摘要 :
The U.S. Naval Research Laboratory (NRL) has been investigating viscous flow through micro-nozzles applicable to advanced spacecraft propulsion systems under a four-year 6.2 Discovery and Invention research effort. The objective o...
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The U.S. Naval Research Laboratory (NRL) has been investigating viscous flow through micro-nozzles applicable to advanced spacecraft propulsion systems under a four-year 6.2 Discovery and Invention research effort. The objective of the program is to create thruster nozzle design tools that perform numerical optimization of the nozzle geometry to minimize viscous losses in the nozzle for challenging viscous flow regimes. This paper summarizes interim results for the program, and provides contextual detail for topics covered more extensively in three related papers from the program by co-investigators Holman, Williams and Rosenberg. The micro-nozzles under investigation produce thrust levels on the order of 1 mN and have throat diameters of less than 0.010 inches. These nozzles are applicable to many micro-propulsion applications for cubesatellites or other small spacecraft. Flow conditions were selected to be representative of resistojet propulsion, but are applicable to many other electro-thermal devices. The nozzles were operated under a variety of pressure, mass flow and temperature conditions, while exhausting to vacuum, over a range of Reynold's Numbers below 2000. Thrust was measured using a precision torsional thrust balance, and other diagnostic measurements were collected for correlation to numerical fluid dynamic simulations. Computational Fluid Dynamics (CFD) was used in the continuum regions of the nozzle flow field and transition to Direct Simulation Monte Carlo (DSMC) in the rarified region of the nozzle exit and near field plume. Advanced DSMC techniques were used to efficiently model the complete flow field. The work demonstrates a new capability to numerically optimize the thruster nozzle geometry allowing the user to tailor thruster geometry to maximize the nozzle thrust or specific impulse performance for challenging viscous flow environments.
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摘要 :
The U.S. Naval Research Laboratory (NRL) has been investigating viscous flow through micro-nozzles applicable to advanced spacecraft propulsion systems under a four-year 6.2 Discovery and Invention research effort. The objective o...
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The U.S. Naval Research Laboratory (NRL) has been investigating viscous flow through micro-nozzles applicable to advanced spacecraft propulsion systems under a four-year 6.2 Discovery and Invention research effort. The objective of the program is to create thruster nozzle design tools that perform numerical optimization of the nozzle geometry to minimize viscous losses in the nozzle for challenging viscous flow regimes. This paper summarizes interim results for the program, and provides contextual detail for topics covered more extensively in three related papers from the program by co-investigators Holman, Williams and Rosenberg. The micro-nozzles under investigation produce thrust levels on the order of 1 mN and have throat diameters of less than 0.010 inches. These nozzles are applicable to many micro-propulsion applications for cubesatellites or other small spacecraft. Flow conditions were selected to be representative of resistojet propulsion, but are applicable to many other electro-thermal devices. The nozzles were operated under a variety of pressure, mass flow and temperature conditions, while exhausting to vacuum, over a range of Reynold's Numbers below 2000. Thrust was measured using a precision torsional thrust balance, and other diagnostic measurements were collected for correlation to numerical fluid dynamic simulations. Computational Fluid Dynamics (CFD) was used in the continuum regions of the nozzle flow field and transition to Direct Simulation Monte Carlo (DSMC) in the rarified region of the nozzle exit and near field plume. Advanced DSMC techniques were used to efficiently model the complete flow field. The work demonstrates a new capability to numerically optimize the thruster nozzle geometry allowing the user to tailor thruster geometry to maximize the nozzle thrust or specific impulse performance for challenging viscous flow environments.
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Schlieren images were gathered from the side, and spray droplet diameter and velocity distributions were obtained along the centerline of an underexpanded jet as it flowed through shocks and an expansion wave. Schlieren images rev...
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Schlieren images were gathered from the side, and spray droplet diameter and velocity distributions were obtained along the centerline of an underexpanded jet as it flowed through shocks and an expansion wave. Schlieren images revealed the location and dimensions of the shock structures, which were then used to determine the locations along the centerline where phase Doppler interferometry measurements were gathered. Schlieren images revealed that, at the liquid flow rates used in this study, there were not dramatic shifts in the compressible flow structures' location or behavior. Liquid spray at the periphery of the jet, produced by the un-entrained liquid film at the nozzle's inside surface that was sheared by the jet's gas flow, was visible in the schlieren images but was not measured in this study. Droplet velocity measurements indicated some of the expected gas velocity behavior: increasing velocity up to the shock then decreasing velocity in the following compressed region. Downstream of the expansion wave, the droplets did not immediately accelerate. Droplet diameter statistics, used to calculate Dio and D32, appeared to suggest both droplet fragmentation and agglomeration as the spray was carried through the abrupt velocity changes at shocks and expansion waves. The differences in Dm and D.u suggest that more careful examination of the droplet diameter probability statistics and shadow microscopy of the spray along the axis of the jet will provide greater clarity of how the spray and shock structures are interacting.
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In order to understand the reasons for the apparent benefits of using a flow-blurring (FB) atomizer in a combustion system, it is necessary to first examine fundamental spray characteristics under non-reacting conditions. Previous...
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In order to understand the reasons for the apparent benefits of using a flow-blurring (FB) atomizer in a combustion system, it is necessary to first examine fundamental spray characteristics under non-reacting conditions. Previous work on FB atomizers, however, has mostly involved only water and a relatively narrow range of parameters. In this study, a phase Doppler anemometry instrument was used to characterize FB atomizer sprays and determine the effects of varying surface tension and viscosity of the liquid. Operating at room pressure and temperature (i.e., a "cold spray"), droplet sizes and velocities were measured for water, a water/surfactant mixture (lower surface tension), a water/glycerol mixture (higher viscosity), and glycerol (much higher viscosity). For all of the tested fluids, with the exception of pure glycerol, the FB atomizer produced small droplets (below 50 μm) whose size did not vary significantly in the radial or axial direction, particularly above a characteristic distance from the atomizer exit. Results show that the spray is essentially unaffected by a 4.5x decrease in surface tension or a 7x increase in viscosity, and that Sauter mean diameter (SMD) only increased by approximately a factor of three when substituting glycerol (750x higher viscosity) for water. The results suggest that the FB atomizer can effectively atomize a wide range of liquids, making it a useful fuel-flexible atomizer for combustion applications.
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Stable and transitional blow off characteristics of turbulent, two-dimensional, bluff body-stabilized premised flames were studied in a rectangular duct with a triangular flame holder in the midspan of the duct cross section in vi...
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Stable and transitional blow off characteristics of turbulent, two-dimensional, bluff body-stabilized premised flames were studied in a rectangular duct with a triangular flame holder in the midspan of the duct cross section in vitiated flow, with and without cooling. The transitional blowoff dynamics were characterized with high-speed imaging and measurements of CH chemiluminescence emissions from the wake. Simultaneous PIV and OH PLEF were used to observe both stable and transitional blow off flame dynamics. With both high-speed imaging and PLIF, the flame sheet was observed to shift from burning on the upstream side of the wake shear layer to within the shear layer, and then finally in the recirculation zone before finally blowing off. Flames in vitiated, uncooled air remained stable at temperature ratios near 2.36 while the shear layer exhibited Kelvm-Helmholtz vortex shedding. Blow off occurred at temperature ratios from 2.08 to 2.18, with blow off temperature ratios increasing with the bulk velocity. An approximate integral length scale was calculated from PIV measurements in the isothermal and reacting flows. Incorporating laminar flame calculations, the turbulent combustion regime was determined to be in the range of corrugated and wrinkled flames.
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