Test Rig Concept for Evaluating the Performance of a CO2 Immersed Electromechanical Rotor System Utilizing Gas Bearings: Part-2 Thermal Systems & Flow Loop Design

Abstract

Abstract Closed-loop supercritical carbon dioxide (sCO2) power cycles and sCO2 turbomachinery hold promise for waste-heat recovery (WHR) in natural gas (NG) pipeline compression stations. A companion paper [2] develops a cascaded sCO2 thermodynamic cycle (operating between 237 bara, 485 °C and 85 bara, 35 °C) using a two-train hermetically sealed turbomachinery power system as a concept for a WHR bottoming cycle at NG compression stations. The key novelty of the hermetic sCO2 turbomachinery was the inclusion of CO2-immersed gas bearings and a CO2-immersed direct-drive permanent magnet (PM) electric machine, both operating in a high-pressure CO2 environment of 27.6 bara. Operation of immersed gas bearings and the immersed PM electric machine in a high-speed, high-pressure CO2 environment is expected to result in high levels of windage heat generation — that can cause undesired structural distortions in thin-film gas bearings, resulting in compromised load carrying capability. Furthermore, high temperature from the windage heat can lead to structural stresses, loss of stator winding laminate glue and overall deterioration of the electric machine performance. To mitigate these risks experimentally, a companion paper [10] proposes a high-speed hermetically sealed test rig that simulates a highspeed 27krpm rotor supported with three radial gas bearings and a thrust gas bearing. High-pressure CO2 is supplied to the test rig with a Gas Supply Flow Loop that includes gas boosters, regulators, flow meters and chillers for a steady state operation. This paper discusses the thermal analysis model for the test rig. Specifically, as part of the thermal model, we present an ASPEN HYSYS model of the external flow loop that tracks the thermodynamic state of the CO2 gas supplied to the rig. Internal to the rig, the thermal model consists of a fluid-advection network that is thermally coupled with a steady-state thermal conduction solver to predict the windage heat, heat transfer coefficients, metal and fluid temperatures in the rig. The internal fluid advection network uses simple 1D fluid elements and real-gas CO2 properties for internal rig flows as well as commercial CFD software for modeling complex thin-film flows in gas bearings. The thermal model uses the principle of energy conservation to couple the heat flow in the advection network with the surrounding rig structural/rotating parts. The thermal model was implemented using an in-house solver using ANSYS workbench and ANSYS APDL script as the underlying platform. We present baseline temperature predictions for the test rig and quantify the amount of cooling mass flow needed to operate the test rig with its temperature constraints. We study the role of cooling in enabling reliable operation of the gas bearing system and electric machine at various rig internal pressures.