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Analysis of Automotive Turbocharger Nonlinear Response Including BifurcationsVistamehr, Arian 2009 August 1900 (has links)
Automotive turbochargers (TCs) increase internal combustion engine power and
efficiency in passenger and commercial vehicles. TC rotors are usually supported on
floating ring bearings (FRBs) or semi-floating ring bearings (SFRBs), both of which are
inexpensive to manufacture. However, fluid film bearings are highly nonlinear
components of TC units and contribute to the complex behavior (i.e. bifurcations and
frequency jumps between a first whirl frequency and a second whirl frequency) of the
entire rotor-bearing system (RBS). The frequency jump phenomenon concerns the TC
manufacturing industry due to increased levels of noise generation. This thesis presents
progress on assessing the effects of some bearing parameters and operating conditions
on the RBS dynamic forced performance and the frequency jump phenomenon.
A fluid film bearing model is integrated into a finite element rotordynamics
computational model for numerical prediction of the TC linear and nonlinear (time
transient) forced response. Since automotive TCs operate with variable rotational speed,
predictions are conducted with shaft acceleration/deceleration.
Over most of its operating speed range, TC rotor nonlinear response predictions
display two subsynchronous whirl frequencies w1 and w
2 representing a conical mode
and a cylindrical bending mode, respectively. At low shaft speeds w1 is present up to a
shaft speed (Omega bifurcation), where there is a frequency jump from w1 to w
2. The second whirl
frequency may persist up to the highest shaft speeds (depending on operating
conditions). Results show during rotor deceleration the Omega bifurcation may be different from
the one during rotor acceleration (hysteresis).
Predictions show the following factors delay the Omega bifurcation: increasing oil supply
pressure, decreasing oil supply temperature, and increasing shaft acceleration. Also, rotor imbalance distribution greatly affects Omega bifurcation and the shaft amplitude of total
motion.
Overall, this study shows the sensitivity of bifurcations and frequency jump
phenomenon in TC nonlinear response due to various bearing parameters and operating
conditions. Further analysis is required to generalize these findings and to assess the
effect of other bearing parameters (i.e. clearances, outer film length, ring rotation, etc.)
on this phenomenon. In addition further validation of the predictions against test data is
required for refinement of the predictive tool.
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Investigation Of The Friction Factor Behavior for Flat Plate Tests Of Smooth And Roughened Surfaces With Supply Pressures Up To 84 BarsKheireddin, Bassem A. 2009 August 1900 (has links)
Annular gas seal clearances were simulated with closely spaced parallel plates
using a Flat?Plate tester. The device is designed to measure the pressure gradient along
the test specimen. The main function of the Flat?Plate tester is to provide friction factor
data and measure dynamic pressure oscillations. A detailed description of the test
facility is described, and a theory for determining the friction factor is reviewed. Three
clearances were investigated: 0.635, 0.381, and 0.254 mm. Tests were conducted at
three different inlet pressures (84, 70, and 55 bars), producing Reynolds numbers range
from 50,000 to 700,000. Three surface configurations were tested including smooth?on-smooth,
smooth?on?hole, and hole?on?hole. The Hole?pattern plates are identical with
the exception of the hole depth. The results indicate that, for the smooth?on?smooth
and smooth?on?hole configurations, the friction factor remains constant or increases
slightly with increasing Reynolds numbers. Moreover, the friction factor increases as
the clearance between the plates increases. However, the results from the hole?on-hole configurations are quite different. A "friction?factor jump" phenomenon was
observed, and the Helmholtz frequency was detected on the frequency spectra.
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