V. Ya. Rudyak

On the thermal conductivity of nanofluids

The dependence of the effective thermal conductivity λ of nanofluids on the properties of dispersed nanoparticles has been studied by the molecular dynamics method. It is established that the thermal conductivity of a nanofluid always exceeds that of the carrier medium, the excess depending on the volume fraction of nanoparticles, their masses, and sizes. An increase in the nanoparticle mass at a constant size leads to a more pronounced increase in λ than does the growth in size at a constant mass, which implies that the density of dispersed nanoparticles is an important factor that determines the thermal conductivity of nanofluids.

A numerical algorithm for modeling laminar flows in an annular channel with eccentricity

We propose a numerical algorithm for simulating steady laminar flows of an incompressible fluid in annular channels with eccentricity and rotation of the inner cylinder. This algorithm enables us to describe this class of flows for wide ranges of the annular channel and flow parameters. We present the implementation details and the results of testing this numerical method. For a series of flows in an annular clearance, we compare the numerical results to available analytic solutions and experimental data. In all cases under consideration the simulated data agree well with the available experimental, analytical, and numerical solutions.

On the Effective Viscosity of Nanosuspensions

The effective viscosity of nanosuspensions is simulated using the molecular dynamics method. It is found that viscosity is controlled not only by the volume concentration of nanoparticles, by also by their mass and diameter. The viscosity of even strongly rarefied nanosuspensions (with a low concentration of nanoparticles) cannot be described by the Einstein relation. This means that the mechanism responsible for the increase in the viscosity of the medium is not of hydrodynamic origin. It is shown that the formation of viscosity of nanosuspensions is associated to a considerable extent with nonequilibrium microfluctuations of density and velocity of the carrier medium, which are induced by the motion of nanoparticles.

On the dependence of the viscosity coefficient of nanofluids on particle size and temperature

The dependence of the viscosity coefficient of nanofluids based on ethylene glycol with SiO2 particles on nanoparticle size and temperature is experimentally studied. The experiments are conducted for nanofluids with an average particle size of 18.1, 28.3, and 45.6 nm. Their volume concentration is varied from 0.2 to 8%. The temperature of the fluid is varied in a range of 20–60°C. It is shown that the viscosity of nanofluids heavily depends on particle size: the smaller the particles, the higher the viscosity. On the other hand, the viscosity of all the studied nanofluids decreases with increasing temperature.

Methods of Measuring the Diffusion Coefficient and Sizes of Nanoparticles in a Rarefied Gas

In view of rapid progress in nanotechnologies, it is necessary to develop new methods of determining the diverse physical properties of nanoparticles. The sizes and diffusion coefficient of particles are basic properties. In practice, these properties are determined by socalled differential mobility (electromobility) analyzers (DMAs). 1 The feasibility of using DMAs in this range was shown in [1]. The interpretation of data obtained by this method is based on the Stokes law of resistance with the Canningham‐Millikan‐Davis corrections. For example, the diffusion coefficient is determined by the formula

Application of new numerical algorithm for solving the Navier—Stokes equations for modelling the work of a viscometer of the physical pendulum type

A model is proposed, which describes the work of the viscometer sensor of the physical pendulum type. The model enables the obtaining of data on fluid viscosity directly from the measurement of the settling frequency of sensor oscillations or the amplitude of these oscillations. To describe the sensor operation a numerical computational algorithm is developed. This method enables the solution of a wide class of three-dimensional laminar fluid flow problems involving moving solids of arbitrary geometry. The results of testing the proposed numerical technique are presented.

Modelling of flows in micromixers

A method is proposed for modelling fluid flows in microchannels. The method is tested on the known experimental data on studying the flows in microchannels with the aid of the micro-PIV. The flow regimes in micromixers of the Y- and T-types are studied. The passive and active mixers are considered. The dependence of the mixing efficiency on the Reynolds and Péclet numbers as well as the possibility of using the hydrophobic and ultra-hydrophobic coatings are analysed. A T-mixer is proposed as an active technique of mixing, in which the flow rate in one of the inlet channels varied according to the harmonic law. The dependence of the mixing efficiency on the frequency of the variation of the flow rate and its amplitude is established.

Diffusion of nanoparticles in a rarefied gas

It is suggested to describe the diffusion of nanoparticles in rarefied gases in terms of the kinetic theory. For this purpose, the potential of interaction between a carrier gas molecule and a dispersed particle is constructed by summing the interactions of the given gas molecule with all atoms (molecules) of the dispersed particle. With this potential, a formula for the diffusion coefficient of the dispersed nanoparticle is derived. The dependence of the diffusion coefficient on the radius and temperature is studied. Analytical results are compared with experimental data. The well-known experimental Cunningham-Millikan correlation is shown to apply only in the range of near-room temperatures, for which the parameters of this correlation were determined.

Diffusion of Nanoparticles and Macromolecules in Dense Gases and Liquids

The method of molecular dynamics is used to study the diffusion of large molecules or nanoparticles in a dense molecular medium (liquid or gas) in a wide range of densities. Particles and molecules are simulated by hard, absolutely elastic spheres. The ratio of the particle and molecule diameters of the medium varies from unity to four, and the mass ratio, from unity to 300. The density of the carrier medium is characterized by the parameter V/V0(V0is the volume of close-packed structure of molecules, and Vis the volume of the calculation cell), which is varied from 2 to 75.3. The dependences of the diffusion coefficient of a particle on its mass and on the density of carrier gas are investigated. It is found that the relaxation of the autocorrelation function of the velocity of a particle is described well by the superposition of two exponential functions with different relaxation times. The obtained data are compared with known theoretical models.

Measurement of the heat transfer coefficient of a nanofluid based on water and copper oxide particles in a cylindrical channel

The heat transfer coefficient of a nanofluid in a cylindrical channel under constant heat flux density at the walls is measured experimentally. The studied fluid was prepared based on distilled water and CuO nanoparticles with an average size of 55 nm. To stabilize the nanofluid, a biopolymer was used. The volume concentration of nanoparticles was in the range from 0.25 to 2%. It is shown that the nanofluid is Newtonian at the lowest concentration of nanoparticles, and in all other cases, its rheology is described well by the model of a power-law fluid. A correlation of the dependence of the parameters of this model on the concentration of nanoparticles is obtained. It is found that the presence of nanoparticles greatly intensifies the heat transfer.