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1.
Ali Benhmidene Béchir Chaouachi Slimane Gabsi Mahmoud Bourouis 《Heat and Mass Transfer》2011,47(11):1341-1347
In the present study, the heat flux received by a bubble pump, which was simulated to a vertical tube 1 m long and with a
variable diameter, was optimized. A numerical study was carried out in order to solve balance equations concerning the water-ammonia
mixture in the up flow. The two-fluid model was used to derive the equations. A numerical study was carried out on a heat
flux between 1 and 70 kW m−2 and the liquid velocity was determined. The optimum flux was determined for a tube diameter equal to 4, 6, 8 and 10 mm and
a mass flow rate ranging from 10 to 90 kg m−2 s−1. The optimum heat flux was correlated as a function of the tube diameter and mass flow rate, while the minimum heat flux
required for pumping was correlated as a function of the tube diameter. 相似文献
2.
This paper presents an experimental and theoretical investigation of drying of moist slab, cylinder and spherical products
to study dimensionless moisture content distributions and their comparisons. Experimental study includes the measurement of
the moisture content distributions of slab and cylindrical carrot, slab and cylindrical pumpkin and spherical blueberry during
drying at various temperatures (e.g., 30, 40, 50 and 60°C) at specific constant velocity (U = 1 m/s) and the relative humidity φ = 30%. In theoretical analysis, two moisture transfer models are used to determine drying process parameters (e.g., drying
coefficient and lag factor) and moisture transfer parameters (e.g., moisture diffusivity and moisture transfer coefficient),
and to calculate the dimensionless moisture content distributions. The calculated results are then compared with the experimental
moisture data. A considerably high agreement is obtained between the calculations and experimental measurements for the cases
considered. The effective diffusivity values were evaluated between 0.741 × 10−5 and 5.981 × 10−5 m2/h for slab products, 0.818 × 10−5 and 6.287 × 10−5 m2/h for cylindrical products and 1.213 × 10−7 and 7.589 × 10−7 m2/h spherical products using the Model-I and 0.316 × 10−5–5.072 × 10−5 m2/h for slab products, 0.580 × 10−5–9.587 × 10−5 m2/h for cylindrical products and 1.408 × 10−7–13.913 × 10−7 m2/h spherical products using the Model-II. 相似文献
3.
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
4.
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
5.
Natural convection in a fluid saturated porous medium has been numerically investigated using a generalized non-Darcy approach.
The governing equations are solved by using Finite Volume approach. First order upwind scheme is employed for convective formulation
and SIMPLE algorithm for pressure velocity coupling. Numerical results are presented to study the influence of parameters
such as Rayleigh number (106 ≤Ra ≤108), Darcy number (10−5 ≤ Da ≤ 10−2), porosity (0.4 ≤ ɛ ≤ 0.9) and Prandtl number (0.01 ≤ Pr ≤ 10) on the flow behavior and heat transfer. By combining the method of matched asymptotic expansions with computational
fluid dynamics (CFD), so called asymptotic computational fluid dynamics (ACFD) technique has been employed to generate correlation
for average Nusselt number. The technique is found to be an attractive option for generating correlation and also in the analysis
of natural convection in porous medium over a fairly wide range of parameters with fewer simulations for numerical solutions. 相似文献
6.
An experimental investigation of flow boiling heat transfer in a commercially available microfin tube with 9.52 mm outer diameter
has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin
height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated.
The experiments have been performed at saturation temperatures between 0 and −20°C. The mass flux was varied between 25 and
150 kg/m2s, the heat flux from 15,000 W/m2 down to 1,000 W/m2. All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. The measured heat transfer
coefficients range from 1,300 to 15,700 W/m2K for R134a and from 912 to 11,451 W/m2K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The mean heat transfer
coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within ±30% and
±15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m2s, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted
to lower mass fluxes for the microfin tube compared to the smooth tube. 相似文献
7.
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
8.
The influence of oil on nucleate pool boiling heat transfer 总被引:1,自引:0,他引:1
The influence of various oil contents in R134a is investigated for nucleate pool boiling on copper tubes either sandblasted
or with enhanced heating surfaces (GEWA-B tube). Polyolester oils (POE) (Reniso Triton) with medium viscosity 55 cSt (SE55)
and high viscosity 170 cSt (SE170) were used. Heat transfer coefficients were obtained for boiling temperatures between −28.6
and +20.1°C. The oil content varied from 0 to 5% mass fraction. For the sandblasted tube and the SE55 oil the heat transfer
coefficients for the refrigerant/oil-mixture can be higher or lower than those for the pure refrigerant, depending on oil
mass fraction, boiling temperature and heat flux. In some cases the highest heat transfer coefficients were obtained at a
mass fraction of 3%. For the 170 cSt oil there is a clear decrease in heat transfer for all variations except for a heat flux
4,000 W/m2 and −10.1°C at 0.5% oil content. The heat transfer coefficients are compared to those in the literature for a smooth stainless
steel tube and a platinum wire. For the enhanced tube and 55 cSt oil the heat transfer coefficients are clearly below those
for pure refrigerant in all cases. The experimental results for the sandblasted tube are compared with the correlation by
Jensen and Jackman. The calculated values are within +20 and −40% for the medium viscosity oil and between +50% and −40% for
the high viscosity oil. A correlation for predicting oil-degradation effects on enhanced surfaces does not exist. 相似文献
9.
Interactive effect of oxygen diffusion and volatiles advection on transient thermal degradation of poly methyl methacrylate (PMMA) 总被引:1,自引:0,他引:1
A transient, one-dimensional model has been presented to formulate the substantial role of polymer gasification in the early
stages of fire growth. The present model comprises the interaction between the oxygen diffusion and the released volatiles
on the rate of polymer gasification, when the polymeric sample is subjected to an external radiative source. The model also
includes different mechanisms affecting the degradation process such as in-depth thermal and oxidative decomposition, in-depth
absorption of radiation and heat and mass transfer in the both gas and solid phases. The results for two different radiative
heat sources (17 and 40 kW m−2) are reported and yielded realistic results, comparing to the published experimental data. It was found that an increase
in the oxygen concentration will lead to a considerable decrease in the surface temperature as well as significant increase
of gasification rate at 17 kW m−2; nevertheless this effect is less apparent at 40 kW m−2. 相似文献
10.
The steady mixed convection boundary-layer flow over a vertical impermeable surface in a porous medium saturated with water
at 4°C (maximum density) when the surface heat flux varies as x
m
and the velocity outside the boundary layer varies as x
(1+2m)/2, where x measures the distance from the leading edge, is discussed. Assisting and opposing flows are considered with numerical solutions
of the governing equations being obtained for general values of the flow parameters. For opposing flows, there are dual solutions
when the mixed convection parameter λ is greater than some critical value λ
c
(dependent on the power-law index m). For assisting flows, solutions are possible for all values of λ. A lower bound on m is found, m > −1 being required for solutions. The nature of the critical point λ
c
is considered as well as various limiting forms; the forced convection limit (λ = 0), the free convection limit (λ → ∞) and
the limits as m → ∞ and as m → −1. 相似文献
11.
A numerical study of laminar forced convective flows of three different nanofluids through a horizontal circular tube with
a constant heat flux condition has been performed. The effect of Al2O3 volume concentration 0 ≤ φ ≤ 0.09 in the pure water, water-ethylene glycol mixture and pure ethylene glycol as base fluids,
and Reynolds number of 100 ≤ Re ≤ 2,000 for different power inputs in the range of 10 ≤ Q(W) ≤ 400 have been investigated. In this study, all of the nanofluid properties are temperature and nanoparticle volume concentration
dependent. The governing equations have been solved using finite volume approach with the SIMPLER algorithm. The results indicate
an increase in the averaged heat transfer coefficient with increasing the mass of ethylene glycol in the water base fluid,
solid concentration and Reynolds number. From the investigations it can be inferred that, the pressure drop and pumping power
in the nanofluids at low solid volumetric concentration (φ < 3%) is approximately the same as in the pure base fluid in the
various Reynolds numbers, but the higher solid nanoparticle volume concentration causes a penalty drop in the pressure. Moreover,
this study shows it is possible to achieve a higher heat transfer rate with lower wall shear stress with the use of proper
nanofluids. 相似文献
12.
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
13.
The present paper focuses on the analysis of unsteady flow and heat transfer regarding an axisymmetric impinging synthetic
jet on a constant heat flux disc. Synthetic jet is a zero net mass flux jet that provides an unsteady flow without any external
source of fluid. Present results are validated against the available experimental data showing that the SST/k − ω turbulence model is more accurate and reliable than the standard and low-Re k − ε models for predicting heat transfer from an impinging synthetic jet. It is found that the time-averaged Nusselt number
enhances as the nozzle-to-plate distance is increased. As the oscillation frequency in the range of 16–400 Hz is increased,
the heat transfer is enhanced. It is shown that the instantaneous Nu distribution along the wall is influenced mainly by the interaction of produced vortex ring and wall boundary layer. Also,
the fluctuation level of Nu decreases as the frequency is raised. 相似文献
14.
The heat transfer, pressure drop and flow patterns during flow boiling of R407C in a horizontal microfin tube have been investigated.
The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and
the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long. It is heated electrically. The experiments have
been performed at saturation temperatures between −30°C and +10°C. The mass flux was varied between 25 and 300 kg/m2/s, the heat flux from 20,000 W/m2 down to 1,000 W/m2. The vapour quality was kept constant at 0.1, 0.3, 0.5, 0.7 at the inlet and 0.8, 1.0 at the outlet, respectively. The measured
heat transfer coefficient is compared with the correlations of Cavallini et al., Shah as well as Zhang et al. Cavallini’s
correlation contains seven experimental constants. After fitting these constants to our measured values, the correlation achieves
good agreement. The measured pressure drop is compared to the correlations of Pierre, Kuo and Wang as well as Müller-Steinhagen
and Heck. The best agreement is achieved with the correlation of Kuo and Wang. Almost all values are calculated within an
accuracy of ±30%. The flow regimes were observed. It is shown, that changes in the flow regime affect the heat transfer coefficient
significantly. 相似文献
15.
A numerical analysis of convective drying of a 3D porous solid of brick material is carried out using the finite element method
and mass lumping technique. The energy equation and moisture transport equations for the porous solid are derived based on
continuum approach following Whitaker’s theory of drying. The governing equations are solved using the Galerkin’s weighted
residual method, which convert the governing equations into discretized form of matrix equations. The resulting capacitance
matrices are made diagonal matrices by following the classical row-sum mass lumping technique. Hence with the use of the Eulerian
time marching scheme, the final equations are reduced to simple algebraic equations, which can be solved directly without
using an equation solver. The proposed numerical scheme is initially validated with experimental results for 1D drying problem
and then tested by application to convective drying of 3D porous solid of brick material for four different aspect ratios
obtained by varying the cross section of the solid. The mass lumping technique could correctly predict the wet bulb temperature
of the solid under evaporative drying conditions. A parametric study carried out for three different values of convective
heat transfer coefficients, 15, 30 and 45 W/m2 K shows an increased drying rate with increase in area of cross section and convective heat transfer coefficient. The proposed
numerical scheme could correctly predict the drying behavior shown in the form of temperature and moisture evolutions. 相似文献
16.
This paper presents the results of an experimental study of convective heat transfer between three parallel vertical plates
symmetrically spaced with and without bubble agitation to ascertain the degree of augmentation of the heat transfer coefficients
due to agitation. The centre plate was electrically heated, while the other side plates were water-cooled forming two successive
parallel vertical rectangular channels of dimensions 20 cm × 3.5 cm × 35 cm (length W, gap L, height H) each. At the bottom of the hot and cold plates air spargers were fitted. Water/ethylene glycol (100%) was used to fill the
channels. The superficial gas velocity ranged from 0.0016 to 0.01 m/s. Top, bottom and sides of the channels were open to
the water/ethylene glycol in the chamber which is the novel aspect of this study. Experimental data have been correlated as
under: Natural convective heat transfer: Nu = 0.60 Gr
0.29, r = 0.96, σ = 0.186, 1.17 E6 < Gr < 1.48 E7; Bubble agitated heat transfer: St = 0.11(ReFrPr
2)−0.23, r = 0.82, σ = 0.002, 1.20 E−2 < (ReFrPr
2) < 1.36 E2. 相似文献
17.
The effects of different drying temperatures on the drying kinetics of tomato slices were investigated using a cabinet-type
dryer. The experimental drying data were fitted best to the to the Page and Modified Page models apart from other theoretical
models to predict the drying kinetics. The effective moisture diffusivities varied from 1.015 × 10−9 to 2.650 × 10−9 m2 s−1over the temperature range studied, and activation energy was 22.981 kJ mol−1. 相似文献
18.
T. C. Jue 《Heat and Mass Transfer》2000,36(4):285-294
Fluid flow and heat transfer phenomena in partially divided cavities filled with porous media have been numerically studied
in this research. A non-Darcy generalized formulation is applied to describe the behavior of fluid flow in porous media. A
splitting semi-implicit finite element method is adopted to solve the governing equations. The range of Ra involved in this
study is between 104 and 106. Three different locations of dividers are investigated to probe the geometrical effect on heat and fluid flow. The results
of Da = 10−2 display a trend similar to the non-porous medium, but those of Da = 10−4 show dramatic decrease in flow strength, as well as heat transfer rate. A different location of divider may change the local
and average Nusselt numbers.
Received on 10 August 1999 相似文献
19.
Natural convective heat transfer from an isothermal narrow flat plate embedded in a plane adiabatic surface and inclined at
moderate positive and negative angles to the vertical has been numerically and experimentally studied. The solution has the
Rayleigh number, the dimensionless plate width, the angle of inclination, and the Prandtl number as parameters. Attention
was restricted to a Prandtl number of 0.7. The numerical results have been obtained for Rayleigh numbers between 103 and 107 for dimensionless plate widths of between 0.3 and 1.2 and for angles of inclination between +45° and −45°. In the experimental
study, results have been obtained for Rayleigh numbers between 4 × 102 and 105 for dimensionless plate widths of 0.4 and 2.5 and for angles of inclination between +45° and −45° to the vertical. Empirical
equations for the heat transfer rate have been derived. 相似文献
20.
The main objective of this article is to describe the drying process of ceramic roof tiles, shaped from red clay, using diffusion
models. Samples of the product with initial moisture content of 0.24 (db) were placed inside an oven in the temperatures of
55.6, 69.7, 82.7 and 98.6°C; and the data of the drying kinetics were obtained. The analytical solutions of the diffusion
equation for the parallelepiped with boundary conditions of the first and third kinds were used to describe the drying processes.
The process parameters were determined using an optimization algorithm based on inverse method coupled to the analytical solutions.
The analysis of the results makes it possible to affirm that the boundary condition of the third kind satisfactorily describes
the drying processes. The values obtained for the convective mass transfer coefficient were between 8.25 × 10−7 and 1.64 × 10−6 m s−1, and for the effective water diffusivity were between 9.21 × 10−9 and 1.80 × 10−8 m2 s−1. 相似文献