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1.
For radiative transfer in complex geometries, Sakami and his co-workers have developed a discrete ordinates method (DOM) exponential
scheme for unstructured meshes which was mainly applied to gray media. The present study investigates the application of the
unstructured exponential scheme to a wider range of non-gray scenarios found in fire and combustion applications, with the
goal to implement it in an in-house Computational Fluid Dynamics (CFD) code for fire simulations. The original unstructured
gray exponential scheme is adapted to non-gray applications by employing a statistical narrow-band/correlated-k (SNB-CK) gas
model and meshes generated using the authors’ own mesh generator. Different non-gray scenarios involving spectral gas absorption
by H2O and CO2 are investigated and a comparative analysis is carried out between heat flux and radiative source terms predicted and literature
data based on ray-tracing and Monte Carlo methods. The maximum discrepancies for total radiative heat flux do not typically
exceed 5%. 相似文献
2.
In the present work, the interaction between non-gray radiation and forced convection in a laminar radiating gas flow over a recess including two backward and forward facing steps in a duct is investigated numerically. Distributions of absorption coefficients across the spectrum (50 cm?1 < η < 20,000 cm?1) are obtained from the HITRAN2008 database. The full-spectrum k-distribution method is used to account for non-gray radiation properties, while the gray radiation calculations are carried out using the Planck mean absorption coefficient. To find the divergence of radiative heat flux distribution, the radiative transfer equation is solved by the discrete ordinates method. The effects of radiation–conduction parameter, wall emissivity, scattering coefficient and recess length on heat transfer behaviors of the convection–radiation system are investigated for both gray and non-gray mediums. In addition, the results of gray medium are compared with non-gray results in order to judge if the differences between these two approaches are significant enough to justify the usage of non-gray models. Results show that for air mixture with 10 % CO2 and 20 % H2O, use of gray model for the radiative properties may cause significant errors and should be avoided. 相似文献
3.
H. H. Liakos E. P. Keramida M. A. Founti N. C. Markatos 《Heat and Mass Transfer》2002,38(4-5):425-432
Impinging jet combusting flows on granite plates are studied. A mathematical model for calculating heat release in turbulent
impinging premixed flames is developed. The combustion including radiative heat transfer and local extinction effects, and
flow characteristics are modeled using a finite volume computational approach. Two different eddy viscosity turbulence models,
namely the standard k–ɛ and the RNG k–ɛ model with and without radiation (discrete transfer model) are assessed. The heat released predictions are compared with
experimental data and the agreement is satisfactory only when both radiative heat transfer and local extinction modeling are
taken into account. The results indicate that the main effect of radiation is the decrease of temperature values near the
jet stagnation point and along the plate surface. Radiation increases temperature gradients and affects predicted turbulence
levels independently of the closure model used. Also, the RNG k–ɛ predicts higher temperatures close the solid plate, with and without radiative heat transfer.
Received on 13 November 2000 / Published online: 29 November 2001 相似文献
4.
Open-pore Ti foam samples with porosity in the range of 10–70% and average pore size of 150–400 μm was fabricated by powder
metallurgy method using polymethyl methacrylate (PMMA) as space holder initially. The resulting foam is anisotropic: the pores
are spheroidal, being shorter along the pressing direction than in the pressing plane. The pore anisotropy decreases as the
size of the polymethyl methacrylate (PMMA) particles used increases and hence with pore size, which leads to a higher conductivity
in the plane of the pressing. As the porosity increases, the conductivity of porous Ti decreases dramatically. The porosity
e{\varepsilon} dependence of the electrical conductivity σ could be well described by Maxwell approximation, while the differential effective medium approximation is only suitable
to porous Ti with finite size of 400 μm in the porosity range of 40–70%, i.e., high porosity metal with randomly oriented
spheroids. 相似文献
5.
Conjugate natural convection-conduction heat transfer in a square porous enclosure with a finite-wall thickness is studied
numerically in this article. The bottom wall is heated and the upper wall is cooled while the verticals walls are kept adiabatic.
The Darcy model is used in the mathematical formulation for the porous layer and the COMSOL Multiphysics software is applied
to solve the dimensionless governing equations. The governing parameters considered are the Rayleigh number (100 ≤ Ra ≤ 1000), the wall to porous thermal conductivity ratio (0.44 ≤ K
r ≤ 9.90) and the ratio of wall thickness to its height (0.02 ≤ D ≤ 0.4). The results are presented to show the effect of these parameters on the heat transfer and fluid flow characteristics.
It is found that the number of contrarotative cells and the strength circulation of each cell can be controlled by the thickness
of the bottom wall, the thermal conductivity ratio and the Rayleigh number. It is also observed that increasing either the
Rayleigh number or the thermal conductivity ratio or both, and decreasing the thickness of the bounded wall can increase the
average Nusselt number for the porous enclosure. 相似文献
6.
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
7.
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
8.
A numerical investigation of the steady-state, laminar, axi-symmetric, mixed convection heat transfer in the annulus between
two concentric vertical cylinders using porous inserts is carried out. The inner cylinder is subjected to constant heat flux
and the outer cylinder is insulated. A finite volume code is used to numerically solve the sets of governing equations. The
Darcy–Brinkman–Forchheimer model along with Boussinesq approximation is used to solve the flow in the porous region. The Navier–Stokes
equation is used to describe the flow in the clear flow region. The dependence of the average Nusselt number on several flow
and geometric parameters is investigated. These include: convective parameter, λ, Darcy number, Da, thermal conductivity ratio,
K
r, and porous-insert thickness to gap ratio (H/D). It is found that, in general, the heat transfer enhances by the presence of porous layers of high thermal conductivity
ratios. It is also found that there is a critical thermal conductivity ratio on which if the values of Kr are higher than
the critical value the average Nusselt number starts to decrease. Also, it found that at low thermal conductivity ratio (K
r ≈ 1) and for all values of λ the porous material acts as thermal insulation. 相似文献
9.
Numerical solutions are obtained for the system of integro-differential equations describing the flow of a viscous, heat-conducting, selectively radiating gas in the region between the shock wave and a blunt body. The calculations are made for bodies of radius from 0.1 to 3 m with stagnation temperature from 6000° to 15 000° K. As a result of the calculations the convective and radiative thermal fluxes in the vicinity of the stagnation point are obtained. The effect of injection on convective and radiative heat transfer is studied.The first calculations of radiative thermal fluxes in air were made about 10 years ago in [1,2]. However, the results did not take account of the effects of emission and reabsorption, nor the interaction of the convective and radiative heating processes. These effects have been studied primarily with the use of simplified models of a radiating gas. Most often the approximation used is that of a gray gas with absorption coefficient which is independent of wavelength ([3–6] and others).The appearance in the literature of quite detailed data on the selective spectral absorption coefficients of air over a wide temperature range [7,8] has made it possible to solve the direct problem of calculating the flow field of a selectively radiating gas behind a shock wave with account for all the effects mentioned above. 相似文献
10.
This paper is devoted to transient heat transfer involving radiation and conduction. Considering a non-gray purely absorbing
media, the radiative heat transfer equation (RTE) is solved iteratively with the Discrete Ordinates Method (DOM) using an
exponential differencing scheme. The energy balance equation is used to compute temperature at each time step with the Crank–Nicholson
technique. Energy equation is coupled to the RTE through the radiative source term. Both equations are discretized with finite
differencing schemes. The energy conservation leads to the sparse system of linear equations A× T=B which is solved with a bi-conjugate stabilized gradient technique (BCSG). Validation of the model with different test cases
is achieved and application to transient heating of glass is also studied. 相似文献
11.
Experimental study of convective heat transfer from a rotating finned tube in transverse air flow 总被引:1,自引:0,他引:1
The convective heat transfer from fins to air has been evaluated using rotating annular fins subjected to an air flow parallel
to the fins. The fin cooling is studied using infrared thermography. The thermal balance in a fin during its cooling process
allows us to obtain the heat transfer coefficient from the temperature time evolution of the fin. Moreover, Particle Image
Velocimetry allows us to obtain the flow field in the mid-plane between two fins. The influence of the fin spacing on the
convective heat transfer is studied for various velocities of the superposed air flow and various fin rotational speeds. These
tests were carried out for air flow Reynolds numbers (based on the shaft diameter and the velocity of the superposed air flow)
between 2550 and 18200 and rotational Reynolds numbers (based on the shaft diameter and the peripheral speed) between 800
and 2.9 × 104, for different fin spacings.
Received: 14 May 1999/Accepted: 8 October 1999 相似文献
12.
A boundary layer analysis has been presented to study the influence of thermal radiation and lateral mass flux on non-Darcy
natural convection over a vertical flat plate in a fluid saturated porous medium. Forchheimer extension is considered in the
flow equations, and the Rosseland approximation is used to describe the radiative heat flux in the energy equation. Similarity
solution for the transformed governing equations is obtained and the combined effect of thermal radiation and fluid suction/injection
on the heat transfer rate is discussed. Numerical results for the details of the velocity and temperature profiles as well
as Nusselt number have been presented.
Received on 7 July 1999 相似文献
13.
Simulations of monodisperse and polydisperse (μ
2(A) = 0.13±0.002) 2D foam samples undergoing simple shear are performed using the 2D viscous froth (VF) model. These simulations
clearly demonstrate shear localisation. The dependence of localisation length on the product λV (shearing velocity V times the wall drag coefficient λ) is examined and is shown to agree qualitatively with published experimental data. A wide range of localisation lengths is
found at low λV, an effect which is attributed to the existence of distinct yield and limit stresses. The general continuum model is extended
to incorporate such an effect and its parameters are subsequently related to those of the VF model. A Herschel–Bulkley exponent
of a = 0.3 is shown to accurately describe the observed behaviour. The localisation length is found to be independent of λV for monodisperse foam samples. 相似文献
14.
The results are given of an investigation of the convective and radiative heat transfer at the leading critical point of a body in the flow of a radiating mixture of carbon dioxide and nitrogen, taking account of viscosity and thermal conductivity. The system of equations is written down under the assumption that the shock layer is thin, and its solution is obtained in the region between the body and the shock wave. It is assumed that there is local thermodynamic equilibrium throughout the compressed layer. The coefficients of absorption of the mixture are assumed to depend on the wavelength, the temperature, and the pressure. From the solution we determine the radiative and convective thermal fluxes at the wall, taking account of their interaction for temperatures behind the shock wave of 9000–12000 deg K and pressures of p=1 and 10 atm. By analyzing these results it is concluded that the effect of radiation on the convective heat transfer is insignificant, the effect being qualitatively different at large and small pressures. The fundamental contributions to the radiant thermal flux at the wall in the versions of the problem considered come from the following spectral interval: 0.128–0.33, where there is a fourth positive system of carbon monoxide bands (~43%), and 0.33–0.66, where there is an ultraviolet system of cyanogen (~40%). The contribution from the spectral interval 0.80–1.15 is ~20%. Only about 15% of the radiant energy comes from the comparatively large interval 0.45–0.80. As the pressure increases, the contribution from the ultraviolet part of the spectrum falls, and the contribution from the visible part of the spectrum increases.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 39–47, March–April, 1971. 相似文献
15.
Gh. Juncu 《Heat and Mass Transfer》1999,35(3):251-257
This paper analyses the influence of the conductivity ratio, Φλ, and the volume heat capacity ratio, Φh, on the conjugate heat transfer from a liquid particle in a liquid environment. Both creeping flow and moderate Re number
domain are considered. Special attention is given to the phenomenon of thermal wake. The occurrence and the evolution of the
thermal wake depend on the values of volume heat capacity ratio and conductivity ratio respectively. The influence of the
Pe numbers on the thermal inversion phenomenon and the interaction (at moderate Re number values) between thermal wake and
flow separation are analysed. The results obtained show that Φλ and Φh influence strongly the conjugate heat transfer.
Received on 11 January 1999 相似文献
16.
A numerical study is performed to analyze steady laminar forced convection in a channel in which discrete heat sources covered
with porous material are placed on the bottom wall. Hydrodynamic and heat transfer results are reported. The flow in the porous
medium is modeled using the Darcy–Brinkman–Forchheimer model. A computer program based on control volume method with appropriate
averaging for diffusion coefficient is developed to solve the coupling between solid, fluid, and porous region. The effects
of parameters such as Reynolds number, Prandtl number, inertia coefficient, and thermal conductivity ratio are considered.
The results reveal that the porous cover with high thermal conductivity enhances the heat transfer from the solid blocks significantly
and decreases the maximum temperature on the heated solid blocks. The mean Nusselt number increases with increase of Reynolds
number and Prandtl number, and decrease of inertia coefficient. The pressure drop along the channel increases rapidly with
the increase of Reynolds number. 相似文献
17.
In this study, a steady, fully developed laminar forced convection heat augmentation via porous fins in isothermal parallel-plate
duct is numerically investigated. High-thermal conductivity porous fins are attached to the inner walls of two parallel-plate
channels to enhance the heat transfer characteristics of the flow under consideration. The Darcy–Brinkman–Forchheimer model
is used to model the flow inside the porous fins. This study reports the effect of several operating parameters on the flow
hydrodynamics and thermal characteristics. This study demonstrates, mainly, the effects of porous fin thickness, Darcy number,
thermal conductivity ratio, Reynolds number, and microscopic inertial coefficient on the thermal performance of the present
flow. It is found that the highest Nusselt number is achieved at fully filled porous duct which requires the highest pumping
pressure. The results show that using porous fins requires less pumping pressure with comparable high heat augmentation weight
against fully filled porous duct. It is found that higher Nusselt numbers are achieved by increasing the microscopic inertial
coefficient (A), the Reynolds number (Re), and the thermal conductivity of the porous substrate k
2. The results show that heat transfer can be enhanced (1) with the use of high thermal conductivity fins, (2) by decreasing
the Darcy number, and (3) by increasing microscopic inertial coefficient. 相似文献
18.
Investigation has been carried out on the thermal performance of sintered miniature heat pipes with 3 mm outer diameter.
In the theoretical analysis, the influence of wick structure parameters is determined by using the theory of capillary limitation.
As a result, the degree of importance is found to be as follows: porosity, powder diameter and thickness of wick structure.
In the experiments, heat pipes with sintered dendritic copper powder wicks were fabricated and tested. The maximum heat transfer
rate is about 13 W with an effective heat pipe length of 20 cm. By adopting the formulae developed for both sintered spherical
powder and fiber and adjusting their proportion, the agreement between experimental results and prediction is found to be
quite good in the tested operation temperature range.
Received on 26 February 2001 相似文献
19.
Ebrahim Momoniat 《Acta Mechanica Sinica》2012,28(2):444-449
A formulation of the fin efficiency based on Newton’s law of cooling is compared with a formulation based on a ratio of heat transferred from the fin surface to the surrounding fluid to the heat conducted through the base.The first formulation requires that the solution of the nonlinear fin equations for constant heat transfer coefficient and constant thermal conductivity is known,whilst the second formulation of the fin efficiency requires only that a first integral of the model equation is known.This paper shows the first formulation of the fin efficiency contains approximation errors as only power series and approximate solutions to the nonlinear fin equations have been determined.The second formulation of the fin efficiency is exact when the first integrals can be determined. 相似文献
20.
This paper presents a new technique for non-contact calorimetry measurement of specific heat capacity and thermal conductivity.
Based on pseudo-white noise modulation and system identification, commonly used in electronics and communication engineering,
this procedure can be used to measure the transfer function of the sample temperature variation due to heating power variation.
The heat capacity and internal heat transfer coefficient are then determined using the equivalence between the identified
transfer functions of the temperatures measured at two locations and the analytical model proposed by Fecht and Johnson (Rev
Sci Instrum 62:1299–1303, 1991) and Wunderlich and Fecht (Measur Sci Technol 16:402–416, 2005). This inverse problem is solved numerically using a Gauss–Seidel algorithm. A numerical simulation of a non-contact modulated
calorimetry experiment is used to demonstrate the relevance of this new technique for indirect measurement of the heat capacity
and heat transfer coefficients of solid samples presenting large Biot numbers (Bi > 0.4). 相似文献