A. V. Minakov

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.

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.

Modeling and Optimization of Y-Type Micromixers

A trend in the global technological progress in the last few decades is the development of microsystem technology, microelectromechanical systems and corresponding technologies. Fluid mixing is an extremely important process widely used in various microfluidic devices (chemical microreactors, chemical and biological analyzers, drug delivery systems, etc.). To increase the mixing rate, it is necessary to use special devices: micromixers. This paper presents the results of a hydrodynamic simulation of Y-shaped micromixers. Flows are analyzed for both low and moderate Reynolds numbers. The passive and active mixers are considered. The dependence of the mixing efficiency on the Reynolds and Peclet numbers, as well as the possibility of using the hydrophobic and ultra-hydrophobic coatings is analyzed. Five different flow regimes were identified: (1) stationary vortex-free flow (Re 400). The maximum mixing efficiency was obtained for stationary asymmetric vortex flow.

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.

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.

Mixing in a T-shaped micromixer at moderate Reynolds numbers

In the present work, the regimes of the flow and mixing of fluids in a T-shaped micromixer in the range of the Reynolds numbers from 1 to 1000 are investigated systematically with the aid of numerical modeling. The flow and mixing regimes are shown to alter substantially with increasing Reynolds numbers. Five different flow regimes have been identified in the total. The dependencies of the friction coefficient and mixing efficiency on the Reynolds number are obtained. A sharp increase in the mixing efficiency at a flow transition from the symmetric to asymmetric steady regime is shown. On the other hand, the mixing efficiency slightly drops in the laminar-turbulent transition region. A substantial influence of the slip presence on walls on flow structure in the channel and mixing efficiency has been revealed.

Numerical modeling of flow in the Francis-99 turbine with Reynolds stress model and detached eddy simulation method

The paper presents numerical simulation of flow in Francis-99 water turbine under three operation modes: part load, best efficiency point and high load. Calculations were performed by means of Reynolds stress model and detached eddy simulation based on k-omega SST model. The paper focuses on the flows in the draft tube. The calculated mean velocity components in the draft tube are in close agreement with experimental results. Calculated r.m.s velocity components agree with experimental pulsations qualitatively.

Measuring the heat-transfer coefficient of nanofluid based on copper oxide in a cylindrical channel

The heat-transfer coefficient of nanofluid during its flow in a cylindrical channel is studied experimentally. The studied nanofluid was prepared based on distilled water and CuO nanoparticles. Nanoparticle concentration varied in the range from 0.25 to 2% in the volume. The nanofluid was stabilized using a xanthane gum biopolymer the mass concentration of which did not exceed 0.03%. Considerable intensification of heat transfer was found. The nanofluid appeared to be Newtonian when particle concentrations exceeded 0.25%. Estimates for rheological parameters of the nanofluid and thermal conductivity coefficient have been obtained.

Measuring of critical density of heat flow during boiling of nanoliquids on a cylindrical heater

Saturated boiling of nanoliquids on a cylindrical heater is experimentally studied. The studied nanoliquids were prepared using distilled water and Fe3O4 iron oxide or diamond nanoparticles. The volume concentration of the nanoparticles was changed from 0.25 to 1%. It has been measured that the critical density of heat flow in nanoliquids is far higher than in water. Moreover, it depends on size of nanoparticles.

Experimental data on the dependence of the viscosity of water- and ethylene glycol-based nanofluids on the size and material of particles

The viscosities of all nanofluids considered are shown to be dependent on the size of nanoparticles. It has been established that the greater the nanofluid viscosity, the smaller the size of particles. The measurements carried out in this study make it possible for the first time to fix experimentally the fact of the dependence of the viscosity of nanofluids on the particle material.