Microturbopump Utilizing Microball Bearings

Abstract

This dissertation presents the development of a microfabricated turbopump capable of delivering fuel with the flow rates and pressures required for portable power generation. The device is composed of a spiral-groove viscous pump that is driven by a radial in-flow air turbine and supported using a novel encapsulated microball bearing.

First, the encapsulated microball bearing and methods to investigate the wear and friction behaviors were developed. Two primary raceway designs, point-contact and planar-contact designs, were developed with the key design factor being wearing of the raceway. A modification to the planar-contact design was made for the final turbopump that reduced both wear and debris generation.

Second, two air turbine platforms were developed using the encapsulated microball bearings to characterize both the bearing and the turbine drive mechanism. A tangential air turbine platform was first developed and characterized using the point-contact bearing mechanism. Rotational speeds >37,000 rpm were demonstrated and long-term operation (>24 hours) using this platform, but with large driving pressures (tens of psi) and large raceway wear (tens of microns). Furthermore, the circumferential asymmetry of the turbine design led to difficulty in measuring pressure distribution and sealing for pump applications.

Results from the tangential air turbine platform led to an axisymmetric radial in-flow air turbine platform using a planar-contact bearing design. Rotational speeds greater than 85,000 rpm with turbine pressure differentials in the range of 1 psi were demonstrated using this platform. The wear of the raceway was observed to be on the order of single microns (a 10x improvement). The radial in-flow air turbine platform allowed an empirical model to be developed relating the friction torque to the rotational speed and load for the planar-contact bearing. This enabled calculation of the power balance for pumping and a method to characterize future bearing designs and materials.

Lastly, a microfabricated turbopump was demonstrated based on a spiral-groove viscous pump and the radial in-flow turbine platform using the planar-contact bearing. Pumping operation was demonstrated with a differential pressure up to +0.3 psi and flow rates ranging from 35 mL/hour to 70 mL/hour, within the range relevant to portable power generation.