UVM Theses and Dissertations
Format:
Print
Author:
Louisos, William Frederick
Dept./Program:
Mechanical Engineering
Year:
2009
Degree:
PhD
Abstract:
The aerospace community along with 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 necessitate precise station-keeping maneuvers. Accordingly, extremely small thrust levels, on the order of mN, and minimum impulse bits of approximately 1-100[mu]N· s will be required in order to correct for small orbital disturbances. The propulsion system itself must be drastically reduced in size, mass, and power consumption in order to meet the demands of a nano-satellite. Micro-Electro-Mechanical Systems (MEMS) based technologies have been proposed and demonstrated as a possible solution to the intricate thruster design and stringent specifications.
Owing to the extremely small geometry, supersonic micronozzle flow is subjected to complex micro-scale effects and can be substantially affected by viscous forces as a result of Reynolds numbers that are well below 1,000. A thruster prototype based on a monopropellant scheme has been developed by the NASA Goddard Space Flight Center (GSFC) in conjunction with the University of Vermont. The research included in this dissertation represents an independent and necessary extension to the work completed at NASAjGSFC. The focus of this dissertation is a Computational Fluid Dynamics (CFD) model of the micronozzle flow-field. Specifically, continuum flow models are used in conjunction with the FLUENT CFD software package in order to perform parametric simulations of micronozzle operation. From this model, the relationship between viscous forces, micronozzle geometry, and thermal effects are analyzed in order to determine nozzle performance and efficiency for varying operating conditions.
The interplay of viscous forces, nozzle geometry, and thermal effects such as heat transfer combine to represent the key operating parameters of supersonic micronozzle flow. It is revealed that the subsonic 'boundary' layer resulting from large viscous forces reduces flow through the nozzle and measurably degrades nozzle performance. To mediate the losses associated with large subsonic layers, nozzle expansion angles are increased sufficiently in order to provide room for subsonic layer growth. However, this results in transverse velocity components; a geometric loss. It is also found that heat loss from the flow-field can reduce subsonic layer growth and improve thrust production. The existence of an optimal expander half-angle in the range of 25° to 30° is a result of the trade-off between viscous subsonic layer growth and geometric 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 micronozzles.
Owing to the extremely small geometry, supersonic micronozzle flow is subjected to complex micro-scale effects and can be substantially affected by viscous forces as a result of Reynolds numbers that are well below 1,000. A thruster prototype based on a monopropellant scheme has been developed by the NASA Goddard Space Flight Center (GSFC) in conjunction with the University of Vermont. The research included in this dissertation represents an independent and necessary extension to the work completed at NASAjGSFC. The focus of this dissertation is a Computational Fluid Dynamics (CFD) model of the micronozzle flow-field. Specifically, continuum flow models are used in conjunction with the FLUENT CFD software package in order to perform parametric simulations of micronozzle operation. From this model, the relationship between viscous forces, micronozzle geometry, and thermal effects are analyzed in order to determine nozzle performance and efficiency for varying operating conditions.
The interplay of viscous forces, nozzle geometry, and thermal effects such as heat transfer combine to represent the key operating parameters of supersonic micronozzle flow. It is revealed that the subsonic 'boundary' layer resulting from large viscous forces reduces flow through the nozzle and measurably degrades nozzle performance. To mediate the losses associated with large subsonic layers, nozzle expansion angles are increased sufficiently in order to provide room for subsonic layer growth. However, this results in transverse velocity components; a geometric loss. It is also found that heat loss from the flow-field can reduce subsonic layer growth and improve thrust production. The existence of an optimal expander half-angle in the range of 25° to 30° is a result of the trade-off between viscous subsonic layer growth and geometric 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 micronozzles.