Valery Ya. Rudyak

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.

Detailed Modeling of Drilling Fluid Flow in a Wellbore Annulus While Drilling

To produce a well safely, the wellbore pressure during drilling must be in a range that prevents collapse yet avoids fracturing. This range is often called “the operating window”. Exceeding the limits of this range can trigger wellbore instability or initiate well control incidents. Pressure prediction requires an understanding of the hydrodynamics processes that occur in a borehole while drilling.Describing these processes is complicated by many factors: the mud rheology is usually non-Newtonian, the flow mode can be laminar or turbulent, and the drillstring can rotate and be positioned eccentrically. Known semi-analytical approaches cannot account for the full range of fluid flows that can arise during drilling. These techniques don’t take into account all factors. Accurate numerical simulation of the flow of drilling fluids is a means to describe the fluid behavior in detail.For numerical solutions of hydrodynamics equations a unique algorithm based on a finite-volume method and a new model of turbulence for non-Newtonian fluids was developed. The model considers string rotation and eccentricity of the drillstring. Newtonian and non-Newtonian fluids as described by the Herschel–Bulkley rheological model have been implemented. Data obtained via systematic parameter studies of the flow in a borehole are available for fast determination of parameters like pressure drop, velocity field, and stresses corresponding to any drilling condition.Applying the new model for the annulus flow and comparing the results to the parallel plate flow approximation enabled us to quantify the error made due to the approximated solution for non-Newtonian fluid rheology.The difference between the solutions grows as the annular gap increases. This situation is a function of the rheological parameters. Secondary flow effects can only be seen when applying the new solution method.Copyright

Thermophysical properties of nanofluids

Abstract.This paper discusses the current state of knowledge of the thermophysical properties of nanofluids. The viscosity, thermal conductivity and heat transfer of nanofluids are considered. Experimental and molecular dynamics data are presented. It is shown that viscosity and thermal conductivity of nanofluids generally cannot be described by classical theories. The transport coefficients of nanofluids depend not only on the volume concentration of the particles but also on their size and material. The viscosity increases with decreasing the particle size while the thermal conductivity increases with increasing the particle size. The reasons for this behavior are discussed. The heat transfer coefficient is determined by the nanofluid flow mode (laminar or turbulent). The use of the nanofluids as a coolant significantly affects the magnitude of the heat transfer coefficient. In laminar flow the heat transfer coefficient of nanofluids in all cases is much more than that of base fluids. It is shown that a 2%-nanofluid intensifies the heat exchange more than twice compared to water. The effect of using nanofluids in turbulent mode depends not only on the thermal conductivity of the nanofluid, but also on its viscosity.Graphical abstract

Fluid Transport Under Confined Conditions

The viscosity and thermal conductivity of the fluid are determined by the transport of the impulse and energy in the system considered. In turn, these transports are defined by and depend on the interaction of the fluid molecules. The situation in the fluid under confined conditions (e.g., in a nanochannel) is more complicated, because the transport of the impulse and energy in fluids is highly dependent on the interaction of the fluid molecules with the wall atoms (or molecules). Therefore, the viscosity and thermal conductivity of such a fluid are the properties of the entire “fluid+wall” system. In this chapter, the statistical theory of transport processes in fluids under confined conditions is proposed. The considered system is the specific two-fluid system consisting of fluid and wall molecules. In the chapter, the new constitutive relations for the fluid under confined conditions are proposed. As a result, the Green-Kubo formulas were generalized. Using this new formula and the molecular dynamics method, the viscosity coefficient of the fluid in a nanochannel was studied. It is shown that the viscosity coefficient depends, to a large extent, on the properties of interaction of fluid molecules with channel wall atoms.

Methods of Modeling of Microflows and Nanoflows

The development and application of methods of numerical simulation of micro- and nanoflows are urgent tasks because of the lack and inconsistency of systematic experimental data. However, interpretation of results and determination of the applicability area of particular methods of modeling such flows should also be treated carefully and cautiously. In addition, precise terminology is important, because inadequate usage of terms can lead not only to misunderstanding, but even to erroneous ideas about the physics of the phenomena being considered. The usual flows of liquids and gases are rather difficult in the general case. This is even more so for micro- and nanoflows. Therefore, such flows should be treated with different methods. The situation becomes even more complicated if multiphase fluid flows are studied. In the present chapter, all of these situations were considered consecutively. It begins with a brief classification of these flows. After that, the methods of the modeling flows of the rarefied and dense gases and liquids are described. In the following two sections, the modeling of dispersed fluids, including nanofluids, is analyzed. The last section is devoted to a brief description of the method of molecular dynamics, the application of which is necessary for the modeling of nanoflows.

Fluid Flows in Microchannels

The chapter describes the results of measurements of friction factors in microchannels of various shapes and various diameters for laminar and turbulent flows, as well as the friction factor for input regions. Much attention is paid in this chapter to technologies of fabrication of test benches, methodical aspects of experiments, and evaluation of reliability of experimental data. The chapter is organized in such a way that all aspects of microflow experiments are consecutively considered: from the development of test benches through choosing measurement techniques to estimating the error of results obtained.

Modeling of Micromixers

Mixing of fluids is an extremely important process, widely used in various microfluidic devices (chemical microreactors, chemical and biological analyzers, drug delivery systems, etc.). Mixing in macroscopic flows usually occurs in the turbulent regime. However, microflows are mainly laminar, and mixing under standard conditions is caused only by molecular diffusion. Because of the extremely low values of the molecular diffusion coefficient, this manner of mixing is very ineffective. To increase the mixing velocity, it is necessary to use special devices: micromixers. For this reason, such devices are key elements of many microelectromechanical systems (MEMS). This chapter describes the results of CFD simulations of the simplest micromixers. The method used to solve the Navier-Stokes equations is described in the first two sections. Sections 4.3 and 4.4 are devoted to the study of the flow and mixing regimes in Y-type micromixers at low and moderate Reynolds numbers. In the next section, the flow in T-type micromixers is studied experimentally and the obtained data is compared with those from modeling. Modeling of two-phase flow and heat transfer in micromixers is considered in the two subsequent sections. One simple active method for mixing is discussed in the last section.

Gas-Dynamic Structure and Stability of Gas Microjets

Microjets are widely used for the mixing of gases and the protection of surfaces from chemically aggressive and high-temperature media. The basic technological characteristics of jets in this case are their penetration capability and the intensity of mixing processes. The goal of the present chapter is to study the structure and stability of microjets. The overview of the works on the study of the gas dynamics of subsonic and supersonic mini- and microjets is given in Sect. 2.1. As tools used in experimental investigations are also very important, they are described in much detail. Diagnostic methods and the results of studying subsonic plane jet stability are described in Sect. 2.2. Experiments aimed at studying the structure and stability of supersonic axisymmetric microjets and the results obtained therein are discussed in Sect. 2.3. Much attention is paid to the techniques used to obtain experimental data. Finally, the problem of microjet modeling with the use of commonly used similarity parameters is discussed in Sect. 2.4.

Modeling of Nanoflows

By definition, nanoflows are flows in channels with a characteristic size (height of a plane channel or diameter of a cylindrical channel) smaller than (or equal to) one hundred nanometers. Depending on the cross-sectional configuration, nanochannels are usually classified as follows. A plane channel is a 2D channel and has only one nanosize (distance between the plates); it is also called a nanoslit. There are also cylindrical nanochannels (1D). Short cylindrical nanochannels are often called nanopores. These flows have been studied for about forty years. However, up to now, there were no algorithms that would permit us to model real nanoflows. In addition, in recent years, many new problems have appeared in this area. To solve these problems, we need correspondent techniques. In this chapter, we propose new molecular dynamics algorithms, which allow one to simulate a real plane Poiseuille-type flow characterized by a certain pressure gradient, and discuss specific features of plane flows in nanochannels. This is the subject of the first four sections of the chapter. In Sects. 5.5 and 5.6, the self-diffusion of the fluid molecules in nanochannel and in porous media is studied. Finally, the last section deals with modeling the separation of nanofluids through the use of nanomembranes.