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An electronics cooler using reflux heat rejection and phase change material (PCM) energy storage has been built and tested. The cooler provides thermal control for an electromechanical actuator motor drive. These aircraft actuator motor drives do not have an available high performance heat sink, and thus must rely on the quiescent air in an actuator bay or the aircraft structure, as heat sinks. Thermal loads on the power electronics of these motor drives can be quite high for short durations, suggesting the use of phase change energy storage to store the heat until it can be rejected to the available heat sinks. The present experiment used Fluorinert®1 compound FC72®2 for the refluxing fluid, and Shellwax®3 paraffin wax with either a 51° C or 61° C melt temperature as a PCM for high power transients.

A two-phase thermosyphon cooler coupled with phase change material (PCM) energy storage was built to demonstrate a concept for cooling a 26 kW actuator motor. FC75®, a Fluorinert® compound, was used as the working fluid to transfer heat to the phase change material, acetamide. The PCM was contained in alternating layers of a plate-fin compact heat exchanger core. At the 90 percent power condition the peak motor temperature was within 90°C of the heat sink, showing good source to sink thermal coupling by the thermosyphon and conductive links. Conversely, when the motor was cooled by natural convection and conduction alone, the peak temperature was 190°C above sink temperature. Testing shows that the PCM material provides additional useful thermal inertia during the melting process. However, test data revealed that the melt temperature of the acetamide had been depressed from 80°C to 68°C by absorbed water, highlighting the need to process the PCM in a dry atmosphere.

A passive cooling approach for aircraft electromechanical actuators is being developed to support the Air Force More Electric Airplane (MEA) program. A two-phase coolant is used in a reflux type cooler to transport heat from the actuator electric motor to a cool aircraft structural surface. Energy storage, in the form of phase change material, is incorporated into the cooler to provide load leveling between peak loads and low loads. Transient thermal analysis, which was used to select the best combination of reflux working fluid and phase change material, indicates that motor temperatures can be reduced by more than 50°C when using thermal energy storage.

A cavity heat pipe experiment has been designed to test the critical issues involved with incorporating thermal energy storage canisters into a heat pipe. The experiment is a replication of the operation of a heat receiver for a Brayton solar dynamic power cycle. The heat receiver is composed of a cylindrical receptor surface and an annular heat pipe with thermal energy storage canisters and gaseous working fluid heat exchanger tubes surrounding it. Hardware for the cavity heat pipe experiment will consist of a sector of the heat pipe, complete with gas tube and thermal energy storage canisters. Thermal cycling tests will be performed on the heat pipe sector to simulate the normal energy charge/discharge cycle of the receiver in a spacecraft application.