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1.
Tzu-Hsiang Ko 《Heat and Mass Transfer》2007,44(2):261-271
This paper investigates the effects of corrugation angle (β) on the developing laminar forced convection and entropy generation in a wavy channel with numerical methods. The studied
cases cover β = 10-, 15-, 20-, 25-, 30- and 35°, whilst Reynolds number (Re) is varied as 100, 200 and 400. The analyzed flow characteristics include recirculating flows, secondary vortices, temperature
distributions, and friction factor as well as Nusselt number. In particular, the effects of corrugation angle on the distributions
and magnitudes of local entropy generation resulted from frictional irreversibility (S
P
′′′) and heat transfer irreversibility (S
T
′′′) are separately discussed in detail in the present paper. Based on the minimal entropy generation principle, the optimal
corrugation angle and favorable Re are reported. 相似文献
2.
A numerical investigation is carried out to study fluid flow and heat transfer characteristics of conjugate mixed convection from a two dimensional horizontal channel with four protruding heat sources mounted on one of the finite thick channel walls. The flow is assumed as laminar, hydrodynamically and thermally developing. Water and FC70 are the fluids under consideration. The geometric parameters such as spacing between the channel walls (S), size of protruding heat sources (Lh×th), thickness of substrate (t) and spacing between heat sources (b) are fixed. Results are presented to show the effect of parameters such as ReS, GrS*, Pr, kp/kf and ks/kf on fluid flow and heat transfer characteristics. Using the method of asymptotic expansions, correlations are also presented for the maximum temperature of heat source. 相似文献
3.
Nabil S. Berbish 《Heat and Mass Transfer》2011,47(3):287-300
Experimental and numerical studies were carried out to investigate forced convection heat transfer and flow features around
the downstream elliptic cylinder in four staggered cylinders in cross flow. The elliptic cylinders examined had an axis ratio
(b/c) of 1:2, and they were arranged with zero angle of attack to the upstream flow. The present heat transfer measurements
were obtained by heating only the downstream elliptic cylinder (test cylinder) under the condition of constant heat flux.
The testing fluid was air and the Reynolds number based on the major axis length (c) was ranged from 4,000 to 45,570. The
tested longitudinal spacing ratio (Sx/c) and the transversal spacing ratio (Sy/b) were in the ranges of 1.5 ≤ Sx/c ≤ 4.0 and 1.5 ≤ Sy/b ≤ 4.0, respectively. The air flow pattern and temperature fields around the four staggered elliptic cylinders were predicted
by using CFD software package. Also, a flow visualization study was made to show the flow features around the elliptic cylinders.
It was observed that Num of the downstream elliptic cylinder in four staggered cylinders was higher than that of three in-line cylinders for all tested
spacing ratios and Reynolds numbers except for Re = 4,000. It was clear that, at lower Reynolds number values (Re < 14,100),
the average Nusselt number of the downstream elliptic cylinder in three staggered arrangement was higher than that of the
downstream cylinder in four staggered arrangement for all tested spacing ratios. On the other hand, at Re > 14,100, the tested
elliptic cylinder in four staggered arrangement had the higher values of the average Nusselt number. Moreover, in four staggered
arrangement, the maximum average Nusselt number enhancement ratio (average Nusselt number of the tested downstream cylinder/average
Nusselt number of a single elliptic cylinder) was found to be about 2.0, and was obtained for spacing ratios of Sx/c = 2.5, Sy/b = 2.5 and at Re = 32,000. Finally, the average Nusselt number of the tested cylinder in four staggered arrangement was
correlated in terms of Reynolds number and cylinder spacing ratios. 相似文献
4.
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
5.
The ‘plug’ flow emerging from a long rotating tube into a large stationary reservoir was used in the experimental investigation
of swirling jets with Reynolds numbers, Re = 600, 1,000 and 2,000, and swirl numbers, S = ΩR/U, in the range 0–1.1, to cover flow regimes from the non-rotating jet to vortex breakdown. Here Ω is the nozzle rotation rate,
R is the radius of the nozzle exit, and U is the mean mass axial velocity. The jet was more turbulent and eddies shed faster at larger Re. However the flow criticality and shear layer morphology remained unchanged with Re. After the introduction of sufficient rotation, co-rotating and counter-winding helical waves replaced vortex rings to become
the dominant vortex structure. The winding direction of the vortex lines suggests that Kelvin–Helmholtz and generalized centrifugal
instability dominated the shear layer. A quantitative visualization study has been carried out for cases where the reservoir
was rotating independently with S
a
= Ω
a
R/U = ±0.35, ±0.51 and ±0.70 at Re = 1,000 and 2000, where Ω
a
is the rotation rate of the reservoir. The criterion for breakdown was found to be mainly dependent on the absolute swirl
number of the jet, S. This critical swirl number was slightly different in stationary and counter-swirl surroundings but obviously smaller when
the reservoir co-rotated, i.e. S
c
= 0.88, 0.85 and 0.70, respectively. These results suggest that the flow criticality depends mainly on the velocity distributions
of the vortex core, while instabilities resulting from the swirl difference between the jet and its ambient seem to have only
a secondary effect. 相似文献
6.
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
7.
Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric,
turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface,
is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric
fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D
c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re
d
≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R
th increased with the confining wall diameter D
c and also with the nozzle diameter d. A minimum value of the thermal resistance of R
th,min=0.4 cm2 K/W was attained for a combination of d=4.42 mm, D
c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of
the thermal resistance and the R
th-predictions. The total fin effectiveness ɛf was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More
than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the
heated round disk.
Received on 30 August 2000 / Published online: 29 November 2001 相似文献
8.
In this study, convective heat transfer and pressure drop in a cross-flow heat exchanger with hexagonal, square and circular
(HSC) pin–fin arrays were studied experimentally. The pin–fins were arranged in an in-line manner. For the applied conditions,
the optimal spacing of the pin–fin in the span-wise and stream-wise directions has been determined. The variable parameters
are the relative longitudinal pitch (S
L
/D = 2, 2.8, 3.5), and the relative transverse pitch was kept constant at S
T
/D = 2. The performances of all pin–fins were compared with each other. The experimental results showed that the use of hexagonal
pin–fins, compared to the square and circular pin–fins, can lead to an advantage in terms of heat transfer enhancement. The
optimal inter-fin pitches are provided based on the largest Nusselt number under the same pumping power, while the optimal
inter-fin pitches of hexagonal pin–fins are S
T
/D = 2 and S
L
/D = 2.8. Empirical equations are derived to correlate the mean Nusselt number and friction coefficient as a function of the
Reynolds number, pin–fin frontal surface area, total surface area, and total number. Consequently, the general empirical formula
is given in the present form.
NuD = a(ReD )b ( \fracNt Af A\texttotal )c \textand f = a(ReD )b Nu_{D} = a(Re_{D} )^{b} \left( {{\frac{{N_{t} A_{f} }}{{A_{\text{total}} }}}} \right)^{c} \quad {\text{and}}\quad f = a(Re_{D} )^{b} 相似文献
9.
A computational study is performed on three-dimensional turbulent flow and heat transfer in a rotating rectangular channel
with aspect ratio (AR) of 10:1, oriented 120° from the direction of rotation. The Focus is on high rotation and high-density
ratios effects on the heat transfer characteristics of the 120° orientation. The Reynolds stress model (RSM), which accounts
for rotational effects are used to compute the turbulent flow and heat transfer in the rotating channel. The effects of rotation
and coolant-to-wall density ratio on the fluid flow and heat transfer characteristics is reported on a range of rotation numbers
and density ratios (0 < Ro < 0.25 and 0.07 < Δρ/ρ < 0.4). The computational results are in good agreement with experimental data within ±15%. The results show that the density
ratio, rotation number and channel orientation significantly affect the flow field and heat transfer characteristics in the
rotating rectangular channel. Flow reversal occurs at high rotation number and density ratio. 相似文献
10.
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
11.
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
12.
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
13.
Large-eddy simulations (LES) of a planar, asymmetric diffuser flow have been performed. The diverging angle of the inclined
wall of the diffuser is chosen as 8.5°, a case for which recent experimental data are available. Reasonable agreement between
the LES and the experiments is obtained. The numerical method is further validated for diffuser flow with the diffuser wall
inclined at a diverging angle of 10°, which has served as a test case for a number of experimental as well as numerical studies
in the literature (LES, RANS). For the present results, the subgrid-scale stresses have been closed using the dynamic Smagorinsky
model. A resolution study has been performed, highlighting the disparity of the relevant temporal and spatial scales and thus
the sensitivity of the simulation results to the specific numerical grids used. The effect of different Reynolds numbers of
the inflowing, fully turbulent channel flow has been studied, in particular, Re
b
= 4,500, Re
b
= 9,000 and Re
b
= 20,000 with Re
b
being the Reynolds number based on the bulk velocity and channel half width. The results consistently show that by increasing
the Reynolds number a clear trend towards a larger separated region is evident; at least for the studied, comparably low Reynolds-number
regime. It is further shown that the small separated region occurring at the diffuser throat shows the opposite behaviour
as the main separation region, i.e. the flow is separating less with higher Re
b
. Moreover, the influence of the Reynolds number on the internal layer occurring at the non-inclined wall described in a recent
study has also been assessed. It can be concluded that this region close to the upper, straight wall, is more distinct for
larger Re
b
. Additionally, the influence of temporal correlations arising from the commonly used periodic turbulent channel flow as inflow
condition (similar to a precursor simulation) for the diffuser is assessed. 相似文献
14.
This work experimentally studies the flow characteristics and forced convective heat transfer in a sintered porous channel
that filled with sintered copper beads of three average diameters (
0.830, and 1.163 mm). The pressure drop and the local temperature measurements can be applied to figure out the distributions
of the friction coefficient and the heat transfer coefficient. Three sintered porous channels differ in the arrangement of
obstacle blocks. Model A has no obstacle. Models B and C have five obstacle blocks facing down and up, respectively, in a
sintered porous channel. The range of experimental parameters, porosity, heat flux, and effect of forced convection are 0.370
≤ ɛ ≤ 0.385, q=0.228, 0.872, 1.862 W/cm2, and 200 ≤ Re
d ≤ 800. The permeability and inertia coefficient of each of the three sintered porous channels are analyzed. The results for
Model A agree with those obtained by previous investigations in C
f distribution. The heat transfer of Model C exceeds that of Model A by approximately 20%. Finally, a series of empirical correlation
equations were obtained for practical applications and engineering problems. 相似文献
15.
D. Estruch D. G. MacManus J. L. Stollery N. J. Lawson K. P. Garry 《Experiments in fluids》2010,49(3):683-699
The understanding of the behaviour of the flow around surface protuberances in hypersonic vehicles is developed and an engineering
approach to predict the location and magnitude of the highest heat transfer rates in their vicinity is presented. To this
end, an experimental investigation was performed in a hypersonic facility at freestream Mach numbers of 8.2 and 12.3 and Reynolds
numbers ranging from Re
∞/m = 3.35 × 106 to Re
∞/m = 9.35 × 106. The effects of protuberance geometry, boundary layer state, freestream Reynolds number and freestream Mach numbers were
assessed based on thin-film heat transfer measurements. Further understanding of the flowfield was obtained through oil-dot
visualizations and high-speed schlieren videos. The local interference interaction was shown to be strongly 3-D and to be
dominated by the incipient separation angle induced by the protuberance. In interactions in which the incoming boundary layer
remains unseparated upstream of the protuberance, the highest heating occurs adjacent to the device. In interactions in which
the incoming boundary layer is fully separated ahead of the protuberance, the highest heating generally occurs on the surface
just upstream of it except for low-deflection protuberances under low Reynolds freestream flow conditions in which case the
heat flux to the side is greater. 相似文献
16.
The ultra-low Reynolds number airfoil wake 总被引:1,自引:0,他引:1
Lift force and the near wake of an NACA 0012 airfoil were measured over the angle (α) of attack of 0°–90° and the chord Reynolds
number (Re
c
), 5.3 × 103–5.1 × 104, with a view to understand thoroughly the near wake of the airfoil at low- to ultra-low Re
c
. While the lift force is measured using a load cell, the detailed flow structure is captured using laser-Doppler anemometry,
particle image velocimetry, and laser-induced fluorescence flow visualization. It has been found that the stall of an airfoil,
characterized by a drop in the lift force, occurs at Re
c
≥ 1.05 × 104 but is absent at Re
c
= 5.3 × 103. The observation is connected to the presence of the separation bubble at high Re
c
but absence of the bubble at ultra-low Re
c
, as evidenced in our wake measurements. The near-wake characteristics are examined and discussed in detail, including the
vortex formation length, wake width, spanwise vorticity, wake bubble size, wavelength of K–H vortices, Strouhal numbers, and
their dependence on α and Re
c
. 相似文献
17.
Numerical modelling of confined flow past a cylinder of square cross-section at various orientations
Results are presented for the unsteady, two-dimensional flow and heat transfer due to a square obstruction of diameter d located asymmetrically between the parallel sliding walls of a channel with length-to-height ratio W/H = 6·44. Analysis is based on the numerical solution of spatially and temporally second-order accurate finite difference approximations of the transport equations expressed in curvilinear co-ordinates. Laminar, constant property flow is assumed for obstruction configurations in which the blockage ratio is d/H = 0·192, the nearest-wall distances are g/d = 0·2, 0·5 and 1, the orientation angles are α=0°, 10° and 20° and the Reynolds numbers are Re=100, 500, and 1000. Preparatory testing of the numerical procedure was performed for a variety of documented flows to verify its physiconumerical accuracy and obtain estimates of the residual grid-dependent uncertainties in the variables calculated. Heat transfer, drag and lift coefficients and Strouhal numbers for the present flow were finally calculated to within 4%–7% of their grid-dependent values using non-uniformly spaced grids consisting of (x=99, y=55) nodes. Above a critical value of the Reynolds number, which depends on the geometrical parameters, the flow is characterized by alternate vortex shedding from the obstruction top and bottom surfaces. Streamline, vorticity and particle streakline plots provide qualitative impressions of the unsteady vortical flow. Especially noteworthy are the extremes in the lift coefficient which ranges from large positive values for an obstruction with g/d=0·2 and α=10° to negative values for one with g/d=0·5 and α=0°. Both the drag and lift coefficients as well as the Strouhal number exhibit non-monotonic variations with respect to the parameters explored. Asymmetries in the obstruction location and orientation account for relatively large vortex-induced periodic variations in heat transfer, especially along the wall nearest the obstruction. Notable differences are also predicted for the heat transfer coefficients of the individual obstruction surfaces as a function of the orientation angle. 相似文献
18.
Vortex mechanism of heat transfer enhancement in a narrow channel with dimples has been investigated numerically using LES
and URANS methods. The flow separation results in a formation of vortex structures which significantly enhance heat transfer
on dimpled surfaces leading to a small increase in pressure loss. The heat transfer can be significantly increased by rounding
the dimple edge and use of oval dimples. To get a deep insight into flow physics LES is performed for single phase flow in
a channel with a spherical dimple. The instantaneous vortex formation and separation are investigated in and around the dimple
area. Considered are Reynolds numbers (based on dimple print diameter) ReD = 20,000 and ReD = 40,000 the depth to print diameter ratio of Δ = 0.26. Frequency analysis of LES data revealed the presence of dominating
frequencies in unsteady flow oscillations. Direct analysis of the flow field revealed the presence of coherent vortex structure
inclined to the mean flow. The structure changes its orientation in time causing the long period oscillations with opposite-of-phase
motion. Three dimensional proper orthogonal decomposition (POD) analysis is carried out on LES pressure and velocity fields
to identify spatio-temporal structures hidden in the random fluctuations. Tornado-like spatial POD structures have been determined
inside dimples. 相似文献
19.
Stereoscopic PIV measurements of a turbulent boundary layer with a large spatial dynamic range 总被引:1,自引:1,他引:0
The flow in a streamwise/wall-normal plane of a turbulent boundary layer at moderate Reynolds number (Re
θ = 2,200) is characterized using two stereo PIV systems just overlapping in the streamwise direction. The aim is to generate
SPIV data for near-wall turbulence with enough spatial dynamic range to resolve most of the coherent structures present in
the flow and to facilitate future comparisons with direct numerical simulations. This is made possibly through the use of
four cameras with large CCD arrays (4,008 px × 2,672 px) and through a rigorous experimental procedure designed to minimize
the impact of measurement noise on the resolution of the small scales. For the first time, both a large field of view [S
x
; S
y
] = [2.6δ; 0.75δ] and a high spatial resolution (with an interrogation window size of 13.6+) have been achieved. The quality of the data is assessed through an analysis of some of the statistical results such as the
mean velocity profile, the rms and the PDF of the fluctuations, and the power spectra. 相似文献
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