Windage and Gas Foil Bearing Losses in a Supercritical Carbon Dioxide Turbine Generator

by Douglas Milone
Knolls Atomic Power Laboratory

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Turbomachinery designed for sealed systems must contend with viscous power loss attributed to bearings and rotor windage. This power loss can reduce turbomachinery performance and necessitate additional cooling mechanisms due to fluid heat build-up. Methods for determining windage and bearing power loss specifically in pressurized carbon dioxide have little validation.

A 100 kWe S-CO2 Brayton cycle turbine-generator utilizing gas foil bearings has been tested over a wide range of shaft speeds (ranging from 25 to 75 krpm) and cavity pressures (ranging from 150 to 850 psia). Testing was performed with the turbine wheel replaced with a simple stub to simplify results, and a novel "coastdown" test approach was used to indirectly measure the total power loss as a function of shaft speed and cavity density. Two thrust bearing sizes were tested, providing insight into the breakdown of power loss between the thrust bearing and the rotor.

Results show that the gas foil thrust bearing for this unit produced the majority of the total power loss, necessitating significant cooling flow to maintain reasonable bearing temperatures. Data was normalized based on cavity density and a single correlation for thrust bearing and total power loss was successfully developed as a function of shaft speed.