Joseph M. Prahl

Dynamic Stiffness and Damping Characteristics of a High-Temperature Air Foil Journal Bearing

Using a high-temperature optically based displacement measurement system, a foil air bearing s stiffness and damping characteristics were experimentally determined. Results were obtained over a range of modified Sommerfeld Number from 1.5E6 to 1.5E7, and at temperatures from 25° to 538°C. An Experimental procedure was developed comparing the error in two curve fitting functions to reveal different modes of physical behavior throughout the operating domain. The maximum change in dimensionless stiffness was 3.0E-2 to 6.5E-2 over the Sommerfeld Number range tested. Stiffness decreased with temperature by as much as a factor of two from 25° to 538°C. Dimensionless damping was a stronger function of Sommerfeld Number ranging from 20 to 300. As the temperature is increased, the damping shifts from a viscous type to a frictional type. Presented as a Society of Tribologists and Lubrication Engineers Paper at the STLE/ASME Tribology Conference in San Francisco, CA October 21–24, 2001

Steady-State Stiffness of Foil Air Journal Bearings at Elevated Temperatures

A previously developed test method for measuring steady-state stiffness of foil air journal bearings is extended to measure trends in bearing stiffness at high temperature. Steady-state stiffness of the tested foil bearing is found to decrease in general as the temperature increases from 25° to 538 °C. The magnitude of stiffness change observed is roughly a factor of two, which is important information for the design of future high speed turbomachinery. It is expected that damping in foil bearings may also be affected by changes in temperature necessitating future testing to evaluate the trends in dynamic bearing characteristics. Presented at the 56th Annual Meeting Orlando, Florida May 20–24, 2001

Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications

ABSTRACT Amethodologyforthedesignandconstructionofsimplefoilthrust bearingsintendedfor parametricperformancetestingandlow marginal costs is presented. Features drawn from a reviewof the openliterature are discussed as they relate to bearingper-formance. The design of fixtures and tooling required to fab-ricate foil thrust bearings is presented, using conventional ma-chining processes where possible. A prototype bearing with di-mensionsdrawnfrom theliteratureis constructed,with allfabri-cationsteps described. Aload-deflectioncurvefor thebearingispresented to illustrate structural stiffness characteristics. Start-stop cycles are performed on the bearing at a temperature of425 ◦ C to demonstrate early-life wear patterns. A test of bearingload capacity demonstrates useful performance when comparedwith data obtained from the open literature. Introduction Foil gas bearings represent an enabling technology foradvanced oil-free turbomachinery systems. Operating athigh speeds and temperatures, these next-generation turboma-chines will present tribological challenges that conventional oil-lubricated rolling element bearings may be unable to meet, in-cluding shaft speeds well above three million DN and bearingtemperatures in excess of 400

Discharge distribution performance for an axisymmetric model of a fire sprinkler head

The discharge flow pattern from fire sprinkler heads in modeled in a nondimensional description which emphasizes a sprinklers ability to evenly distribute spray over a maximum possible floor area. An axisymmetric model sprinkler, consisting of a jet impinging normal to and at the center of a flat disk, employs the disintegration of the resulting film to produce the droplet sprays. Measurements of the sheet breakup radius show that it is proportional to the negative one-third power of the Weber number based on jet diameter. The discharge distribution is varied by controlled axial vibrations of the disk. Nondimensional sheet breakup radii and radial flow distributions correlate with the ratio of the disk driving frequency to the frequency of maximum growth of sinuous waves. Flow patterns from the model apparatus are compared with those from a commercial fire sprinkler. An analytical model to predict droplet trajectories and consequent discharge distributions shows qualitative agreement with the measured flow patterns for an assumed Rosin-Rammler droplet size distribution about a volume mean droplet diameter in the accepted range of previous investigators.

Preliminary Investigations of Forced Convection on Flame Propagation along Paper and Matchstick Arrays

Abstract Experimental observations are made on the burning of a linear, uniformly spaced, single row of vertically oriented matchsticks, and on rows of continuous paperstrips, with an imposed wind, up to three feet per second, blowing with and opposed to the direction of fire spread. Matchstick height and spacing are varied. One height and row spacing are used for the paperstrips, and the number of rows is varied. Necessary conditions for flame propagation in the matchstick arrays are presented in a non-dimensional plot. Linear rates of flame propagation for the paper and matchstick arrays are presented as functions of the wind velocities for various spacings and heights. A correlation of the matchstick results is made by use of a semi-empirical model based on an ignition temperature and convective heat transfer. The experimental results are compared with previous work with no convection, of Emmons and Shen (1971) and Vogel and Williams (1970).

Parched Elasto Hydrodynamic Lubrication Film Thickness Measurement in an Instrument Ball Bearing

Parched Elasto Hydrodynamic Lubricant (PEHL) film thickness in a large instrument ball bearing is measured by electrical capacitance across its ball set. Correlation is shown between changes in film thickness and changes in Basic Speed Ratio (BSR) measured at the same time. BSR is confirmed as a sensitive, non-intrusive measure of transients in film thickness in a real bearing. Presented at the 43rd Annual Meeting In Cleveland, Ohio May 9–12, 1988

Observations of the Kelvin-Helmholtz Instability in Laboratory Models and Field Examples of Thermal Plumes

Abstract Buoyant, rectangular thermal plumes were modelled in a 2 by 7 meter laboratory water table facility. The plume enters at right angles to a constant temperature, uniform crossflow of depth equal to that of the plume. Highly buoyant plumes (0.16 o A Kelvin-Helmholtz instability is observed in the plume thermocline for Fr o o a cold water wedge penetrates beneath the plume into the plume discharge channel. A series of transverse line vortices quickly grow in the thermocline to cover the vertical extent of the plume, moving nearly with a mean flow. These vortices are visible both by dye injection into the thermocline and surface infrared imagery. Local gradient Richardson numbers fall below 1/4 in the thermocline for all plumes observed (Fr o o = 0.50 the Kelvin-Helmholtz instability is dominated by randomized turbulent mixing. This vortex instability was also observed in aerial infrared photographs taken by NASA of the Cuyahoga River mouth entering Lake Erie. Field measurements indicate that Ri These results are compared with related experimental observations and previous theoretical inviscid stability analyses of stably stratified shear flows by other authors.

Parched elastohydrodynamic lubrication: Instrumentation and procedure

A counter rotating bearing rig has been designed and construced to study transient elastohydrodynamic lubrication phenomena. This paper describes new instrumentation, and documents test procedures. Ball and race speed measurement systems and the capitance (film thickness) measurement) system were upgraded. Methods for measuring bearing torque and race temperatures were implemented

Thermal Management Phenomena in Foil Gas Thrust Bearings

Thrust foil gas bearings operate at high speeds on a very thin fluid film which experiences high shear. Shear induced viscous losses result in localized heating which must be managed to prevent thermal distortions and failure. The current work examines the need for thermal management in thrust foil bearings as evidenced by reduced performance in uncooled bearings. Measured bearing power loss is a few hundred watts from twenty-five to fifty-five krpm (89–196 m/s runner surface velocity based on mean bearing diameter). Modeling of the thrust runner demonstrates a potential for stress-induced running surface deformations greater than the gas film thickness (>10 μm), and radial temperature gradients in the bearing foils can exceed a few degrees Celcius per millimeter. Air flow forced through the foil structure achieves improvements in bearing load capacity and demonstrates a need for increased understanding of the thermal environment in these bearings.© 2006 ASME

Buoyant Rectangular Surface Thermal Plumes

Abstract A near-field integral model of a buoyant surface thermal plume entering a crossflow is developed which includes the effects of buoyancy through the empirical assumption that the slope of the plume thermocline is proportional to the local plume depth times the local plume excess temperature and inversely proportional to the initial plume internal Froude number, Fr o . The model is compared with laboratory data taken by Kuhlman and Prahl (1974), and field data taken by Frigo and Frye (1972). Data generally agree well with the model using single values of the empirical constants in the theory, varying only the initial internal Froude number, Fr o , and the crossflow-to-initia1-plume-velocity ratio, R, to match the data values. Matching is achieved for distances along the plume center line of approximately 20 initial plume widths for the laboratory data and 50 widths for the field data.