Gaseous and multiphase flows in MEMS

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Recent trends in the space community for smaller, cheaper and more frequent space missions have driven the development of micro- and nanosatellites. The use of small spacecraft constellations is an attempt to enhance the overall performance of communication and remote sensing tasks currently done by a relatively small number of large platforms. Because micro-technologies have the advantage of reducing the total resources required on a spacecraft, the continued development of micro-technologies for space applications will further enable small satellite missions. Nanosatellites (mass between 1 and 10 kg) impose significant limitations on mass, power and volume available for all subsystems including propulsion. In recent years, micropropulsion systems have been developed to address this need. A wide array of concepts will require the expansion of propellant gases through microscale nozzles. Because many micropropulsion systems will also operate at relatively low pressures, the investigation of low-Reynolds-number nozzle and jet flows has become increasingly important for realistic evaluation of new concepts.

It is planned to perform a series of experimental and numerical studies of supersonic flows in micronozzles, which are an important element of thrusters of micro- and nanosatellites. Research work includes the development of technologies for manufacturing of supersonic micron-sized nozzle and the development of diagnostics of mass flow and momentum flux distribution in the micronozzles and the exhausting jets of gas mixtures. In addition, it is expected to obtain the integral characteristics of the micronozzles, such as specific impulse, thrust and time of engine thrust. The goal of this work is to increase the specific thrust of microjet engines, which should prolong the time of functioning of micro-and nanosatellites and increase the accuracy of their control. Numerically, Navier-Stokes, BGK and DSMC codes will be used to simulate inert, chemically reacting, and multi-phase flows in micronozzles and microjets. Plane, axisymmetric, and 3D micronozzles will be studied. Numerical simulations will be aimed at elucidating flow features in micronozzles and exhausting jets. For chemical and electrothermal micropropulsion devices, the fluid mechanics of reduced length scales (low Reynolds numbers) results in a significant degradation of the thrust efficiency as a result of increased viscous and heat-transfer losses. Numerical and experimental investigation of fluid flow and performance of micronozzles will be conducted in order to realistically evaluate the new micropropulsion concepts. Careful attention will be paid to the determination of characteristics of propulsion systems that scale favorably with reduced size. Advanced gas-surface interaction models will be implemented into the GPU-based DSMC code, validated against experiments and effects of nozzle surface roughness on the microthruster performance will be studied.

A rapid progress in micromachining techniques during the last two decades has resulted in the fabrication and utilization of micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS). Many micromachines, such as micropumps, microturbines, microvalves, etc. involve flows of gases both in the subsonic and supersonic speed ranges. Due to very small sizes of MEMS, flows in them have many important features that differ from those in macromachines. Of particular interest in the development of MEMS is the design of the devices that are able to produce mechanical work from chemical heat release, i.e. micro-engines. At the microlevel, the time scales associated with heat loss mechanisms are reduced dramatically while the characteristic time scales for heat release stay virtually independent of the size. Thus, the efficiency of conventional devices such as internal combustion engines and gas turbines is seriously degraded when they are scaled to small sizes. One possibility to overcome this difficulty is to increase the rapidity of heat release using shock-induced (and/or shock-assisted) combustion. However, this technique requires a deeper insight into the mechanisms governing the microscale shock wave phenomena. Effects of viscosity and heat conduction, heat losses due to the wall heat transfer as well as non-equilibrium phenomena observed in rarefied flows are of importance for the shock wave propagation and interaction at microscales while they can be usually neglected for macro-scale shock waves. This requires better understanding of physical and chemical processes associated with propagation of shock and detonation waves at small scales. There is a significant current effort in this direction; in particular, the beginning of activities aimed at creating a microscopic test facility (shock tube 10 μm in diameter) was announced. Some recent publications also deal with microdetonics – processes that occur during detonation of microscopic amounts of explosives. Interesting results were obtained in experimental research of propagation of detonation waves in capillary tubes (V.I. Manzhalei, Lavrentyev Institute of Hydrodynamics, Siberian Branch, Russian Academy of Sciences, 1992-1999); in particular, Manzhalei found that detonation waves under such conditions can propagate with velocities that are only 0.45-0.6 of the Chapman-Jouguet velocity. It should be noted that propagation of detonation waves in thin capillary tubes is important for explosion safety problems.

For Knudsen numbers Kn ~ 0.01 and higher, the continuum approach has to be used with caution, and the results have to be verified through comparisons with the kinetic approach. Thus, it is necessary to use the kinetic approach, namely, the DSMC method, to study detonation at microscopic scales, where the characteristic Knudsen numbers can exceed 0.01. In the framework of the proposed research laboratory the structure of an unsteady detonation wave in microchannels will be studied at the kinetic level with the use of the DSMC method. For this reason, the further development and numerical implementation of collision models and algorithms for the description of the kinetic mechanism for a hydrogen-oxygen mixture in the DSMC method will be carried out. In particular, recombination processes that require simulations of ternary collisions will be considered in detail.

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