Finite element profile optimization of nanocrystalline aluminum flywheel under rotation

Keywords

Aluminum alloys, Finite element method

Abstract

The main contributions of this study are (a) a validated finite element model and model development procedure for a rotating disk, and (b) use of the model in seeking a disk profile which maximizes specific kinetic energy and the specific angular momentum. Until recent years, the primary materials for lightweight space application flywheels have been carbon fiber composites. These materials have the disadvantage of being brittle, prone to contain defects causing fiber separations, and high degree of difficulty in manufacturing [4]. Nanocrystalline Aluminum has a very high ratio of strength and toughness to density, which is comparable to the fiber composite materials, and it is a good candidate for flywheel design. For flywheel energy storage in space, a simple optimization criterion is maximum (rotational) kinetic energy relative to the disk weight. For inertial guidance in space, a corresponding optimization criterion is maximum specific angular momentum. Kinetic energy and angular momentum depend on the mass and rotational velocity, and have a maximum value when the stresses caused by rotation satisfy the strength criterion. It will be seen that the maximum rotational velocity is limited by the strength to density ratio. Several candidate disk profiles are studied in seeking the optimized profile for a nanocrystalline aluminum flywheel. We start with the constant stress disk, (Case 1) which is a classical but simplified case with an analytical solution [1 ]. It exhibits equal and constant values of the radial and tangential stresses, and ignores shear and axial stresses. Further the stresses are set equal to the strength. Initially, we consider the interference case (i.e. stress free hole in the center of disk) for theoretical variable thickness disk profile under rotational velocity compared to a uniform thickness disk profile (with a hole) of the same mass. Then, following Collins [2], we add an outer rim to the constant stress interference profile while keeping the mass a constant design parameter (Case 2). For comparison, the non-interference case (no central hole) of the constant stress variable thickness disk profile is also examined (Case 3). Additional specially formulated one-parameter "custom" disk profiles are also analyzed (Case 4). The procedure to optimize the disk profile includes closed form equation derivations, spreadsheet optimization, CAD model generation and finite element analysis. The study evaluates profiles in terms of the two performance parameters: specific kinetic energy and specific angular momentum. The optimal profile should show significant improvement over the uniform thickness disk. However, the optimal flywheel profile using the two performance parameters may not attain the performance of the classical constant stress profile owing to simplifications and unrealistic aspects the latter incorporates. For example, the constant stress profile neglects shear and axial stresses and leads to an exceedingly low thickness at its outer diameter. Finally, the optimal profile attains a constant value of the Von Mises or Tresca stress, equal to the corresponding strength, over the disk.

Notes

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Graduation Date

2004

Degree

Master of Science (M.S.)

College

College of Engineering

Format

PDF

Pages

74 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0029490

Subjects

Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic

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