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1.
Three-dimensional turbulent forced convective heat transfer and flow characteristics, and the non-dimensional entropy generation
number in a helical coiled tube subjected to uniform wall temperature are simulated using the k–ε standard turbulence model. A finite volume method is employed to solve the governing equations. The effects of Reynolds number,
curvature ratio, and coil pitch on the average friction factor and Nusselt number are discussed. The results presented in
this paper cover a Reynolds number range of 2 × 104 to 6 × 104, a pitch range of 0.1–0.2 and a curvature ratio range of 0.1–0.3. The results show that the coil pitch, curvature ratio and
Reynolds number have different effects on the average friction factor and Nusselt number at different cross-sections. In addition,
the flow and heat transfer characteristics in a helical coiled tube with a larger curvature ratio for turbulent flow are different
from that of smaller curvature ratio for laminar and turbulent flow in certain ways. Some new features that are not obtained
in previous researches are revealed. Moreover, the effects of Reynolds number, curvature ratio, and coil pitch on the non-dimensional
entropy generation number of turbulent forced convection in a helical coiled tube are also discussed. 相似文献
2.
G. Juncu 《Heat and Mass Transfer》1998,34(2-3):203-208
The reverse of the transfer direction in the unsteady conjugate heat transfer between a spherical particle and a surrounding
fluid flow has been analysed. The aspect this work is focused on is the influence of the continuous phase convection on the
occurrence and development of this phenomenon. The energy equations are solved by the ADI finite difference method. The range
of the Pe numbers investigated is between 0 and 10. The ratios of the thermal conductivity and volume heat capacity between
the particle and its ambient flow belong to the interval 0.01–100. It was found that, in creeping flow, the thermal wake occurs
at Pe=0.690·10−3. Increasing the Pe number up to 1 the dimension of thermal wake increases. For Pe>1, the increase in Pe decreases thermal
wake.
Received on 13 January 1998 相似文献
3.
The problem of heat convection from a vertically oscillating cylinder in a quiescent fluid is investigated. The governing
equations of motion and energy are solved numerically in a non-inertial frame of references to determine the flow field and
heat transfer characteristics under different conditions. The main dominating parameters are Keulegan–Carpenter number, KC,
frequency parameter, β, Grashof number, Gr and Prandtl number, Pr. The ranges considered for these parameters are KC ≤ 10,
β≤40 and Gr ≤ 105 while Prandtl number is kept constant. The study revealed that the effect of amplitude and frequency of oscillation on heat
transfer is strongly influenced by the Grashof number range. In the forced convection regime (Gr = 0), the increase of KC
creates extensive vortex motion at all cylinder positions that leads to a significant increase in heat transfer. A similar
trend, but with a lesser extent, is also observed for the increase of β. However, at high Grashof numbers, the effect of oscillation
on heat convection is only significant at large values of KC.
Received on 5 June 2000 / Published online: 29 November 2001 相似文献
4.
Theoretical analysis of convective heat transfer enhancement of microencapsulated phase change material slurries 总被引:3,自引:0,他引:3
This paper analyzes the convective heat transfer enhancement mechanism of microencapsulated phase change material slurries based on the analogy between convective heat transfer and thermal conduction with thermal sources. The influence of each factor affecting the heat transfer enhancement for laminar flow in a circular tube with constant wall temperature is analyzed using an effective specific heat capacity model. The model is validated with results available in the literature. The analysis and the results clarify the heat transfer enhancement mechanism and the main factors influencing the heat transfer. In addition, the conventional Nusselt number definition of phase change slurries for internal flow is modified to describe the degree of heat transfer enhancement of microencapsulated phase change material slurries. The modification is also consistent evaluation of the convective heat transfer of internal and external flows.c volumetric concentration of microcapsules - cm mass concentration of microcapsules - cp specific heat, kJ kg–1 K–1 - hfs phase change material heat of fusion, kJ kg–1 - hm* modified convective heat transfer coefficient, W m–2 K–1 - k thermal conductivity, W m–1 K–1 - ke effective thermal conductivity of slurry, W m–1 K–1 - kb slurry bulk thermal conductivity, W m–1 K–1 - ML dimensionless initial subcooling - Mr dimensionless phase change temperature range - Nu conventional Nusselt number - Nu* improved Nusselt number - qwn wall heat flux, Wm–2 - Pe Peclet number - Pr Prandtl number - Re Reynolds number - r radial coordinate, m - r0 duct radius, m - r1 dimensionless radial coordinate - Ste Stefan number - T temperature, K - T1 lower phase change temperature limit, K - T2 upper phase change temperature limit, K - Ti slurry inlet temperature, K - u axial velocity, m/s - v radial velocity, m/s - x axial coordinate, m - x1 dimensionless axial coordinate - thermal diffusivity, m2/s - dimensionless temperature - dynamic viscosity, N·s/m2 - kinematic viscosity, m2/s - t width of thermal boundary, m - degree of heat transfer enhancement, = hm*/(hm*)single - b bulk fluid (slurry) - b0 slurry without phase change - l liquid - m mean - s solid - f suspending fluid - p microcapsule particles - w wall - single single-phase fluid 相似文献
5.
Free convection heat transfer along an isothermal vertical wavy surface was studied experimentally and numerically. A Mach-Zehnder
Interferometer was used in the experiment to determine the local heat transfer coefficients. Experiments were done for three
different amplitude–wavelength ratios of α = 0.05, 0.1, 0.2 and the Rayleigh numbers ranging from Ra
l
= 2.9 × 105 to 5.8 × 105. A finite-volume based code was developed to verify the experimental study and obtain the results for all the amplitude–wavelength
ratios between α = 0 to 0.2. It is found that the numerical results agree well with the experimental data. Results indicate
that the frequency of the local heat transfer rate is the same as that of the wavy surface. The average heat transfer coefficient
decreases as the amplitude–wavelength ratio increases and there is a significant difference between the average heat transfer
coefficients of the surface with α = 0.2 and those surfaces with α = 0.05 and 0.1. The experimental data are correlated
with a single equation which gives the local Nusselt number along the wavy surface as a function of the amplitude–wavelength
ratio and the Rayleigh number. 相似文献
6.
Numerical studies were conducted to investigate the natural convection heat transfer around a uniformly heated thin plate
with arbitrary inclination in an infinite space. The numerical approach was based on the finite volume technique with a nonstaggered
grid arrangement. For handling the pressure–velocity coupling the SIMPLE-algorithm was used. QUICK scheme and first order
upwind scheme were employed for discretization of the momentum and energy convective terms respectively. Plate width and heating
rate were used to vary the modified Rayleigh number over the range of 4.8×106 to 1.87×108. Local and average heat transfer characteristics were compared with regarding to the inclination angle. The empirical expressions
for local and average Nusselt number were correlated. It has been found that for inclination angle less than 10∘, the flow and heat transfer characteristics are complicated and the average Nusselt number can not be correlated by one equation
while for inclination angle larger than 10∘, the average Nusselt number can be correlated into an elegant correlation.
Received on 18 April 2001 / Published online: 29 November 2001 相似文献
7.
T. Aihara K. Yamamoto K. Narusawa T. Haraguchi M. Ukaku A. Lasek F. Feuillebois 《Heat and Mass Transfer》1997,33(1-2):109-120
Heat transfer characteristics of a turbulent, dilute air-solids suspension flow in thermally developing/developed regions
were experimentally studied, using a uniformly heated, horizontal 54.5 mm-ID pipe and 43-μm-diameter glass beads. The local
heat transfer was measured at 27 locations from the inlet to 120-dia downstream of the heated section over a range of Reynolds
numbers 3×104−1.2×105 and solids loading ratio 0–3, and the fully developed profiles of air velocity/temperature and particle mass flux were measured
at a location 140-dia downstream of the heated section using specially designed probes, inserted into the suspension flow.
The effects of the Reynolds number, solids loading ratio, and azimuthal/longitudinal locations on the heat transfer characteristics
and their interactions are discussed through comparison of the present results with the data obtained by other investigators.
Received on 14 October 1996 相似文献
8.
The main objective of this article is to study the effect of discrete heating on free convection heat transfer in a rectangular
porous enclosure containing a heat-generating substance. The left wall of the enclosure has two discrete heat sources and
the right wall is isothermally cooled at a lower temperature. The top and bottom walls, and the unheated portions of the left
wall are adiabatic. The vorticity–stream function formulation of the governing equations is numerically solved using an implicit
finite difference method. The effects of aspect ratio, Darcy number, heat source length, and modified Rayleigh number on the
flow and heat transfer are analyzed. The numerical results reveal that the rate of heat transfer increases as the modified
Rayleigh number and the Darcy number increases, but decreases on increasing the aspect ratio. The average heat transfer rate
is found to be higher at the bottom heater than at the top heater in almost all considered parameter cases except for ε = 0.5. Also, the maximum temperature takes place generally at the top heater except for the case ε = 0.5, where the maximum temperature is found at the bottom heater. Further, the numerical results reveal that the maximum
temperature decreases with the modified Rayleigh number and increases with the aspect ratio. 相似文献
9.
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
10.
Natural convection in a partially filled porous square cavity is numerically investigated using SIMPLEC method. The Brinkman-Forchheimer
extended model was used to govern the flow in the porous medium region. At the porous-fluid interface, the flow boundary condition
imposed is a shear stress jump, which includes both the viscous and inertial effects, together with a continuity of normal
stress. The thermal boundary condition is continuity of temperature and heat flux. The results are presented with flow configurations
and isotherms, local and average Nusselt number along the cold wall for different Darcy numbers from 10−1 to 10−6, porosity values from 0.2 to 0.8, Rayleigh numbers from 103 to 107, and the ratio of porous layer thickness to cavity height from 0 to 0.50. The flow pattern inside the cavity is affected
with these parameters and hence the local and global heat transfer. A modified Darcy–Rayleigh number is proposed for the heat
convection intensity in porous/fluid filled domains. When its value is less than unit, global heat transfer keeps unchanged.
The interfacial stress jump coefficients β
1 and β
2 were varied from −1 to +1, and their effects on the local and average Nusselt numbers, velocity and temperature profiles
in the mid-width of the cavity are investigated. 相似文献
11.
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
12.
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
13.
Experiments were performed by using water and paraffin slurry to investigate thermal characteristics from a test multichip
module. The parameters were the mass fraction of paraffin slurry (0, 2.5, 5, 7.5%), heat flux (10, 20, 30, 40 W/cm2) and channel Reynolds numbers. The size of paraffin slurry particles was within 10–40 μm. The local heat transfer coefficients
for the paraffin slurry were larger than those for water. Thermally fully developed conditions were observed after the third
or fourth row. The paraffin slurry with a mass fraction of 5% showed the most efficient cooling performance when the heat
transfer and the pressure drop in the test section were considered simultaneously. A new correlation for the water and the
paraffin slurry with a mass fraction of 5% was obtained for a channel Reynolds number over 5300.
Received on 25 January 1999 相似文献
14.
The buoyancy effects on the boundary layers induced by continuous surfaces stretched with rapidly decreasing velocities 总被引:1,自引:0,他引:1
Mohamed E. Ali 《Heat and Mass Transfer》2004,40(3-4):285-291
Heat and mass transfer characteristics of the self-similar boundary layer flows induced by continuous surfaces stretched with rapidly decreasing power law velocities Uw xm, m < –1 are considered for mixed convection flow. The effect of various governing parameters, such as Prandtl number Pr, temperature exponent n, dimensionless injection/suction velocity fw, and the mixed convection parameter = s Gr/Re2 are studied. These parameters have great effects on velocity and temperature profiles, heat transfer coefficient, and skin friction coefficient at the moving surface. Results show that similarity solutions exist only when the condition n = 2m – 1 is satisfied. Critical values of , Nu/Re0.5 and Cf Re0.5 are obtained for predominate natural convection for different Prandtl numbers at m = –2, –6 and n = –5, and –13 respectively. Results also show that the effect of buoyancy is more significant for weak than for strong suction. Furthermore, critical Prandtl numbers where fw profiles have minimums are obtained for m = –2 and –6. Finally, critical values of , Cf Re0.5 are also obtained for predominate natural convection for both m = –2 and –6. 相似文献
15.
Tzer-Ming Jeng Sheng-Chung Tzeng Yen-Chan Wang Chao-Hsien Lin 《Heat and Mass Transfer》2008,44(5):527-533
This work experimentally investigates the heat transfer characteristics in the annulus with an inner rotating rib-roughness
cylinder, whose flow and thermal behaviors are associated with Taylor number (Ta) and centrifugal buoyancy parameter (Gr
Ω/Ta). The operating range of Ta is from 4.90 × 102 to 5.80 × 105, while the surface of the inner cylinder is heated up with several constant heat fluxes (279, 425 and 597 W/m2) to obtain various values of Gr
Ω/Ta. Besides, three modes of the inner cylinder without/with longitudinal ribs are considered. The end of the annular channel
is connected to a side chamber to fit practical applications (such as in the rotary blade coupling of a four-wheel-drive vehicle).
The experimental results show that the average Nusselt number
was almost constant at low Ta, but increased rapidly with Ta when Ta exceeded some critical value (3,000–5,200 for present study). Additionally, the Gr
Ω/Ta effect on the heat transfer was negligible herein. Furthermore, by comparing with the inner cylinder without longitudinal
ribs, stalling ribs on the inner cylinder increases the transport of heat by a factor of 1.22 at 105 < Ta < 106, and embedding cavities into the ribs increases the transport of heat by a factor of 1.16 at 105 < Ta < 106. Finally, the relationships between the and the Ta for various modes of test sections were proposed. 相似文献
16.
Javad Rostami 《Heat and Mass Transfer》2008,44(9):1079-1087
In this paper, unsteady heat transfer and fluid flow characteristics in an enclosure are investigated. The enclosure consists
of two vertical wavy and two horizontal straight walls. The top and the bottom walls are considered adiabatic. Two wavy walls
are kept isothermal and their boundaries are approximated by a cosine function. Governing equations including continuity,
momentum and energy were discretized using the finite-volume method and solved by SIMPLE method in curvilinear coordinate.
Simulation was carried out for a range of Grashof number Gr = 103–106, Prandtl number Pr = 0.5–4.0, wave ratio A (defined by amplitude/wavelength) 0.0–0.35 and aspect ratio W (defined by average width/wavelength) 0.5–1.0. Streamlines and isothermal lines are presented to corresponding flow and thermal
fields. Local and average Nusselt number distributions are presented. The obtained results are in good agreement with available
numerical and experimental data. 相似文献
17.
The three-dimensional unsteady problem of the action of a localized radiation pulse on an aluminum plate of finite thickness
in a tangential airflow is solved on the basis of the Euler equations in the gas phase and the heat conduction equation in
the solid. The process is accompanied by the intense evaporation of plate material and the subsequent (after the action of
the radiation pulse ceases) intense condensation of aluminum vapor and condensate particles on the surface. The problem is
solved on the assumption that the surface is not screened by its disintegration products and the processes associated with
the self-radiation of the gas are disregarded. Heat pulses of intensityq=106−107 W/cm2 and durationt
i
≈10−4 sec are investigated. Various regimes are calculated and the effect of the governing parameters on the impulse transmitted
to the surface and ablation of the target material is established. A nontraditional approach, based on the idea of statistical
modeling of the motion of an inviscid non-heat-conducting gas at the kinetic level, is used for solving the gas dynamic problem.
The application of the approach in question to the solution of problems of this class is justified.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 139–146, January–February,
1994. 相似文献
18.
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
19.
Mixed convection flow in a two-sided lid-driven cavity filled with heat-generating porous medium is numerically investigated.
The top and bottom walls are moving in opposite directions at different temperatures, while the side vertical walls are considered
adiabatic. The governing equations are solved using the finite-volume method with the SIMPLE algorithm. The numerical procedure
adopted in this study yields a consistent performance over a wide range of parameters that were 10−4 ≤ Da ≤ 10−1 and 0 ≤ Ra
I
≤ 104. The effects of the parameters involved on the heat transfer characteristics are studied in detail. It is found that the
variation of the average Nusselt number is non-linear for increasing values of the Darcy number with uniform or non-uniform
heating condition. 相似文献
20.
A numerical and experimental study has been made on the flow and heat transfer in inclined air-filled cavities with aspect
ratios 1–18 at Ra numbers from 2·104–5·105 and angles of inclination from 40 to 90°. Core stratification influences the flow. Due to this there arises a torque with
two components depending on angle of inclination. On basis of the two torques the computed effects on flow and temperature
fields can be explained. For the heat transfer a scaling law could be derived. Experimental data validate the numerical studies. 相似文献