高浓度过氧化氢中AlCoCrFeNiCu 的摩擦学性能研究

为研究AlCoCrFeNiCu高熵合金在强氧化的过氧化氢介质中的摩擦学性能,采用销盘磨损试验机测试了AlCoCrFeNiCu合金与3种工程陶瓷组成摩擦副在90％过氧化氢介质中的摩擦学性能,采用SEM、EDS、白光共焦显微镜等分析了磨损表面,并且探讨了高熵合金与3种陶瓷配副在高浓度过氧化氢中的磨损机理.结果表明:在高浓度过氧化氢中,AlCoCrFeNiCu合金与碳化硅和氮化硅陶瓷配副具有较低的摩擦系数和较小的磨损;AlCoCrFeNiCu/ZrO₂摩擦副的主要磨损机制为黏着磨损和磨粒磨损,同时伴随有氧化磨损;AlCoCrFeNiCu/SiC摩擦副和AlCoCrFeNiCu/Si₃N₄摩擦副的主要磨损机制为抛光型氧化磨损并伴随有轻微的三体磨粒磨损;AlCoCrFeNiCu/Si₃N₄摩擦副还伴随有边界润滑效应.     

Transport coefficients of air in the region of temperatures from 3000 to 25,000 ° K and pressures of 0.1, 1, 10, and 100 atm

In the article a calculation is made of the transport properties of air in the region of temperatures from 3000 to 25,000 °K and pressures of 0.1, 1, 10, and 100 atm. The calculations are made within the framework of molecular-kinetic theory in a first approximation of the distribution function by the Chapman-Enskog method, taking account of the first four terms of a series expansion by Sonin polynomials. The collision integrals _ij ^l,s of the air components (N₂-N₂, O₂-O₂, NO-NO, N₂-O₂, N₂-NO, O₂-NO, N₂-N, N₂-O, O₂-N, O₂-O, N-NO, O-NO, N-N, O-O, O-N, e-N, e-O, e-N₂, e-O₂, e-NO, N-N⁺, O-O⁺, N-O⁺, O-N⁺, electron-ion, ion-ion, electron-electron) of the ordersl=1, 4 and s=l, (8-l) are calculated. As far as possible the collision integrals presented are calculated on the basis of experimentally measured interaction potentials obtained through the direct scattering of atomic and molecular beams on gas targets or other beams. The missing information on interaction potentials is borrowed from works on spectroscopic analysis. The scattering of an electron on an oxygen atom is calculated in the framework of a quantum-mechanical analysis. The collision integrals presented for charged components of the air were calculated numerically for the Debye-Hückel screening potential. The collision integral moments obtained in the work can be used to calculate the kinetic coefficients for an arbitrary mixture of nitrogen and oxygen, including the case of a variable elementary composition.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 2, pp. 80–90, March–April, 1973.In conclusion the author thanks V. G. Sevast'yanenko under whose guidance the work was carried out.     

Flow of rarefied gases in a capillary screen at different temperatures

An experimental study was made of the flow of the gases He, Ne, Ar, Kr, Xe, H₂, D₂, N₂, CO₂, and CH₄ in the range of Knudsen numbers of 10⁴–10^?1 at room temperature in a capillary screen. A study was also made of the flow of a number of inert and diatomic gases at temperatures of 77.2 and 194.7 °K in an orifice and in a capillary screen. The relative flow rates were determined in the free-molecular mode of flow. The coefficients of accomodation of tangential momentum are calculated for the gases studied at different temperatures.     

A collisional-radiative code for computing air plasma in high enthalpy flow

A time-dependent collisional-radiative model for air plasma has been developed to study the effects of nonequilibrium atomic and molecular processes on population densities in a weakly ionized high enthalpy flow. This model consists of 15 species: e^-,N, N⁺,N²⁺,O, O⁺,O²⁺,O^-,N₂,N₂⁺,NO, NO⁺,O₂,O₂⁺{{\rm e}^{-},{\rm N, N}^{+},{\rm N}^{2+},{\rm O, O}^{+},{\rm O}^{2+},{\rm O}^{-},{\rm N}_{2},{{\rm N}_{2}}^{+},{\rm NO, NO}^{+},{\rm O}_{2},{{\rm O}_{2}}^{+}}, and O₂^-{{{\rm O}_{2}}^{-}} with their major electronic excited states. Many elementary processes are considered in the number density range of 10¹²/cm³ ≤ N ≤ 10¹⁹/cm³ and the temperature range of 300 K ≤ T ≤ 40,000 K. We then compare our results with an existing collisional-radiative code to validate our model. Additionally, the unsteady nature of pulsively heated air plasma is investigated. When the ionization relaxation time is of the same order as the time scale of a heating pulse, the effects of unsteady ionization are important for estimating air plasma states. From parametric computations, we determine the appropriate conditions for the collisional-radiative steady state, local thermodynamic equilibrium, and corona equilibrium models in that density and temperature range.     

Validity of the bead-spring model for describing the linear viscoelastic properties of single-strand DNA under strongly denaturing conditions

Using a normal mode analysis, we predict the infinite dilution linear viscoelastic properties of single-strand (ss) DNA molecules and compare the results to the linear viscoelastic data of Shusterman et al. (Phys Rev Let 92(4):048303, 2004) obtained by monitoring the diffusion of a fluorescently labeled terminus of the molecule. To compute the overall best global fit, we constrain the hydrodynamic interaction parameter, h*, equilibrium root mean square spring extension, b, and the number of Kuhn steps per spring, N _K,S, to be equal for the strands compared. The fits using the bead-spring model for all but 23,100 base ss-DNA strands match the experimental data at long times with significant deviations at intermediate and short times. However, parameters fitted separately to all individual strand lengths predict results well. The best fits to data for 2,400 and 6,700 base pairs yield N _K,S ∼12 and h* = 0.12. These values are similar to those found for conventional polymers such as polystyrene which have been successfully modeled with N _K,S ∼7 and h* = 0.15, indicating ss-DNA and polystyrene exhibit analogous hydrodynamic behavior.     

Crack growth rate and cracking velocity in fatigue for a class of ceramics

Fatigue crack growth rate data and cracking velocity data are studied for a class of ceramics including SiC, TiB₂, Si₃N₄, ZrO₂ and Al₂O₃. Both sine and square wave cyclic loading are combined such that the data could be converted to cracking velocity for a given frequency of cyclic load. An effective stress intensity factor range is defined and used in an relation for computing the crack growth rate and cracking velocity. As for the metal alloys, the data for ceramics also fall into three regimes identified with near-threshold, stable growth and rapid crack extension, except that the slope of the da/dN (the crack growth rate) curves for ceramics are steeper in comparison with that for metals. Reported are the empirical constants in the relations for the crack growth rate and the cracking velocity for a variety of ceramics.     

On the N-wave equations and soliton interactions in two and three dimensions

Several important examples of the N-wave equations are studied. These integrable equations can be linearized by formulation of the inverse scattering as a local Riemann-Hilbert problem (RHP). Several nontrivial reductions are presented. Such reductions can be applied to the generic N-wave equations but mainly the 3- and 4-wave interactions are presented as examples. Their one and two-soliton solutions are derived and their soliton interactions are analyzed. It is shown that additional reductions may lead to new types of soliton solutions. In particular the 4-wave equations with ?₂ × ?₂ reduction group allow breather-like solitons. Finally it is demonstrated that RHP with sewing function depending on three variables t, x and y provides some special solutions of the N-wave equations in three dimensions.     

Diffusion processes in the mixing zone of a supersonic jet of low density

Results of an investigation into the diffusion processes in a jet of low density behind a strongly underexpanded sonic nozzle, in the zone of mixing with the surrounding gas, are presented. By means of electron-beam methods, the structure of the jet was studied in the case of expanding N₂, into an atmosphere of CO₂ + N₂ in transient regimes of flow varying from solid to rarefied. The results of an analysis of the fields of concentration of the separate components are given in a generalized form.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 121–127, January–February, 1973.     

Microhardness indentation application and limitation in fracture-toughness evaluation of ceramics

The use of a microhardness indentation as a controlled flaw in a flexural specimen for measuring fracture toughness in advanced ceramics is critically evaluated. From a review of fracture-toughness testing methodologies for ceramics it is shown that currently there are not any standardized or widely agreed uponK _Ic test procedures. The problems associated with the controlled flaw test are discussed and analyzed. An experimental program is used to investigate the effects of residual stress on the measurement of fracture toughness in a wide variety of sintered and hot-pressed advanced ceramics including TiB₂, Al₂O₃, SiC, ZrO₂, Si₃N₄. The validity of the test procedure for evaluatingK _Ic of advanced ceramics in general is put forth.     

Calculation of convective heat flux in the vicinity of the stagnation point for partially ionized gas flow past a body

From numerical solutions of the boundary layer equations for a four-component gas mixture (E, N⁺, N₂, and N) with gas injection, approximate formulas for the heat flux as a function of the variation of λρ/c_p and h^* across the boundary layer and the magnitude of the objection are obtained (λ is the thermal conductivity of the mixture,ρ is density, c_p is the specific heat, and h^* is the enthalpy of the ideal gas state of the mixture). An effective ambipolar diffusion coefficient D^(a)(i) is introduced, making possible finite formulas for the convective heat fluxes in the “frozen” boundary layer. We study the behavior of these coefficients within the boundary layer. A formula is obtained for convective heat flux to the wall from partially ionized air for a nine-component mixture (E, O⁺, N⁺, NO⁺, O, N, NO, O₂ N₂). Even for simpler four-component gas model three effective ambipolar diffusion coefficients are necessary: $$\begin{gathered} D^{(a)} (A) = D (A, M) D^{(a)} (I) = 2D (A, M), \hfill \\ D^{(a)} (M) = [ 1 + c_e (I)] D(A, M). \hfill \\ \end{gathered} $$ Here D(A, M) is the binary diffusion coefficient of the atoms into molecules, and c_e(I) is the ion concentration at the outer edge of the boundary layer. The assumption of an infinitely large charge-exchange cross section and the other simplifying assumptions used in [1] lead to overestimation of the magnitude of the dimensionless heat flux by 7–15% for the “frozen” boundary layer case.     

Effect of nonequilibrium processes in the infrasonic and ultrasonic regions of a nozzle on population inversion in flow of CO2 - N - O2 - H2O gas mixtures

The processes of oscillatory relaxation in flow through nozzles of CO₂, N₂, O₂, H₂O gas mixtures are studied. The effect of various oscillatory-oscillatory and osciallatory-translational exchange channels between mixture components and CO₂ intermode energy-exchange Channels on population inversion is examined. The importance of considering nonequilibrium processes in the infrasonic region of the nozzle is noted. The effect of inclined shock waves on flow characteristics in the supersonic nozzle region and population inversion is studied.     

The dependence of viscoelastic flow properties on the structure for styrene-butadiene copolymers

The flow properties for 300 kg/m³ solutions of four-arm, star-branched random and block styrene-butadiene-styrene copolymers in n-butylbenzene are presented. The viscosity, η, first and second normal stress differences, N₁ and N₂, and the steady sher compliance, J, were determined as a function of the shear rate from cone-plate shearing data obtained with a stiffened Model R17 Weissenberg Rheogoniometer. The normal stress differences were determined from total normal force and plate pressure distribution data. Four sensitive, miniature, variable-capacitance pressure transducers mounted in the 7.4-cm plate with their approximately 2.4 mm in diameter pressure-sensing diaphragms flush with the plate surface provided data for the pressure distribution on the plate. In general, the data extend from the zero shear-rate viscosity region somewhat into the shear thinning region. Based on limited data, the zero shear viscosities for the random copolymers increased with (M₃)^3.2, whereas those for the 28% block and 38% block polymers increased with (M_w)^?;6, respectively. The latter high exponents are believed to be a consequence of a network with junctions formed by dispersed phase polystyrene block domains. The sign for N₂ was opposite that of N₁ and the ratio N₂/M₁ for all of the star copolymers averaged –0.214 with a standard deviation of 1.5%. This value is within ±1% of the ratios for tetrachain polybutadienes and polystyrenes and is significantly lower than the –0.29 for linear polybutadiene and polystyrene solutions in normal butylbenzene. The N₂/N₁ ratio did not vary significantly over the shear-rate range investigated.     

Numerical study of the detonation structure in rich H2?NO2/N2O4 and very lean H2?N2O mixtures

In some mixtures and under certain conditions, detonation soot records show substructures. In nitromethane and nitrogen tetroxide mixtures, particular cellular structures can be observed. This kind of structures has been reported as the so-called double cellular structure. One- and two-dimensional simulations of detonation have shown that the double cellular structure is related to a non-monotonous energy release. Two-step energy release is also observed in rich H₂−NO₂/N₂O₄ and in very lean H₂−N₂O mixtures. The present study aims at the investigation of the effect of the energy release profile on the detonation structure in these two mixtures through numerical simulations. The origin of the non-monotonous energy release is explained in both mixtures using one-dimensional simulations with detailed chemistry. Reduced kinetic schemes are obtained and used to perform two-dimensional simulations. It is shown that in rich H₂−NO₂/N₂O₄ mixtures, the double cellular structure appears, whereas in very lean H₂−N₂O mixtures, classical substructures are observed. Both behaviours are explained based on ZND calculations and previous stability results. Phenomenological considerations led the authors to link the formation of the double cellular structure with the appearance of a large scale instability mode (a super cellular structure).     

Non-similar Solution for Natural Convective Boundary Layer Flow Over a Sphere Embedded in a Porous Medium Saturated with a Nanofluid

A boundary layer analysis is presented for the natural convection past an isothermal sphere in a Darcy porous medium saturated with a nanofluid. Numerical results for friction factor, surface heat transfer rate, and mass transfer rate have been presented for parametric variations of the buoyancy ratio parameter N _r, Brownian motion parameter N _b, thermophoresis parameter N _t, and Lewis number L _e. The dependency of the friction factor, surface heat transfer rate (Nusselt number), and mass transfer rate (Sherwood number) on these parameters has been discussed.     

Effect of flow conditions on the amplification and generation of pulsed gasdynamic lasers in N2O and CO2

The weak signal amplification is studied in jets involving N₂O-N₂He, N₂O-N₂ and CO₂-CO-N₂-He mixtures with installations combining a shock-tube and slit-throat (1.5×40 and 2.5×70 mm²) or two-dimensional wedge (=15°, A/A _* = 15) nozzles. The relationships between amplification and gasdynamic characteristics are elucidated.     

A computational study of some rheological influences on the “splashing experiment”

In various attempts to relate the behaviour of highly-elastic liquids in complex flows to their rheometrical behaviour, obvious candidates for study have been the variation of shear viscosity with shear rate, the two normal stress differences N₁ and N₂, especially N₁, the extensional viscosity, and the dynamic moduli G′ and G″. In this paper, we shall confine attention to ‘constant-viscosity’ Boger fluids, and, accordingly, we shall limit attention to N₁, η_E, G′ and G″.We shall concentrate on the “splashing” problem (particularly that which arises when a liquid drop falls onto the free surface of the same liquid). Modern numerical techniques are employed to provide the theoretical predictions. We show that high η_E can certainly reduce the height of the so-called Worthington jet, thus confirming earlier suggestions, but other rheometrical influences (steady and transient) can also have a role to play and the overall picture may not be as clear as it was once envisaged. We argue that this is due in the main to the fact that splashing is a manifestly unsteady flow. To confirm this proposition, we obtain numerical simulations for the linear Jeffreys model.     

Natural convection of nanofluid over vertical plate embedded in porous medium: prescribed surface heat flux

The aim of the present paper is to analyze the natural convection heat and mass transfer of nanofluids over a vertical plate embedded in a saturated Darcy porous medium subjected to surface heat and nanoparticle fluxes. To carry out the numerical solution, two steps are performed. The governing partial differential equations are firstly simplified into a set of highly coupled nonlinear ordinary differential equations by appropriate similarity variables, and then numerically solved by the finite difference method. The obtained similarity solution depends on four non-dimensional parameters, i.e., the Brownian motion parameter (N _b), the Buoyancy ratio (N _r), the thermophoresis parameter (N _t), and the Lewis number (Le). The variations of the reduced Nusselt number and the reduced Sherwood number with N _b and N _t for various values of Le and N _r are discussed in detail. Simulation results depict that the increase in N _b, N _t, or N _r decreases the reduced Nusselt number. An increase in the Lewis number increases both of the reduced Nusselt number and the Sherwood number. The results also reveal that the nanoparticle concentration boundary layer thickness is much thinner than those of the thermal and hydrodynamic boundary layers.     

Simulation of the rheological properties of suspensions of oblate spheroidal particles in a Newtonian fluid

A simulation algorithm was developed to predict the rheological properties of oblate spheroidal suspensions. The motion of each particle is described by Jeffery’s solution, which is then modified by the interactions between the particles. The interactions are considered to be short range and are described by results from lubrication theory and by approximating locally the spheroid surface by an equivalent spherical surface. The simulation is first tested on a sphere suspension, results are compared with known experimental and numerical data, and good agreement is found. Results are then presented for suspensions of oblate spheroids of two mean aspect ratios of 0.3 and 0.2. Results for the relative viscosity η _r, normal stress differences N ₁ and N ₂ are reported and compared with the few available results on oblate particle suspensions in a hydrodynamic regime. Evolution of the orientation of the particles is also observed, and a clear alignment with the flow is found to occur after a transient period. A change of sign of N ₁ from negative to positive as the particle concentration is increased is observed. This phenomenon is more significant as the particle aspect ratio increases. It is believed to arise from a change in the suspension microstructure as the particle alignment increases.     

Characterization of the flow over a cylinder moving harmonically in the cross-flow direction

The flow over a stationary cylinder is self-excited with a specific natural frequency f_N. When the cylinder is moved harmonically in the cross-flow direction, the response of the flow (in terms of the lift force) will contain two frequencies, namely, the natural frequency f_N and the excitation frequency f_E. When f_E is close to f_N, the natural flow response will be entrained by the excitation, and the response will be periodic with frequency f_E, and dynamicists refer to this phenomenon as lock-in or synchronization. When f_E is away from f_N, the flow will be either periodic with a period that is multiple of the excitation period (i.e., period-n) and dynamicists refer to this phenomenon as secondary synchronization or quasiperiodic consisting of two incommensurate frequencies, or chaotic. We use modern methods of non-linear dynamics to characterize these responses and show that the route to chaos is torus breakdown.