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1.
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
2.
杨翔翔 《应用数学和力学(英文版)》1988,9(3):267-278
Circular fins are used extensively in heat exchange devices to increase the heat transfer. For economic purposes, the traditional
approach to the optimization of fins consists of minimizing the comsumption (investment) of fin material for the excution
of a specified heat transfer task. The minimum weight cooling fin has optional profile to be a concave parabola. Therefore,
the optimum geometric dimensions of circular fins of parabolic profile with variable thermal parameters are studied. The effect
of the two pertinent physical parameters-thermal conductivity variation parameter α and the index of the heat transfer coefficient
variationm upon the optimum geometric dimensions is also studied. The results pressented can be used as the design guideline for engineering
practice. 相似文献
3.
This work theoretically investigated the thermal performance and stability characteristics of a straight pin fin subject
to boiling considering a temperature-dependent thermal conductivity of fin, k=k
sat(1+b(T−T
sat)). Steady-state temperature distribution and the associated fin base heat flow were for the first time analytically found,
whose stability characteristics were evaluated by linear stability analysis. A positive temperature coefficient b will raise both the fin's temperature and base heat flow. The corresponding stability for stable fin boiling was enhanced.
A negative b results in an opposite trend. The use of a mean thermal conductivity in fin boiling calculations is discussed.
Received on 3 November 1997 相似文献
4.
The problem of the self-similar boundary flow of a “Darcy-Boussinesq fluid” on a vertical plate with temperature distribution
T
w(x) = T
∞+A·x
λ and lateral mass flux v
w(x) = a·x
(λ−1)/2, embedded in a saturated porous medium is revisited. For the parameter values λ = 1,−1/3 and −1/2 exact analytic solutions
are written down and the characteristics of the corresponding boundary layers are discussed as functions of the suction/ injection
parameter in detail. The results are compared with the numerical findings of previous authors.
Received on 8 March 1999 相似文献
5.
Tensile impact experiments of EC8.0−24×7 glass fiber bundles at different low temperaturesT(14°C, −40°C and −10°C) and strain rates ɛ were carried out, and complete stress-strain curves were obtained. Within the range
of the experiment temperatures and strain rates, it is found that the initial modulusE, the ultimate strength σmax and the unstable strain ɛ
b
of the glass fiber bundles all increase with ɛ at an identicalT. At an identical ɛ, with the decrease ofT, E and σmax increase; but ɛ
b
increases when 10°C>T>−40°C and decreases when −40°C>T>−100°C. The strain-rate- and temperature-dependent bimodal Weibull statistical constitutive theory was adopted for the statistical
analysis of the experimental results, and the Weibull parameters of single fiber were obtained. The results show that the
bimodal Weibull distribution function is suitable to represent the strength distribution of the glass fiber at low temperature
and different strain rates. The differences in the mechanical properties between EC8.0−24×7 and EC5.5−12 ×14 glass fiber bundles
were also discussed.
Project supported by the National Natural Science Foundation of China (No. 19772058). 相似文献
6.
C.-H. Chen 《Heat and Mass Transfer》1998,33(5-6):471-476
The analysis of laminar mixed convection in boundary layers adjacent to a vertical, continuously stretching sheet has been
presented. The velocity and temperature of the sheet were assumed to vary in a power-law form, that is, u
w
(x)=Bx
m
and T
w
(x)−T
∞=Ax
n
. In the presence of buoyancy force effects, similarity solutions were reported for the following two cases: (a) n=0 and m=0.5, which corresponds to an isothermal sheet moving with a velocity of the form u
w
=Bx
0.5 and (b) n=1 and m=1, which corresponds to a continuous, linearly stretching sheet with a linear surface temperature distribution, i.e. T
w
−T
∞=Ax. Formulation of the present problem shows that the heat transfer characteristics depends on four governing parameters, namely,
the velocity exponent parameter m, the temperature exponent parameter n, the buoyancy force parameter G
*, and Prandtl number of the fluid. Numerical solutions were generated from a finite difference method. Results for the local
Nusselt number, the local friction coefficient, and temperature profiles are presented for different governing parameters.
Effects of buoyancy force and Prandtl number on the flow and heat transfer characteristics are thoroughly examined.
Received on 17 July 1997 相似文献
7.
Hiroshi Watanabe Tomohiro Sato Motoyuki Hirose Kunihiro Osaki Ming-Long Yao 《Rheologica Acta》1999,38(2):100-107
Rheo-dielectric behavior was examined for 4−4′−n-octyl-cyanobiphenyl (8CB) having large dipoles parallel to its principal axis (in the direction of the C≡N bond). In the
quiescent state at all temperatures (T) examined, orientational fluctuation of the 8CB molecules was observed as dielectric dispersions at characteristic frequencies
ωc>106 s−1. In the isotropic state at high T, no detectable changes of the complex dielectric constant ɛ*(ω) were found under slow flow at shear rates ˙γ≫ωc. In the nematic state at intermediate T, the terminal relaxation intensity of ɛ*(ω) was decreased under such slow flow. In the smectic state at lower T, the flow effect became much less significant. These results were related to the flow-induced changes of the liquid crystalline
textures in the nematic and smectic states, and the differences of the rheo-dielectric behavior in these states are discussed
in relation to a difference of the symmetry of molecular arrangements in the nematic and smectic textures.
Received: 1 October 1998 Accepted: 13 January 1999 相似文献
8.
Márcio José Horta Dantas 《Journal of Elasticity》2006,82(1):1-7
Let W(F) = φ(λ
1
s
+ λ
2
s
+ λ
3
s
) + ψ(λ
1
r
λ
2
r
+ λ
1
r
λ
3
r
+ λ
2
r
λ
3
r
) + f(λ
1
λ
2
λ
3) be a stored energy function. We prove that, for this function, rank-one convexity is equivalent to polyconvexity.under suitable assumptions on φ, ψ and f. 相似文献
9.
The problem of the free convection boundary-layer flow over a semi-infinite vertical flat surface in a porous medium is considered,
in which the surface temperature has a constant value T1 at the leading edge, where T1 is above the ambient temperature, and takes a value T2 at a given distance L along the surface, varying linearly between these two values and remaining constant afterwards. Numerical solutions of the
boundary-layer equations are obtained as well as solutions valid for both small and large distance along the surface. Results
are presented for the three cases, when the temperature T2 is greater, equal or less than the ambient temperature T∞. In the first case, T2 > T∞, a boundary-layer flow develops along the surface starting with a flow associated with the temperature difference T1 − T∞ at the leading edge and approaching a flow associated with the temperature difference T2 − T∞ at large distances. In the second case, T2 = T∞, the convective flow set up on the initial part of the surface drives a wall jet in the region where the surface temperature
is the same as ambient. In the final case, T2 < T∞, a singularity develops in the numerical solution at the point where the surface temperature becomes T∞. The nature of this singularity is discussed. 相似文献
10.
Crack linkup: An experimental analysis 总被引:1,自引:0,他引:1
TheT
ɛ
*
integral was used to assess stable crack growth and crack linkup in 0.8 mm thick 2024-T3 aluminum tension specimens with
multiple site damage (MSD) under monotonic and cyclic loads. TheT
ɛ
*
values were obtained directly from the recorded moiré fringes on the fracture specimens with and without MSD. TheT
ɛ
*
resistance curves of these fracture specimens of different geometries were in excellent agreement with each other. The results
suggest thatT
ɛ
*
is a material parameter which can be used to characterize crack growth and linkup in the absence of large overloading.T
ɛ
*
based crack growth and net-section-yield based crack linkup criteria for MSD specimens are proposed. The crack tip opening
angle (CTOA) criterion can also be used to correlate crack growth larger than 2 mm. 相似文献
11.
Non-Darcy mixed convection in a porous medium from horizontal surfaces with variable surface heat flux of the power-law distribution
is analyzed. The entire mixed convection regime is divided into two regions. The first region covers the forced convection
dominated regime where the dimensionless parameter ζ
f
=Ra*
x
/Pe2
x
is found to characterize the effect of buoyancy forces on the forced convection with K
′
U
∞/ν characterizing the effect of inertia resistance. The second region covers the natural convection dominated regime where
the dimensionless parameter ζ
n
=Pe
x
/Ra*1/2
x
is found to characterize the effect of the forced flow on the natural convection, with (K
′
U
∞/ν)Ra*1/2
x
/Pe
x
characterizing the effect of inertia resistance. To obtain the solution that covers the entire mixed convection regime the
solution of the first regime is carried out for ζ
f
=0, the pure forced convection limit, to ζ
f
=1 and the solution of the second is carried out for ζ
n
=0, the pure natural convection limit, to ζ
n
=1. The two solutions meet and match at ζ
f
=ζ
n
=1, and R
*
h
=G
*
h
.
Also a non-Darcy model was used to analyze mixed convection in a porous medium from horizontal surfaces with variable wall
temperature of the power-law form. The entire mixed convection regime is divided into two regions. The first region covers
the forced convection dominated regime where the dimensionless parameter ξ
f
=Ra
x
/Pe
x
3/2 is found to measure the buoyancy effects on mixed convection with Da
x
Pe
x
/ɛ as the wall effects. The second region covers the natural convection dominated region where ξ
n
=Pe
x
/Ra
x
2/3 is found to measure the force effects on mixed convection with Da
x
Ra
x
2/3/ɛ as the wall effects. Numerical results for different inertia, wall, variable surface heat flux and variable wall temperature
exponents are presented.
Received on 8 July 1996 相似文献
12.
P. Razelos 《Heat and Mass Transfer》1979,12(2):113-119
In this paper a generalized approach to the problem of heat transfer through convective fins is given. The proper dimensionless variables, which specify the general problem are identified, and upper bounds of the values of the dimensionless number Nr defined as “the ratio of the heat transferred by the fin to that of the corresponding bare surface” are derived. It was shwon that these limiting values of the Nr are 1/√B1 and √2/B1 for longitudinal fins and spines respectively, where B1 is the Biot number hb/k, while for annular fins of constant thickness and hyperbolic profile, Nr? K(β)/√Bi, where K(β) is a number determined by the profile of the fin and the ratio β=x2/x1 of the outside to the inside radii. It was also shown that for longitudinal fins and spinces the possible adverse insulating effect by the use of the fin is avoided, if one selects the value of √hA/KC < 1, which is a rather stricter criterion than the one reported in the literature, namely that of hA/kC < 1 [2–5]. An example is given to show how one may utilize the appropriate value of Nr and the fin effectiveness e, to obtain the dimensions of the fin. 相似文献
13.
Control of Flow and Heat Transfer in a Porous Enclosure due to an Adiabatic Thin fin on the Hot Wall
Natural convection flow in a differentially heated square enclosure filled with porous matrix with a solid adiabatic thin
fin attached at the hot left wall is studied numerically. The Brinkman–Forchheimer-extended Darcy model is used to solve the
momentum equations, in the porous medium. The numerical investigation is done through streamlines, isotherms, and heat transfer
rates. A parametric study is carried out using the following parameters: Darcy number (Da) from 10−4 to 10−2, dimensionless thin fin lengths (L
p) 0.3, 0.5, and 0.7, dimensionless positions (S
p) 0.25, 0.5, and 0.75 with Prandtl numbers (Pr) 0.7 and 100 for Ra = 106. For Da = 10−3 and Pr = 0.7, it is observed that there is a counter clock-wise secondary flow formation around the tip of the fin for S
p = 0.5 for all lengths of L
p. Moreover when Da = 10−2 the secondary circulation behavior has been observed for S
p = 0.25 and 0.75 and there is another circulation between the top wall and the fin that is separated from the primary circulation.
However, these secondary circulations features are not observed for Pr = 100. It is also found that the average Nusselt number decreases as the length of the fin increases for all locations. However,
the rate of decrease of average Nusselt number becomes slower as the location of fin moves from the bottom wall to the top
wall. The overall heat transfer rate can be controlled with a suitable selection of the fin location and length. 相似文献
14.
Heat transfer for laminar flow in internally finned pipes with different fin heights and uniform wall temperature 总被引:3,自引:0,他引:3
An analysis is presented for fully developed laminar convective heat transfer in a pipe provided with internal longitudinal fins, and with uniform outside wall temperature. The fins are arranged in two groups of different heights. The governing equations have been solved numerically to obtain the velocity and temperature distributions. The results obtained for different pipe-fins geometries show that the fin heights affect greatly flow and heat transfer characteristics. Reducing the height of one fin group decreases the friction coefficient significantly. At the same time Nusselt number decreases inappreciably so that such reduction is justified. Thus, the use of different fin heights in internally finned pipes enables the enhancement of heat transfer at reasonably low friction coefficient.Nomenclature Af dimensionless flow area of the finned pipe, Eq. (8) - af flow area of the finned pipe - Cp specific heat at constant pressure - f coefficient of friction, Eq. (12) - H1, H2 dimensionless fin height h1/ro h2/ro - h1, h2 fin heights -
average heat transfer coefficient at solid-fluid interface - KR fin conductance parameter, ks/kf - kf thermal conductivity of fluid - ks thermal conductivity of fin - l pipe length - mass flow rate - N number of fins - Nu Nusselt number, Eqs. (15) and (16) - P pressure - Q total heat transfer rate at solid fluid interface - Qf1, Qf2 heat transfer rate at fin surface - qw average heat flux at pipe-wall, Q/(2 rol) - R dimensionless radial coordinate r/ro - Re Reynolds Number, Eq. (13) - r radial coordinate - ro radius of pipe - r1, r2 radii of fin tips - T temperature - Tb bulk temperature - U dimensionless velocity, Eq. (2) - Ub dimensionless bulk velocity - uz axial velocity - z axial coordinate - angle between the flanks of two adjacent fins - half the angle subtended by a fin - angle between the center-lines of two adjacent fins - angular coordinate - dynamic viscosity - density - dimensionless temperature, Eq. (6) - b dimensionless bulk temperature 相似文献
15.
A linear stability analysis is used to study the conditions marking the onset of secondary flow in the form of longitudinal
vortices for plane Poiseuille flow of water in the thermal entrance region of a horizontal parallel-plate channel by a numerical
method. The water temperature range under consideration is 0∼30°C and the maximum density effect at 4°C is of primary interest.
The basic flow solution for temperature includes axial heat conduction effect and the entrance temperature is taken to be
uniform at far upstream location jackie=−∞ to allow for the upstream heat penetration through thermal entrance jackie=0. Numerical results for critical Rayleigh number are obtained for Peclet numbers 1, 10, 50 and thermal condition parameters
(λ
1, λ
2) in the range of −2.0≤λ
1≤−0.5 and −1.0≤λ
2≤1.4. The analysis is motivated by a desire to determine the free convection effect on freezing or thawing in channel flow
of water. 相似文献
16.
The mixing length theory is employed to simulate the fully developed turbulent heat transfer in annular-sector ducts with
five apex angles (θ0=18,20,24,30,40∘) and four radius ratios (R
o/R
i=2,3,4,5). The Reynolds number range is 104105. The numerical results agree well with an available correlation which was obtained in following parameter range: θ0=18,20,24,30,40∘, R
o/R
i=4 and Re=1045×104. The present work demonstrates that the application range of the correlation can be much extended. Apart from the mixing
length theory, the kɛ model with wall function and the Reynolds stress model are also employed. None of the friction factor results predicted
by the three models agrees well with the test data. For the heat transfer prediction the mixing length theory seems the best
for the cases studied.
Received on 17 July 2000 / Published online: 29 November 2001 相似文献
17.
At the same solid volume fraction (Φ) the relative viscosity (η
r
) of a concentrated noncolloidal bidisperse suspension of hard spherical particles is lower than that of a monodisperse suspension.
In this paper a semi-analytical viscosity model of noncolloidal bidisperse suspensions is derived using an integration method.
In this model the random loose packing density obtained by computer simulation is taken as the limit of solid volume fraction
Φ
m
which depends upon both the diameter ratio (λ) of large to small particles and the volume fraction of large particles (ξ=Φ
l
/Φ). This model shows that at high solid volume fraction, Φ > 0.40, both λ and ξ significantly influence η
r
. For example, at Φ=0.5, it predicts that for monodisperse suspensions η
r
=70, while for bidisperse suspensions (λ=2 and ξ=0.7) η
r
=40. Comparison shows that, at high solid volume fraction (0.4–0.5), the relative viscosity predicted by this model is in
good agreement with that predicted by the work of Shapiro and Probstein (1992) and of Patlazhan (1993), but is higher than
that predicted by the work of others.
Received: 27 February 2001 Accepted: 25 April 2001 相似文献
18.
The near-field flow structure of a tip vortex behind a sweptback and tapered NACA 0015 wing was investigated and compared
with a rectangular wing at the same lift force and Re=1.81×105. The tangential velocity decreased with the downstream distance while increased with the airfoil incidence. The core radius
was about 3% of the root chord c
r, regardless of the downstream distance and α for α<8°. The core axial velocity was always wake-like. The core Γc and total Γo circulation of the tip vortex remained nearly constant up to x/c
r=3.5 and had a Γc/Γo ratio of 0.63. The total circulation of the tip vortex accounted for only about 40% of the bound root circulation Γb. For a rectangular wing, the axial flow exhibited islands of wake- and jet-like velocity distributions with Γc/Γo=0.75 and Γo/Γb=0.70. For the sweptback and tapered wing tested, the inner region of the tip vortex flow exhibited a self-similar behavior
for x/c
r≥1.0. The lift force computed from the spanwise circulation distributions agreed well with the force-balance data. A large
difference in the lift-induced drag was, however, observed between the wake integral method and the inviscid lifting-line
theory. 相似文献
19.
A closed-form model for the second-law-based thermoeconomic optimization of constant cross-sectional area fins, is discussed
with an example problem. In this approach, different monetary values are attached to the irreversible losses caused by the
finite temperature difference heat transfer (T
0Δ
T
) and pressure drop (T
0Δ
P
) in the fin application. In addition, a simplified closed-form solution is presented for the case when the capital cost of
the fin is negligible and only operational costs are considered. To illustrate the usefulness of the present analytical approach,
the simplified cost optimized results are compared with the numerical results obtained from Poulikakos and Bejan's analysis,
who have assumed same monetary values for T
0Δ
T
and T
0Δ
P
. Furthermore, the influence of important fin thermal, physical, geometrical and cost parameters on the optimum Reynolds numbers
Re
Dopt
and Re
Lopt
are presented in algebraic forms, and also graphical results are shown for the case of pin and plate fin, as examples.
Received on 26 January 1998 相似文献
20.
Robert McKibbin 《Transport in Porous Media》1986,1(3):271-292
The theory describing the onset of convection in a homogeneous porous layer bounded above and below by isothermal surfaces
is extended to consider an upper boundary which is partly permeable. The general boundary condition p + λ ∂p/∂n = constant is applied at the top surface and the flow is investigated for various λ in the range 0 ⩽ λ < ∞. Estimates of the magnitude and horizontal distribution of the vertical mass and heat fluxes at the surface, the horizontally-averaged
heat flux (Nusselt number) and the fraction of the fluid which recirculates within the layer are found for slightly supercritical
conditions. Comparisons are made with the two limiting cases λ → ∞, where the surface is completely impermeable, and λ = 0, where the surface is at constant pressure. Also studied are the effects of anisotropy in permeability, ξ = K
H
/K
V
, and anisotropy is thermal conductivity, η = k
H
/k
V
, both parameters being ratios of horizontal to vertical quantities. Quantitative results are given for a wide variety of
the parameters λ, ξ and η. In the limit ξ/η → 0 there is no recirculation, all fluid being converted out of the top surface, while in the limit ξ/η → ∞ there is full recirculation. 相似文献
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