It is generally accepted that the central differences scheme with a stabilizing correction for the transport equation in the 3D case is conditionally stable. This article shows that, strictly ...speaking, this scheme is absolutely unstable. However, the region of unstable harmonics in the wave vector space and their increments quickly tend to zero as the Courant parameter tends to zero, which makes it possible to successfully use this scheme. Therefore, it is more correct to talk about the practically conditional stability of this scheme.
Abstract
Applying the variable separation method, an exact analytical solution is found for thermal conductivity equation, established based on the modified Fourier’s law with thermal flux relaxation ...and temperature gradient under Newton’s boundary conditions (BC). An alternative method is proposed for modeling the transient heat conduction in the plate washed at the boundaries by a liquid or gas, which differs from the traditional one in the absence of the need to determine the heat transfer coefficients from the criterial heat transfer equations, instead of which it is proposed to introduce summands into the classical heat conductivity equation. The summands should take into account the relaxation nature of solids, the features of heat transfer at the boundary, acceleration of heat flux and temperature gradient during the process.
•The heat transfer coefficient increases many times upon boiling on a capillary-porous coating.•Heat transfer on a cоating is lower than on a smooth surface during evaporation.•Pressure determines ...the mechanism of crises during evaporation and boiling on a coating.•The CHF increases significantly on the microstructured surface produced by a 3D laser printer.
Heat transfer was experimentally studied under evaporation/boiling conditions in thin horizontal layers of n-dodecane at low relative pressures. A capillary-porous coating from a stainless steel powder with a sinusoidal 2D modulated profile was applied using a 3D laser printer. The selective laser sintering (SLS) additive technique was applied in the study. The dependence of a heat transfer coefficient on the heat flux under different pressures was studied for liquid layers various heights. The resulting experimental data were compared with the values obtained during evaporation/boiling on a smooth surface under the same conditions. A rise in the relative pressure results in an increase of the heat transfer coefficient at bubble boiling. The study demonstrates that the use of a capillary-porous coating leads to a 3.5-fold increase of the heat transfer coefficient in the regimes of bubble boiling. The heat transfer coefficient on the capillary-porous coating was lower compared to a smooth surface in heat transfer regimes with evaporation at low relative pressures. The critical heat fluxes (CHF) on the capillary-porous coating increase with the increasing height of the liquid layer.
Periodic modulation of the deposition angle (PMDA) is a new method to deposit nanostructured and continuous layers with controllable periodic density fluctuation. The method is used for the magnetron ...sputtering of a WO3 layer for an electrochromic device (ECD). An experimental study indicates that the electrochromic coloration-bleaching rate nearly doubles and the electrochromic efficiency grows by about 25% in comparison with the traditional method. The ECD efficiency rises with the increasing degree of nanostructure ordering, surface roughness, and homogeneity of the WO3 layer. The method is promising for coating deposition techniques needed to produce versatile devices with specific requirements for ion transport in surface layers, coatings, and interfaces, such as fuel cells, batteries, and supercapacitors.
The problem of sudden flame acceleration in narrow channels and subsequent Deflagration to Detonation Transition (DDT) is revisited. The hydrogen–oxygen combustion system is considered both ...experimentally and numerically. The flame is initiated at one open end of a square narrow channel of different widths (4, 8, 10 and 20 mm) and propagates to another open end. Experimental results show a high sensitivity to the mixture composition: for higher mixture reactivity, an abrupt flame acceleration is reported. In 4 × 4 mm2 channel, when the mixture composition is close to stoichiometry (2:1), the DDT is observed without any evidence of shock waves prior to the detonation transition. Two complementary sub-models are combined to account for the effect of walls on flow and on flame front geometry. A pseudo-spectral numerical scheme is used to integrate the system of equations both in time and in space. The simulation results show how the suggested model reproduces main features of the phenomenon. A simple criterion is suggested for the onset of the flame self-acceleration. There exists a critical flame folding factor for the DDT, which is an invariant of the system. The reported critical folding factor is compared to the theoretical estimation introduced by Gordon et al (2020).
Autoignition delay time measurements were performed at equivalence ratios of 0.5, 1 and 2 for butan-1-ol at reflected shock pressures of 1, 2.6 and 8
atm at temperatures from 1100 to 1800
K. ...High-level ab initio calculations were used to determine enthalpies of formation and consequently bond dissociation energies for each bond in the alcohol. A detailed chemical kinetic model consisting of 1399 reactions involving 234 species was constructed and tested against the delay times and also against recent jet-stirred reactor speciation data with encouraging results. The importance of enol chemistry is highlighted.
Electronic excitations have a profound impact on molecular motion induced by femtosecond laser pulses and play a large role in catalytic reactions as well as in spin transport within bulk metals, ...across interfaces, and at surfaces. Chulkov et al present a study of the decay mechanisms of electron excitations on surfaces in which several experimental and theoretical methods were used including photoelectron spectroscopy, inverse photoemission, and self-energy formalism of many-body theory.
Recently, human mesenchymal stem cells (hMSc) have attracted a great deal of attention as potential therapeutic agents in the treatment of socially significant diseases. Despite substantial advances ...in stem-cell therapy, the biological mechanisms of hMSc action after transplantation remain unclear. The use of magnetic resonance imaging (MRI) as a non-invasive method for tracking stem cells in the body is very important for analysing their distribution in tissues and organs, as well as for ensuring control of their lifetime after injection. Herein, detailed experimental data are reported on the biocompatibility towards hMSc of heavily gadolinium-doped cerium oxide nanoparticles (Ce
Gd
O
) synthesised using two synthetic protocols. The relaxivity of the nanoparticles was measured in a magnetic field range from 1 mT to 16.4 T. The relaxivity values (
= 11 ± 1.2 mM
s
and
= 7 ± 1.2 mM
s
in magnetic fields typical of 1.5 and 3 T MRI scanners, respectively) are considerably higher than those of the commercial Omniscan MRI contrast agent. The low toxicity of gadolinium-doped ceria nanoparticles to hMSc enables their use as an effective theranostic tool with improved MRI-contrasting properties.
A model of binary programs implementing the functions of the algebra of logic (Boolean functions) is considered. The programs consist of one or several modules containing instructions of three types: ...computational and redirecting instructions and instructions for summoning the procedures. In contrast to earleir models of binary programs, a model is introduced that admits the recursive summoning of procedures; i.e., the procedures can directly summon themselves while executing a binary program, or through other procedures. The functioning of this model of programs is described, as is its relationship to other discrete control systems (e.g., circuits made of functional elements or binary decision diagrams). Ways are presented for obtaining lower and upper estimates of the Shannon function for the complexity of using Boolean functions in the class of binary programs. The proposed technique allows the asymptotics of the Shannon function to be established under certain structural and parametric limitations imposed on the model of binary programs.
Volumetric modification of dielectrics by ultrashort laser pulses is a complex dynamic phenomenon involving material photoexcitation and associated nonlinear processes. To achieve control over ...modification, it is necessary to gain a deep insight into the dynamics of laser-excited processes that can be realized using double-laser-pulse experiments with different time separations supported by numerical simulations. In this paper, we apply this approach to investigate fused silica modification with femtosecond laser pulses that provides time-resolved information about the dynamic behavior of the laser-excited bandgap material. It is shown that the laser-generated free-electron plasma causes a shielding effect for the following pulse with a characteristic duration of ∼600 fs after the pulse action. Within this time interval, the second pulse produces a reduced modification as compared to a longer time separation between pulses. For double pulses with different energies, it was found that the volumetric modification is stronger when a lower-energy pulse couples with material first. This is explained by the combination of the effects of the re-excitation of self-trapped excitons, which are generated as a result of free electron recombination and associated light shielding. Experimental results are supported by numerical simulations of double laser pulse propagation in nonlinear media based on Maxwell's equations. Our findings offer a route for better controlling the inscription of 3D photonic structures in bulk optical materials.
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