Sergei Stepanovich Sazhin

The effects of Knudsen numbers (Kn), particle sizes, their relative velocities, and particle and gas properties on the drag force acting on micro- and nano-particles, moving in a gas, are investigated using the molecular dynamics approach. The slip correction factor C and the slip parameter A are shown to depend not only on Kn, as commonly assumed, but also on the physical properties of the gases and particles. Quantitative analysis of the ratio between local (near the particle surface) and ambient gas number densities showed that increased gas–particle interactions at micro- and nano-scales lead to an increase in the local gas number density, which leads to an increase in the drag force. A comprehensive potential depth parameter εggp is introduced to take into account both gas–particle and gas–gas interactions. This parameter allowed us to provide a unified framework for interpreting slip correction factor behavior for different particle–gas combinations. A universal predictive approximation of the slip parameter is suggested as a function of εggp and Kn. The prediction of this approximation is shown to be in good agreement with experimental data referring to the drag forces experienced by polystyrene latex particles in nitrogen at Kn>10.

A new model for heating and evaporation of a multi-component liquid film that takes into account the contribution of the heat flux from/to the wall (the Neumann boundary condition) and supply/removal of liquid to/from the film is described. The Robin boundary condition at the outer surface of the film is used. The film is assumed to be sufficiently thin to allow for the description of the heat transfer and component diffusion processes using one-dimensional heat transfer and component diffusion equations. These equations are used at each time step of the calculation. The solutions to these equations at the end of the time step are used as initial conditions for the next time step with updated values of the input parameters. The predictions of the numerical code are validated using in-house experimental data that refers to water spray impact on a vertical wall. The verification of the model is performed by comparing the predictions of two in-house numerical codes, one based on the new model and the other on the numerical solutions of the heat transfer and component diffusion equations, for a water–ethanol mixture with initial equal mass fractions of the components.

The convection rate (velocity) fields in a droplet colliding with a stationary particle (steel ball) are constructed using particle image velocimetry. The collision process is shown to lead to both a change in the nature of the velocity field in the droplet (most vectors change their original direction) and an intensification of the convection velocity by a factor of 3–10. The convection rate is shown to decrease as the ratio of droplet to particle diameters increases. A new correlation for the convection rate as a function of the initial diameter of the droplet is suggested. A phase field method, implemented in COMSOL Multiphysics, is used for numerical simulation of the collision between a droplet and a spherical solid particle. The difference between the predicted and observed average values of convection velocities is shown to be less than 12%. Potential applications of the results to water droplets that contain unregulated impurities with diameters of up to 500 μm, and to the development of thermal water treatment technologies, are discussed.