Joshua Feldman with Advisors Prof. Kiruba S. Haran and Prof. Nenad Miljkovic

Aviation is a growing contributor to the global climate crisis. One way to reduce greenhouse gas emissions from aviation is to replace the jet engine propulsion systems with electric motor-driven propellers. The Center for High Efficiency Electric Technologies for Aircraft (CHEETA) has proposed a hydrogen-electric commercial plane where liquid hydrogen (LH2) would act as the energy carrier and the heat sink for the plane’s motors. Liquid nitrogen (LN2) could be an intermediary between the motors and the LH2, thereby minimizing potential LH2 hazards. At LN2 temperatures, the motor could benefit from low-resistivity aluminum windings and be highly efficient.

Proper thermal modeling of such a motor requires correlations equations for predicting the heat transfer coefficient (HTC) if flow-boiling heat transfer is present. These correlations are typically derived by correlating experimental data. However, gaps exist in the literature for some cryogenic flow-boiling conditions. These conditions include subcooled flow boiling with low heat flux and saturated boiling with high vapor content and high mass flux. Also, there are no experimental data for flow-boiling in partially heated tubes. Modelling partially heated tubes could be important in motor heat exchangers where the coolant tube is only partially embedded in the heat sink.

This project has filled these gaps by measuring flow boiling HTC data for uniformly heated and partially heated tubes. A picture and schematic of the experimental setup are shown in Figure 1. The experimental data was used to establish a set of experimentally verified HTC correlations for subcooled, saturated, and partially heated tubes. This correlation set allows engineers to accurately predict HTC for many LN2 flow-boiling conditions for the first time, enabling better thermal modeling of cryogenic motors for electric aircraft propulsion.

Drawing of experimental flow loop for measuring nitrogen flow-boiling HTC. Components labeled

Photographs of flow loop with components labeled: (left) Photograph outside the vacuum chamber, showing the liquid nitrogen tanks that drive the flow and flow meter. (right) Photograph inside the vacuum chamber, showing preheat and test sections.