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1.
Kalabin et al. (Numer. Heat Transfer A 47, 621-631, 2005) studied the unsteady natural convection for the sinusoidal oscillating wall temperature on one side wall
and constant average temperature on the opposing side wall. The present article is on the unsteady natural convective heat
transfer in an inclined porous cavity with similar temperature boundary conditions as those of Kalabin et al. The inclined
angle of the cavity is varied from 0° to 80°. The flow field is modeled with the Brinkman-extended Darcy model. The combined effects
of inclination angle of the enclosure and oscillation frequency of wall temperature are studied for Ra* = 103, Da = 10−3, , and Pr=1. Some results are also obtained with the Darcy–Brinkman–Forchheimer model and Darcy’s law and are compared with the present
Brinkman-extended Darcy model. The maximal heat transfer rate is attained at the oscillating frequency f = 46.7π and the inclined angle . 相似文献
2.
In this study, a unique combination of a vibrating plate and a cross-flow passage is proposed as a means of enhancing natural
convection cooling. The enhancement potential was estimated based on numerical studies involving a representative model which
includes a short, transversely oscillating plate, placed over a transverse cross-flow opening in a uniformly heated vertical
channel wall dividing two adjacent vertical channels. The resulting velocity and temperature fields are analyzed, with the
focus on the local thermal effects near the opening. The simulation indicates up to a 50% enhancement in the local heat transfer
coefficient for vibrating plate amplitudes of at least 30% of the mean clearance space and frequencies of over 82 rad/s. 相似文献
3.
C. Y. Wang 《Transport in Porous Media》2008,73(3):369-378
Fully developed laminar forced convection inside a semi-circular channel filled with a Brinkman-Darcy porous medium is studied.
Analytical solutions for flow and constant flux heat transfer are found using a mixture of Cartesian and cylindrical coordinates.
The problem depends on a parameter s, which is proportional to the inverse square of the Darcy number. Velocity boundary layers exist when s is large. Both friction factor-Reynolds number product and Nusselt number are determined. Closed form expressions for the
clear fluid () limit are found. Rare analytical solutions not only describe fundamental channel flows, but also serve as a check for more
complicated numerical solutions. 相似文献
4.
Unsteady Natural Convection Flow in a Square Cavity Filled with a Porous Medium Due to Impulsive Change in Wall Temperature 总被引:1,自引:0,他引:1
Unsteady natural convection flow in a two-dimensional square cavity filled with a porous material has been studied. The flow
is initially steady where the left-hand vertical wall has temperature T
h and the right-hand vertical wall is maintained at temperature T
c (T
h > T
c) and the horizontal walls are insulated. At time t > 0, the left-hand vertical wall temperature is suddenly raised to which introduces unsteadiness in the flow field. The partial differential equations governing the unsteady natural convection
flow have been solved numerically using a finite control volume method. The computation has been carried out until the final
steady state is reached. It is found that the average Nusselt number attains a minimum during the transient period and that
the time required to reach the final steady state is longer for low Rayleigh number and shorter for high Rayleigh number. 相似文献
5.
An analysis is made of the transient free convection from a vertical flat plate which is embedded in a fluid-saturated porous medium. It is assumed that for time
a steady state temperature or velocity has been obtained in the boundary-layer which occurs due to a uniform flux dissipation rate
. Then at time
the heat flux on the plate is suddenly changed to
and maintained at this value for
0$$
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. An analytical solution has been obtained for the temperature/velocity field for small times in which the transport effects are confined within an inner layer adjacent to the plate. These effects cause a new steady boundary layer. A numerical solution of the full boundary-layer equations is then obtained for the whole transient from
to the steady state, firstly by means of a step-by-step method and then by a matching technique. The transition between the two distinct solution methods is always observed to occur very near to the turning point of the plate surface temperature, a time at which the fluid temperature is close to its steady state profile. The solution obtained using the step-by-step method shows excellent agreement with the small time analytical solution. Results are presented to illustrate the occurrence of transients from both small and large increases and decreases in the levels of existing energy inputs. 相似文献
6.
The Darcy Model with the Boussinesq approximation is used to study natural convection in a shallow porous layer, with variable
permeability, filled with a binary fluid. The permeability of the medium is assumed to vary exponentially with the depth of
the layer. The two horizontal walls of the cavity are subject to constant fluxes of heat and solute while the two vertical
ones are impermeable and adiabatic. The governing parameters for the problem are the thermal Rayleigh number, R
T, the Lewis number, Le, the buoyancy ratio, φ, the aspect ratio of the cavity, A, the normalized porosity, ε, the variable permeability constant, c, and parameter a defining double-diffusive convection (a = 0) or Soret induced convection (a = 1). For convection in an infinite layer, an analytical solution of the steady form of the governing equations is obtained
on the basis of the parallel flow approximation. The onset of supercritical convection, or subcritical, convection are predicted by the present theory. A linear stability analysis of the parallel flow model is conducted and the
critical Rayleigh number for the onset of Hopf’s bifurcation is predicted numerically. Numerical solutions of the full governing
equations are found to be in excellent agreement with the analytical predictions. 相似文献
7.
J. van der Burgh 《Applied Scientific Research》1960,9(1):293-296
Summary Heat transfer by thermal free convection at the surface of a sphere has been studied experimentally by melting a sphere of
solid benzene in a large quantity of liquid benzene of homogeneous temperature. The influence of cold liquid produced by the
melting process is taken into account to yield results that are representative for the pure effect of heat transfer without
melting. In the general formula for heat transfer by thermal convection,
=C(GrPr)1/4, we found C=0.525. 相似文献
8.
A laser specklegram or speckle photography technique allows a direct measurement of surface temperature gradients and provides a full field interrogation with an extremely high resolution from a single data taking. The specklegram technique has been successfully applied to investigate the natural convection heat transfer from an upward-facing isothermal plate. For a plate with a large aspect ratio of 15, both local and global Nusselt numbers have been determined from the direct measurement of local temperature gradients. The Rayleigh number, based on the length scale equivalent to the ratio of the surface area to the perimeter, has been varied from 9.0 × 103 to 4.0 × 104. The present result for the global heat transfer has shown that a 1/5-power law, i.e., Nu = C1
Ra
1/5, correlates the data more properly whilst previously published results showed a large scatter in the exponent, ranging from 1/8-power to 1/4-power. The proportional constant, C1 has been determined to be 0.56 which shows a fairly good agreement with previously published theoretical results. The laser specklegram technique has shown a strong potential as a powerful and convenient method for an experimental assessment of natural convection heat transfer problems. The specklegram technique at the same time has eliminated the deficiencies of both the mass transfer analogy technique and the classical heat transfer measurement technique.List of symbols
a
characteristic length scale defined as a = A/P where A is the surface area and P is the perimeter of the plate edge [mm]
- AR
aspect ratio [L/H]
-
c
defocusing distance [mm]
-
d
image distance of Young's fringes from speckle negative
-
h
thermal convection coefficient [W/m2 · K]
-
average thermal convection coefficient [W/m2 · °C]
-
H
width of the test section measured perpendicular to the optic axis [mm]
-
k
thermal conductivity [W/m · K]
-
L
length of the test section measured parallel to the optical axis [mm]
-
n
index of refraction
-
Nu
local Nusselt number [ha/k]
-
global Nusselt number
-
Pr
Prandtl number [v/]
-
q
heat flux per unit area [W/m2 · s]
-
Ra
Rayleigh number
-
s
fringe spacing [mm]
-
Sc
Schmidt number [v/D]
-
T
temperature [K]
Greek symbols
thermal diffusivity [m2/s]
-
volumetric coefficient of expansion (1/T)
-
v
kinematic viscosity of air [m2/s]
-
wavelength of helium-neon laser [632.8 nm]
-
amount of speckle dislocation 相似文献
9.
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
10.
Heat transfer enhancement for laminar natural convection along a vertical plate due to sub-millimeter-bubble injection 总被引:1,自引:0,他引:1
Atsuhide Kitagawa Keita Kosuge Kenji Uchida Yoshimichi Hagiwara 《Experiments in fluids》2008,45(3):473-484
Sub-millimeter-bubble injection is one of the most promising techniques for enhancing heat transfer for the laminar natural
convection of liquids. However, flow and heat transfer characteristics for laminar natural convection of water with sub-millimeter
bubbles have not yet been fully understood. The purpose of this study is to experimentally clarify the effects of sub-millimeter-bubble
injection on the laminar natural convection of water along a heated vertical plate. The use of thermocouples and a particle
tracking velocimetry (PTV) technique are applied to temperature and velocity measurements, respectively. The temperature measurement
shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection increases
with an increase in the bubble flow rate or a decrease in the wall heat flux and that the ratio ranges from 1.35 to 1.85.
Moreover, it is concluded from simultaneous measurement of temperature and velocity that the heat transfer enhancement is
directly affected by flow modification due to bubbles rising near the heated vertical plate. 相似文献
11.
Natural convection heat transfer in a square cavity induced by heated electronic board (as a thin plate at constant temperature) is investigated using the lattice Boltzmann method. Lattice Boltzmann simulation of natural convective heat transfer in a cavity in the presence of internal straight obstacle has not been considered completely in the literature and this challenge is generally considered to be an open research topic that may require more study. The present work is an extension to our previous paper (see Nazari and Ramzani in Modares. Mech. Eng. 11(2):119–133, 2011) in which the effects of position and dimensions of obstacle on the flow pattern and heat transfer rate are completely studied. A suitable forcing term is represented in the Boltzmann equation. With the representation, the Navier–Stokes equation can be derived from the lattice Boltzmann equation through the Chapman-Enskog expansion. Top and bottom of the cavity are adiabatic; the two vertical walls of the cavity have constant temperatures lower than the plate’s temperature. The study is performed for different values of Grashof number ranging from 103 to 105 for different aspect ratios and position of heated plate. The effect of the position and aspect ratio of heated plate on heat transfer are discussed and the position of the obstacle in which the maximum rate of heat transfer is investigated in both vertical and horizontal situation. The obtained results of the lattice Boltzmann method are validated with those presented in the literature. 相似文献
12.
Atsuhide Kitagawa Kenji Uchida Yoshimichi Hagiwara 《International Journal of Heat and Fluid Flow》2009,30(4):778-788
Injection of sub-millimeter bubbles is considered a promising technique for enhancing natural convection heat transfer for liquids. So far, we have experimentally investigated heat transfer characteristics of laminar natural convection flows with sub-millimeter bubbles. However, the effects of the bubble size on the heat transfer have not yet been understood. The purpose of this study is to clarify the effects of the bubble size on the heat transfer enhancement for the laminar natural convection of water along a vertical heated plate with uniform heat flux. Temperature and velocity measurements, in which thermocouples and a particle tracking velocimetry technique are, respectively used, are conducted to investigate heat transfer and flow characteristics for different bubble sizes. Moreover, two-dimensional numerical simulations are performed to comprehensively understand the effects of bubble injection on the flow near the heated plate. The result shows that the ratio of the heat transfer coefficient with sub-millimeter-bubble injection to that without injection ranges from 1.3 to 2.2. The result also shows that for a constant bubble flow rate, the heat transfer coefficient ratio increases with a decrease in the mean bubble diameter. It is expected from our estimation based on both experimental data and simulation results that this increase results from an increase in the advection effect due to bubbles. 相似文献
13.
The Darcy flow model with the Boussinesq approximation is used to investigate numerically the natural convection inside an
inclined wavy cavity filled with a porous medium. Finite Element Method is used to discretize the governing differential equations
with non-staggered variable arrangement. Results are presented for
and
, where ϕ, Ra, A and λ correspond to the cavity inclination angle, Rayleigh number, aspect ratio and surface waviness parameter, respectively.
Stream and isotherm lines representing the corresponding flow and thermal fields, and local and average Nusselt numbers distribution
expressing the rate of heat transfer are determined and shown on graphs and tables. A good agreement is observed between the
present results and those known from the open literature. The flow and thermal structures found to be highly dependent on
surface waviness for inclination angles less than 45°, especially for high Rayleigh numbers. 相似文献
14.
This paper presents the results of an experimental study of buoyancy-driven convective heat transfer between three parallel
vertical plates, symmetrically spaced with water as the intervening medium. The centre plate was electrically heated, while
the other side plates were water-cooled forming two successive parallel vertical channels of dimensions 20 cm × 3.5 cm × 35 cm
(length W, gap L, height H) each. Top, bottom and sides of the channels were open to water in the chamber which is the novel aspect of this study. Plate
surface temperature and bath temperature at different levels of height from the bottom of channel were measured by K-type
thermocouples. Experimental data have been correlated as under:
相似文献
15.
Experimental Study on Oscillatory Natural Convection in a Hele-Shaw Cell due to Unstably Heated Side
The oscillatory motion of natural convection in a porous medium has been investigated experimentally using a Hele-Shaw cell
technique. The cell has been heated on the lower half and cooled on the upper half along the same vertical sidewall. Flows
have been visualized using the pH indicator method. Photographs of natural convection patterns as well as average Nusselt
number data have been presented for different Rayleigh numbers. Oscillatory motion of natural convection has been observed
for large enough Rayleigh numbers and the critical Rayleigh number has been estimated to be between 120 and 450. Scaling analysis
has been conducted to understand the heat transfer and the oscillating mechanism. According to the scaling analysis, it has
been found that the average Nusselt number is proportional to the square root of the Rayleigh number, and that the oscillation
frequency is proportional to the Rayleigh number. Obtained experimental data support the scaling analysis. 相似文献
16.
M. -Y. Wen 《Heat and Mass Transfer》2005,41(10):921-930
An experiment was carried out to investigate the characteristics of the heat transfer and pressure drop for forced convection airflow over tube bundles that are inclined relative to the on-coming flow in a rectangular package with one outlet and two inlets. The experiments included a wide range of angles of attack and were extended over a Reynolds number range from about 250 to 12,500. Correlations for the Nusselt number and pressure drop factor are reported and discussed. As a result, it was found that at a fixed Re, for the tube bundles with attack angle of 45 ° has the best heat transfer coefficient, followed by 60, 75 and 90 °, respectively. This investigation also introduces the factors
which can be used for finding the heat transfer and the pressure drop factor on the tube bundles positioned at different angles to the flow direction. Moreover, no perceptible dependence of Cand C on Re was detected. In addition, flow visualizations were explored to broaden our fundamental understanding of the heat transfer for the present study. 相似文献
17.
This paper studies combined heat and mass transfer by laminar natural convection from a vertical plate maintained with uniform surface heat flux and species concentration. Very accurate finite-difference solutions of a set of nonsimilarity equations have been obtained for most practical gaseous solutions (Pr?=?0.7, 0.21 ≤ Sc ≤ 2.1) and aqueous solutions (Pr?=?7, 140 ≤ Sc ≤ 1400). Variations of heat and mass transfer rates with buoyancy ratio and Lewis number are presented. Precise correlations have been developed for predicting heat and mass transfer rates of natural convection arising from single (solutal or thermal) buoyancy force and dual buoyancy forces. 相似文献
18.
The development of the mixed convection flow of an incompressible laminar viscous fluid over a semi-infinite vertical plate has been investigated when the fluid in the external stream is set into motion impulsively, and at the same the surface temperature is suddenly raised from its ambient temperature. The problem is formulated in such a way that at time t = 0, it reduces to Rayleigh type of equation and as time t , it tends to Blasius type of equation. The scale of time has been selected such that the traditional infinite region of integration becomes finite which significantly reduces the computational time. The nonlinear coupled singular parabolic partial differential equations governing the unsteady mixed convection flow have been solved numerically by using an implicit finite-difference scheme. The surface shear stress and the heat transfer increase or decrease with time when the buoyancy parameter is greater or less than a certain valve. There is a smooth transition from the initial steady state to the final steady state. The skin friction and heat transfer for the constant heat flux case are more than those of the constant wall temperature case. Also they increase with the buoyancy force. 相似文献
19.
An experimental investigation of the convective heat transfer on a flat surface in a multiple-jet system is described. A thin
metal sheet was heated electrically and cooled from one side. On the other black coated side the temperature field was measured
using an IR camera. Varied parameters were the jet Reynolds number in the range from 1,400 to 41,400, the normalized distance
nozzle to sheet H/d from 1 to 10, and the normalized nozzle spacing S/d from 2 to 10. A geometrical arrangement of nine nozzle in-line arrays was tested. The results show that the multiple-jet
system enhances the local and average heat transfer in comparison with that of a single nozzle. A maximum of the heat transfer
was found for the normalized spacing S/d = 6.0. The normalized distance H/d has nearly no effect on the heat transfer in the range 2 ≤ H/d ≤ 4. The maximum average Nusselt number was correlated as a function of the jet Reynolds number 相似文献
20.
The stability of the conduction regime of natural convection in a porous vertical slab saturated with an Oldroyd-B fluid has been studied. A modified Darcy’s law is utilized to describe the flow in a porous medium. The eigenvalue problem is solved using Chebyshev collocation method and the critical Darcy–Rayleigh number with respect to the wave number is extracted for different values of physical parameters. Despite the basic state being the same for Newtonian and Oldroyd-B fluids, it is observed that the basic flow is unstable for viscoelastic fluids—a result of contrast compared to Newtonian as well as for power-law fluids. It is found that the viscoelasticity parameters exhibit both stabilizing and destabilizing influence on the system. Increase in the value of strain retardation parameter \(\Lambda _2 \) portrays stabilizing influence on the system while increasing stress relaxation parameter \(\Lambda _1\) displays an opposite trend. Also, the effect of increasing ratio of heat capacities is to delay the onset of instability. The results for Maxwell fluid obtained as a particular case from the present study indicate that the system is more unstable compared to Oldroyd-B fluid. 相似文献