Darren L. Hitt

Viscous Effects on Performance of Two-Dimensional Supersonic Linear Micronozzles

A comprehensive numerical investigation of a steady viscous flow through a two-dimensional supersonic linear micronozzle has been performed. The baseline model for the study is derived from the NASAGoddard Space Flight Center microelectromechanical systems-based hydrogen peroxide prototype microthruster. On the microscale, substantial viscous subsonic layers may form on the nozzle expander walls, which reduce thrust and efficiency. One approach to compensate for the presence of these layers has been to designmicronozzleswith expansion angles larger than traditional macroscale nozzles. Numerical simulations have been conducted for a range of Reynolds numbers (Re 15–800) and for expander half-angles of 10–50 deg. Twodifferentmonopropellant fuels have been considered: decomposed 85% pure hydrogen peroxide and decomposed hydrazine. It is found that an inherent tradeoff exists between the viscous losses and the losses resulting from the nonaxial exit flow at larger expansion angles. Our simulations indicate that themaximumnozzle efficiency occurs for bothmonopropellants at a nozzle expansion halfangle of approximately 30 deg, which is significantly larger than that of traditional conical nozzle designs.

Control of ruthenium oxide nanorod length in reactive sputtering

Ruthenium oxide nanorods have been grown on Si wafer substrates under a variety of pre-existing surface conditions by reactive radio frequency sputtering in an electron cyclotron resonant plasma process. Nanorod formation by this method is fast relative to that observed in other processes reported in the literature, with nucleation being the rate determining step. Growth in the axial direction is limited by the availability of ruthenium precursors which competes with their consumption in the lateral growth of the nanorods. The availability of Ru precursors at the top of the nanorods can be controlled by surface diffusion and therefore substrate temperature. The ultimate length of the nanorods is determined by the mole fraction of oxygen used in the reactor ambient through the production of mobile Ru hyperoxide precursors. The results of this investigation show the way to develop a process for producing a high density field of nanorods with a specified length.

Influence of Wall Heat Transfer on Supersonic MicroNozzle Performance

Anumericalmodel to characterize the influence of wall heat transfer on performance of amicroelectromechanical systems (MEMS)-based supersonic nozzle is reported. Owing to the large surface-area-to-volume ratio and inherently lowReynolds numbers of aMEMSdevice, wall phenomena, such as viscous forces and heat transfer, play critical roles in shaping performance characteristics of themicronozzle. Viscous subsonic layers inhibit flow and can grow sufficiently large on the nozzle expanderwalls, potentiallymerging to cause the flow to be subsonic at the nozzle exit, and result in reduced efficiency and performance. Heat flux from the flow into the surrounding substrate can mitigate subsonic layer growth and improve overall thrust production. In this study, subsonic layer growth is quantified to characterize the impact on performance of micronozzles with a flowfield that is subject to wall heat transfer. Both twoand three-dimensional (3-D) simulations are performed for varying expanderhalf-angles (15 deg, 30 deg, and 45 deg) and varying throat Reynolds numbers (30–800), whereas the depth of the 3-D nozzle is varied (25–300 m). Simulation results and nozzle efficiencies are compared with inviscid theory, previous adiabatic results, and existing numerical and experimental data. It is found that heat loss to the substratewill further accelerate the supersonic core flowviaRayleigh flow theory and can reduce subsonic layer growth. These effects can combine to alter the micronozzle expansion angle, which maximizes thrust production and specific impulse efficiency.

Numerical Simulations of Supersonic Flow in a Linear Aerospike Micronozzle

In this study, we numerically examine thrust performance of the linear aerospike nozzle micro-thruster for various nozzle spike lengths and flow parameters in order to identify optimal geometry(s) and operating conditions. Decomposed hydrogen-peroxide is used as the monopropellant in the studies. Performance is characterized for different flow rates (Reynolds numbers) and aerospike lengths, and the impact of micro-scale viscous forces is assessed. It is found that 2-D full micro-aerospike efficiencies can exceed axisymmetric micro-nozzle efficiencies by as much as 10%; however, severe penalties are found to occur for truncated spikes at low Reynolds numbers.

Numerical simulations of laminar mixing surfaces in pulsatile microchannel flows

Laminar mixing on the micro-scale is difficult to achieve owing to the inherently low Reynolds numbers of the flows and lack of turbulence. Alternative strategies, both passive and active, have been proposed to enhance laminar mixing. In this study the effect of flow pulsatility on the dynamics of a laminar mixing surface formed between converging microchannel flows is studied numerically. The results indicate that complex wave-like interfacial distortions are possible for high-frequency pulsations and are more significant when the frequencies of the converging flows are related by and irrational ratio.

Viscous Effects on Performance of Three-Dimensional Supersonic Micronozzles

A numerical investigation of three-dimensional flow in a converging–diverging microelectromechanical-systemsbased supersonic nozzle design is reported. Based on microfabrication techniques, the nozzle geometry is a twodimensional pattern etched into a substrate to a specified depth, thus resulting in a three-dimensional ductlike geometry and flowfield. Owing to the low Reynolds numbers associated with the microscale, viscous effects result in substantial subsonic layers that develop along expander walls and reduce nozzle efficiency. In this study, the threedimensional flowfield is simulated with a continuummodel and decomposed hydrogen peroxide as the working fluid monopropellant. Numerical simulations are performed over a range of nozzle expander half-angles (15–45 ), for several nozzle depths (25–400 m), and for varying throat Reynolds numbers (15–800). Simulation results are analyzed to determine subsonic layer growth rates and to delineate their impact on micronozzle thrust and performance. Specific impulse efficiencies are compared with quasi-one-dimensional theory, previous twodimensional simulations, and data from the micronozzle literature, including direct simulation Monte Carlo and experimental studies. It is found that three-dimensional impulse efficiencies can be much less than corresponding two-dimensional model results owing to the presence of viscous, subsonic layers on the additional walls in a threedimensional device. Themicronozzle depth is a critical parameter that impacts thrust and efficiency. For sufficiently shallow nozzles, it is possible for the subsonic layers to merge and decelerate the flow to subsonic conditions, thus degrading nozzle performance.

Analysis of Transient Flow in Supersonic Micronozzles

A numerical investigation of transient supersonic flow through a two-dimensional linear micronozzle has been performed. The baseline model for the study is derived from the NASA Goddard Space Flight Center microelectromechanical-systems-based hydrogen peroxide prototype microthruster. A hyperbolic-tangent actuation profile is used to simulate the opening and closing of a microvalve with a maximum inlet stagnation pressure of 250 kPa, which generates a maximum throat Reynolds number of Re 800. The complete duty cycle occurs over 1.7ms.Numerical simulations have been conducted for expanderhalf-angles of 10–50 , andboth slip and no-slip wall boundary conditions have been examined. The propulsion scheme employs 85%-pure hydrogen peroxide as the monopropellant fuel. Simulation results have been analyzed, and thrust production as a function of time has been quantified, along with the total impulse delivered. Micronozzle impulse efficiency has also been determined based on a theoretical maximum impulse achieved by a quasi-1-D inviscid flow responding instantaneously to the actuation profile. It is found that both the flow and thrust exhibit a response lag to the timevarying inlet pressure profile. Simulations indicate that a maximum efficiency and impulse occur for an expander half-angle of 30 for the no-slip wall boundaries, and the slip simulations demonstrate a maximum plateau in the range of 20–30 ; these angles are significantly larger than with traditional conical nozzle designs.

Role of Heat Transfer on Performance of 3-D Supersonic MicroNozzles

A numerical model for the characterization of heat transfer effects on performance and operation of a MEMS-based supersonic nozzle is reported. Owing to the large surface area to volume ratio and inherently low Reynolds numbers (Re < 1000) of a MEMS device, wall phenomena such as heat transfer and viscous forces play critical roles in shaping performance characteristics of the micronozzle. Substantial viscous subsonic layers can grow sufficiently large on the expander walls, inhibit flow through the nozzle, and potentially merge together causing the flow to be subsonic at the nozzle exit; this causes a reduction in thruster efficiency and performance. Heat transfer from the flow into the surrounding substrate can significantly alter this subsonic layer and impact overall thrust production. In this study the flow of decomposed hydrogen peroxide monopropellant is simulated in a converging-diverging supersonic microscale nozzle. The flow field is analyzed to determine subsonic layer growth on the micronozzle walls with varying degrees of heat transfer to the surrounding substrate in order to characterize the impact on nozzle performance. 3D simulations are performed for several micrnozzle depths (25μm, 50μm, and 200μm) at Reynolds number between 30−800 while varying the amount of heat transfer. Simulation results and efficiencies are compared to inviscid theory and existing data. It is shown that heat loss from the flow can reduce subsonic layer growth and thus improve nozzle performance. Similarly, thrust production is shown to be enhanced with heat transfer via Rayleigh flow and an increased gas density at the nozzle exit.

Numerical Simulations of Viscous Flow in 3D Supersonic Bell Micronozzles

In this study we numerically examine thrust production and efficiency of 3D supersonic micronozzles with bell-shaped expanders. Three different expander configurations are simulated (100% ‘full-bell’, 80% and a 60%) for varying nozzle depths (25μm− 400μm) and Reynolds numbers (15− 800) with decomposed hydrogen peroxide as the working gas. Owing to the inherently low Reynolds numbers on the microscale, substantial viscous subsonic ‘boundary’ layers are able to develop along the walls of the nozzle expander. These layers retard the bulk flow field and reduce nozzle performance. It was found that the subsonic layers on the expander walls can grow sufficiently large so as to merge together and revert the entire cross section of the expander flow field to subsonic conditions markedly reducing thrust performance. Thrust production and specific impulse efficiency is computed for the various flow scenarios and nozzle geometries to delineate the impact of viscous forces on nozzle performance. Results are also compared to inviscid theory and previous 3D linear nozzle simulations. It is found that the flow alignment of the bell nozzle comes at the expense of an increased expander length and with that an increase in the corresponding viscous losses via subsonic layer. As such, the 30 ◦ linear nozzle produces more thrust and has a higher efficiency than the bell nozzle at a given Reynolds number.

Performance Characteristics of 3D Supersonic Micronozzles

A 3D numerical investigation of the flow in a converging-diverging MEMS-based supersonic nozzle is reported. Owing to the large surface area to volume ratio of a MEMS device along with inherently low Reynolds numbers, substantial viscous subsonic ‘boundary’ layers develop along the expander walls which reduce thrust production and nozzle efficiency. Decomposed hydrogen peroxide monopropellant flow is simulated using continuum 3D flow models. The flow field is analyzed to determine subsonic layer growth rate and to characterize this impact on micronozzle performance. Simulations are performed over a range of nozzle expander halfangles (15 ◦−45 ◦) and for several nozzle depths (25μm− 400μm). Simulation results are analyzed and thrust production, subsonic layer development, and efficiencies are compared to inviscid theory and previous 2D simulations. It is shown that the subsonic layers on the expander walls can grow sufficiently large and merge together causing subsonic flow across the entire crosssection of the flow field up to the nozzle exit. The consequence is reduced thrust output and efficiency. Viscous losses are quantified in 3D and are shown to be greater than prior 2D models owing to the addition of nozzle side-walls in a 3D simulation.