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1.
This study experimentally examines the forced convective flow over two sequentially heated blocks mounted on one principal wall of a channel. The experiments, involving mass transfer, were carried out via the naphthalene sublimation technique (NST). By virtue of the analogy between heat and mass transfer, the results can then be converted to determine the heat transfer. In the experiments, the block spacings were set at 2, 4, 6, 8, 12, 16, and 22 and the Reynolds numbers were set at 1300 and 104 which correspond to the laminar and the turbulent convective flow cases, respectively. Results show that the Sherwood number increases or decreases monotonically along the block surfaces in the laminar convection cases; while the hump and sharp increase in the Sherwood number can be found in the turbulent convection cases. This is attributed to the reattachment of the separating bubble and the flow impingement, respectively. Comparison between the experimental and numerical results is made and the effect of the block spacing on heat transfer is discussed. 相似文献
2.
This article reports a numerical study of double-diffusive convection in a fluid-saturated vertical porous annulus subjected
to discrete heat and mass fluxes from a portion of the inner wall. The outer wall is maintained at uniform temperature and
concentration, while the top and bottom walls are adiabatic and impermeable to mass transfer. The physical model for the momentum
equation is formulated using the Darcy law, and the resulting governing equations are solved using an implicit finite difference
technique. The influence of physical and geometrical parameters on the streamlines, isotherms, isoconcentrations, average
Nusselt and Sherwood numbers has been numerically investigated in detail. The location of heat and solute source has a profound
influence on the flow pattern, heat and mass transfer rates in the porous annulus. For the segment located at the bottom portion
of inner wall, the flow rate is found to be higher, whereas the heat and mass transfer rates are higher when the source is
placed near the middle of the inner wall. Further, the average Sherwood number increases with Lewis number, while for the
average Nusselt number the effect is opposite. The average Nusselt number increases with radius ratio (λ); however, the average Sherwood number increases with radius ratio only up to λ = 5, and for λ > 5 , the average Sherwood number does not increase significantly. 相似文献
3.
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
4.
Absorber is an important component in absorption machines and its characteristics have significant effects on the overall
efficiency of absorption machines. This article reports a model of simultaneous heat and mass transfer process in absorption
of refrigerant vapor into a lithium bromide solution of water––cooled vertical plate absorber in the Reynolds number range
of 5 < Re < 150. The boundary layer assumptions were used for the transport of mass, momentum and energy equations and the fully implicit
finite difference method was employed to solve the governing equations in the film flow. Dependence of lithium bromide aqueous
properties to the temperature and concentration and film thickness to vapor absorption was employed. This model can predict
temperature, concentration and properties of aqueous profiles as well as the absorption heat and mass fluxes, heat and mass
transfer coefficients, Nusslet and Sherwood number of absorber. An analysis for linear distribution of wall temperature condition
carries out to investigation the reliability of the present numerical method through comparing with previous investigation. 相似文献
5.
K. A. Yih 《Heat and Mass Transfer》1998,34(1):55-61
A boundary layer analysis is used to investigate the heat and mass transfer characteristics of mixed convection about a vertical
flat plate embedded in a saturated porous medium under the coupled effects of thermal and mass diffusion. The plate is maintained
at prescribed surface temperature/concentration (PST/PSC) or prescribed heat/mass flux (PHF/PMF). The nonsimilar governing
equations are obtained by using a suitable transformation and solved by Keller box method. Numerical results for the local
heat transfer rate and the local mass transfer rate are presented for various parameters. The local heat and mass transfer
rates increase with increasing n and m and buoyancy parameter ξ. When buoyancy parameter ξ is very small (large) the value of local Nusselt and the local Sherwood
number correspond with the pure forced (free) convection, respectively. Increasing buoyancy ratio N (or N
*) increases the local heat and mass transfer rates. It is apparent that Lewis number has a pronounced effect on the local
mass transfer rate than it does on the local heat transfer rate. Furthermore, increasing Lewis number decreases (increases)
the local heat (mass) transfer rate.
Received on 8 December 1997 相似文献
6.
Among major components of LiBr–H2O absorption chillers is the absorber, which has a direct effect on the chillier size and whose characteristics have significant
effects on the overall efficiency of absorption machines. In this article, heat and mass transfer process in absorption of
refrigerant vapor into a lithium bromide solution of water-cooled incline plate absorber in the Reynolds number range of 5 < Re < 150 is performed numerically. The boundary layer assumptions are used for the mass, momentum and energy transport equations
and the fully implicit finite difference method is employed to solve the governing equations. Dependence of lithium bromide
aqueous properties to the temperature and concentration is employed as well as dependence of film thickness to vapor absorption.
An analysis for linear distribution of wall temperature condition carries out to investigate the reliability of the present
numerical method through comparing with previous investigation. The effect of plate angle on heat and mass transfer parameters
is investigated and the results show that absorption mass flux and heat and mass transfer coefficient increase as the angle
of the plate increase. The main parameters of absorber design, namely Nusselt and Sherwood numbers, are correlated as a function
of Reynolds Number and the plate angle. 相似文献
7.
The present paper deals with the prediction of three-dimensional fluid flow and heat transfer in rib-roughened ducts of square
cross-section, which are either stationary, or rotate in orthogonal mode. The main objective is to assess how a recently developed
variant of a cubic non-linear k−ε model (proposed by Craft et al. Flow Turbul Combust 63:59–80, 1999) can predict three-dimensional flow and heat transfer characteristics through stationary and rotating ribbed ducts. The present
paper discusses turbulent air flow and heat transfer through two different configurations, namely: (I) a stationary square
duct with “in-line” normal and (II) a square duct with normal ribs in a “staggered” arrangement under stationary and rotating
conditions, with the axis of rotation normal to the flow direction and parallel to the ribs. In this paper the flow and thermal
predictions of the linear k−ε model (EVM) are also included, as a set of baseline predictions. The mean flow predictions show that both linear and non-linear
k−ε models can successfully reproduce most of the measured data for stream-wise and cross-stream velocity components. Moreover,
the non-linear model is able to produce better results for the turbulent stresses. The heat transfer predictions show that
both EVM and NLEVM2, the more recent variant of the non-linear k−ε, with the algebraic length-scale correction term, overestimate the measured Nusselt numbers for both geometries examined.
While the EVM with the differential length-scale correction term underestimates heat transfer levels, the Nusselt number predictions
with the NLEVM2 and the ‘NYP’ term are in close agreements with the measured data. Comparisons with our earlier work, Iacovides
and Raisee (Int J Heat Fluid Flow, 20:320–328, 1999), show that the NLEVM2 thermal predictions are of similar quality to those of a second-moment closure. 相似文献
8.
In the present case, the conjugate heat transfer involving a turbulent plane offset jet is considered. The bottom wall of
the solid block is maintained at an isothermal temperature higher than the jet inlet temperature. The parameters considered
are the offset ratio (OR), the conductivity ratio (K), the solid slab thickness (S) and the Prandtl number (Pr). The Reynolds number considered is 15,000 because the flow becomes fully turbulent and then it becomes independent of the
Reynolds number. The ranges of parameters considered are: OR = 3, 7 and 11, K = 1–1,000, S = 1–10 and Pr = 0.01–100. High Reynolds number two-equation model (k–ε) has been used for turbulence modeling. Results for the solid–fluid interface temperature, local Nusselt number, local
heat flux, average Nusselt number and average heat transfer have been presented and discussed. 相似文献
9.
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
10.
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
11.
A numerical study of the steady forced convection heat transfer from an unconfined circular cylinder
Forced convection heat transfer from an unconfined circular cylinder in the steady cross-flow regime has been studied using
a finite volume method (FVM) implemented on a Cartesian grid system in the range as 10 ≤ Re ≤ 45 and 0.7 ≤ Pr ≤ 400. The numerical results are used to develop simple correlations for Nusselt number as a function of the pertinent dimensionless
variables. In addition to average Nusselt number, the effects of Re, Pr and thermal boundary conditions on the temperature field near the cylinder and on the local Nusselt number distributions
have also been presented to provide further physical insights into the nature of the flow. The rate of heat transfer increases
with an increase in the Reynolds and/or Prandtl numbers. The uniform heat flux condition always shows higher value of heat
transfer coefficient than the constant wall temperature at the surface of the cylinder for the same Reynolds and Prandtl numbers.
The maximum difference between the two values is around 15–20%. 相似文献
12.
A. A. Tawfek 《Heat and Mass Transfer》1999,35(4):327-333
An experimental investigations of heat transfer for a stationary isothermal circular cylinder exposed normal to an impinging
round air-jet has been reported. The circumferential heat transfer distributions as well as axial Nusselt number is measured.
The measurements are taken as a function of the Reynolds number ranging from 3.8 × 103 to 4 × 104, the cylinder separation distance to the nozzle diameter (z/d) varying from 7 to 30, and the nozzle to cylinder diameter ratio (d/D) changing from 0.06 to 0.14. The output results indicated that the axial and radial distributions of the local heat transfer
peaked at the impingement point. The heat transfer rate increases as the values of z decreases, for the same d and Re. The drop-off of the Nusselt number with increasing axial distance or radial angle from the impingement point was
more pronounced for smaller z and d. The peripheral and surface average Nusselt numbers were determined by integration. The experimental data was used to produce
correlations for both average and stagnation point heat transfer.
Received on 4 January 1999 相似文献
13.
A boundary layer analysis is presented for the natural convection past an isothermal sphere in a Darcy porous medium saturated
with a nanofluid. Numerical results for friction factor, surface heat transfer rate, and mass transfer rate have been presented
for parametric variations of the buoyancy ratio parameter N
r, Brownian motion parameter N
b, thermophoresis parameter N
t, and Lewis number L
e. The dependency of the friction factor, surface heat transfer rate (Nusselt number), and mass transfer rate (Sherwood number)
on these parameters has been discussed. 相似文献
14.
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
15.
The unsteady natural convection boundary layer flow over a semi-infinite vertical cylinder is considered with combined buoyancy
force effects, for the situation in which the surface temperature T
′
w(x) and C
′
w(x) are subjected to the power-law surface heat and mass flux as K(T
′/r) = −ax
n
and D(C
′/r) = −bx
m
. The governing equations are solved by an implicit finite difference scheme of Crank-Nicolson method. Numerical results are
obtained for different values of Prandtl number, Schmidt number ‘n’ and ‘m’. The velocity, temperature and concentration profiles, local and average skin-friction, Nusselt and Sherwood numbers are
shown graphically. The local Nusselt and Sherwood number of the present study are compared with the available result and a
good agreement is found to exist.
Received on 7 July 1998 相似文献
16.
Air-side performance of louver-finned flat aluminum heat exchangers at a low velocity region 总被引:1,自引:0,他引:1
The heat transfer and pressure drop characteristics of heat exchangers having louver fins were experimentally investigated.
The samples had small fin pitches (1.0–1.4 mm), and experiments were conducted up to a very low frontal air velocity (as low
as 0.3 m/s). Below a certain Reynolds number (critical Reynolds number), the fall-off of the heat transfer coefficient curve
was observed. The critical Reynolds number was insensitive to the louver angle, and decreased as the louver pitch to fin pitch
ratio (L
p
/F
p
) decreased. Existing correlations on the critical Reynolds number did not adequately predict the data. The heat transfer
coefficient curves crossed over as the Reynolds number decreased. Possible explanation is provided considering the louver
pattern between neighboring rows. Different from the heat transfer coefficient, the friction factor did not show the fall-off
characteristic. The reason was attributed to the form drag by louvers, which offsets the decreased skin friction at low Reynolds
numbers. The friction factor increased as the fin pitch decreased and the louver angle increased. A new correlation predicted
92% of the heat transfer coefficient and 94% of the friction factor within ±10%. 相似文献
17.
A boundary layer analysis is used to investigate the effect of lateral mass flux on mixed convection heat and mass transfer over inclined permeable surfaces in porous media. The conservation equations that govern the problem are reduced to a system of non-linear ordinary differential equations and then the resulting equations is solved by numerical method. The numerical results for heat and mass transfer in terms of Nusselt and Sherwood number are presented in x-y plots for the buoyancy ratio (N) and Lewis number (Le) with mass flux pammeter (Fw). The obtained results are validated against previously published results with on special case of the problem. 相似文献
18.
Andrew J. Onstad Christopher J. Elkins Frank Medina Ryan B. Wicker John K. Eaton 《Experiments in fluids》2011,50(6):1571-1585
Open-celled foam geometries show great promise in heat/mass transfer, chemical treatment, and enhanced mixing applications.
Flow measurements on these geometries have consisted primarily of observations of the upstream and downstream effects the
foam has on the velocity field. Unfortunately, these observations give little insight into the flow inside the foam. We have
performed quantitative flow measurements inside a scaled replica of a metal foam, ϕ = 0.921, D
Cell = 2.5 mm, by Magnetic Resonance Velocimetry to better understand the fluid motion inside the foam and give an alternative
method to determine the foam cell and pore sizes. Through these 3-D, spatially resolved measurements of the flow field for
a cell Reynolds number of 840, we have shown that the transverse motion of the fluid has velocities 20–30% of the superficial
velocity passing through the foam. This strong transverse motion creates and dissipates streamwise jets with peak velocities
2–3 times the superficial velocity and whose coherence length is strongly correlated to the cell size of the foam. This complex
fluid motion is described as “mechanical mixing” and is attributed to the geometry of the foam. A mechanical dispersion coefficient,
D
M, was formed which demonstrates the transverse dispersion of this geometry to be 14 times the kinematic viscosity and 10 times
the thermal diffusivity of air at 20°C and 1 atm. 相似文献
19.
We investigated the influence of elastic material compressibility on parameters of an expanding spherical stress wave. The
material compressibility is represented by Poisson’s ratio, ν, in this paper. The stress wave is generated by a pressure produced inside a spherical cavity surrounded by the isotropic
elastic material. The analytical closed form formulae determining the dynamic state of the mechanical parameters (displacement,
particle velocity, strains, stresses, and material density) in the material have been derived. These formulae were obtained
for surge pressure p(t) = p
0 = const inside the cavity. From analysis of these formulae, it is shown that the Poisson’s ratio substantially influences
the course of material parameters in space and time. All parameters intensively decrease in space together with an increase
of the Lagrangian coordinate, r. On the contrary, these parameters oscillate versus time around their static values. These oscillations decay in the course
of time. We can mark out two ranges of parameter ν values in which vibrations of the parameters are “damped” at a different rate. Thus, Poisson’s ratio in the range below about
0.4 causes intense decay of parameter oscillations. On the other hand in the range 0.4 < ν < 0.5, i.e. in quasi-incompressible materials, the “damping” of parameter vibrations is very low. In the limiting case when
ν = 0.5, i.e. in the incompressible material, “damping” vanishes, and the parameters harmonically oscillate around their static
values. The abnormal behaviour of the material occurs in the range 0.4 < ν < 0.5. In this case, an insignificant increase of Poisson’s ratio causes a considerable increase of the parameter vibration
amplitude and decrease of vibration “damping”.
相似文献
20.
Heat and mass transfer for micropolar flow with radiation effect past a nonlinearly stretching sheet
Kai-Long Hsiao 《Heat and Mass Transfer》2010,46(4):413-419
In this study, an analysis has been performed for heat and mass transfer with radiation effect of a steady laminar boundary-layer
flow of a micropolar flow past a nonlinearly stretching sheet. Parameters n, K, k
0, Pr, Ec, and Sc represent the dominance of the nonlinearly effect, material effect, radiation effect, heat and mass transfer effects which
have presented in governing equations, respectively. The similar transformation, the finite-difference method and Runge–Kutta
method have been used to analyze the present problem. The numerical solutions of the flow velocity distributions, temperature
profiles, the wall unknown values of θ′(0) and ϕ′(0) for calculating the heat and mass transfer of the similar boundary-layer flow are carried out as functions of n, Ec, k
0, Pr, Sc. The value of n, k
0, Pr and Sc parameters are important factors in this study. It will produce greater heat transfer efficiency with a larger value of those
parameters, but the viscous dissipation parameter Ec and material parameter K may reduce the heat transfer efficiency. On the other hand, for mass transfer, the value of Sc parameter is important factor in this study. It will produce greater heat transfer efficiency with a larger value of Sc. 相似文献