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1.
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
2.
Jatuporn Kaew-On Kittipong SakamatapanSomchai Wongwises 《Experimental Thermal and Fluid Science》2011,35(2):364-374
The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1 mm for 14 numbers of channels and 1.2 mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979 mm2 and 6171 m2, while the inner surface areas are 13,545 mm2 and 8856 mm2 for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at the heat fluxes between 15 and 65 kW/m2, mass flux of refrigerant between 300 and 800 kg/m2 s and saturation pressure ranging from 4 to 6 bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed. 相似文献
3.
A finite-difference scheme has been developed to solve the equations governing the laminar forced convection heat transfer
around and inside a spherical fluid droplet moving steadily in another immiscible fluid for both steady and transient thermal
conditions. For large values of the external flow Reynolds number (Re), results not available in the literature have been
obtained for circulating droplets at intermediate and high interior-to-exterior viscosity ratios (μ*). Detailed results over a wide range of viscosity ratio (μ*) and for 200≤Re≤1000 are presented for the temperature profiles outside and inside the sphere, Nusselt number, the time required
to attain a uniform surface temperature and the time required to reach the steady-state temperature. Results show that convective
heating is dependent on the external flow Reynolds number (Re) and the interior-to-exterior viscosity ratio (μ*) where increasing Re or decreasing μ* result in increasing heat transfer rate convected to the liquid sphere.
Received on 1 March 1999 相似文献
4.
The development of steady, turbulent flow in a 90° section of a curved square duct was studied at a Reynolds number of 4 × 104 by hot-wire anemometer. The curved duct has a cross-section measuring 80 × 80 mm and a curvature radius ratio of 4 and is
connected with a long, straight duct at its both ends. The longitudinal and lateral components of mean and fluctuating velocities,
and the Reynolds stresses were measured by the method of rotating a probe with an inclined hot-wire. The velocity fields of
the primary and secondary flows, and the Reynolds stress distributions in the cross-section were illustrated in the form of
contour map. The development of the primary flow was found to be connected with a strong pressure gradient near the outer
and inner wall and a secondary flow induced in the cross-section of the bend by a pressure difference between the outer and
inner wall and a centrifugal force acting on the fluid; the fluid is accelerated near the inner wall and decelerated near
the outer wall between the bend angle ϕ ≅ 0° and ϕ ≅ 30°, but an increase and decrease of the fluid velocity are reversed between ϕ ≅ 30° and ϕ ≅ 90°. The fluctuating velocity correlations, i.e. the Reynolds stresses follow a complicated progress according to the complex
development of the primary flow. The results obtained can be available to verify various types of turbulence models and to
develop new models.
Received: 10 May 1999/Accepted: 15 March 2000 相似文献
5.
Experimental investigation of mixed convection heat transfer in a horizontal and inclined rectangular channel 总被引:2,自引:0,他引:2
Mixed convection heat transfer in rectangular channels has been investigated experimentally under various operating conditions.
The lower surface of the channel is subjected to a uniform heat flux, sidewalls are insulated and adiabatic, and the upper
surface is exposed to the surrounding fluid. Experiments were conducted for Pr=0.7, aspect ratios AR=5 and 10, inclination
angles 0° ≤ θ ≤ 30°, Reynolds numbers 50 ≤ Re ≤ 1000, and modified Grashof numbers Gr*=7.0 × 105 to 4.0 × 107. From the parametric study, local Nusselt number distributions were obtained and effects of channel inclination, surface
heat flux and Reynolds number on the onset of instability were investigated. Results related to the buoyancy affected secondary
flow and the onset of instability have been discussed. Some of the results obtained from the experimental measurements are
also compared with the literature, and a good agreement was observed. The onset of instability was found to move upstream
for increasing Grashof number and increasing aspect ratio. On the other hand, onset of instability was delayed for increasing
Reynolds number and increasing inclination angle.
Received on 19 March 2001 / Published online: 29 November 2001 相似文献
6.
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
7.
A. Klaczak 《Heat and Mass Transfer》2001,37(4-5):443-448
This article presents the results of laboratory research on heat exchange while heating water in horizontal and vertical
tubes with twisted-tape inserts.
The scope of the research:
70 ≤ Re ≤ 4000
3.6 ≤ Pr ≤ 5.9
8.6 ≤ Gz ≤ 540
The research was held for three cases:
– horizontal experimental tube
– vertical experimental tube, the direction of flow according to the free convection vector
– vertical experimental tube, the direction of flow not in accordance with the free convection vector
For such cases the correlation equation was defined NuT=f(Gz; y), Nu = f(Gz) and the proportion NuT/Nu was analysed.
Received on 30 March 2000 相似文献
8.
Mixing by secondary flow is studied by particle image velocimetry (PIV) in a developing laminar pulsating flow through a circular curved pipe. The pipe curvature ratio is η = r
0/r
c
= 0.09, and the curvature angle is 90°. Different secondary flow patterns are formed during an oscillation period due to
competition among the centrifugal, inertial, and viscous forces. These different secondary-flow structures lead to different
transverse-mixing schemes in the flow. Here, transverse mixing enhancement is investigated by imposing different pulsating
conditions (Dean number, velocity ratio, and frequency parameter); favorable pulsating conditions for mixing are introduced.
To obviate light-refraction effects during PIV measurements, a T-shaped structure is installed downstream of the curved pipe.
Experiments are carried out for the Reynolds numbers range 420 ≤ Rest ≤ 1,000 (Dean numbers 126.6 ≤ Dn ≤ 301.5) corresponding to non-oscillating flow, velocity component ratios 1 ≤ (β = U
max,osc/U
m,st) ≤ 4 (the ratio of velocity amplitude of oscillations to the mean velocity without oscillations), and frequency parameters
8.37 < (α = r
0(ω/ν)0.5) < 24.5, where α2 is the ratio of viscous diffusion time over the pipe radius to the characteristic oscillation time. The variations in cross-sectional
average values of absolute axial vorticity (|ζ|) and transverse strain rate (|ε|) are analyzed in order to quantify mixing.
The effects of each parameter (Rest, β, and α) on transverse mixing are discussed by comparing the dimensionless vorticities (|ζ
P
|/|ζ
S
|) and dimensionless transverse strain rates (|ε
P
|/|ε
S
|) during a complete oscillation period. 相似文献
9.
The effects of solid particles on the flow structure in the near field region of a coaxial water jet are investigated non-intrusively
using molecular tagging velocimetry. Glass beads of 240 μm and specific gravity SG of 2.46 are used at three volume loadings
of γv=0.03, 0.06, and 0.09% in the central water jet with a Reynolds number of 4.1×104. Measurements are acquired for four annular to central jet velocity ratios in the range 0.11≤ U
o/U
i≤1.15 at downstream distances up to six inner jet diameters and the results are analyzed for the effects of solid particles
on the characteristics of flow. It is found that the addition of particles does not affect the mean fluid velocity profile
in this region. The results also indicate a small and moderate enhancement of axial turbulent velocity and radial gradients
of velocity fluctuations, respectively, due to the presence of particles. 相似文献
10.
Natural convection in a fluid saturated porous medium has been numerically investigated using a generalized non-Darcy approach.
The governing equations are solved by using Finite Volume approach. First order upwind scheme is employed for convective formulation
and SIMPLE algorithm for pressure velocity coupling. Numerical results are presented to study the influence of parameters
such as Rayleigh number (106 ≤Ra ≤108), Darcy number (10−5 ≤ Da ≤ 10−2), porosity (0.4 ≤ ɛ ≤ 0.9) and Prandtl number (0.01 ≤ Pr ≤ 10) on the flow behavior and heat transfer. By combining the method of matched asymptotic expansions with computational
fluid dynamics (CFD), so called asymptotic computational fluid dynamics (ACFD) technique has been employed to generate correlation
for average Nusselt number. The technique is found to be an attractive option for generating correlation and also in the analysis
of natural convection in porous medium over a fairly wide range of parameters with fewer simulations for numerical solutions. 相似文献
11.
Three-dimensional turbulent forced convective heat transfer and flow characteristics, and the non-dimensional entropy generation
number in a helical coiled tube subjected to uniform wall temperature are simulated using the k–ε standard turbulence model. A finite volume method is employed to solve the governing equations. The effects of Reynolds number,
curvature ratio, and coil pitch on the average friction factor and Nusselt number are discussed. The results presented in
this paper cover a Reynolds number range of 2 × 104 to 6 × 104, a pitch range of 0.1–0.2 and a curvature ratio range of 0.1–0.3. The results show that the coil pitch, curvature ratio and
Reynolds number have different effects on the average friction factor and Nusselt number at different cross-sections. In addition,
the flow and heat transfer characteristics in a helical coiled tube with a larger curvature ratio for turbulent flow are different
from that of smaller curvature ratio for laminar and turbulent flow in certain ways. Some new features that are not obtained
in previous researches are revealed. Moreover, the effects of Reynolds number, curvature ratio, and coil pitch on the non-dimensional
entropy generation number of turbulent forced convection in a helical coiled tube are also discussed. 相似文献
12.
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
13.
An experimental investigation of flow boiling heat transfer in a commercially available microfin tube with 9.52 mm outer diameter
has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin
height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated.
The experiments have been performed at saturation temperatures between 0 and −20°C. The mass flux was varied between 25 and
150 kg/m2s, the heat flux from 15,000 W/m2 down to 1,000 W/m2. All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. The measured heat transfer
coefficients range from 1,300 to 15,700 W/m2K for R134a and from 912 to 11,451 W/m2K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The mean heat transfer
coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within ±30% and
±15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m2s, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted
to lower mass fluxes for the microfin tube compared to the smooth tube. 相似文献
14.
Mixed convection flow in a two-sided lid-driven cavity filled with heat-generating porous medium is numerically investigated.
The top and bottom walls are moving in opposite directions at different temperatures, while the side vertical walls are considered
adiabatic. The governing equations are solved using the finite-volume method with the SIMPLE algorithm. The numerical procedure
adopted in this study yields a consistent performance over a wide range of parameters that were 10−4 ≤ Da ≤ 10−1 and 0 ≤ Ra
I
≤ 104. The effects of the parameters involved on the heat transfer characteristics are studied in detail. It is found that the
variation of the average Nusselt number is non-linear for increasing values of the Darcy number with uniform or non-uniform
heating condition. 相似文献
15.
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
16.
The free convective flow and heat transfer, within the framework of Boussinesq approximation, in an anisotropic fluid filled
porous rectangular enclosure subjected to end-to-end temperature difference have been investigated using Brinkman extended
non-Darcy flow model. The studies involve simultaneous consideration of hydrodynamic and thermal anisotropy. The flow and
temperature fields in general are governed by, Ra, the Rayleigh number, AR, the aspect ratio of the slab, K*, the permeability ratio and k*, the thermal conductivity ratio, and Da, Darcy number. Numerical solutions employing the successive accelerated replacement (SAR) scheme have been obtained for 100 ≤
Ra ≤ 1000, 0.5 ≤ AR ≤ 5, 0.5 ≤ K* ≤ 5, 0.5 ≤ k* ≤ 5, and 0 ≤ Da ≤ 0.1. It has been found that [`(Nu)]{\overline {Nu}}, average Nusselt number increases with increase in K* and decreases as k* increases. However, the magnitude of the change in [`(Nu)]{\overline {Nu}} depends on the parameter Da, characterizing the Brinkman extended non-Darcy flow. 相似文献
17.
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
18.
In this paper we consider a fully developed turbulent flow in a round pipe with a small inner annulus. The diameter of the
inner annulus is less than 10% of the diameter of the outer pipe. As a consequence, the surface area of the inner pipe compared
to the outer pipe is small. The friction exerted by the wall on the flow is proportional to the surface area and the wall
shear stress. Due to the small surface area of the inner annulus the additional stress on the flow due to the presence of
the annulus may expected to be negligible. However, it will be shown that the inner annulus drastically changes the flow patterns
and gives rise to unexpected scaling properties. In previous studies (Chung et al., Int J Heat Fluid Flow 23:426–440, 2002; Churchill and Chan, AIChE J 41:2513–2521, 1995) it was argued that radial position of the point of zero shear stress does not coincide with the radial location of the point
of maximum axial velocity. In our direct numerical simulations we observe a coincidence of these points within the numerical
accuracy of our model. It is shown that the velocity profile close to the inner annulus is logarithmic. 相似文献
19.
Duangkamon Baowan James M. Hill 《International Journal of Solids and Structures》2007,44(25-26):8297-8312
Carbon nanostructures such as nanotubes and fullerenes, represent future materials because of their remarkable mechanical, electrical and thermal properties. Double-walled carbon nanotubes are widely studied as possible gigahertz oscillators, where the inner tube oscillates within the outer tube. These oscillators are believed to generate frequencies in the gigahertz range and typically of the order of 1–74 GHz. They are also known to generate wave-like formations on the outer surface. In this paper, we study such induced deformations on the surface of the outer tube, as generated by the moving inner tube. Following previous authors we assume that double-walled carbon nanotubes can be modelled as transversely isotropic linearly elastic materials. Using a previously derived approximate force distribution for the resultant van der Waals forces arising from the interatomic interactions, we solve a dynamic linearly elastic problem, and show that the resulting solution exhibits wave-like behaviour. 相似文献
20.
The transient critical flow experiment was carried out in the high-pressure test loop. The break sections were the sharp-edged
tubes with inner diameter of nearly 4 mm but with quite different lengths. The initial pressure was up to 22.0 MPa and the
inlet subcooling covered the range of 0 to 60∘C. New critical flow data were provided and the effects of inlet liquid subcooling and tube lengths were described. The proposed
empirical nonequilibrium correlation was used to calculate the critical mass flow rate for such small diameter tube.
Received on 9 September 1998 相似文献