MODELING OF MICROSCALE GAS FLOWS USING THE DIRECT SIMULATION MONTE CARLO METHOD

Open Access
Author:
Alexeenko, Alina A.
Graduate Program:
Aerospace Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 09, 2003
Committee Members:
  • Deborah Levin, Committee Chair
  • Michael Matthew Micci, Committee Member
  • Lyle Norman Long, Committee Member
  • John Harlan Mahaffy, Committee Member
Keywords:
  • microflow
  • direct simulation Monte Carlo
  • rarefied gas flow
  • finite element analysis
  • transient heat transfer
Abstract:
Microflows are defined as fluid flow phenomena associated with microscale mechanical devices. A great number of such devices have been manufactured over the last few decades using surface and bulk silicon micro-machining. Many of micro-electro-mechanical systems (MEMS) involve gaseous flows. Gas flows in MEMS with characteristic size on the order of microns differ from their larger counterparts. Three important flow parameters: Knudsen and Reynolds numbers and surface-to-volume ratio, are drastically different from those encountered in large scale flows. Accurate numerical modeling of microflows is indispensable for providing design capability for MEMS by predicting the flow field and performance characteristics. The main goal of the thesis research is the development, implementation and application of comprehensive direct simulation Monte Carlo approach to microscale gas flows. Flows in microthrusters and microchannels are commonly encountered in MEMS and are the focus of the study. The investigation of physical processes in three-dimensional micronozzles and the influence of Reynolds number, geometrical configuration and temperature regime have been carried out in the thesis. The impact of wall effects on thrust is examined for axisymmetric and two- and three-dimensional cold gas micronozzles. The flow in a flat micronozzle has a 3D structure and is strongly influenced by the end walls. The additional friction losses on the side walls cause a reduction in thrust of about 20\% compared to the two-dimensional and axisymmetric nozzles. The work on coupled analysis of a microthruster is aimed at the developing a numerical simulation code capable of modeling the temporal variation of microthruster material temperature and performance characteristics. The application of the developed approach to two-dimensional and three-dimensional microthrusters gives several important insights into the dependence of performance characteristics on Reynolds number, thermal conditions and thruster geometry. The mass discharge of the microthruster have been found to decrease significantly in time due to increasing wall temperature. Such behavior of the mass discharge coefficient is obtained for both 2D and 3D models as well as for different stagnation pressures and geometrical shapes. Investigation of gas flows in microchannels with constriction have been carried out both analytically and numerically in order to understand the flow phenomena observed in experiments. An analytical model is developed to predict pressure drop and mass flow rate. The validation of the model is conducted by comparison with 2D DSMC calculations. The model accurately predicts the mass flow rate and pressure drop at the constriction section and compares well with the DSMC results. The DSMC simulations have shown that the flow separation may occur at the constriction. The presence of the separation zone does not influence the pressure distribution and the mass flow rate. The DSMC method have been applied for calibration of micro-Newton thrust stand and investigation of effects of the facility background. The DSMC calculations have been conducted for orifice flow for $Kn=0.01$ to $40$. It is found that for low Knudsen numbers the background gas contribution to the total force becomes significant. This is attributed to the jet shadowing effect, and, therefore, it must be included in modeling to permit a comparison with experiment.