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1.
This paper is concerned with the melting of a vertical ice plate into a calcium chloride aqueous solution inside a rectangular cavity. The initial temperature of the ice plate and the mixture are both −5°C and the initial concentration of the mixture is 20 wt%. The effect of the liquid height H, the width W, the aspect ratio of the liquid region A (=H/W) and the initial ice plate thickness δ i on the transient melting mass per unit melting front area, M, is mainly considered numerically. M keeps a similar value in spite of H varied for A = 1 at early melting stage, however, becomes considerably influenced by H as melting progresses. The ice plate melts influenced by A for H = 20 mm fixed at early melting stage due to the fast development of the stagnant region and M decreases with increasing A (=1∼ 10). A dimensionless correlation of the transient melting mass, the aspect ratio and the melting time was presented under the restricted condition of H = 20 mm.  相似文献   

2.
This paper is concerned with the melting of snow layer by showering the Calcium Chloride aqueous solution at the temperature of 0°C to obtain the effects of initial concentration of the solution, showering amount of the solution and density of packed snow sample on melting of snow, in the short period after snow melting being initiated. The rate of internal melting in the snow layer is great compared with that of the surface melting. As melting time passes, the temperature of snow layer gradually decreases to the solid-liquid equilibrium temperature. This phenomenon corresponds to the solidification temperature drop. The temperature distribution in the normal direction in snow layer becomes uniform at the equilibrium temperature. Therefore, after this period the major part of melt amount is surface melting only. From this fact the typical melting behaviour may be mainly induced by the effect of chemical reaction. It is demonstrated that the results obtained in this experiment could give an useful basic information for accelerating the melting of snow layer.
Schnee schmelzen mit einer wäßrigen Lösung niedriger Erstarrungstemperatur
Zusammenfassung Der Aufsatz behandelt das Schmelzen einer Schneeschicht mittels Übersprühen mit einer wäßrigen Kalziumchloridlösung einer Temperatur von 0°C mit dem Ziel, die Einflüsse unterschiedlicher Anfangskonzentrationen der Lösung, verschiedener Sprühmengen und unterschiedlich dichte Packung des Schnees in der Anfangsphase des Aufschmelzens zu untersuchen. Die Schmelzrate im Inneren der Schneeschicht ist groß, verglichen mit der auf der Schneeoberfläche. Im Verlauf der Aufschmelzzeit sinkt die Temperatur der Schneeschicht langsam auf die fest-flüssige Gleichgewichtstemperatur. Dies entspricht der Temperaturabsenkung bei der Erstarrung. Die Temperaturverteilung senkrecht zur Schneeschicht vergleichmäßigt sich bei der Gleichgewichtstemperatur. Deshalb resultiert nach dieser Anfangsperiode die Schmelzmasse hauptsächlich nur aus dem Aufschmelzen an der Oberfläche. Aus diesen Tatsachen kann auf ein Schmelzverhalten in der Anfangsperiode geschlossen werden, das hauptsächlich durch die chemische Reaktion bestimmt ist. Es wird gezeigt, daß die in dem vorgestellten Experiment ermittelten Ergebnisse nützliche, grundlegende Hinweise für die Beschleunigung des Abschmelzens einer Schneeschicht geben können.
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3.
The development of the thermo-viscous fingering instability of miscible displacements in homogeneous porous media is examined. In this first part of the study dealing with stability analysis, the basic equations and the parameters governing the problem in a rectilinear geometry are developed. An exponential dependence of viscosity on temperature and concentration is represented by two parameters, thermal mobility ratio β T and a solutal mobility ratio β C , respectively. Other parameters involved are the Lewis number Le and a thermal-lag coefficient λ. The governing equations are linearized and solved to obtain instability characteristics using either a quasi-steady-state approximation (QSSA) or initial value calculations (IVC). Exact analytical solutions are also obtained for very weakly diffusing systems. Using the QSSA approach, it was found that an increase in thermal mobility ratio β T is seen to enhance the instability for fixed β C , Le and λ. For fixed β C and β T , a decrease in the thermal-lag coefficient and/or an increase in the Lewis number always decrease the instability. Moreover, strong thermal diffusion at large Le as well as enhanced redistribution of heat between the solid and fluid phases at small λ is seen to alleviate the destabilizing effects of positive β T . Consequently, the instability gets strictly dominated by the solutal front. The linear stability analysis using IVC approach leads to conclusions similar to the QSSA approach except for the case of large Le and unity λ flow where the instability is seen to get even less pronounced than in the case of a reference isothermal flow of the same β C , but β T  = 0. At practically, small value of λ, however, the instability ultimately approaches that due to β C only.  相似文献   

4.
The turbulent film boiling from a vertical non-isothermal surface is formulated with due consideration to thermal radiation from its lateral face. It is observed that the application of Reynolds analogy together with thermal conduction in the test surface has yielded a conjugate solution from which the case of an isothermal condition can be generated as a special case. The analysis has further paved the way in establishing a functional relation between the Nusselt numberNu, radiation parameterN R , fin parameterM, temperature ratio termT s /(T w,0?T s ), and a product of characteristic modified Grashof, Prandtl and superheating parameter defined as (Gr 2 Pr S). In a fully developed turbulent film boiling i.e., modified Grashof number being greater than 1010, the temperature ratio term accounts for the non-linearities arising due to the inclusion of radiation from the lateral face of the fin. The results are in good agreement with experimental data over a wide range of system conditions.  相似文献   

5.
Using objective computerized statistical procedures, we have examined high precisionC p data by DSC reported by Wunderlich and Gaur for a series of alkyl acrylate and methacrylate polymers. Although they claimed the data to be linear inT aboveT g , our results do not support the linear model. One or two endothermic slope changes are revealed aboveT g in lowT f polymers (T f < 20 °C) and at least one exothermic slope change in highT f polymers (T f > 20 °C).T f is the flow temperature of Ueberreiter. Both the first endotherm and the first exotherm occur near (1.22 ± 0.07)T g , suggesting aT ll type phenomenon.T ll varies as \(1/\bar M_n \) . The first exotherm is associated by us with wetting of the DSC pan by molten polymer on the first heating of particulate highT f polymers. The rate of wetting, and presumably the magnitude of the exotherm, depends in part on the ratio,γ/η, whereγ is surface tension andη is melt viscosity of the molten polymer. Sinceγ is relatively constant, the molecular weight and temperature dependence for rate of wetting resides inη, which depends on \(\bar M_w \) . For \(\bar M_n > > \bar M_c \) , a second exothermic event caused by sintering, and also controlled by η, may be present. The interactive roles of \(\bar M_n ,\bar M_w ,\bar M_w /\bar M_n \) ;M c (entanglement molecular weight); particle size, and heating rate onC p T behaviour are delineated for the first time. LowT f hydrocarbon polymers, namely atactic polyalphaolefins,C 3 ,C 5 ,C 6 ; PIB; and dienes, PBD and cis-PI, exhibit single or double endotherms. Other results on highT f polymers showing exothermic effects, notably PS, PnBMA and polyglycidylmethacrylate are cited.  相似文献   

6.
We consider an ordinary differential equation E, x(t) = F(t, x(t)), with time periodic right hand side, with period T. The translation mapping Θ is the one which transforms an initial point Y at time t0 into the value at time t0 + T of the solution of E with initial conditions (t0, Y). It is known that the solution with initial conditions (t0, Z) is periodic with period T if and only if Z is a fixed point of Θ. In this paper the Newton's method is applied to locate the fixed points of the translation mapping Θ.  相似文献   

7.
This paper presents an analytical and numerical study of natural convection of a double-diffusive fluid contained in a rectangular slot subject to uniform heat and mass fluxes along the vertical sides. Governing parameters of the problem under study are the thermal Rayleigh number, Ra T ; buoyancy ratio, N; Lewis number, Le; Prandtl number, Pr and aspect ratio of the cavity, A. In the first part of the analytical study a scale analysis is applied to the two extreme cases of heat-transfer and mass-transfer-driven flows. In the second part, an analytical solution, based on the parallel flow approximation, is reported for tall enclosures (A?1). Solutions for the flow fields, temperature and concentration distributions and Nusselt and Sherwood numbers are obtained in terms of the governing parameters of the problem. In the limits of heat-driven and solute-driven flows a good agreement is obtained between the prediction of the scale analysis and those of the analytical solution. The numerical solutions are based on the complete governing equations for two-dimensional flows, and cover the range 1≤Ra T ≤107, 0≤N≤105, 10-3Le≤103, 1≤A≤20 and Pr=7. A good agreement is found between the analytical predictions and the numerical simulation.  相似文献   

8.
Lift coefficients, CL, of single bubbles in linear shear flows are measured to investigate effects of the bubble shape, the liquid velocity gradient and the fluid property on CL. The range of the Morton number, M, tested is from logM = − 6.6 to − 3.2. The shapes of bubbles are spherical and ellipsoidal. A correlation of bubble aspect ratio for single bubbles in infinite stagnant liquids proposed in our previous study can give good evaluations for bubbles in the linear shear flows. The CL of spherical bubbles at low bubble Reynolds numbers, Re, depend on the dimensionless shear rate Sr and Re and decrease with increasing Re. These characteristics agree with the Legendre-Magnaudet correlation. The use of a single dimensionless group such as Re, the Eötvös number, the Weber number and the Capillary number cannot correlate CL of non-spherical bubbles. The trend of the critical Re for the reversal of the sign of CL is the same as that for the onset of oscillation of bubble motion, which supports the mechanism proposed by Adoua et al., at least within the range of −6.6 ≤ logM ≤ −3.2. An experimental database of CL is provided for validation of available CL models and CFD.  相似文献   

9.
We establish an existence principle for the impulsive periodic boundary-value problem {fx029-01}, where gC(0, ∞) can have a strong singularity at the origin. Furthermore, we assume that 0 < t 1 < … < t m < T, eL 1[0, T], c ∈ ℝ, J i and M i , i = 1, 2, …, m, are continuous mappings of G[0, T] × G[0, T] into ℝ, and G[0, T] denotes the space of functions regulated on [0, T]. The presented principle is based on an averaging procedure similar to that introduced by Manásevich and Mawhin for singular periodic problems with p-Laplacian. Published in Neliniini Kolyvannya, Vol. 11, No. 1, pp. 32–44, January–March, 2007.  相似文献   

10.
A simplified analytical model of the Mach reflection of a planar shock wave over a concave cylindrical wedge has been used to predict the triple point trajectory and the triple point trajectory angle at glancing incidence.Comparison with experimental results suggests that the simplified approach for predicting the triple point trajectory is good for an incident shock wave Mach number, Mi, less than 1.1. However, the prediction of the triple point trajectory angle at glancing incidence is good for any value of Mi as long as the assumption of a perfect gas is valid.  相似文献   

11.
M. Peleg 《Rheologica Acta》1993,32(6):575-580
Mechanical changes in biomaterials at and around their glass transition are key factors in their functionality and/or stability. They are described in terms of a relationship betwen a relative stiffness R(T,M) defined as the ratio between a modulus or storage modulus at a temperature T, and moisture M, and its magnitude in the glassy state. The relationship, in turn, is expressed by the model R(T,M) = 1/[1+exp [{T-T c,(M))/a(M)]} where T c(M) is a critical temperature identifying the transition temperature range and a(M) a constant representing the relationship's slope. The proposed model correctly accounts for the downward concavity of the stiffness vs temperature relationship at the transition onset. Published data on biosolids indicate that T c(M) can be described by a single exponential decay term, and so most probably also a(M). Incorporation of these terms into the model enables the creation of realistic three-dimensional maps of the relative stiffness-temperature-moisture relationship at and in the neighborhood of the glass transition region. In principle, the same method can also be used to describe the effect of plasticizers other than water if their influence on T cand the steepness parameter can be formulated as an algebraic expression.  相似文献   

12.
The propensity of the transition of fracture type in either brittle or ductile cracked solid under mixed-mode I and III loading conditions is investigated. A fracture criterion based on the competition of the maximum normal stress and maximum shear stress is utilized. The prediction of the fracture type is determined by comparing τmax/σmax at a critical distance from the crack tip to the material strength ratio τC/σC, i.e., (τmax/σmax)<(τC/σC) for tensile fracture and (τmax/σmax)>(τC/σC) for shear fracture, where σC (τC) is the fracture strength of materials in tension (shear). Mixed mode I/III fracture tests were performed using circumferentially notched cylindrical bars made of PMMA and 7050 aluminum alloy. Fracture surface morphology of the specimens reveals that: (1) for the brittle material, PMMA, only tensile type of fracture occurs, and (2) for the ductile material, 7050 aluminum alloy, either tensile or shear type of fracture occurs depending on the mode mixity. The transition (in ductile material) or non-transition (in brittle material) of the fracture type and the fracture path observed in experiments were properly predicted by the theory. Additional test data from open literature are also included to validate the proposed theory.  相似文献   

13.
This paper describes the measurement of thermal strains and temperatures in a 0.25-scale aluminum-filled epoxy model of a double-flow large-steam-turbine nozzle chamber. A temperature gradient was induced by circulating chilled methyl alcohol through the interior. Strain gages and thermocouples were used to determine surface strains and temperatures at various locations for comparison with a finite-element analysis under development. A uniform cylinder was included on the inlet section of the model for calibration. The maximum measured tensile strain on the interior surface was 0.96 αΔT n , where ΔT n was the average temperature difference between the interior surface and the initial temperature. A maximum compressive strain of 0.50 αΔT n was measured on the outside surface of the nozzle-bowl sidewall.  相似文献   

14.
The mechanical and thermal characteristics of the self-similar boundary-layer flows induced by continuous surfaces stretched with rapidly decreasing power-law velocities U wx m , m<?1 are considered. Comparing to the well studied cases of the increasing stretching velocities (m>0) several new features of basic significance have been found. Thus: (i) for m<?1 the boundary layer equations admit self-similar solutions only if a lateral suction is applied; (ii) the dimensionless suction velocity f w<0 must be strong enough, i.e. f w<f w,max(m) where f w,max(m) depends on m so that its absolute maximum max (f w,max(m))=?2.279 is reached for m→?∞, while for m→?1, f w,max(m)→?∞; (iii) the case {m→?∞, f w,max(m)=?2.279} of the flow boundary value problem is isomorphic to the stretching problems with exponentially decreasing velocities U w∝e ax with arbitrary a<0; (iv) for any fixed m<?1 and f w<f w,max(m) the flow problem admits a non-denumerable infinity of multiple solutions corresponding to the values of the dimensionless skin friction f (0)≡s belonging to a finite interval s∈ [s min(f w,m), s max(f w,m)]; (v) the solution is only unique for f w=f w,max(m) where s=s min(f w,m)= s max(f w,m) holds; (vi) to every one of the multiple solutions of the flow problem there corresponds a unique solution of the heat transfer problem with a wall temperature distribution T w?T x n and a well defined and distinct value of the dimensionless wall temperature gradient ?(0), except for the cases n=(|m|?1)/2 where ?(0) has the same value ?(0)=Pr·f w for the whole class of flow solutions with s∈[s min(f w,m), s max(f w,m)]; (vii) for f w→?∞ one obtains the `asymptotic suction profiles' corresponding to s=s min(f w,m)?f w and ?(0)?Pr·f w in an explicit analytic form. The paper includes several examples which illustrate the dependence of the heat and fluid flows induced by surfaces stretching with rapidly decreasing velocities on the physical parameters f w, m, n and Pr.  相似文献   

15.
The rheology of tetrafluoroethylene/hexafluoropropylene (TFE/HFP) copolymers, also known as Teflon FEP polymers, having different molecular weight and composition (HFP content) was studied by means of a parallel-plate rheometer. Two groups of polymers having different molecular weights with nearly constant polydispersity (around 2.5) were considered; namely, one group having a relatively low melting temperature (amorphous with a high content of HFP) and a second group having a higher melting point (semi-crystalline with a lower content of HFP). The relaxation time spectrum, H(λ), calculated by use of the BSW model (developed for monodisperse linear polymers) followed a scaling relationship in the terminal zone with scaling exponent of 0.13. However, at higher frequencies the model fails to predict adequately the experimental data. The longest relaxation time calculated from both the BSW model and discrete relaxation spectra (λ i ,g i ), which was determined by use of a parsimonious fitting software, depends on the molecular weight in a similar way as the zero-shear viscosity does with the well-established scaling factor of 3.4. The critical molecular weight for the onset of entanglements, M c , was found to be about 100000, a value much higher than those previously reported in literature for other polymers. The rheology of resins in the second group (higher melting point) was found to exhibit a strong dependence on thermal history during oscillatory-shear measurements. The data obtained in experiments at different temperatures without a preheating to a certain value (330°C) exhibited a violation of the time-temperature superposition principle and no well-defined values of the zero-shear viscosity. This is attributed to residual crystallinity even at temperatures well above their melting point (260°C). However, the same experiments with preheating and subsequent cooling to desired temperature resulted into a very good time-temperature scaling. Received: 13 January 1998 Accepted: 6 April 1998  相似文献   

16.
The development of the momentum and thermal boundary layers over a semi-infinite flat plate has been studied when the external stream as well as the plate are impulsively moved with constant velocities. At the same time the temperature of the wall is suddenly raised from T, the temperature of the surrounding fluid, toT w and maintained at this temperature. The problem has been formulated in a new system of scaled coordinates such that fort?=0 it reduces to Rayleigh type of equation and fort? → ∞ it reduces to Blasius or Sakiadis type of equation. A new scale of time has been used which reduces the region of integration from an infinite region to a finite region which reduces the computational time considerably. The governing singular parabolic partial differential equations have been solved numerically using an implicit finite difference scheme. For some particular cases, analytical solutions have been obtained. The results show that there is a smooth transition from Rayleigh solution to Blasius or Sakiadis solution as the dimensionless timeξ increases from zero to one. The shear stress at the wall is negative for the friction parameterλ<0.5, positive forλ>0.5 and zero forλ=0.5. The zero shear stress at the wall does not imply separation but corresponds to the parallel flow. The surface heat transfer is strongly dependent on the Prandtl numberPr and increases with it. Also forPr<Pr 0, the surface heat transfer is enhanced as the friction parameterλ increases, but forPr>Pr 0 it get reduced.  相似文献   

17.
The weak Mach reflection phenomenon has been analyzed by applying both the shock dynamics approach and the disturbance propagation concept. The analysis which is based on modified Whitham's ray shock theory results in analytical expressions for the triple point trajectory angle,, and the shape of the curved Mach stem, which are functions of the incident shock wave Mach number,M i, and the reflecting wedge angle, w. The analytical results were found to be in good agreement with experimental results.This article was processed using Springer-Verlag TEX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.  相似文献   

18.
A theoretical analysis is made of the heat, mass and momentum transfer from an evaporative liquid sphere which is suddenly introduced into a parallel stream of fluid at a higher temperature. The velocity field around the liquid sphere is assumed to be steady and of the Hadamard-Rybczynski type. Numerical solutions of energy and the vapour mass continuity equations have been carried out using the alternate direction implicit scheme of finite difference method. Temporal histories of the average Nusselt and Sherwood numbers (Nu, Sh) alongwith the drag coefficient (C D ) during the life time of an evaporating drop have been predicted in terms of the pertinent input parameters, namely, initial and instantaneous Peclet number (Pe i ,Pe), Lewis number (Le), and the ratio of free stream to initial droplet temperature (T a /T i ). Variations of local Nusselt and Sherwood numbers withPe, in the region of steady state evaporation, have also been presented. Values ofNu for steady state droplet evaporation are found to be in fair agreement with the corresponding values evaluated from the empirical equation of Eisenklam [5].Es wurde eine theoretische Untersuchung der Wärme-, Massen- und Impulsübertragung eines verdampfenden kugelförmigen Fluidtropfens, welcher plötzlich in eine gleichgerichtete Fluidströmung höherer Temperatur eingeleitet wird, untersucht. Das Geschwindigkeitsprofil um den Fluidtropfen herum wurde als konstant und als ein Hadamard-Rybczynski-Profil angenommen. Unter Benutzung eines ADI-Schemas der Finiten-Differenzen-Methode wurden numerische Lösungen der Erhaltungsgleichungen für Energie und Dampfmasse gewonnen. Zeitliche Gesetzmäßigkeiten der durchschnittlichen Nusselt und Sherwood-Zahlen (Nu, Sh) und des Widerstandsbeiwertes (C D ) bis zur vollständigen Verdampfung des Tropfens wurden in Abhängigkeit von den zugehörigen Eingabeparametern nämlich der Anfangs-und momentanen Peclet-Zahl (Pe i ,Pe) der Lewis-Zahl und dem Verhältnis von freier Strömungstemperatur zur Eintrittstemperatur des Tropfens (T a /T i ) berechnet. Ebenso werden die lokalen Nusselt und Sherwood-Zahlen in Abhängigkeit von der Peclet-Zahl im Bereich der stationären Verdampfung dargestellt. Es wurde festgestellt, daß Werte der Nusselt-Zahl im Bereich der stationären Verdampfung von Tropfen in guter Übereinstimmung mit den entsprechenden berechneten Größen aus der empirischen Gleichung von Eisenklam liegen.  相似文献   

19.
This paper reports on the results of a numerical study of convection flow and heat transfer in a rectangular porous cavity filled with a phase change material under steady state conditions. The two vertical walls of the cavity are subject respectively to temperatures below and above the melting point of the PCM while adiabatic conditions are imposed on the horizontal walls. The porous medium is characterized by an anisotropic permeability tensor with the principal axes arbitrarily oriented with respect to the gravity vector. The problem is governed by the aspect ratioA, the Rayleigh numberRa, the anisotropy ratioR and the orientation angle θ of the permeability tensor. Attention is focused on these two latter parameters in order to investigate the effects of the anisotropic permeability on the fluid flow and heat transfer of the liquid/solid phase change process. The method of solution is based on the control volume approach in conjunction with the Landau-transformation to map the irregular flow domain into a rectangular one. The results are obtained for the flow field, temperature distribution, interface position and heat transfer rate forA=2.5,Ra=40, 0≤θ≤π, 0.25≤R≤4. It was found that the equilibrium state of the solid/liquid phase change process may be strongly influenced by the anisotropy ratioR as well as by the orientation angle θ of the permeability tensor. First, for a given set of parametersA,Ra andR, there exists an optimum orientation θmax for which the flow strength, the liquid volume and the heat transfer rate are maximum. There also exists an orientation θminmax+π/2 for which these quantities are minimum. Second, when an anisotropic medium is oriented along the optimum direction θmax, an increase of the permeability component along that direction will increase the flow and heat transfer rate in a same order while an increase of the other permeability component only has a negligible effect. For the parameter ranges considered in the present study, it was found that the optimum direction is lying between the gravity vector and the dominant flow direction.  相似文献   

20.
The results of experimental study of volume osmotic flows in a double-membrane system are presented in this article. The double-membrane system consists of two membranes (M u, M d) oriented in horizontal planes and three identical compartments (u, m, d), containing unstirred binary or ternary ionic solutions. In this system concentrations of the solutions fulfil the following conditions C us  = C ds  < C ms (s = 1 or 2). Solutions of aqueous potassium chloride or ammonia were used as binary solutions, whereas potassium chloride dissolved in aqueous ammonia solution or ammonia dissolved in aqueous potassium chloride solution were used as ternary solutions. For binary solutions, the dependencies of a volume flux (J v) on potassium chloride or ammonia concentration (C ms ) are linear, whereas for ternary solutions these dependencies are nonlinear. The volume flux amplification and the osmotic conductivity coefficients were calculated on the basis of experimental data. The coefficient of the volume flux amplification for ternary solutions in comparison to binary ones depends on solutes concentrations and has maximum values dependent on solutes concentrations. Similarly, the osmotic conductivity coefficient has maximal values dependent on solutes concentrations. Moreover, the thermodynamic model of the osmotic volume flux was developed and the results were interpreted within the gravitational instability category.  相似文献   

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