1. TURBOMACHINERY: Computational analysis of automotive turbochargers.
Turbochargers (TCs) improve performance in internal combustion engines. Due to low production costs, TC assemblies are supported on floating ring bearings (FRBs) and show subsynchronous motions of significant amplitudes over a wide speed range. However, the subsynchronous whirl motions generally reach a limit cycle enabling continuous operation. The project advances on computational analysis for prediction of the linear and nonlinear rotordynamic response of shaft motions in automotive TCs. A comprehensive thermohydrodynamic model predicts the floating ring speeds, inner and outer film temperatures and lubricant viscosity changes, clearances thermal growth, operating eccentricities for the floating ring and journal, and linearized force coefficients. A nonlinear rotordynamics program integrates the FRB lubrication model for prediction of system time responses under actual operating conditions. Predictions will be compared with measurements of shaft motion in a TC unit driven by pressurized air demonstrating typical oil-whirl induced instabilities The computational tools aid to accelerate TC prototype development and product troubleshooting.
TASK: The student will perform measurements of the free-free mode natural frequencies and mode shapes of a typical turbocharger using miniature accelerometers. The results of the measurements will be used to validate the structural Finite Element model of a turbocharger.

2. TURBOMACHINERY: Gas bearings for oil-free turbomachinery
Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings (FPTPBs) demonstrate superior static and dynamic forced performance than other geometries as measured in a high speed rotor-bearing test rig. The bearings comprise four pivoted pads, offset 60%, with 1 mm feed holes for external pressurization, and nominal clearance and preload equal to 0.040 mm and 20%, respectively. The test rotor, (0.825 kg, 28.6 mm diameter) integrates an AC motor with top speed of 100 krpm. For various imbalance conditions, coasts down tests from 60 krpm characterize the rotor response on its bearings. The experimental results show that external pressurization stiffens the bearings, increasing the system critical speed while reducing the modal damping. Most importantly, the measurements evidence that external pressurization is not needed for super critical speed operation. In practice, the feed pressure could be shut off at high speeds with substantial savings in machine efficiency, unless thermal management is an issue. In addition, controlling the feed pressure while the rotor passes through speed zones of critical speed generation could eliminate high amplitude motions because of lack of damping.
TASK: The main objective is to advance the technology of gas film bearings for applications to oil-free turbomachinery by demonstrating their rotordynamic performance, reliability and durability. The student will conduct measurements of rotor response for increasing imbalances with comparison to predictions from computational models.

3. TURBOMACHINERY: Rotordynamic performance of foil gas bearings: tests and analysis.
In 2003, NSF funded a three-year research program, analytical and experimental, to advance the technology (design and operation) of gas foil bearings for oil-free turbomachinery. To date, the research has advanced:

• Extended computational tools (and GUIs) with two FE models for the top foil supported on bump strip layers. Predictions for load capacity and minimum film thickness have been validated to the only known reliable test data available in the open literature.
• Coast down rotor response measurements from 50 krpm conducted on a hollow rigid rotor supported on the foil bearings. The tests show persistent and severe subsynchronous motions from 50 to 27 krpm. The whirl frequency ranges from 20% to 27% of rotor speed and coincides with the system natural frequency. At lower rotor speeds, subsynchronous motions are negligible. Measurements are obtained for increasing imbalance masses in coast down tests from 25 krpm where no subsynchronous vibrations exist. The peak amplitudes of motion are proportional to the mass imbalance. The bearings tolerated without damage a persistent rubbing condition!

4. TURBOMACHINERY: Experimental studies of high speed miniature tilting pad gas bearing.
Flexure pivot tilting pad gas bearings with 8 mm diameter were designed and manufactured using precision wire EDM.
TASK: Participating student will modify dental drill turbine to drive a small rotor supported by the miniature tilting pad gas bearing and measure imbalance response up to 300,000 rpm.

5. TURBOMACHINERY: Load capacity and drag measurement of hybrid air foil bearing.
Air foil bearings have been used extensively for many oil-free micro-turbomachinery applications, such as turbo chargers, compressors, air management systems in the aircrafts, and micro gas turbines for distributed power generation. Hybrid air foil bearing was designed and constructed to increase load capacity and reduce friction drag. Load capacity and drag estimations require sophisticated instrumentation and experimental skills.
TASK: Participating student will run the hybrid air foil bearing with a graduate student and extract important design parameters and performance characteristics

6. TURBOMACHINERY: Analysis and design of miniature gas bearing.
Gas rarefaction effects should be considered when the miniature gas bearings are operated at elevated temperature. At this condition the molecular mean free path, that is proportional to the temperature, becomes comparable to gas film thickness. Analysis of miniature gas bearings for micro turbines will employ slip boundary conditions, derived from linearized Boltzman equation, at the rotor and bearing surfaces to calculate load capacity correctly. Current existing codes will be improved to accommodate the new boundary conditions.
TASK: Students will simulate the effect of different boundary conditions on bearing load capacity.

7. MANUFACTURING: Fabrication of miniature gas bearings.
Very small micro gas turbine generators (100~200W) and micro engines for micro unmanned air vehicles are expected to appear in the emerging markets. These small gas turbines are expected to be operated at above 500,000 rpm and these miniature gas bearings are backbone of the small gas turbines. Traditional precision machining cannot meet the design specifications of the small gas bearings and novel manufacturing method should be sought. LIGA process is a kind of micro manufacturing method of mesoscale metallic structure using X-ray lithography and electroplating.
TASK: Participating student will join with a graduate student and work on the LIGA process to make miniature tilting pad and air foil bearings with diameter of 5 mm.

8. MANUFACTURING: Stress analysis and failure prediction for micro cutting tools.
High performance materials are required for micro turbomachinery that operates at high stress and high temperature. Poor machinability of these materials is known, and the issue is worsened when miniature tools are used to fabricate components of a micro turbomachine. Excessive stress leading to frequent catastrophic failure of miniature tools has been observed. This project investigates the effects of tool geometry and micromachining conditions on tool failure.
TASK: Students will develop a stress model for miniature cutting tool. Theoretical calculation, finite element analysis, and experiments will be performed to validate the accuracy of the model.

9. MANUFACTURING: Development of investment microcasting technology.
Turbomachinery operates at high stress and high temperature exceeding the limits for most metals. Although superalloys such as inconel (Ni 15.5Cr 8Fe), hastelloy (Ni 28Mo), or stellite (Co 30Cr 4.5W) and manufacturing processes have been developed for large-scale turbomachinery, development of new micromanufacturing processes is needed to fabricate miniature 3D components using these superalloys. Investment microcasting is a suitable process, but research work on microfluidics on how liquid metal flows across a microchannel and effect of process parameters on resulted grain sizes of a cast component is yet to be found.
TASK: Participating students will study microfluidic phenomenon and simulate the flow using computation fluid dynamics and finite element method. Since grain size affects creep resistance and long term reliability of a microturbine, another student group will model the grain growth to effectively controlling the casting parameters. Experimental results will be used to validate the proposed model.

10. MANUFACTURING: Optimization of fiber laser micromachining of superalloys.
Since superalloys are difficult to be machined using conventional methods, a nonconventional material removal technology such as fiber laser micromachining is an alternative.
TASK: Students will optimize the laser parameters to remove the material using factorial experiments and available software. YOu will then section a laser machined sample to measure geometrical features such as straightness and roundness of drilled microholes with high aspect ratio, observe other fine details using scanning electron microscope, and analyze the recast layer using energy dispersive X-ray technique.

11. MANUFACTURING: Electrical-discharge drilling of high aspect ratio microholes.
Superalloys were developed for a system operating in demanding environments such as turbomachinery. To improve the system efficiency and reliability, microholes must be designed help cooling or lubricating the system. Since such features are difficult to fabricate by conventional machining, microEDM is a suitable solution.
TASK: Students will design an experiment to drill high aspect ratio microholes on a supperalloy at an oblique angle on a microEDM system. Hole dimension, straightness, and surface integrity of an EDM'ed microhole will be investigated. Scanning electron microscopy and energy dispersive X-ray analysis will be used to evaluate the results.

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