Print

Louisos, William F.

Mechanical Engineering

2005

MS

NASA and Department of Defense agencies are actively conducting research initiatives aimed at the development of miniaturized satellites that will be capable of operating in distributed networks and performing missions not possible with traditional satellite architectures. Commonly referred to as "nanosats," these small scale spacecraft feature a mass of less than 10kg and will require extremely small thrust levels to correct for small disturbances via precise station-keeping maneuvers. As such, they have unique propulsion systems requirements including extremely low thrust levels and/or extremely low minimum impulse requirements for orbital maneuvers and attitude control. In fact, thrust on the order of mN and impulse bits of approximately 1-100(mu)N · sec are expected. The propulsion system itself must be drastically reduced in size and weight. Micro-Electro-Mechanical Systems (MEMS) based technologies have been proposed and demonstrated as a possible solution to the intricate thruster design and specifications. Owing to the extremely small geometry, the thruster nozzle flow is subjected to complex micro-scale effects and can be substantially affected by viscous losses as a result of Reynolds numbers that are well below 1,000. A thruster prototype has been developed by the NASA Goddard Space Flight Center (GSFC) for which this thesis examines numerical simulations of the micro-scale geometry effects on the supersonic nozzle performance.
The research represents an independent and necessary extension to the work completed by NASA/GSFC including characterization of the existing micro-nozzle prototype through optimization of thruster performance and efficiency. Specifically, the focus of this thesis is a numerical model of steady-state micro-nozzle operation that is capable of analyzing the relationship between viscous effects and micro-nozzle performance under varying operating conditions. This includes underexpanded and perfectly expanded nozzle operation as well as a Reynolds number parametric analysis of the viscous effects in the flow regime. Two separate monopropellants, high purity hydrogen peroxide (H₂O₂) and hydrazine (N₂H₄), will be modeled as the working fluid propellant in this study. Two-dimensional continuum flow models are used in conjunction with the FLUENT6.1® computational fluid dynamics software to perform parametric simulations of varying mass flow rates and varying expander angles in an optimization study. The interplay of nozzle geometry and viscous effects represent an important trade-off in supersonic micro-nozzle flow. It is shown that the subsonic boundary layer has a significant impact on micro-nozzle performance and can even choke off the flow at low mass flow rates. The existence of an optimal half-angle in the range of 25° to 30° is a result of trade-offs between the size of the viscous boundary layer and losses in thrust due to non-axial flow components. The fact that this optimal angle is substantially larger (2x) than traditional conical thrusters is a direct indication of the important role viscous effects play in supersonic micro-nozzles.