Pedro I. Espina

Particle size measurement of inert-gas-atomized powder

Abstract Metal powder produced by supersonic inert gas metal atomization (SiGMA) has been analyzed using several diagnostic methods. This analysis has brought to our attention several interesting and unexpected results. Some of these unexpected results concern the reliability of the various particle-measuring techniques, the procedure for proper (reproducible) particle size analysis, and the graphical representation of the data that best shows the powders characteristics. This study has also shown that gas-atomized powders produced in the SiGMA facility have distinct size distribution characteristics that do not follow the log-normal pattern. The fragmentation mechanisms leading to droplet formation which explain the SiGMA powder size distribution data are examined. We conclude that these new powder analysis procedures are applicable to all inert-gas-atomized powder and can lead to a better understanding of an atomizing systems operative liquid disruption mechanisms.

Temperature Characterization in the Collection Tank of the NIST 26 m(3) PVTt Gas Flow Standard

Gas temperature gradients created during the filling stage of a pressure–volume–temperature–time (PVTt) calibration cycle, and those imposed by inhomogeneous room conditions, lead to uncertainties in the average gas temperature in the collection tank. Because these temperature uncertainties dominate the overall flow uncertainty, NIST upgraded the temperature-averaging scheme used in its 26 m3 PVTt system. Instead of arithmetically averaging 10 thermistors to obtain the mean gas temperature, we now calculate this value via a volume-weighted trapezoidal integration procedure using 35 thermistors. Applying the new temperature-averaging scheme, the mean gas temperature can be determined with a standard uncertainty of 89 mK after only 2700 s of fan mixing. As a result, the flow uncertainty in the NIST 26 m3 PVTt system has decreased from 0.22% to 0.13% (with a coverage factor of 2). This paper highlights the temperature improvements and presents a detailed analysis for estimating the lower temperature uncertainty.

An Uncertainty Analysis of a NIST Hydrocarbon Liquid Flow Calibration Facility

This paper presents the uncertainty characterization of NIST’s new hydrocarbon liquid flow calibrator (HLFC). This facility uses a passive piston prover technique where fluid is driven by pumps while the measuring piston is passively stroked through the calibration interval. This facility is typically operated using MIL-C-7024C fluid (also known as Stoddard solvent – a surrogate liquid for JP-4 and JP-5 jet fuels), but using a variety of other fluids offers a wider range of measurements. The range of flows for this facility is 0.19 to 5.7 liters per minute – lpm (0.05 to 1.5 gallons per minute – gpm). Over this range, the expanded uncertainty claim for this facility is ±0.01%, at 95% confidence level. The uncertainty of a dual-turbine meter tested in the system is also reported. In addition, NIST is working to incorporate additional piston provers so that the flow for hydrocarbon liquids calibration service will reach 760 lpm (200 gpm).Copyright

The intelligent control of an inert-gas atomization process

Intelligent control is an attempt to specify the function of a controller in ways which mimic the decision-making capabilities of humans. Traditionally, issues relating to the emulation of human-like capabilities have fallen in the domain of artificial intelligence. Intelligent processing is a specific form of intelligent control in which the system to be controlled is a process rather than the more conventional mechanical or electrical system. The National Institute of Standards and Technology’s program on intelligent processing of metal powders is a multi-disciplinary research initiative investigating the application of intelligent control technologies to improve the state of the art of metal powder manufacturing. This paper reviews the design of the institute’s supersonic inert-gas metal-atomizer control system.

Error Free Liquid Flow Diverters for Calibration Facilities

A design for diverter valves in gravimetric liquid flow calibration facilities is proposed. The concept makes use of repeated unidirectional motions of the diverter valve to reduce errors associated with asymmetry in the diverter valve motion and in the liquid jet velocity profile. Various implementation examples are provided and their benefits are discussed.Copyright

A flow visualization study of supersonic inert gas-metal atomization

Abstract Fine metal powders (less than 45 μm for many alloys) have been shown to have unique properties due to their homogeneity, novel microstructures and metastable phases. The primary factor that determines these features is solidification rate, which for powder processing depends on the amount of undercooling achieved by the metal droplets prior to solidification (which is inversely related to particle size). Particle size, in turn, depends on the process that produces these powders. One such process-supersonic inert gas-metal atomization-has been found to be particularly efficient in producing these powders for a wide range of alloys. For a selected atomizer configuration the gas and metal flows have been studied to characterize the salient features so that design changes leading to increased process efficiency can be realized. Using flow visualization techniques (schlieren, shadowgraph and 0.5 μs flash photography), flow fields in gas and liquid streams have been studied. Emphasis has been placed on showing how aspiration conditions in the liquid delivery nozzle combine with the design of the inner bore of this same nozzle to promote liquid stream instability and, consequently, to improve atomization efficiency.