Thermomolecular pressure difference under arbitrary accommodation at a surface

The phenomenon of thermomolecular pressure difference is described in terms of the solution to the linearized Bubnov-Galerkin-Knudsen model, assuming an arbitrary tangential-momentum accomodation coefficient for molecules impinging on a surface. A comparison of theoretical results with test data shows a close agreement between them.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 24, No. 2, pp. 227–232, February, 1973.     

Statistics of specular points at a randomly rough surface

Statistical parameters are obtained for an ensemble of specular points at a randomly rough Gaussian statistically isotropic surface at normal incidence. The joint probability density functions (PDFs) of specular point heights and total curvatures are derived separately for maxima, minima, and saddle points. The joint PDFs of brightness and surface elevations of specular points of different types are obtained analytically in an explicit form.     

Direct simultaneous measurement of rotational and translational accomodation coefficients

When molecules of a low density diatomic gas strike a solid surface both translational energy and the internal energy modes of rotation and vibration will contribute the energy exchange that occurs. Theoretical studies indicate that accomodation coefficient for rotational energy should be less than that for translational energy, and this is confirmed by experimental results. The experimental apparatus described in this paper uses the electron bream fluorescence detector to measure simultaneously both rotational and translational energy accommodation coefficients of room temperature nitrogen reflecting from a solid surface. A bakeable ultra high vacuum system was built to provide a clean vacuum environment for control of the solid surface properties. In addition to being the only known direct measurement of rotational accomodation coefficient the system offers an advantage over some previous methods of translational accommodation measurement in that there are few restrictions on solid surface temperature or composition.     

Backscatter cross sections for randomly oriented metallic flakes at optical frequencies: full wave approach

The backscatter cross sections for randomly oriented metallic flakes are derived using the full wave approach. The metallic flakes are characterized by their surface height spectral density function. Both specular point and Bragg scattering at optical frequencies are accounted for in a self-consistent manner. It is shown that the average normalized backscatter cross sections (per unit volume) for the randomly oriented metallic flakes are larger than that of metallic spheres.     

Reconstruction of the net emission distribution from the total radiance distribution on a reflecting surface

The problem is considered of reconstructing, from a measurement of the total radiance distribution on an emitting surface, the radiance distribution that would be observed in the absence of reflected radiation. An explicit solution of the implied inverse problem is derived for the case in which the reflective properties of the surface are given in terms of a bidirectional reflection distribution function. Also considered are the limiting cases of diffuse and specular reflection. Practical considerations are discussed for application of the theory to the nonintrusive and remote measurement of temperatures and pressures on concave surfaces, either by traditional radiometry or by the use of thermographic phosphors and temperature- and pressure-sensitive paints.     

Calculation of the statistical characteristics of the light reflected by a rough random cylindrical homogeneous Gaussian surface

Statistical characteristics of light reflected by a rough random cylindrical homogeneous Gaussian surface are investigated using a modified method of specular points, as developed by Gardashov. In this proposed method, a special procedure for determining the light intensity near the caustics has been formulated. The probability distribution of the intensity of reflected light is expressed in terms of a special function, which is determined by the characteristic function of distribution of radii of curvature at the specular points and the distribution density of the number of specular points. The distribution of radii of curvature, derived by Gardashov, and expressed in terms of dimensionless radii of curvature, has a simple expression which does not contain any parameter of the surface (as a surface rms deviation, etc.). Consequently, it is universally valid and applicable to any cylindrical homogeneous Gaussian surface. After modification, the infinite dispersion of the reflected light intensity turns into a finite. The relationship between the distributions of reflected light intensity and the number of specular points, in the form of a Fredholm integral equation of the first kind is obtained. The kernel of the integral equation is expressed in terms of a characteristic function of the radii of curvature at specular points. The validity of formulae and relationships, thus derived, is tested by numerical simulations.     

Heat transfer from a spherical source in a rarefied gas

The heat-transfer problem from a sphere in a rarefied gas is solved using a model kinetic equation. Diffuse scattering of gas molecules by the sphere surface with arbitrary energy accomodation is assumed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 3, pp. 428–434, September, 1980.     

Sensitivity of high vacuum standard parameters to the share of molecule specular reflections in the gas scattering

The molecular gas flow parameters were evaluated using cosine form of the scattering function of the molecules incident on the surface. This gas scattering model is commonly used for calculating the vacuum system parameters, but it is theoretically valid only if the vacuum system is in equilibrium. This also means that there is no flow in the system. We applied the Maxwell's model of gas scattering with the contribution of specular reflections to study how these reflections influence the flow parameters. In typical calibration systems with dynamic expansion of gases, a 10% contribution of the specular reflections results in the 0.18% decrease of the computed value of the gas density inside the gauge to be calibrated as compared with that when the cosine law is employed.     

Pressure build-up at the metal delivery tube orifice in ultrasonic gas atomization

The effects of geometry (diameter and tip design) and position (relative to the gas nozzles) of the metal delivery tube in an ultrasonic gas atomization (USGA) device on the pressure condition in the gas-metal interaction zone at the tube orifice have been studied. Simulation of ultrasonic gas (argon or nitrogen) atomization has been conducted, both at low (3.5 to 14 atm) and high (15 to 75 atm) atomization pressures. Low gas atomization pressures are generally used in spray deposition processes such as liquid dynamic compaction (LDC), while high pressures are used for powder production. Depending on the experimental conditions, i.e. the shape and angle of the taper at the metal delivery tube orifice or its position with respect to the nozzles' gas exit common plane, either partial vacuum (equivalent to downward aspiration of the melt) or overpressures (equivalent to back-pressurization of the melt) at the metal delivery tube was detected. Underpressure and overpressure effects were found to increase with gas atomization pressure. The maximum pressure differences measured with respect to the atomization chamber pressure were about 0.15 to 0.25 atm for the low-pressure experiments, and 0.50 to 0.60 atm for the high-pressure experiments. Underpressures or overpressures of these magnitudes have a large effect on the metal flow rate during gas atomization, either enhancing or reducing it, and thus changing significantly the gas to metal flow ratio. Because this is a crucial parameter for both the USGA and the LDC processes, the state of pressure at the delivery tube's orifice has to be monitored carefully, in order to ensure optimal processing conditions.     

原位光学法研究PET-CO2系统的玻璃化转变

采用原位光学法研究聚酯(PET)/CO2系统的玻璃化转变温度(Tg)与压力间的关系。在温度为50℃～120℃,压力为0.1MPa～20MPa范围内,PET薄膜样品的玻璃化转变温度的降低(Tg)与气体的压力(p)成线性关系,其斜率dTg/dp≈-1.35℃/MPa,上述条件下的最大降幅ΔTg,max=-26.7℃。在30MPa的文献值明显偏离ΔTg～p直线,意味着在高压阶段,静压起主导作用,使PET样品的玻璃化转变温度升高。     

Split-type free-displacer Stirling refrigerator design using linear network analysis

A linear network analysis is developed for the system design of split-type free-displacer Stirling refrigerators. The dynamics models are derived to describe the input/output relation of each component by use of the governing equations in conjunction with linearization and approximation. By connecting the equivalent circuits of the components together, a linear network consisting of a fluid network and a displacer network is obtained. The fluid network accounts for the propagation of gas flow induced by the pressure wave; while the displacer network accounts for the displacer motion induced by the pressure force exerted on the displacer. Two transfer functions are further derived for the displacer motion and the gas pressure of the expansion space with respect to the piston motion. The system performance is then evaluated from the frequency response of the transfer functions by using the sinusoidal signal analysis. The performance prediction of a split-type free-displacer Stirling refrigerator using the linear network analysis is shown to be satisfactory.     

Finite element analysis of non-isothermal multiphase geomaterials with application to strain localization simulation

Finite element analysis of non-isothermal elasto-plastic multiphase geomaterials is presented. The multiphase material is modelled as a deforming porous continuum where heat, water and gas flow are taken into account. The independent variables are the solid displacements, the capillary and the gas pressure and the temperature. The modified effective stress state is limited by the Drucker-Prager yield surface for simplicity. Small strains and quasi-static loading conditions are assumed. Numerical results of strain localization in globally undrained samples of dense, medium dense and loose sands and isochoric geomaterial are presented. A biaxial compression test is simulated assuming plane strain condition during the computations. Vapour pressure below the saturation water pressure (cavitation) develops at localization in case of dense sands, as experimentally observed. A case of strain localization induced by a thermal load where evaporation takes place is also analysed. Dedicated to Professor S. Valliappan in occasion of his retirement     

Reflection from bound microbubbles at high ultrasound frequencies

Targeted contrast agents and ultrasound imaging are now used in combination for the assessment and tracking of biomarkers in animal models in vivo. These applications have triggered interest in the understanding and prediction of the ultrasound echoes from contrast agents attached to cells. This study describes the reflection enhancement due to microbubbles bound on a gelatin surface. The reflection enhancement was measured using ultrasound pulses at high frequency (40 MHz) and low pressure (38 kPa peak-negative pressure) allowing a linear approximation to be applied. The observed reflection coefficient increased with the number of microbubbles, until reaching saturation at 0.9 when the surface coverage fraction was 35%. A multiple scattering model assuming that the targeted microbubbles are confined within an infinitesimally thin layer appeared suitable in predicting the reflection coefficient even at very high surface densities. These results could permit the optimization of the sensitivity of high frequency ultrasound to targeted contrast agents.     

Effect of surface roughness and subsurface damage on grazing-incidence x-ray scattering and specular reflectance

Grazing-incidence specular reflectance and near-specular scattering were measured at Al-K(alpha) (1.486-keV, 8.34-?) radiation on uncoated dielectric substrates whose surface topography had been measured with a scanning probe microscope and a mechanical profiler. Grazing-incidence specular reflectance was also measured on selected substrates at the Cu-K(alpha) (8.047-keV, 1.54-?) wavelength. Substrates included superpolished and conventionally polished fused silica; SiO(2) wafers; superpolished and precision-ground Zerodur; conventionally polished, float-polished, and precision-ground BK-7 glass; and superpolished and precision-ground silicon carbide. Roughnesses derived from x-ray specular reflectance and scattering measurements were in good agreement with topographic roughness values measured with a scanning probe microscope (atomic force microscope) and a mechanical profiler that included similar ranges of surface spatial wavelengths. The specular reflectance was also found to be sensitive to the density of polished surface layers and subsurface damage down to the penetration depth of the x rays. Density gradients and subsurface damage were found in the superpolished fused-silica and precision-ground Zerodur samples. These results suggest that one can nondestructively evaluate subsurface damage in transparent materials using grazing-incidence x-ray specular reflectance in the 1.5-8-keV range.     

Split off-specular reflection and surface scattering from woven materials

We measured radiance distributions for black lining cloth and copper gauze using the convenient technique of wrapping the materials around a circular cylinder, irradiating it with a parallel light source and collecting the scattered radiance by a digital camera. One family of parallel threads (weave or weft) was parallel to the cylinder generator. The most salient features for such glossy plane weaves are a splitting up of the reflection peak due to the wavy variations in local slopes of the threads around the cylinders and a surface scattering lobe due to the threads that run along the cylinder. These scattering characteristics are quite different from the (off-)specular peaks and lobes that were found before for random rough specular surfaces. The split off-specular reflection is due to the regular structures in our samples of man-made materials. We derived simple approximations for these reflectance characteristics using geometrical optics.     

Patterns of recruitment and injury in a heterogeneous airway network model

In respiratory distress, lung airways become flooded with liquid and may collapse due to surface-tension forces acting on air–liquid interfaces, inhibiting gas exchange. This paper proposes a mathematical multiscale model for the mechanical ventilation of a network of occluded airways, where air is forced into the network at a fixed tidal volume, allowing investigation of optimal recruitment strategies. The temporal response is derived from mechanistic models of individual airway reopening, incorporating feedback on the airway pressure due to recruitment. The model accounts for stochastic variability in airway diameter and stiffness across and between generations. For weak heterogeneity, the network is completely ventilated via one or more avalanches of recruitment (with airways recruited in quick succession), each characterized by a transient decrease in the airway pressure; avalanches become more erratic for airways that are initially more flooded. However, the time taken for complete ventilation of the network increases significantly as the network becomes more heterogeneous, leading to increased stresses on airway walls. The model predicts that the most peripheral airways are most at risk of ventilation-induced damage. A positive-end-expiratory pressure reduces the total recruitment time but at the cost of larger stresses exerted on airway walls.     

Kinematics of a multi-dimensional shock of arbitrary strength in an ideal gas

Summary It has been shown that the kinematics of a shock front in an ideal gas with constant specific heat can be completely described by a first order nonlinear partial differential equation (called here — shock manifold equation or SME) which reduces to the characteristic partial differential equation as the shock strength tends to zero. The condition for the existence of a nontrivial solution of the jump relations across the shock turns out to be the Prandtl relation. Continuing the functions representing the state on the either side of the shock to the other side as infinitely differentiable functions and embedding the shock in a one parameter family of surfaces, it has been shown that the Prandtl relation can be treated as a required shock manifold equation for a function , where =0 is the shock surface. We also show that there are other forms of the SME and prove an important result that they are equivalent. Shock rays are defined to be the characteristic curves of a SME and it has been shown that when the flow on either side of the shock is at rest, the shock rays are orthogonal to the successive positions of the shock surface. Certain results have been derived for a weak shock, in which case the complete history of the curved shock can be determined for a class of problems.With 4 Figures     

Fabrication of Al/TiB2 composites through gas pressure infiltration

Titanium diboride is being evaluated as a replacement of SiC in metal/ceramic composites for thermal management. Albeit these composites are produced by gas pressure infiltration, no study of the process is available. This work presents an analysis of the threshold pressure for infiltration of Al into TiB₂ compacts and of the infiltration kinetics. Particles of diameters 14 and 20 μm, and spans of 1.62 and 1.77, were packed obtaining a volume fraction of 0.63. The compacts were infiltrated in air with pure Al at 700 °C. Despite of the fact that the average particle radii differ in a 50%, the threshold pressure for the coarser particles is only a 4% lower, while intrinsic compact permeabilities (derived from infiltration of a wetting organic liquid) are identical. The origin of this apparently odd behavior is the almost identical specific surface areas that those two particles have. Contact angles derived from infiltration experiments are shown to be compatible with sessile drop results.     

Thermocapillary flow in a liquid layer at minimum in surface tension

Summary In the present paper a class of similarity solutions for the two-dimensional Navier-Stokes and energy equations describing thermocapillary flows in a liquid layer of constant width and infinite extent is presented. The layer is bounded by a horizontal rigid plate from one side and opened to the ambient gas from the other one. The physical properties of the liquid are assumed to be constant except the surface tension which varies as a quadratic function with temperature. It is supposed that a constant temperature gradient exists along either the liquid free surface (case I) or the rigid boundary (case II).In both cases, by means of a similarity transformation, the equations of motion and energy are reduced to a system of three ordinary differential equations, one for the velocity and two for the temperature. The equation for the velocity can be solved separately from the other equations and its solution, found numerically, exists only for the Marangoni number less than a certain finite value. The solution of the whole system depends also on the Prandtl number. The solution of one of the temperature equations is presented in an analytical form and the other equation is solved numerically. Asymptotic formulas of the functions are also obtained for small and large Marangoni numbers. Flow pattern and temperature fields are presented. One convective roll exists in every semi-infinite layer. Fluid velocities at different points of the free surface are evaluated for an aqueous solution of n-heptanol and compared with those measured in the experiments.     

Atomic layer deposition of Al(2)O(3) and ZnO at atmospheric pressure in a flow tube reactor

Improving nanoscale thin film deposition techniques such as atomic layer deposition (ALD) to permit operation at ambient pressure is important for high-throughput roll-to-roll processing of emerging flexible substrates, including polymer sheets and textiles. We present and investigate a novel reactor design for inorganic materials growth by ALD at atmospheric pressure. The reactor uses a custom "pressure boost" approach for delivery of low vapor pressure ALD precursors that controls precursor dose independent of reactor pressure. Analysis of continuum gas flow in the reactor shows key relations among reactor pressure, inert gas flow rate, and species diffusion that define conditions needed to efficiently remove product and adsorbed reactive species from the substrate surface during the inert gas purge cycle. Experimental results, including in situ quartz crystal microbalance (QCM) characterization and film thickness measurements for deposition of ZnO and Al(2)O(3) are presented and analyzed as a function of pressure and gas flow rates at 100 °C. At atmospheric pressure and high gas flow, ZnO deposition can proceed at the same mass uptake and growth rate as observed during more typical low pressure ALD. However, under the same high pressure and flow conditions the mass uptake and growth rate for Al(2)O(3) is a factor of ～1.5-2 larger than at low pressure. Under these conditions, Al(2)O(3) growth at atmospheric pressure in a "flow-through" geometry on complex high surface area textile materials is sufficiently uniform to yield functional uniform coatings.