TEST RIG CONCEPT FOR EVALUATING THE PERFORMANCE OF A CO₂ IMMERSED ELECTRO-MECHANICAL ROTOR SYSTEM UTILIZING GAS BEARINGS: PART-1 MECHANICAL AND ELECTRIC MACHINE DESIGN

This paper presents the mechanical and electric machine design of a 27krpm rotating test rig aimed at reducing technical risks with hermetic oil-free super-critical carbon dioxide (sCO₂) turbomachinery. The test rig rotor system is based on a conceptual turbomachine design discussed in a companion paper that evaluates a sCO₂ waste heat recovery (WHR) unit for land-based gas turbines at natural gas (NG) compressor stations. The key novelty of the hermetic sCO2 turbomachine concept is the utilization of additively manufactured CO₂ gas bearings and a CO₂-immersed direct-drive permanent magnet (PM) electric machine. The previous effort on conceptual design identified 3 main technical risks, which included thrust bearing load capacity, rotordynamics, and thermal performance of the system. Therefore, key requirements of the test vehicle include the ability to test the rotor system in a 400psi (27.6 bar) hermetic CO₂ operating environment, apply up to 1,500 lbs (6.7 kN) of rotor thrust loads, capability to assess rotordynamics with radial gas bearings, inclusion of instrumentation to assess thrust bearing load capability, and flexibility for varying secondary cooling flows to confirm thermal model predictions. Design topics such as rotordynamics, bearing design, and electric machine design are addressed in the paper, while highlighting critical elements. In addition, additive build trials for Inconel 718 radial and thrust bearings were performed to prove the manufacturability of the bearing design concepts. The results of the design efforts yield a test rig concept with ability to operate at 27,000 rpm, apply thrust loads to 1,500 lbs, deliver CO₂ up to 800psi, and modulate cooling flows.