Session: 11-44-01: Heat and Mass Transfer for Renewable Energy Conversion Processes

Paper Number: 94832

94832 - Heat Transfer in Liquid Nitrogen Cooled Superconducting Transformers: An Experimental Investigation 

A single-phase, 15-kilowatt electric transformer was upgraded to replace the secondary copper winding with one formed of Yttrium Barium Copper Oxide [YBCO] superconducting ribbon.  YBCO material offers exceptional electrical efficiency at current densities which require water cooling in conventional conductors.  These conditions are encountered when converting fossil-fuel combustion to electrical heating for industrial processes transitioning to low carbon sources.  A superconducting winding of two turns resulted in secondary currents over 200 amperes which exploited the benefits of the YBCO material.  This winding must operate below its critical temperature of 92K to avoid damage and was thus routed within a tube through which liquid nitrogen flowed.  Published research on superconducting transformers often implements submersion cooling of a complete transformer assembly for its relative simplicity and stability in operation.  This method ensures electrical properties, but generates significant cryogen boil off as the steel transformer core heat the fluid with negligible benefit to operation.  To maximize cooling efficiency between the YBCO critical temperature of 92K and the atmospheric boiling point of nitrogen at 77K, the latent heat of nitrogen was exploited.   The resulting multi-phase system required accurate heat transfer analysis to optimize fluid loss against superconductor operation.

Steady state enthalpy variation of the cryogenic fluid was measured by recording inlet and outlet temperatures via platinum RTD’s.  Additionally, the transformer assembly was placed within a vacuum vessel, and system mass measured continuously via 4 load sensors to quantify the rate of nitrogen boil-off.  By operating the experimental setup without electrical power at a range of cryogenic flow rates, the contributions from each heat transfer mechanism were effectively isolated.  Controlling steady state vacuum pressure within the tank permitted evaluation of conductive and convective heat transfer into the cryogenic fluid.  Detailed experimental measurements were captured across the range of temperature, pressure, and cryogenic flow rates, and comparisons with published critical heat flux research indicated strong agreement.  Considerable literature exists for multi-phase cryogens flowing in tubes with swirlers and other features intended to enhance heat transfer.  This research applied proven experimental methods to observe heat transfer into the cryogen at substantially lower flux rates than previously published and thus expanded the scope of understanding for these fluids.   

This research provided a comprehensive heat transfer model for multiphase cryogenic cooling systems which are well suited to high-temperature superconductors.  These experimentally derived results are useful when selecting superconducting materials as a replacement for conventional conductors in a range of electrical devices.  Practical application of superconducting devices at industrial scale and a detailed experimental summary with challenges and recommendations was provided in the research. 

Presenting Author: Sean Orchuk The University of Toronto

Presenting Author Biography: Sean Orchuk B.A.Sc. M.S. P.Eng. is a lifetime scholar of mechanical engineering and currently a Ph.D. student at The University of Toronto. <br/><br/>Sean completed a bachelor&#39;s degree in Mechanical Engineering at The University of British Columbia followed by a Master&#39;s Degree in Aerospace Engineering at The Ohio State University. After 4 years of lean jet-engine design for Rolls-Royce Energy in Montreal, Canada, Sean migrated to power-generation and was employed by Electric Machinery (WEG) for 15 years as Global Field Service Manager. After visiting 106 countries and over a thousand industrial plants, Sean opened Interlock Consulting LLC and is currently the Vice President of Engineering. Interlock Consulting focuses on thermonuclear fusion and clean energy projects worldwide.<br/><br/>Sean mentors students at several universities and offers STEM courses at primary schools in greater Vancouver, Canada. An avid collaborator, Sean is a member of the Institute of Electrical and Electronics Engineers, Fusion Industry Association, American Society of Mechanical Engineers, and many other organizations.

Authors:

Sean Orchuk The University of Toronto
Sanjeev Chandra University of Toronto