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1.
An ablation model for a hydrogen pellet accelerating in a thermonuclear facility fuel-injector bore is developed. The model
is based on the Lagrange internal-ballistic problem complemented by the Stefan conditions on the ablating surface of the pellet
Calculations demonstrate that, during pellet motion in the bore under the action of the gas, the pellet length remains almost
unchanged, while due to the heat exchange with the bore walls the pellet diameter decreases by 4–10%
St. Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 3–11, January–February,
1999. 相似文献
2.
Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent
fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement
in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure
terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation,
are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard
convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by
modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion.
The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results
in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy
diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance
of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable
and unstable stratification and that it allows for the required counter gradient energy flux. 相似文献
3.
Björn Johannesson 《Transport in Porous Media》2010,85(2):565-592
A numerical scheme for the transient solution of a generalized version of the Poisson–Nernst–Planck (PNP) equations is presented.
The finite element method is used to establish the coupled non-linear matrix system of equations capable of solving the present
problem iteratively. The PNP equations represent a set of diffusion equations for charged species, i.e. dissolved ions, present
in the pore solution of a rigid porous material in which the surface charge can be assumed neglectable. These equations are
coupled to the ‘internally’ induced electrical field and to the velocity field of the fluid. The Nernst–Planck equations describing
the diffusion of the ionic species and Gauss’ law in use are, however, coupled in both directions. The governing set of equations
is derived from a simplified version of the so-called hybrid mixture theory (HMT). The simplifications used here mainly concerns
ignoring the deformation and stresses in the porous material in which the ionic diffusion occurs. The HMT is a special version
of the more ‘classical’ continuum mixture theories in the sense that it works with averaged equations at macroscale and that
it includes the volume fractions of phases in its structure. The background to the PNP equations can by the HMT approach be
described by using the postulates of mass conservation of constituents together with Gauss’ law used together with consistent
constitutive laws. The HMT theory includes the constituent forms of the quasistatic version of Maxwell’s equations making
it suitable for analyses of the kind addressed in this work. Within the framework of HTM, constitutive equations have been
derived using the postulate of entropy inequality together with the technique of identifying properties by Lagrange multipliers.
These results will be used in obtaining a closed set of equations for the present problem. 相似文献
4.
《应用数学和力学(英文版)》2021,(9)
The magnetohydrodynamic Sutterby fluid flow instigated by a spinning stretchable disk is modeled in this study. The Stefan blowing and heat and mass flux aspects are incorporated in the thermal phenomenon. The conventional models for heat and mass flux, i.e., Fourier and Fick models, are modified using the Cattaneo-Christov(CC)model for the more accurate modeling of the process. The boundary layer equations that govern this problem are solved using the apt similarity variables. The subsequent system of equations is tackled by the Runge-Kutta-Fehlberg(RKF) scheme. The graphical visualizations of the results are discussed with the physical significance. The rates of mass and heat transmission are evaluated for the augmentation in the pertinent parameters. The Stefan blowing leads to more species diffusion which in turn increases the concentration field of the fluid. The external magnetism is observed to decrease the velocity field. Also,more thermal relaxation leads to a lower thermal field which is due to the increased time required to transfer the heat among fluid particles. The heat transport is enhanced by the stretching of the rotating disk. 相似文献
5.
Generalizations of Fick's law for the diffusion flux are often considered in the literature by analogy with those for the heat flux. The paper reviews the balance equations for a fluid mixture and provides the equations for the diffusion fluxes. As a consequence, the mass densities are shown to satisfy a system of hyperbolic equations. Moreover, for a binary mixture of ideal gases in stationary conditions, Fick's law is recovered. Next, diffusion fluxes are regarded as constitutive functions and a whole set of thermodynamic restrictions are determined which account for diffusion, heat conduction, viscosity and inhomogeneities. Hyperbolic models for diffusion and heat fluxes are established which involve the co-rotational derivative. The driving term of diffusion turns out to be the gradient of chemical potential rescaled by the temperature. 相似文献
6.
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8.
A. N. Kalarakis V. K. Michalis E. D. Skouras V. N. Burganos 《Transport in Porous Media》2012,94(1):385-398
Rarefied gas flow in channels and computer-aided reconstructions of porous media is simulated using the direct simulation Monte Carlo (DSMC) method and a modified lattice-Boltzmann (LB) method that can account for rarefaction effects. An increase of two orders of magnitude was noted for the gas permeability as the Knudsen number increased from 0.1 to 10. It was found that incorporation of a Bosanquet expression for the viscosity in the dusty gas flux equations leads to the recovery of the well known Klinkenberg expression for the gas permeability, revealing an explicit relation of the, thus far empirical, permeability correction factor to the fluid and structure properties. The expression for the effective gas viscosity in the transition flow regime is also incorporated in the LB method, which is then validated against the DSMC method by comparing predictions for the velocity profiles in straight channels over the entire transition flow regime. The new LB method offers the additional advantages of simplicity in the code implementation and great savings in computational time and memory compared to the DSMC method. It is shown that the rough adjustment of a single parameter suffices to make the LB method suitable for the reliable prediction of the gas permeability in porous media over the entire transition flow regime. 相似文献
9.
V. M. Agranat 《Fluid Dynamics》1988,23(5):729-732
Approximate analytic expressions for the local friction and heat transfer coefficients in a dusty laminar boundary layer are
obtained and tested in the case of an incompressible carrier phase, power-law variation of the external gas flow velocity
and small velocity and temperature phase disequilibrium. These expressions supplement the numerical analysis of the dusty
boundary layer on a blunt body [1, 2] and the asymptotic calculation of the friction and heat transfer in a quasiequilibrium
dusty gas boundary layer on a plate [3]. The combined effect of dustiness and pressure gradient on the friction and heat transfer
coefficients is discussed. The results obtained can be used for the practical calculation of the friction and heat transfer
in a quasiequilibrium dusty laminar boundary layer and for interpreting the corresponding experimental data.
Tomsk. Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 105–108, September–October, 1988. 相似文献
10.
Conditional Moment Closure for Large Eddy Simulations 总被引:1,自引:0,他引:1
A conditional moment closure (CMC) based combustion model for large-eddy simulations (LES) of turbulent reacting flow is proposed
and evaluated. Transport equations for the conditionally filtered species are derived that are consistent with the LES formulation
and closures are suggested for the modelling of the conditional velocity, conditional scalar dissipation and the fluctuations
around the conditional mean. A conventional β-probability density distribution of the scalar is used together with dynamic
modelling for the sub-grid fluxes. The model is validated by comparison of simulations with measurements of a piloted, turbulent
methane-air jet diffusion flame. 相似文献
11.
The present study is devoted to investigate the influences of mass transfer on buoyancy induced flow over vertical flat plate
embedded in a non-Newtonian fluid saturated porous medium. The Ostwald–de Waele power-law model is used to characterize the
non-Newtonian fluid behavior. Similarity solution for the transformed governing equations is obtained with prescribed variable
surface heat flux. Numerical results for the details of the velocity, temperature and concentration profiles are shown on
graphs. Excess surface temperature as well as concentration gradient at the wall associated with heat flux distributions,
which are entered in tables, have been presented for different values of the power-law index n, buoyancy ration B and the exponent λ as well as Lewis number Le.
Received on 26 April 2000 相似文献
12.
Steady convective mass transfer to or from fluid interfaces in pores of angular cross-section is theoretically investigated.
This situation is relevant to a variety of mass transport process in porous media, including the fate of residual non-aqueous
phase liquid ganglia and gas bubbles. The model incorporates the essential physics of capillarity and solute mass transfer
by convection and diffusion in corner fluid filaments. The geometry of the corner filaments, characterized by the fluid–fluid
contact angle, the corner half-angle and the interface meniscus curvature, is accounted for. Boundary conditions of zero surface
shear (‘perfect-slip’) and infinite surface shear (‘no-slip’) at the fluid–fluid interface are considered. The governing equations
for laminar flow within the corner filament and convective diffusion to or from the fluid–fluid interface are solved using
finite-element methods. Flow computations are verified by comparing the dimensionless resistance factor and hydraulic conductance
of corner filaments against recent numerical solutions by Patzek and Kristensen (J. Colloid Interface Sci 236, 305–317 2001). Novel results are obtained for the average effluent concentration as a function of flow geometry and pore-scale
Peclet number. These results are correlated to a characteristic corner length and local pore-scale Peclet number using empirical
equations appropriate for implementation in pore network models. Finally, a previously published “2D-slit” approximation to
the problem at hand is checked and found to be in considerable error. 相似文献
13.
Use of the Fick’s law of diffusion is inappropriate in multi-component systems comprised of more than two species since it
results in local violation of overall mass conservation. One approach that is often used to circumvent this problem is the
dilute approximation, in which the conservation of the diluent is not enforced. It is generally believed that if the mass
fraction of the diluent is “large” and it is a non-reacting species, the dilute approximation yields accurate results. In
this study, the accuracy of the dilute approximation is quantitatively assessed by comparing its predictions against a rigorous
multi-component diffusion formulation, derived from the Stefan–Maxwell equation. Both mass and heat flux errors are computed
for a 1D pure diffusion problem. It is found that even for quite dilute mixtures with mass fraction of the diluent exceeding
70%, the errors are ∼15% for mass fluxes and as high as 35% for heat fluxes. In addition, the errors are found to be strongly
dependent on the type of species in the mixture, implying that the validity of the dilute approximation cannot be judged a
priori. Additional results are presented to show that in systems with combined heat and mass diffusion, net heat transfer
can occur against the imposed temperature gradient. A Second Law analysis is presented and performed to corroborate these
non-intuitive results. 相似文献
14.
Hameda Mohammed Shawky 《Heat and Mass Transfer》2009,45(10):1261-1269
The flow due to the pulsatile pressure gradient of dusty non-Newtonian fluid with heat transfer in a channel is considered.
The system is stressed by an external magnetic field. The non-Newtonian fluid under consideration is obeying the rheological
equation of state due to Ree-Eyring’s stress–strain relation. The equations of momentum and energy have been solved by using
Lightill method. The velocity and temperature distributions of the two phase of the dusty fluid are obtained. The effects
of various physical parameters of distributions the problem on these distributions are discussed and illustrated graphically
through a set of figure. 相似文献
15.
The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine
blade film cooling. A cooling fluid is injected from an inclined pipe at α=30° into a turbulent boundary layer profile at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations
comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas
mixture. A variation of the density ratio is simulated by the heat-mass transfer analogy, i.e., gases of different density
are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows
based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The numerical results
and experimental velocity data measured using two-component particle-image velocimetry (PIV) are in excellent agreement. The
results show the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination
of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream
of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio between the fluids
and the velocity ratio have to be considered. 相似文献
16.
V. M. Agranat 《Fluid Dynamics》1986,21(6):983-985
The nonisothermal Blasius problem for a gas suspension is considered on the basis of the equations of a quasiequilibrium two-phase
laminar boundary layer [1–3]. Approximate analytical expressions are obtained for the friction and heat transfer coefficients
and their region of applicability is estimated; the Reynolds analogy between friction and convective heat transfer processes
[4] is extended to the case of a dusty quasiequilibrium laminar boundary layer.
Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 160–162, November–December, 1986. 相似文献
17.
We present the theory of space–time elasticity and demonstrate that it is the extended reversible thermodynamics and gives the coupled model of thermoelasticity and heat conductivity and involves traditional thermoelasticity. We formulate the generally covariant variational model’s dynamic thermoelasticity and heat conductivity in which the basic kinematic and static variables are unified tensor objects (subject, matter). Variation statement defines the whole set of the initial-boundary problems for the 4D vector governing equation (Euler equation), the spatial projections of which define motion equations and the time projection gives the heat conductivity equation. We show that space–time elasticity directly implies the Fourier and the Maxwell–Cattaneo laws of heat conduction. However, space–time elasticity is richer than classical thermoelasticity, and it advocates its own equations of motion for coupled thermoelasticity. Moreover, we establish that the Maxwell–Cattaneo law and Fourier law can be defined for the reversible processes as compatibility equations without introducing dissipation. We argue that the present framework of space–time elasticity should prove adequate to describe the thermoelastic phenomena at low temperatures for interpreting the results of molecular simulations of heat conduction in solids and for the optimal heat and stress management in the microelectronic components and the thermoelectric devices. 相似文献
18.
Laminar boundary layer flows behind constant speed shock waves moving into a dusty gas are analyzed numerically. The basic equations of two-phase flows are derived in shock fixed coordinates and solved by an implicit finite-difference method for the side wall boundary layer in a dusty gas shock tube. The development of the boundary layer and resulting velocity and temperature profiles, respectively, for the gas and particles are given from the shock front to far downstream. The effects of diaphragm pressure ratio, mass loading ratio of particles and particle size upon the flow properties are discussed in detail.This article was processed using Springer-Verlag TEX Shock Waves macro package 1990. 相似文献
19.
Hot-wire anemometry is a measuring technique that is widely employed in fluid mechanics research to study the velocity fields
of gas flows. It is general practice to calibrate hot-wire sensors against velocity. Calibrations are usually carried out
under atmospheric pressure conditions and these suggest that the wire is sensitive to the instantaneous local volume flow
rate. It is pointed out, however, that hot wires are sensitive to the instantaneous local mass flow rate and, of course, also
to the gas heat conductivity. To calibrate hot wires with respect to mass flow rates per unit area, i.e., with respect to
(ρU), requires special calibration test rigs. Such a device is described and its application is summarized within the (ρU) range 0.1–25 kg/m2 s. Calibrations are shown to yield the same hot-wire response curves for density variations in the range 1–7 kg/m3. The application of the calibrated wires to measure pulsating mass flows is demonstrated, and suggestions are made for carrying
out extensive calibrations to yield the (ρU) wire response as a basis for advanced fluid mechanics research on (ρU) data in density-varying flows. 相似文献
20.
Optical distortions have previously prevented non-intrusive measurements of dissolved oxygen concentration profiles by Laser
induced fluorescence (LIF) to within 200 μm of the air–water interface. It is shown that by careful experimental design, reliable
measurements can be obtained within 28 μm of moving air–water interfaces. Consideration of previously unidentified optical
distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental
design. Phase resolved gas flux measurements have now been accomplished along wind forced microscale waves and indicate that
the highest mean gas fluxes are located in the wave troughs. The local mean oxygen fluxes as determined by LIF techniques
can be reconciled to within 40% of those obtained by bulk measurement in the water. These data provide a new perspective on
wind-wave enhancement of low solubility gas transfer across the air–water interface. 相似文献