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1.
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
2.
The flow and heat transfer characteristics of an unconfined air jet that is impinged normally onto a heated flat plate have
been experimentally investigated for high Reynolds numbers ranging from 30,000 to 70,000 and a nozzle-to-plate spacing range
of 1–10. The mean and turbulence velocities by using hot-wire anemometry and impingement surface pressures with pressure transducer
are measured. Surface temperature measurements are made by means of an infrared thermal imaging technique. The effects of
Reynolds number and nozzle-to-plate spacing on the flow structure and heat transfer characteristics are described and compared
with similar experiments. It was seen that the locations of the second peaks in Nusselt number distributions slightly vary
with Reynolds number and nozzle-to-plate spacing. The peaks in distributions of Nusselt numbers and radial turbulence intensity
are compatible for spacings up to 3. The stagnation Nusselt number was correlated for the jet Reynolds number and the nozzle-to-plate
spacing as Nu
st∝Re
0.69(H/D)0.019. 相似文献
3.
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
4.
Comparative evaluation of three heat transfer enhancement strategies in a grooved channel 总被引:1,自引:0,他引:1
Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel
plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported.
Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic
grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block.
In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed
cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations
of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified
by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining
insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities
in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding
to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic
grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At
Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated
grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat
transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5,
respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties
also increased significantly.
Received on 5 April 2001 相似文献
5.
A detailed numerical study is carried out to investigate fluid flow and heat transfer characteristics in a channel with heated
V corrugated upper and lower plates. The parameters studied include the Reynolds number (Re = 2,000–5,500), angles of V corrugated plates (θ = 20°, 40°, 60°), and constant heat fluxs (q″ = 580, 830, 1,090 W/m2). Numerical results have been validated using the experimented data reported by Naphon, and a good agreement has been found.
The angles of V corrugated plates (θ) and the Reynolds number are demonstrated to significantly affect the fluid flow and the heat transfer rate. Increasing the
angles of V corrugated plates can make the heat transfer performance become better. The increasing Reynolds number leads to
a more complex fluid flow and heat transfer rate. The numerical calculations with a non-equilibrium wall function have a better
accuracy than with a standard wall function for solving high Reynolds numbers or complex flow problems. 相似文献
6.
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
7.
Local heat transfer coefficients and temperature distributions within the fluid for air flow around a 180° square-sectioned bend have been measured. The ratio of bend radius to hydraulic diameter of the duct is 3.35:1 and the flow entering the bend is sensibly fully developed. Measurements of air and wall temperatures span a range of Reynolds numbers from 9.9 × 103 to 9.2 × 104 with the principal emphasis given to the case of Re ? 5.6 × 104. This Reynolds number and geometric configuration coincide with that of a companion LDA study carried out by Chang et al1 which provides detailed maps of the mean and turbulent velocity fields. The data show that by 45° into the bend the heat transfer coefficients on the inner convex wall of the bend drop markedly while those on the other walls increase. By 90° the ratio of the heat transfer coefficients at the mid positions of the concave and convex walls is more than 2:1. Nevertheless this ratio is less than would be anticipated from considering two-dimensional flow on weakly curved surfaces. There is a general consistency between the velocity and the temperatyre field data in the heated fluid 相似文献
8.
A general heat transfer correlation for non-boiling gas–liquid flow with different flow patterns in horizontal pipes is proposed. In order to overcome the effect of flow pattern on heat transfer, a flow pattern factor (effective wetted-perimeter) is developed and introduced into our proposed correlation. To verify the correlation, local heat transfer coefficients and flow parameters were measured for air–water flow in a pipe in the horizontal position with different flow patterns. The test section was a 27.9 mm ID stainless steel pipe with a length to diameter ratio of 100. A total of 114 data points were taken by carefully coordinating the liquid and gas superficial Reynolds number combinations. The heat transfer data were measured under a uniform wall heat flux boundary condition ranging from about 3000 W/m2 to 10,600 W/m2. The superficial Reynolds numbers ranged from about 820 to 26,000 for water and from about 560 to 48,000 for air. These experimental data including different flow patterns were successfully correlated by the proposed general two-phase heat transfer correlation with an overall mean deviation of 5.5%, a standard deviation of 11.7%, and a deviation range of −18.3% to 37.0%. Ninety three percent (93%) of the data were predicted within ±20% deviation. 相似文献
9.
The velocity distribution in laminar upward flow of water (Pr 7.25) in the entry of a vertical internally heated annulus (radius ratio 4:1) has been determined by visual observation. Photographic measurements have been made of the motion of hydrogen bubble clusters, which were generated by a carefully controlled process of electrolysis, to assess the effects of free convection effects on the forced flow.For heat fluxes up to 2500 W/m2 and at a Reynolds number of approximately 450, local heat transfer coefficients have been obtained in a length of about 23 equivalent diameters. Heat transfer rate in the immediate entry was found to be insensitive to change in heat flux over the range of variables considered. As the distance downstream increased, the heat transfer rate was found to be dependent on the heat flux. 相似文献
10.
H.W. Byun N.H. Kim 《Experimental Thermal and Fluid Science》2011,35(6):920-932
Refrigerant R-410a flow distribution is experimentally studied in a test section simulating a parallel flow heat exchanger having vertical headers with two pass configuration. Tubes are heated to yield a test section outlet superheat of 5 °C with inlet quality of 0.3. Mass flux is varied from 50 kg/m2 s to 70 kg/m2 s. Effects of inlet and outlet locations are investigated in a search for an optimum configuration. Results show that, significant liquid flows through bottom channels, and less liquid is supplied to top channels. As for the inlet location, better flow distribution (pressure drop as well) is obtained for top inlet as compared with middle inlet. As for the outlet location, top or bottom outlet is better than middle outlet. Correlations are developed for the fraction of liquid or gas taken off by downstream channel as a function of header gas Reynolds number at immediate upstream. The correlations may be used to predict the liquid or gas distribution in a parallel flow heat exchanger having vertical headers. A novel thermal performance evaluation method, which accounts for tube-side flow mal-distribution is proposed. 相似文献
11.
Experimental study of convective heat transfer from a rotating finned tube in transverse air flow 总被引:1,自引:0,他引:1
The convective heat transfer from fins to air has been evaluated using rotating annular fins subjected to an air flow parallel
to the fins. The fin cooling is studied using infrared thermography. The thermal balance in a fin during its cooling process
allows us to obtain the heat transfer coefficient from the temperature time evolution of the fin. Moreover, Particle Image
Velocimetry allows us to obtain the flow field in the mid-plane between two fins. The influence of the fin spacing on the
convective heat transfer is studied for various velocities of the superposed air flow and various fin rotational speeds. These
tests were carried out for air flow Reynolds numbers (based on the shaft diameter and the velocity of the superposed air flow)
between 2550 and 18200 and rotational Reynolds numbers (based on the shaft diameter and the peripheral speed) between 800
and 2.9 × 104, for different fin spacings.
Received: 14 May 1999/Accepted: 8 October 1999 相似文献
12.
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
13.
Experimental investigation on turbulent flow in a circular-sectioned 90-degree bend 总被引:11,自引:0,他引:11
The steady, turbulent flow in a circular-sectioned 90° bend with smooth walls has been investigated experimentally. The bend
had a curvature radius ratio of 4.0 with long, straight upstream and downstream pipes. The longitudinal, circumferential and
radial components of mean and fluctuating velocities, and the Reynolds stresses in the pipe cross section at several longitudinal
stations were obtained with the technique of rotating a probe with an inclined hot wire at a Reynolds number of 6×104. The velocity fields of the primary and secondary flows, and the Reynolds stress distributions in the cross section were
illustrated. Moreover, other characteristics of the bend flow, such as deviation of the primary flow and intensity of the
secondary flow, were presented. Simultaneously, discussions were given on the transition of phenomena in the longitudinal
direction and the structures of turbulence in the 90° bend.
Received: 21 April 1997/Accepted: 14 November 1997 相似文献
14.
Toshio Aihara Wu-Shung Fu Mitsuo Hongoh Toshiyuki Shimoyama 《Experimental Thermal and Fluid Science》1990,3(6):623-631
An experimental study was made on convective heat and mass transfer from a horizontal heated cylinder in a downward flow of air-water mist at a blockage ratio of 0.4. The measured local heat transfer coefficients agree fairly well with the authors' numerical solutions obtained previously for the front surface of a cylinder over the ranges mass flow ratio 0–4.5×10−2, a temperature difference between the cylinder and air 10–43 K, gas Reynolds number (7.9–23)×103, Rosin-Rammler size parameter 105–168 μm, and dispersion parameter 3.4–3.7. Heat transfer augmentation, two-pahse to single-phase of greater than 19 was attained at the forward stagnation point. For heat transfer in the rear part of the cylinder, an empirical formula is derived by taking into account the dimensionless governing variables, that is, coolant-feed and evaporation parameters. 相似文献
15.
An experimental study was conducted of incompressible, moderate Reynolds number flow of air over heated rectangular blocks
in a two-dimensional, horizontal channel. Holographic interferometry combined with high-speed cinematography was used to visualize
the unsteady temperature fields in self- sustained oscillatory flow. Experiments were conducted in the laminar, transitional
and turbulent flow regimes for Reynolds numbers in the range from Re = 520 to Re = 6600. Interferometric measurements were
obtained in the thermally and fluiddynamically periodically fully developed flow region on the ninth heated block. Flow oscillations
were first observed between Re = 1054 and Re = 1318. The period of oscillations, wavelength and propagation speed of the Tollmien–Schlichting
waves in the main channel were measured at two characteristic flow velocities, Re = 1580 and Re = 2370. For these Reynolds
numbers it was observed that two to three waves span one geometric periodicity length. At Re = 1580 the dominant oscillation
frequency was found to be around 26 Hz and at Re = 2370 the frequency distribution formed a band around 125 Hz. Results regarding
heat transfer and pressure drop are presented as a function of the Reynolds number, in terms of the block-average Nusselt
number and the local Nusselt number as well as the friction factor. Measurements of the local Nusselt number together with
visual observations indicate that the lateral mixing caused by flow instabilities is most pronounced along the upstream vertical
wall of the heated block in the groove region, and it is accompanied by high heat transfer coefficients. At Reynolds numbers
beyond the onset of oscillations the heat transfer in the grooved channel exceeds the performance of the reference geometry,
the asymmetrically heated parallel plate channel.
Received on 26 April 2000 相似文献
16.
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
17.
Experimental study on the pressure drop and heat transfer characteristics of tubes with internal wave-like longitudinal fins 总被引:11,自引:0,他引:11
Experiments were performed to determine the heat transfer and pressure drop characteristics in the entrance and fully developed
regions of tubes with internal wave-like longitudinal fins. The test tube has a double-pipe structure, with the inner tube
as an insertion. The wave-like fins are in the annulus and span its full width. Experiments were conducted for two cases:
one with the inner tube blocked (no air flowing through it) and the other with the inner tube unblocked. The outer tube was
electrically heated. Local and average heat transfer coefficients and friction factors were measured. The friction factor
and Nusselt number correlations in the fully developed region were obtained in the Reynolds number range of 9×102 to 3.5×103. It has been found that the wave-like fins enhance heat transfer significantly with the blocked case being superior. In addition,
the in-tube heat transfer process is characterized by an earlier transition from laminar to turbulent flow and Reynolds number-dependent
thermal entrance length.
Received on 12 May 1998 相似文献
18.
An experimental study had been carried out to investigate the buoyancy-opposed mixed convection from an upward flow of hot
air to a vertical pipe with a cooled surface. The investigation covered a wide range of flow regime, ranging from the “free
convection significant” to the “forced convection significant” conditions. Reynolds number of the flow extended from 966 to
14780, whereas the Buoyancy parameter, Ω [=Grd/(Red)2], varied from 0.008 to 2.77.
A steady stream of hot air at a moderate pressure and a Prandtl number of 0.707 was arranged to flow upward through a vertical
steel pipe, whose external wall was cooled uniformly by ambient air at 20°C. Test section of the vertical pipe was 1625 mm
long with an internal diameter of 156 mm and an external diameter of 166 mm. Air temperature at inlet of the test section
was varied from 40°C to 70°C. Both radial temperature and velocity profiles of the airflow were measured at inlet and exit
of the test section respectively. Temperatures along the pipe wall were also measured. Non-dimensional expression for the
prediction of the average heat transfer coefficient of the mixed convection from an upward flow of hot air to a vertical pipe
with a cooled surface was developed from the experimental results. Convection heat transfer was found to impair when the flow
is laminar and was enhanced for turbulent flow condition.
Received on 20 July 1998 相似文献
19.
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
20.
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