Nonstationary Gas Filtration Caused by an Intense Thermal Action on a Damp Porous Medium

A mathematical model and an algorithm are proposed for evaluating nonstationary heat and mass transfer in a porous medium that contains a mechanically absorbed liquid and a two–component gas (vapor—inert gas mixture). The case of an intense thermal action on a damp porous mixture caused by an external heat flux and convective heat transfer is considered. Typical flow regions and typical regions of the interaction between the phases are described.     

Experimental study on the performance of a novel structure for two-phase flow distribution in parallel vertical channels

Uniform flow distribution is critical to obtain high thermal performance in many heat and mass transfer devices. It especially plays an important role in a compact heat exchanger. In this paper, a two-phase flow distributor is proposed for the evaporator unit of the plate-fin heat exchanger to alleviate the phase maldistribution in the multiphase flow. Air and water mixture was adopted as two-phase medium and distributions into ten parallel channels were measured in detail. The results show that the proposed distributor can improve the two-phase flow distribution of the plate-fin heat exchanger.     

Phase discontinuities in water flows through a porous medium

Flow of a fluid through a porous medium is considered with allowance for heat conduction processes and phase transitions. Discontinuities in flows between both single-phase zones saturated with water and steam and single-and two-phase zones saturated with an equilibrium steam-water mixture are studied. It is shown that only the evaporation fronts are evolutionary for a convex-downward shock adiabat of the discontinuity inside the steam-water mixture. The structure of these fronts is considered and a condition supplementary to the conservation laws and necessary for the well-posed formulation of problems whose solution contains this front is found from the condition of existence of a discontinuity structure between the water (steam) and the steam-water mixture.     

Radiative-convective heat transfer in a flow past an evaporating semitransparent melt film

A conjugate problem of nonstationary radiative-convective heat transfer in a turbulent flow of a mixture of gases with solid particles around a horizontal evaporating semitransparent melt film is numerically solved. The moving film is subjected to intense radiative heating by an external source whose radiation interacts with the gas-particle medium and the film in a bounded spectral range. The temperature fields and velocities in the boundary layer and the film are calculated. The computational results given allow determination of the impact of radiation on heat transfer and film dynamics in the boundary layer-film system.     

Triple-Diffusive Mixed Convection in a Porous Open Cavity

The triple-diffusive mixed convection heat and mass transfer of a mixture is analyzed in an enclosure filled with a Darcy porous medium. The mass transfer buoyancy effects due to concentration gradients of the dispersed components (pollutant components) are taken into account using the Boussinesq approximation model. The governing equations are transformed into a non-dimensional form, and six groups of non-dimensional parameters, including Darcy–Rayleigh number, Peclet number, two Lewis numbers for pollutant components 1 and 2 and two buoyancy ratio parameters for pollutant components 1 and 2, are introduced. The governing equations are numerically solved for various combinations of non-dimensional parameters using the finite element method. The effect of each group of non-dimensional parameters on the pollutant distribution and the heat transfer in the cavity is discussed. The results indicate that the presence of one pollutant component can significantly affect the pollutant distribution of the other component. When the Lewis number of a pollutant component is small, the increase in the bouncy ratio parameter of the proposed component always increases the Nusselt and Sherwood numbers in the cavity.     

Features of the dynamics of hydrothermodynamic perturbations in a stably stratified fluid

The evolution of a certain class of arbitrary-amplitude perturbations in a stratified binary mixture is investigated analytically and numerically. The radical differences between the process in question and the process of hydrostatic adaptation in an ordinary single-component medium are demonstrated. In particular, a long-life “wake“ with nonzero temperature and admixture concentration perturbations that compensate one another in the density field may be formed when the process is completed. The slow process of dissipation of this wake, associated with heat and mass transfer, is investigated.     

Unsteady radiative-convective heat transfer in a high-temperature gas-particle flow past a semi-transparent plate

Numerical simulations of unsteady radiative-convective heat transfer in a turbulent flow of a mixture of gases and solid particles past a semi-transparent plate are performed. An ablation process is demonstrated to occur on the plate surface in the case of intense radiative heating of the plate by an external source with emission in a limited spectral range. Temperature fields and distributions of heat fluxes in the boundary layer and in the plate are calculated. Calculation results are presented, which allow determining the effect of ablation and reflecting properties of the plate surface on the thermal state of the medium in the system containing the boundary layer and the plate under conditions of plate heating by a high-temperature source of radiation. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 3, pp. 140–146, May–June, 2009.     

Self-similar flow of a mixture of a non-ideal gas and small solid particles behind a shock wave in presence of heat conduction, radiation heat flux and a gravitational field

Similarity solutions are obtained for one-dimensional unsteady flow of a dusty gas behind a spherical shock wave with heat conduction and radiation heat flux under a gravitational field of heavy nucleus at the centre (Roche model). The dusty gas is assumed to be a mixture of small solid particles and a non-ideal gas. The equilibrium flow conditions are assumed to be maintained, and the heat conduction is expressed in terms of Fourier’s law and the radiation is considered to be of the diffusion type for an optically thick grey gas model. The thermal conductivity K and the absorption coefficient α _R are assumed to vary with temperature and density. In order to obtain similarity solutions the density of the undisturbed medium is assumed to be constant. The effects of an increase in the value of the parameter of non-idealness of the gas in the mixture $\bar{b}$ , the mass concentration of the solid particles in the mixture K _p , the ratio of the density of the solid particles to the initial density of the gas G ₁ and the variation of the heat transfer parameters Γ _R and Γ _c are obtained.     

Flow of a viscous heat-conducting gas and binary mixtures in a sink

The class of exact solutions of the one-dimensional Navier-Stokes equations corresponding to gas flows from a spherical source or sink has been investigated analytically and numerically on a number of occasions (see, for example, [1, 2]). Here, the solution for a sink is considered in the presence of heat transfer from the ambient medium. Apart from seeking the solution itself, the object of the investigation was to establish the conditions of transi tion from viscous to inviscid flow in the sink as the Reynolds number tends to infinity. As shown in [3], for zero heat flux at an infinitely remote point there is no such transition for flow in a sink. The sink flow characteristics of a binary gas mixture are investigated in detail. In the transonic flow region an asymptotic solution is obtained.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 56–62, January–February, 1989.     

Free convection in a saturated porous medium adjacent to a vertical impermeable wall subjected to a non-uniform heat flux

Boundary layer analysis is performed for free convection in a fluid-saturated porous medium adjacent to a vertical impermeable wall subjected to a non-uniform heat flux. The wall heat flux is assumed to be an arbitrary function of the distance along the surface. The solutions are obtained in the form of perturbations to the uniform heat flux case. Using the differentials of the wall heat flux, which are functions of the distance along the surface, as perturbation elements, universal functions are obtained. These universal functions can be used to estimate the heat transfer for any type of wall heat flux variation. For the wall heat flux variation as a power function of the distance from the origin, the solutions obtained by using these universal functions have been compared with those obtained by similarity analysis and the agreement is found to be good. Further, solutions are presented for wall heat flux varying exponentially and sinusoidally but comparison could not be drawn due to non-availability of solutions in the literature.     

Heat transfer in nucleate boiling of R134a/R152a mixtures

Heat transfer coefficients were measured on a horizontal platinum wire and converted to data on horizontal copper tubes. The measurements spanned a large region of pressures p* = p/p_crit = 0.05–0.50 and heat fluxes of q = 10³–1.5 × 10⁵ W/m². The preparation of the test equipment is described. The effects of pressure and concentration on the heat transfer coefficients are shown. The mixture behaves very much like an azeotropic mixture; concentration has only a small effect, the heat transfer coefficients can be obtained from the heat transfer coefficients of the pure components according to their molar fractions. The conversion steps from wire- to tube-data are presented. A comparison of wire-data with correlations given in literature is shown. It renders good agreement.     

Theory of flame propagation in a gas and drop mixture

The equations of a reacting multiphase continuous medium [1] are used to investigate the problem of steady-state flame front propagation in a gas mixture with evaporating drops. A simple model for ignition of the liquid drops is proposed which is based on the application of the method of equally accessible surfaces [2] to the heat and mass exchange processes between the microflames surrounding the separate drops, the drops, and the carrying gas medium. The parameter distributions in the macroscopic flame front as well as the dependences of the flame propagation velocity in the gas suspension on a number of parameters governing the process under investigation are represented.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 101–108, July–August, 1973.     

Phase Behavior of Methane-Pentane Mixture in Bulk and in Porous Media

The adiabatic calorimetry method is used to measure the isochoric heat capacity and the dew-bubble curves for the methane-normal pentane mixtures in bulk for three different mixture compositions of n-pentane. The near-critical behavior of the mixture heat capacity indicates that at low n-pentane concentrations, this system is close to the special point of the critical locus. The effect of porous medium has been investigated for one of the mixtures studied in bulk. It has been shown that in porous medium the essential shift of the dew-bubble curve takes place. In our opinion this shift is caused by the formation of wetting film on the surface of porous medium. The estimation of film thickness resulting from the data obtained yields the value 7–8 nm.     

Specific features of unsteady exhaustion of a gas–particle medium into vacuum

Results of an analysis of two-dimensional unsteady exhaustion of a one-velocity gas–particle medium into vacuum for limiting equilibrium cases of heat transfer between the phases are reported. Domains of existence of a one-dimensional Riemann wave and a lateral expansion wave, as well as boundaries of the flow expansion region are determined. Under thermal equilibrium conditions, the reverse flow is found to occupy a large domain extending beyond the boundaries defined by angles of expansion for an ideal gas and for a gas–particle mixture with thermally insulated phases. Exhaustion of a nonequilibrium (in terms of velocities and temperatures) two-phase medium into vacuum is numerically simulated. It is demonstrated that a barrel-shaped structure with wave expansion of the gas and a combined discontinuity in the expanding gas–particle mixture is formed.     

Convection in a stratified binary mixture near a thermally inhomogeneous vertical surface

The stationary convection in a stratified two-component medium, for example, saline sea water, near a thermally inhomogeneous vertical surface is investigated analytically. Physically different cases of thermal inhomogeneities extended in the vertical or horizontal direction are considered. The solutions obtained can be applied to problems of convection in semibounded horizontal or vertical layers in the presence of thermal inhomogeneities at the “ends” of the layer. The solutions show that in two-component media convection is very specific. In particular, the spatial pattern of the thermal response to inhomogeneous heatingmay significantly differ from the case of an ordinary single-component medium: additional perturbation modes that penetrate deeply into the stably stratified medium appear. For an arbitrarily strong hydrostatic stability of the medium there exists an unexplored mechanism of convective instability related with the difference in the boundary conditions for the two substances. Weak variations of the background stratification of the admixture concentration (salinity) may significantly affect the heat exchange between a vertical surface and the medium. Even a very weak presence of the second component (a small contribution of the admixture stratification to the background density stratification) may lead not only to a significant quantitative change in the thermal response but also to a change in its sign, for example, to a significant decrease in the temperature of the medium in response to a heat influx from the vertical boundary.     

Evaporation and Condensation Fronts in Porous Media

Flow of a fluid through a porous medium is considered with allowance for heat conduction. Both fronts at which the liquid is transformed into steam or a liquid-steam mixture and fronts with inverse transformations are studied. The evolutionarity conditions of these fronts are considered and a model of their structure is proposed.     

Determination of heat fluxes in the neighborhood of a double curvature stagnation point in a flow of dissociating gas of arbitrary chemical composition

Numerical solutions are obtained for the equations of a uniform compressible boundary layer with variable physical properties in the vicinity of a stagnation point with different principal curvatures in the presence of an injected gas with the same properties as the incident flow. The results of the numerical solutions are approximated for the heat flux in the form of a relation that depends on the variation of the product of viscosity and density across the boundary layer and on the ratio of the principal radii of curvature.Using the concepts of effective diffusion coefficients in a multicomponent boundary layer, previously introduced by the author in [1], and the generalized analogy between heat and mass transfer in the presence of injection, together with the numerical solutions obtained, it is always possible, even without additional solutions of the boundary-layer equations, to derive final formulas for the heat fluxes in a flow of dissociating gas of arbitrary chemical composition, provided that we make the fundamental assumption that all recombination reactions take place at the surface.By way of example, formulas are given for the heat transfer to the surface of a body from dissociating air, regarded as a five-component mixture of the gases O, N, NO, O₂, N₂, and from a dissociating mixture of carbon dioxide and molecular nitrogen of arbitrary composition, regarded as an eleven-component mixture of the gases O, N, C, NO, C₂, O₂, N₂, CO, CN, C₃, CO₂.In the process of obtaining and analyzing these solutions it was found that, in computing the heat flux, a multicomponent mixture can be replaced with an effective binary mixture with a single diffusion coefficient only when the former can be divided into two groups of components with different (but similar) diffusion properties. In this case the concentrations of one group at the surface must be zero, while the diffusion flows of the second group at the surface are expressible, using the laws of mass conservation of the chemical elements, in terms of the diffusion flows of the first. Then the single effective diffusion coefficient is the binary diffusion coefficient D(A,M), where A relates to one group of components and M to the other.In view of the small amount of NO(c(NO) < 0.05), the diffusion transport of energy in dissociated air maybe described with the aid of a single binary diffusion coefficient D(A, M)(A=O, N, M=O₂, N₂, NO). However even in the case of complete dissociation into O and C atoms at the outer edge of the boundary layer, the diffusion transport of energy in dissociated carbon dioxide can not be described accurately enough by means of a model of a binary mixture with a single diffusion coefficient, since the diffusion properties of the O and C atoms are distinctly different.     

Motion of a nonisothermal adsorbable mixture with enhanced values of the concentrations through a porous medium

The equations of motion of a nonisothermal adsorbable mixture with enhanced values of the concentrations of the components in the case of infinitely large coefficients of heat and mass transfer reduce to a hyperbolic quasilinear system of equations. The invariant solutions of this system are analyzed. Convexity conditions are obtained under which a traveling-wave regime is realized in the porous medium. A system of equations is found for determining the concentrations of the adsorbable components of the mixture when a self-similar regime of dispersing waves is realized. For the case of finite values of the coefficients of heat and mass transfer, expressions are given for the width of the stationary front in the traveling-wave regime.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 5, pp. 76–86, September–October, 1980.     

Thermal performance of heat pipe with suspended nano-particles

Nanofluids are employed as the working medium for a conventional cylindrical heat pipe. A cylindrical copper heat pipe of 19.5?mm outer diameter and 400?mm length was fabricated and tested with two different working fluids. The working fluids used in this study are DI-water and Nano-particles suspension (mixture of copper nano particle and DI-water). The overall heat transfer coefficient of the heat pipe was calculated based on the lumped thermal resistance network and compared with the heat transfer coefficient of base fluid filled heat pipe. There is a quantitative improvement in the heat transfer coefficient using nano-particles suspension as the working medium. A heat transfer correlation was also developed based on multiple regression least square method and the results were compared with that obtained by the experiment.     

Analytical solution of non-fourier temperature response in a finite medium under laser-pulse heating

A temperature wave solution predicted by the hyperbolic heat equation is developed for a finite medium exposed to a surface heat flux of laser with an actual temporal profile. By using the analytical solution, the temperature response, the propagation and the reflection of the temperature wave due to such heat pulse are investigated for different pulse duration, thickness of the medium, and energy absorption depth.