Microchannel Flow Boiling for Embedded Cooling of Electric Machines

As the world continues to adopt electric alternatives in transportation in an effort to reduce CO₂ emissions, commercial aviation remains a difficult area to decarbonize. One of the largest barriers to electric propulsive flight is achieving a sufficient motor power density (kW/kg). This power-to-weight ratio of the motor is critical during takeoff where the plane’s weight and required motor power are at a maximum. State-of-the-art commercial motors cannot operate at a high enough current density, and therefore power density, required for takeoff of large aircraft without overheating. The copper windings within an electric machine are coated with enamel to electrically insulate each wire, and further, the windings are collectively insulated from the stator housing with dielectric sheets. Both of these insulation materials typically define the maximum operating temperature of the motor, as critically, the windings themselves produce a majority of the waste heat. Ultimately, the performance of electric machines is thermally limited due to the need to remove heat from the windings through these insulating layers, which prevents operation at necessary powers for electric propulsion architectures.

Current commercial motor cooling techniques are focused on exterior convective cooling of the stator. This is not sufficient for high power density usage due to the large conduction thermal resistances across the stator iron laminates and winding electrical insulation layers. By embedding cooling solutions within the motor, thermal resistance between the windings and coolant can be reduced. One such method is called slot cooling in which coolant flows between the windings in the stator slots. Prior demonstrations of this technique utilize single-phase flow and demonstrate significant power increases. This opens new pathways for further enhancement in slot cooling channels by increasing the heat transfer coefficient. Increased heat transfer coefficients in embedded cooling schemes offer more significant thermal performance gains compared to stator exterior cooling, due to the much larger conduction resistance in exterior cooling which dominates overall heat transfer regardless of the heat transfer coefficient. To achieve higher heat transfer coefficients, flow boiling can be applied to slot cooling. However, two-phase flow is susceptible to various instabilities and must be integrated into a thermal management system with proper mitigation strategies to ensure stable flow.

In this dissertation, integration of two-phase flow into an embedded cooling solution is investigated. A novel motor design has been developed with a unique slot cooling approach. In this motor, a multi-functional additively manufactured structural stator backbone holds toroidally-wound electrical coils, but it also has internal microchannels with a hierarchical manifold to deliver coolant between each coil. The coolant used in these studies is a low global warming potential (GWP), low-pressure, non-flammable, dielectric refrigerant R-1233zd(E). These integrated cooling tubes, with seven microchannels each, are interspersed between each coil throughout the motor. All tubes are hydraulically connected in parallel, and orifices are included at the inlet to each tube to suppress flow maldistribution resulting from the Ledinegg instability, a common instability arising in multichannel arrays with two-phase flow.

Two test sections are created for applied demonstration. The first is a four-coil, five-tube stator subsection prototype for assembly viability and performance testing. The applied potting technique is successful with minimal voids. Four repeated steady-state tests show excellent repeatability. A continuous current density of 35 A/mm² was reached at an average coil temperature of 133 °C, greater than the maximum current density of 29 A/mm² required by the motor to operate at the target power density for takeoff. Transient current densities are applied to the prototype in accordance with two six-cycle flight mission profiles applied to the prototype at 1.0× (baseline) and 1.3× (elevated) current densities. No temperature drift or unexpected behavior was observed across all cycles. The results matched steady-state temperatures. A test-to-failure is conducted to determine the failure mode of the prototype. At 42 A/mm² and an average coil temperature of 172 °C, the outer turn of Litz wire delaminated and charred. No sudden or catastrophic failure was observed. The stator subsection prototype achieved all target performance metrics while maintaining all materials below their temperature limits, across both steady-state and transient testing. The continuous current densities reached in these tests far exceed current state-of-the-art by eliminating end windings and evenly cooling the entire surface of the winding bundle. The test section manufacturing techniques demonstrate potential for full-scale motor construction.

The second demonstration prototype is a two-coil, one-tube test section. This prototype has an increased tube wall thickness to enable wall-embedded thermocouples for extraction and interpretation of additional thermal performance characteristics. Specifically, the convection and conduction thermal resistances are able to be delineated and measured independently with the additional of the wall thermocouples. For steady-state testing at baseline operating conditions, a heat transfer coefficient of 2530 W/m²K and a ground insulation thermal resistance of 1.4 K/W are measured. A two-phase flow sensitivity study to mass flow rate is conducted. The high heat transfer coefficients achieved with flow boiling dramatically reduce convective resistance, which makes the cooling solution conduction-dominated. Because conduction resistances are independent of flow rate, the average coil temperature is independent of flow rate as well. A one-dimensional heat transfer model is developed to predict heat transfer coefficients, and a mean average error of 10.4% is achieved. A three-dimensional steady-state simulation for thermal resistance matches experimental results. Two-phase flow in this embedded motor cooling solution reaches up to six times the heat transfer coefficient achieved with single-phase flow at an equivalent pumping power, pushing the performance up to the limit of diminishing returns. These high heat transfer coefficients associated with two-phase flow are also with respect to a constant fluid saturation temperature regardless of flow rate, resulting in a robust cooling solution unaffected by deviations in the flow rate from the nominal operating point.

Lastly, a study is conducted to assess the uniformity and stability of the two-phase flow through the microchannel tubes and hierarchical manifold. A set of test sections are developed to characterize the flow behavior by isolating the impact of orifices in the design. The first is a single-tube test section for pressure drop load curve measurement and baseline temperature performance. The second is a multi-tube test section to measure flow non-uniformity between tubes at different operating conditions. These test sections are tested for three inlet temperatures and ten flow rates. Measurements show an increase in average tube temperature in the multi-tube test section compared to the single-tube test section after boiling incipience. Heat loads are increased from 0 W to 50 W during testing while waiting for steady-state at each point. Boiling incipience does not occur in every tube at one heat load, but rather, incipience occurs across a range heat loads due to some tubes remaining in single-phase flow. Once boiling is initiated in all tubes, the system remains fully stable during these tested operating conditions. Two benchmark test sections are designed without orifices, but otherwise identical to the single-tube and multi-tube test sections, to investigate the impact of the orifices on flow stability and temperature uniformity. R-1233zd(E) is found to be stable without orifices at these operating conditions. Without flow isolation from the orifices, incipience is observed in all tubes at the same heat load operating at the design point flow rate and inlet temperature. Minimal difference is observed between the resulting average wall temperatures when comparing test sections with and without orifices. The hierarchical manifold integrated into this motor cooling architecture reduces backflow to the header for a minor pressure drop penalty, but in the process, it decouples boiling incipience between parallel tubes and creates incipience overshoot-induced maldistribution. This reveals an interesting design tradeoff where orifices are necessary to ensure uniform flow distribution during two-phase operation at heat power inputs, but may slightly compromise temperature uniformity at lower heat inputs near boiling incipience.

These studies have advanced high-power-density motor cooling by incorporating two-phase flow into embedded geometry for high heat removal at low pumping power. This work has also improved the understanding of stability and flow uniformity in large parallel channel networks with hierarchical manifolding through experimental testing of thermally isolated microchannel arrays. This work creates a launching point for further applied testing to assess physical responses seen in aviation, such as orientation and vibration, and fundamental studies on the effects of surface roughness and orifice sizing.