiments.
Received: 18 June 1996/Accepted: 29 April 1997 相似文献
14.
The position of the free surface is calculated numerically for a porous slab which is partly filled with a liquid and differentially heated from its sides. A coordinate transformation is used to transform the original problem from a physical coordinate system to a non‐orthogonal system where the free surface becomes a fixed straightline. The transformed problem is then solved using a finite difference method. Results are obtained for Rayleigh numbers up to 1000. The Nusselt numbers increase slightly with medium Rayleigh numbers (convection‐dominated region) as expected. Since at low Ra the conduction is dominant and at high Ra radiation is dominant. Hadizadeh and Tien (Int. J. Heat Mass Transfer 2004; 17 (6):799–804) studied the forced convection on the surface of porous layer. In that paper they dealt with in detail the boundary regime of liquid in the channel and modelled the flow and heat transfer. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
15.
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
16.
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
17.
Velocity profile measurements in zero pressure gradient, turbulent boundary layer flow were made on a smooth wall and on two
types of rough walls with a wide range of roughness heights. The ratio of the boundary layer thickness (δ) to the roughness
height (k) was 16≤δ/k≤110 in the present study, while the ratio of δ to the equivalent sand roughness height (k
s) ranged from 6≤δ/k
s≤91. The results show that the mean velocity profiles for all the test surfaces agree within experimental uncertainty in velocity-defect
form in the overlap and outer layer when normalized by the friction velocity obtained using two different methods. The velocity-defect
profiles also agree when normalized with the velocity scale proposed by Zagarola and Smits (J Fluid Mech 373:33–70, 1998). The results provide evidence that roughness effects on the mean flow are confined to the inner layer, and outer layer similarity
of the mean velocity profile applies even for relatively large roughness. 相似文献
18.
Natural convection in a partially filled porous square cavity is numerically investigated using SIMPLEC method. The Brinkman-Forchheimer
extended model was used to govern the flow in the porous medium region. At the porous-fluid interface, the flow boundary condition
imposed is a shear stress jump, which includes both the viscous and inertial effects, together with a continuity of normal
stress. The thermal boundary condition is continuity of temperature and heat flux. The results are presented with flow configurations
and isotherms, local and average Nusselt number along the cold wall for different Darcy numbers from 10−1 to 10−6, porosity values from 0.2 to 0.8, Rayleigh numbers from 103 to 107, and the ratio of porous layer thickness to cavity height from 0 to 0.50. The flow pattern inside the cavity is affected
with these parameters and hence the local and global heat transfer. A modified Darcy–Rayleigh number is proposed for the heat
convection intensity in porous/fluid filled domains. When its value is less than unit, global heat transfer keeps unchanged.
The interfacial stress jump coefficients β
1 and β
2 were varied from −1 to +1, and their effects on the local and average Nusselt numbers, velocity and temperature profiles
in the mid-width of the cavity are investigated. 相似文献
19.
S. A. Nada 《Heat and Mass Transfer》2006,43(2):135-148
Heat transfer characteristics of a slot/slots jet air impinging on a cylinder have been experimentally investigated for two different orientations of slot/slots jet plan with respect to cylinder axis. The experiments were carried out to study the effects of orientation of slot/slots jet plan with respect to cylinder axis on the rate of heat transfer from the cylinder. Two different jet–cylinder configurations were studied: (1) single slot jet aligned with cylinder axis (slot length = cylinder length), and (2) multiple slot jets equally spaces distributed orthogonal to cylinder axis (each slot length = cylinder diameter and sum of slots lengths = cylinder length). For each configuration, parametric effects of Reynolds numbers (Re) ranging from 1,000 to 10,000, dimensionless slot widths (W/d) ranging from 0.125 to 0.5, and dimensionless slot orifice-to-cylinder spacing (Z/W) ranging from 1 to 12 on local and average Nusselt numbers around cylinder surface have been investigated. The results showed that: (1) cooling the cylinder by multiple slots jets situated orthogonal to cylinder axis gave more uniform surface temperature distributions and higher heat transfer rate than the case of cooling the cylinder by single slot jet aligned with cylinder axis, (2) for both configurations the Nusselt number around the cylinder increased with increasing Re and W/d, and (3) for both configurations there was a certain Z/W in the range 4<Z/W<6 at which the stagnation and mean Nusselt number were maximum. Correlations for the mean Nusselt numbers around cylinder surface have been presented for both configurations. Comparisons between the correlations predictions and the present and other previous experimental data have been conducted. 相似文献
20.
N. Samyuktha R. Ravindran M. Ganapathirao 《Journal of Applied Mechanics and Technical Physics》2017,58(1):116-128
An analysis is performed to study the effects of the chemical reaction and heat generation or absorption on a steady mixed convection boundary layer flow over a vertical stretching sheet with nonuniform slot mass transfer. The governing boundary layer equations with boundary conditions are transformed into the dimensionless form by a group of nonsimilar transformations. Nonsimilar solutions are obtained numerically by solving the coupled nonlinear partial differential equations using the quasi-linearization technique combined with an implicit finite difference scheme. The numerical computations are carried out for different values of dimensionless parameters to display the distributions of the velocity, temperature, concentration, local skin friction coefficient, local Nusselt number, and local Sherwood number. The results obtained indicate that the local Nusselt and Sherwood numbers increase with nonuniform slot suction, but nonuniform slot injection produces the opposite effect. The local Nusselt number decreases with heat generation and increases with heat absorption. 相似文献