Simulating fluid flows in micro and nano devices : the challenge of non-equilibrium behaviour

We review some recent developments in the modelling of non-equilibrium (rarefied) gas flows at the micro- and nano-scale, concentrating on two different but promising approaches: extended hydrodynamic models, and lattice Boltzmann methods. Following a brief exposition of the challenges that non-equilibrium poses in micro- and nano-scale gas flows, we turn first to extended hydrodynamics, outlining the effective abandonment of Burnett-type models in favour of high-order regularised moment equations. We show that the latter models, with properly-constituted boundary conditions, can capture critical non-equilibrium flow phenomena quite well. We then review the boundary conditions required if the conventional Navier-Stokes-Fourier (NSF) fluid dynamic model is applied at the micro scale, describing how 2nd-order Maxwell-type conditions can be used to compensate for some of the non-equilibrium flow behaviour near solid surfaces. While extended hydrodynamics is not yet widely-used for real flow problems because of its inherent complexity, we finish this section with an outline of recent 'phenomenological extended hydrodynamics' (PEH) techniques-essentially the NSF equations scaled to incorporate non-equilibrium behaviour close to solid surfaces-which offer promise as engineering models. Understanding non-equilibrium within lattice Boltzmann (LB) framework is not as advanced as in the hydrodynamic framework, although LB can borrow some of the techniques which are being developed in the latter-in particular, the near-wall scaling of certain fluid properties that has proven effective in PEH. We describe how, with this modification, the standard 2nd-order LB method is showing promise in predicting some rarefaction phenomena, indicating that instead of developing higher-order off-lattice LB methods with a large number of discrete velocities, a simplified high-order LB method with near-wall scaling may prove to be just as effective as a simulation tool

Plasma texturing for enhanced tribological performance of cast iron

Cathodic plasma electrolysis (CPE) was used to create surface texturing on gray iron samples, which could reduce the friction and increase the wear resistance. During the treating process, cast iron sample served as a cathode where the plasma discharging occurred, increasing the surface hardness and leaving an irregular array of micro craters on the surface. Modified surface morphology was determined from scanning electron microscope (SEM) and surface profiler. Recessed and protruded surface textures were observed when the CPE was applied at low and high voltages, respectively. Pin-on-disk tribotests were conducted on CPE-treated samples and untextured sample. The friction of as-treated samples could be reduced in boundary lubrication regime at low sliding speed due to the ability to store lubricant. Besides that, the surface texture generated extra hydrodynamic pressure that separated two sliding surfaces, increased the oil film thickness and accelerated the transition from boundary to mixed lubrication at high sliding speeds

Microfabricated rankine cycle steam turbine for power generation and methods of making the same

In accordance with the present invention, an integrated micro steam turbine power plant on-a-chip has been provided. The integrated micro steam turbine power plant on-a-chip of the present invention comprises a miniature electric power generation system fabricated using silicon microfabrication technology and lithographic patterning. The present invention converts heat to electricity by implementing a thermodynamic power cycle on a chip. The steam turbine power plant on-a-chip generally comprises a turbine, a pump, an electric generator, an evaporator, and a condenser. The turbine is formed by a rotatable, disk-shaped rotor having a plurality of rotor blades disposed thereon and a plurality of stator blades. The plurality of stator blades are interdigitated with the plurality of rotor blades to form the turbine. The generator is driven by the turbine and converts mechanical energy into electrical energy

Tribology of Machine Elements

Tribology is a branch of science that deals with machine elements and their friction, wear, and lubrication. Tribology of Machine Elements - Fundamentals and Applications presents the fundamentals of tribology, with chapters on its applications in engines, metal forming, seals, blasting, sintering, laser texture, biomaterials, and grinding

Tribology of power train systems

Tribology of power train system

Topology and Shape Optimization of Hydrodynamically–Lubricated Bearings for Enhanced Load-Carrying Capacity

Bearings are basic and essential components of nearly all machinery. They must be designed to work under different loads, speeds, and environments. Of all the performance parameters, load-carrying capacity (LCC) is often the most crucial design constraint. The objective of this research is to investigate different design methodologies that significantly improve the LCC of liquid-lubricated bearings. This goal can be achieved by either altering the surface texture or the bearing geometrical configuration. The methodology used here is based on mathematical topological/shape optimization algorithms. These methods can effectively improve the design performance while avoiding time-consuming trial-and-error design techniques. The first category of design studied is a micro-scale mechanical self-adaptive type which can provide “flexible surface texturing”. An accurate 3D model based on the classic plate theory and thin film lubrication is developed and a shape optimization analysis is carried out. Special attention is given to the cavitation phenomena and its numerical analysis. Also proposed is a numerical procedure to improve the convergence rate and stability of the Elrod cavitation algorithm. The idea of using self-adaptive mechanism to improve LCC is also adopted for thrust bearings. Novel flexible-pad thrust bearing designs that provide an optimum load-responsive mechanism are presented and an accurate multi-physics model that considers the coupled mechanism between the lubricant pressure and the pad deformation is developed. The optimum shapes for different bearing geometries are given and a detailed design guideline is provided for optimum performance. The second category of design studied focuses on bearing geometrical configuration. The optimum shape of finite width sectorial sliders, which is an open problem in the field, is determined for the first time in this research using topological optimization algorithms. Also three suboptimum solutions for special cases of 2D step profile, constant film thickness in the radial direction and constant film depth with quadrilateral shape are presented. These configurations are particularly attractive because they can be easily manufactured. The optimum shape of bearings with periodic surface grooves is also determined in this research. It is shown that the optimum shape is dependent to the aspect ratio of the grooves and it can change from elongated “heart-like” shapes to spiral-like shapes. A series of laboratory tests to authenticate the theoretical development is carried out. Results show very good agreement with the theory validating the accuracy of the model. Finally, the optimum geometry of spiral grooves that provide the highest LCC in liquid-lubricated parallel flat surface bearings is determined and a detailed design guideline is provided. The thermal effects are also considered and an approximate thermo-hydrodynamic model is developed for a range of seal geometries and operating conditions

A multi-physics multi-scale approach in engine design analysis

Vibration behaviour of an internal combustion engine depends on rigid body inertial dynamics, structural modal characteristics of its elastic members, tribological behaviour of loadbearing contacts, and piston-cylinder interactions. Therefore, it is essential to use a multi-physics approach that addresses all these physical properties in a single integrative model as presented in this paper. This approach can be regarded as holistic and a good aid for detailed design. Particular attention is paid to the critical elements in the system, such as load-bearing conjunctions (crankshaft main bearings) and piston-cylinder wall interactions. Another important feature is the integrated analysis across the physics of motion from microscale fluid film formation to submillimetre structural deformations and onto large displacements of inertial members. In order to succeed in predictions within sensible industrial time scales, analytical methods have been used as far as possible rather than numerical approaches. Model predictions show good agreement with fired engine test data

Gas Flows in Microsystems

International audienc

Steady Characteristics of High- Speed Micro-Gas Journal Bearings With Different Gaseous Lubricants and Extreme Temperature Difference

High-speed micro-gas journal bearing is one of the essential components of micro-gas turbines. As for the operating conditions of bearings, the high-speed, high-temperature, ultra-high temperature difference along the axial direction and the species of gaseous lubricants are extremely essential to be taken into account, and the effects of these factors are examined in this paper. The first-order modified Reynolds equation including the thermal creep, which results from the extremely large temperature gradient along the axial direction, is first derived and coupled with the simplified energy equation to investigate the steady hydrodynamic characteristics of the micro-gas bearings. Under the isothermal condition, it is found that CO 2 can not only improve the stability of bearings but also generate a relatively higher load capacity by some comparisons. Thus, CO 2 is chosen as the lubricant to further explore the influence of thermal creep. As the rotation speed and eccentricity ratio change, the thermal creep hardly has any effect on the gas film pressure. However, the shorter bearing length can augment the thermal creep. Compared with the cases without the thermal creep, the thermal creep could remarkably destroy the stability of gas bearing, but it might slightly enhance the load capacity