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1.
Natural convection heat transfer phenomena on horizontal cylinders were investigated experimentally in order to explore the
applicability of analogy experimental method using the copper electroplating system and to visualize the local heat transfer
depending on the angular position and the diameter of the horizontal cylinder. The diameters of the cylinders are varied from
0.01 to 0.15 m, which correspond to the Rayleigh numbers of 1.73 × 107–5.69 × 1011. The measured mass transfer coefficients show good agreements with the existing heat transfer correlations. The patterns
of copper plated on the aluminum cathodes for various Rayleigh numbers reveal and visualize the local heat transfer depending
on the angular position and show good agreement with the works of Kitamura et al. The hydrogen bubbles produced at higher
applied potential visualize the plumes appeared on top region of the cylinders. 相似文献
2.
S. A. Nada 《Heat and Mass Transfer》2008,44(8):929-936
In the present study, an experimental investigation of heat transfer and fluid flow characteristics of buoyancy-driven flow
in horizontal and inclined annuli bounded by concentric tubes has been carried out. The annulus inner surface is maintained
at high temperature by applying heat flux to the inner tube while the annulus outer surface is maintained at low temperature
by circulating cooling water at high mass flow rate around the outer tube. The experiments were carried out at a wide range
of Rayleigh number (5 × 104 < Ra < 5 × 105) for different annulus gap widths (L/D
o = 0.23, 0.3, and 0.37) and different inclination of the annulus (α = 0°, 30° and 60°). The results showed that: (1) increasing
the annulus gap width strongly increases the heat transfer rate, (2) the heat transfer rate slightly decreases with increasing
the inclination of the annulus from the horizontal, and (3) increasing Ra increases the heat transfer rate for any L/D
o and at any inclination. Correlations of the heat transfer enhancement due to buoyancy driven flow in an annulus has been
developed in terms of Ra, L/D
o and α. The prediction of the correlation has been compared with the present and previous data and fair agreement was found. 相似文献
3.
S. Morsli A. Sabeur-Bendehina 《Journal of Applied Mechanics and Technical Physics》2013,54(6):913-920
The aim of the present work is to study the entropy generation in the natural convection process in square cavities with hot wavy walls through numerical simulations for different undulations and Rayleigh numbers, while keeping the Prandtl number constant. The results show that the hot wall geometry affects notably the heat transfer rate in the cavity. It has been found in the present numerical study that the mean Nusselt number in the case of heat transfer in a cavity with wavy walls is lower, as compared to heat transfer in a cavity without undulations. Based on the obtained dimensionless velocity and temperature values, the distributions of the local entropy generation due to heat transfer and fluid friction, the local Bejan number, and the local entropy generation are determined and plotted for different undulations and Rayleigh numbers. The study is performed for Rayleigh numbers 103 < Ra < 105, irreversibility coefficients 10?4 < φ < 10?2, and Prandtl numbers Pr = 0.71. The total entropy generation is found to increase with increasing undulation number. 相似文献
4.
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
5.
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
6.
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
7.
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
8.
Radiation-conduction interaction on mixed convection from a horizontal circular cylinder 总被引:1,自引:0,他引:1
A mixed convection flow of an optically dense viscous incompressible fluid along a horizontal circular cylinder has been
studied with the effect of radiation when the surface temperature is uniform. Using appropriate transformations, the boundary
layer equations governing the flow are reduced to local nonsimilarity form. Solutions of the governing equations are obtained
employing the implicit finite difference method. Effects of varying the pertinent parameters, such as, the Planck number,
R
w the surface temperature parameter, θw and the buoyancy parameter, α on the local skin-friction and local heat transfer coefficients are shown graphically as well
as in tabular form against the curvature parameter ξ, while taking Prandtl number Pr = 1.0. It is found that an increase of
R
d,θw or α leads to increases in the values of the local skin-friction and the local rate of heat transfer coefficients. At the
stagnation point asymptotic solutions for large value of α are also obtained and the effect of the other pertinent parameters
on the formation of the flow separation are studied.
Received on 28 July 1998 相似文献
9.
The effect of the surface thermal radiation in tall cavities with turbulent natural convection regime was analyzed and quantified
numerically. The parameters considered were: the Rayleigh number 109–1012, the aspect ratio 20, 40 and 80 and the emmisivity 0.0–1.0. The percentage contribution of the radiative surface to the total
heat transfer has a maximum value of 15.19% (Ra = 109, A = 20) with emissivity equal to 1.0 and a minimum of 0.5% (Ra = 1012, A = 80) with ε* = 0.2. The average radiative Nusselt number for a fixed emissivity is independent of the Rayleigh number, but for a fixed
Rayleigh number diminishes with the increase of the aspect ratio. The results indicate that the surface thermal radiation
does not modify significantly the flow pattern in the cavity, just negligible effects in the bottom and top of the cavity
were observed. Two different temperature patterns were observed a conductive regime Ra = 109 and a boundary layer regime Ra = 1012. 相似文献
10.
A. A. Tawfek 《Heat and Mass Transfer》1999,35(4):327-333
An experimental investigations of heat transfer for a stationary isothermal circular cylinder exposed normal to an impinging
round air-jet has been reported. The circumferential heat transfer distributions as well as axial Nusselt number is measured.
The measurements are taken as a function of the Reynolds number ranging from 3.8 × 103 to 4 × 104, the cylinder separation distance to the nozzle diameter (z/d) varying from 7 to 30, and the nozzle to cylinder diameter ratio (d/D) changing from 0.06 to 0.14. The output results indicated that the axial and radial distributions of the local heat transfer
peaked at the impingement point. The heat transfer rate increases as the values of z decreases, for the same d and Re. The drop-off of the Nusselt number with increasing axial distance or radial angle from the impingement point was
more pronounced for smaller z and d. The peripheral and surface average Nusselt numbers were determined by integration. The experimental data was used to produce
correlations for both average and stagnation point heat transfer.
Received on 4 January 1999 相似文献
11.
Two-dimensional numerical studies of flow and temperature fields for turbulent natural convection and surface radiation in
inclined differentially heated enclosures are performed. Investigations are carried out over a wide range of Rayleigh numbers
from 108 to 1012, with the angle of inclination varying between 0° and 90°. Turbulence is modeled with a novel variant of the k–ε closure model. The predicted results are validated against experimental and numerical results reported in literature. The
effect of the inclination of the enclosure on pure turbulent natural convection and the latter’s interaction with surface
radiation are brought out. Profiles of turbulent kinetic energy and effective viscosity are studied to observe the net effect
on the intensity of turbulence caused by the interaction of natural convection and surface radiation. The variations of local
Nusselt number and average Nusselt number are presented for various inclination angles. Marked change in the convective Nusselt
number is found with the orientation of enclosure. Also analyzed is the influence of change in emissivity on the flow and
heat transfer. A correlation relevant to practical applications in the form of average Nusselt number, as a function of Rayleigh
number, Ra, radiation convection parameter, N
RC and inclination angle of the enclosure, φ is proposed. 相似文献
12.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献
13.
We describe how outer flow turbulence phenomena depend on the interaction with the wall. We investigate coherent structures
in turbulent flows over different wavy surfaces and specify the influence of the different surface geometries on the coherent
structures. The most important contribution to the turbulent momentum transport is attributed to these structures, therefore
this flow configuration is of large engineering interest. In order to achieve a homogeneous and inhomogeneous reference flow
situation two different types of surface geometries are considered: (1) three sinusoidal bottom wall profiles with different
amplitude-to-wavelength ratios of α = 2a/Λ = 0.2 (Λ = 30 mm), α = 0.2 (Λ = 15 mm), and α = 0.1 (Λ = 30 mm); and (2) a profile consisting of two superimposed sinusoidal
waves with α = 0.1 (Λ = 30 mm). Measurements are carried out in a wide water channel facility (aspect ratio 12:1). Digital
particle image velocimetry (PIV) is performed to examine the spatial variation of the streamwise, spanwise and wall-normal
velocity components in three measurement planes. Measurements are performed at a Reynolds number of 11,200, defined with the
half channel height h and the bulk velocity U
B. We apply the method of snapshots and perform a proper orthogonal decomposition (POD) of the streamwise, spanwise, and wall-normal
velocity components to extract the most dominant flow structures. The structure of the most dominant eigenmode is related
to counter-rotating, streamwise-oriented vortices. A qualitative comparison of the eigenfunctions for different sinusoidal
wall profiles shows similar structures and comparable characteristic spanwise scales Λ
z
= 1.5 H in the spanwise direction for each mode. The scale is observed to be slightly smaller for α = 0.2 (Λ = 15 mm) and slightly
larger for α = 0.2 (Λ = 30 mm). This scaling for the flow over the basic wave geometries indicates that the size of the largest
structures is neither directly linked to the solid wave amplitude, nor to the wavelength. The characteristic spanwise scale
of the dominant eigenmode for the developed flow over the surface consisting of two superimposed waves reduces to 0.85 H. However, a scale in the order of 1.3 H is identified for the second mode. The eigenvalue spectra for the superimposed waves is much broader, more modes contribute
to the energy-containing range. The turbulent flow with increased complexity of the bottom surface is characterized by an
increased number of dominant large-scale structures with different spanwise scales. 相似文献
14.
Experiments of flow boiling heat transfer and two-phase flow frictional pressure drop in a spirally internally ribbed tube
(φ22×5.5 mm) and a smooth tube (φ19×2 mm) were conducted, respectively, under the condition of 6×105 Pa (absolute atmosphere pressure). The available heated length of the test sections was 2500 mm. The mass fluxes were selected,
respectively, at 410, 610 and 810 kg/m2 s. The maximum heat flux was controlled according to exit quality, which was no more than 0.3 in each test run. The experimental
results in the spirally internally ribbed tube were compared with that in the smooth tube. It shows that flow boiling heat
transfer coefficients in the spirally internally ribbed tube are 1.4–2 times that in the smooth tube, and the flow boiling
heat transfer under the condition of smaller temperature differences can be achieved in the spirally internally ribbed tube.
Also, the two-phase flow frictional pressure drop in the spirally internally ribbed tube increases a factor of 1.6–2 as compared
with that in the smooth tube. The effects of mass flux and pressure on the flow boiling heat transfer were presented. The
effect of diameters on flow boiling heat transfer in smooth tubes was analyzed. Based on the fits of the experimental data,
correlations of flow boiling heat transfer coefficient and two-phase flow frictional factor were proposed, respectively. The
mechanisms of enhanced flow boiling heat transfer in the spirally internally ribbed tube were analyzed.
Received on 1 December 1999 相似文献
15.
Based on the mass transfer theory, a new mass transfer model of ion-exchange process on zeolite under liquid film diffusion
control is established, and the kinetic curves and the mass transfer coefficients of –K+ ion-exchange under different conditions were systemically determined using the shallow-bed experimental method. The results
showed that the –K+ ion-exchange rates and transfer coefficients are directly proportional to solution flow rate and temperature, and inversely
proportional to solution viscosity and the size of zeolite granules. It also showed that the transfer coefficient is not influenced
by the ion concentrations. For a large ranges of operational conditions including temperatures (10 − 75°C), flow rates (0.031 m s−1 −0.26 m s−1), liquid viscosities (1.002 × 10−3 N s m−2 − 4.44 × 10−3 N s m−2), and zeolite granular sizes (0.2 − 1.45 mm), the average mass transfer coefficients calculated by the model agree with the
experimental results very well. 相似文献
16.
In order to evaluate characteristics of the liquid film flow and their influences on heat and mass transfer, measurements
of the instantaneous film thickness using a capacitance method and observation of film breakdown are performed. Experimental
results are reported in the paper. Experiments are carried out at Re = 250–10000, T
in = 20–50°C and three axial positions of vertically falling liquid films for film thickness measurements. Instantaneous surface
waveshapes are given by the interpretation of the test data using the cubic spline method. The correlation of the mean film
thickness versus the film Reynolds number is also given by fitting the test data. It is revealed that the surface wave has
nonlinear behavior. Observation of film breakdown is performed at Re = 1.40 × 103–1.75 × 104 and T
in = 85–95°C. From experimental results, the correlation of the film breakdown criterion can be obtained as follows: Bd = 1.567 × 10−6
Re
1.183 相似文献
17.
18.
Natural convective heat transfer from an isothermal narrow flat plate embedded in a plane adiabatic surface and inclined at
moderate positive and negative angles to the vertical has been numerically and experimentally studied. The solution has the
Rayleigh number, the dimensionless plate width, the angle of inclination, and the Prandtl number as parameters. Attention
was restricted to a Prandtl number of 0.7. The numerical results have been obtained for Rayleigh numbers between 103 and 107 for dimensionless plate widths of between 0.3 and 1.2 and for angles of inclination between +45° and −45°. In the experimental
study, results have been obtained for Rayleigh numbers between 4 × 102 and 105 for dimensionless plate widths of 0.4 and 2.5 and for angles of inclination between +45° and −45° to the vertical. Empirical
equations for the heat transfer rate have been derived. 相似文献
19.
Heat transfer coefficients were measured on a horizontal platinum wire and converted to data on horizontal copper tubes. The
measurements spanned a large region of pressures p* = p/pcrit = 0.05–0.50 and heat fluxes of q = 103–1.5 × 105 W/m2. The preparation of the test equipment is described. The effects of pressure and concentration on the heat transfer coefficients
are shown. The mixture behaves very much like an azeotropic mixture; concentration has only a small effect, the heat transfer
coefficients can be obtained from the heat transfer coefficients of the pure components according to their molar fractions.
The conversion steps from wire- to tube-data are presented. A comparison of wire-data with correlations given in literature
is shown. It renders good agreement. 相似文献
20.
The effect of a vertical adiabatic wall on the natural convection heat transfer from vertical array of attached cylinders, which can be considered as wavy surface, was investigated experimentally and numerically. The experiments were carried out using Mach-Zehnder interferometer and the commercial FLUENT code was used for numerical study. This paper focuses on the effect of wall-wavy surface spacing and Rayleigh number variation on the local and average free convection heat transfer coefficients from the each cylinder and the wavy surface. Rayleigh number ranges from 2400 to 10,000 and from 300,000 to 1,250,000 based on cylinder diameter and wavy surface height respectively. The local and average Nusselt numbers were determined for the different Rayleigh numbers, and the ratio of wall- wavy surface spacing to cylinder diameter 0.75, 1, 1.5, 2, 3, 4, 5, and ∞. Results are indicated with a single correlation which gives the average Nusselt number as a function of the ratio of the wall-wavy surface spacing to cylinder diameter and the Rayleigh numbers. There is an optimum distance between the wall and wavy surface in which the Nusselt number attain its maximum value. This optimum distance depends on the Rayleigh number. 相似文献