Unstructured adaptive Monte Carlo simulations of flows in micronozzles

Abstract

A direct simulation Monte Carlo (DSMC) coed is developed using unstructured tetrahedral grids with adaptation. The numerical code is used to simulate rarefied flows in micronozzles. The grid generation methodology and adaptation are reviewed. The simulations include cold-gas micronozzles and a MEMS-manufactured micronozzle. The results are compared with experimental data and previous DSMC computations. Introduction The growing interest of microspacecraft has caused increased need in onboard micropropulsion systems. The miniaturization of electronics and improved fabrication techniques, such as deep reactive ion etching, has enabled the manufacture of smaller propulsion devices than ever before. Micropropulsion is a mission enabling technology for microspacecraft, providing precision maneuvering and thrust at reduced sizes and weight' . This work is motivated by the increasing interest in space applications of rarefied gas and plasma dynamics, primarily in the simulation of internal flows in micropropulsion devices as well as plume flows and their interaction with spacecraft surfaces. Flows in micro thrusters exhibit distinct flow characteristics that can include transition from continuum to slip to rarefied regimes. Rarefaction effects may be a direct result of reduced spatial dimensions or operating conditions. See for example, previous work on cold-gas rnicrothrusters, the simplest available onboard propulsion device used primarily for precise station-keeping and drag nullification'. Under such conditions the use of Navier-Stokes formulations for the nozzle flow becomes questionable and even break down. Prior to MEMS manufacturing techniques, low thrust levels were achieved by running small nozzles at reduced chamber pressures . However, this regime of operation is associated with considerable viscous losses, as the Reynolds number (ratio of inertial force to viscous force) is low for these conditions. The Graduate Research Assistant. 100 Institute Rd. Student Member AIAA. ' Graduate Research Assistant. * Associate Professor. Senior Member AIAA Copyright © 2001 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission ability to manufacture micron-scale nozzles with MEMS-type fabrication techniques may reduce viscous losses for the same thrust regime, as the chamber may operate at higher pressures and the throat size is reduced. This increases thruster efficiency. In the case of plume flows, rarefaction and non-equilibrium effects occur regardless of the operating conditions of the microthruster. In certain cases rarefaction effects can persist well inside the nozzle. The modeling of micronozzle and plume flows that exhibit transition and/or non-equilibrium characteristics is achieved by modeling techniques based on kinetic descriptions of the gas or techniques that combine the continuum and kinetic approaches. The Direct Simulation Monte Carlo method (DSMC) has been proven a method capable of handling a wide range of rarefied gas flows and has been applied successfully to the characterization of flows in micronozzles.''''' A small traditionally-machined helium thruster was modeled for the purpose of obtaining precise flow characteristics desirable for complete drag nullification' . This thruster produced thrusts of the order of 1 mN, which is similar to the regime of MEMS rnicrothrusters, but a chamber pressure of only 7 Pa. Normalized mass flux and discharge coefficient compared favorably with experimental measurements. This study demonstrated the applicability of the DSMC method to the study of small nozzles for engineering purposes. The DSMC had been applied subsequently to MEMS-fabricated micronozzles. Piekos and Breuer' 11 presented 2-D DSMC calculations of a parabolic micronozzle with atmospheric chamber conditions and a vacuum at the exit. Temperature and Mach contours showed considerable slip (> M=0.5) at the exit. The performance characteristics *of MEMS nozzles were studied experimentally in a vacuum chamber and compared with 2-D Navier-Stokes results. These conical micronozzles were manufactured using deep reactive ion etching. At lower chamber pressures,