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The oxygen absorbance was studied at wavelengths 200–270 nm in Schumann-Runge system behind the front of a strong shock wave. Using these data, the vibrational temperature Tv behind the front of shock waves was measured at temperatures 4000–10 800 K in undiluted oxygen. Determination of Tv was based on the measurements of time histories of absorbance for two wavelengths behind the shock front and on the results of detail calculations of oxygen absorption spectrum. Solving the system of standard quasi-one-dimensional gas dynamics equations and using the measured vibrational temperature, the time evolution of oxygen concentration and other gas parameters in each experiment were calculated. Based on these data, the oxygen dissociation rate constants were obtained for thermal equilibrium and thermal non-equilibrium conditions. Furthermore, the oxygen vibrational relaxation time was also determined at high temperatures. Using the experimental data, various theoretical and empirical models of high-temperature dissociation were tested, including the empirical model proposed in the present work.

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Boundary conditions required for numerical solution of the Boltzmann kinetic equation (BKE) for mass/heat transfer between evaporation and condensation surfaces are analyzed by comparison of BKE results with molecular dynamics (MD) simulations. Lennard-Jones potential with parameters corresponding to solid argon is used to simulate evaporation from the hot side, nonequilibrium vapor flow with a Knudsen number of about 0.02, and condensation on the cold side of the condensed phase. The equilibrium density of vapor obtained in MD simulation of phase coexistence is used in BKE calculations for consistency of BKE results with MD data. The collision cross-section is also adjusted to provide a thermal flux in vapor identical to that in MD. Our MD simulations of evaporation toward a nonreflective absorbing boundary show that the velocity distribution function (VDF) of evaporated atoms has the nearly semi-Maxwellian shape because the binding energy of atoms evaporated from the interphase layer between bulk phase and vapor is much smaller than the cohesive energy in the condensed phase. Indeed, the calculated temperature and density profiles within the interphase layer indicate that the averaged kinetic energy of atoms remains near-constant with decreasing density almost until the interphase edge. Using consistent BKE and MD methods, the profiles of gas density, mass velocity, and temperatures together with VDFs in a gap of many mean free paths between the evaporation and condensation surfaces are obtained and compared. We demonstrate that the best fit of BKE results with MD simulations can be achieved with the evaporation and condensation coefficients both close to unity.

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Many viruses, such as coronaviruses, tend to spread airborne inside water microdroplets. Evaporation of the microdroplets may result in a reduction of their contagiousness. However, the evaporation of small droplets is a complex process involving mass and heat transfer, diffusion, convection and solar radiation absorption. Virological studies indicate that airborne virus survival is very sensitive to air humidity and temperature. We employ a model of droplet evaporation with the account for the Knudsen layer. This model suggests that evaporation is sensitive to both temperature and the relative humidity (RH) of the ambient air. We also discuss various mechanisms such as the effect of solar irradiation, the dynamic relaxation of moving droplets in ambient air and the gravitational sedimentation of the droplets. The maximum estimate for the spectral radiative flux in the case of cloudless sky showed that the radiation contribution to evaporation of single water droplets is insignificant. We conclude that at small and even at moderately high levels of RH, microdroplets evaporate within dozens of seconds with the convective heat flux from the air being the dominant mechanism in every case. The numerical results obtained in the paper are in good qualitative agreement with both the published laboratory experiments and seasonal nature of many viral infections. Sophisticated experimental techniques may be needed for in situ observation of interaction of viruses with organic particles and living cells within microdroplets. The novel controlled droplet cluster technology is suggested as a promising candidate for such experimental methodology.

Flows of two-component mixtures with vapor condensation on cooled surfaces are analyzed by the methods of molecular-kinetic theory. The mixture contains a noncondensable component whose average density remains constant in the region under study. The influence of the gas on the process of recondensation of the vapor and the interaction of the components of the mixture in the cases of equal and different molecular weights are studied. The problems posed are investigated using the method of direct numerical solution of the kinetic Boltzmann equation modified for the mixture of gases. Special emphasis is placed on compution of direct and cross collision integrals.Problems in solving which one must take into account the nonequilibrium of transfer processes are topical for many situations of practical importance.In a number of practical applications, one frequently has such regimes of flow in which the regularities of flows of a continuous medium, on the one hand, and those of a free-molecular medium, on the other, cease to hold.Under such conditions, intermolecular collisions turn out to be insufficient for the superposition of a large number of random interactions to completely counterbalance their probabilistic character and to make it possible to use the regularities of a continuous medium. At the same time, collisions between gas or vapor particles are rather frequent and they cannot be disregarded, as in the case of a free-molecular regime of flow. Therefore, it is expedient to describe rarefied-gas flows at the level of the velocity-distribution function of molecules.The problem of calculation of the parameters of gas or vapor flows is also complicated by the presence of at least two components of the gas which interact with each other and by the phase transitions on cooled surfaces.Correct investigation of such flows is possible by the methods of molecular-kinetic theory. The motion and interaction of gas or vapor molecules are described based on the kinetic equation. In the present work, we use, as such an equation, the traditional Boltzmann equation, which, for a two-component mixture, becomes the system of equations ∂f a ∂t + ξ a ∂f a ∂r = J aa + J ab , ∂f b ∂t + ξ b ∂f b ∂r = J bb + J ba .In this work, we use collision integrals written in the following form: