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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.

Test results are described for a coldplate heat exchanger designed to remove electronics heat loads of up to 100 W/cm2 while maintaining device junction temperatures of 90 °C. The heat exchanger was designed to operate within a notional fighter aircraft environment with the following constraints: 0 °C minimum coolant temperature, poly alpha olefin (PAO) coolant, and minimized flow rate and pressure drop. The heat exchanger was configured to Standard Electronics Module, Format “E” (SEM-E) specifications. High heat flux capability was achieved by combining the high heat transfer characteristics of multiple jet impingement with the compact extended surface area enhancement of laminated construction. Thermal tests verified 100 W/cm2 local capability, and 2000 W total module heat load capability, with wall-to-fluid thermal resistance of 0.281 °C/(W/cm2). Repeatable thermal and hydraulic performance was obtained over a one-month period of testing, including 22 hours of flowing coolant.

Numerous studies have concluded that improved high power density cooling methods are required for future avionics due to the trends in increasing heat dissipation, device miniaturization and higher packaging density. A high flux heat exchanger (HFHE) has been designed to optimally meet the thermal performance and envelope requirements selected as appropriate for future avionics, at a minimum impact to the host aircraft. This paper summarizes the design and fabrication of the prototype HFHE. The HFHE has been successfully performance tested with results included in Part II of this paper. Design objectives for the HFHE included a local heat flux capability of 100 W/cm2 (at 20 one-cm2 sites), a total SEM-E size module heat load of 2,000 W, and a maximum device junction temperature of 90°C with PAO coolant at not less than 0°C. An aluminum frame coldplate with a laminated copper heat transfer insert was designed and fabricated which met all of these performance requirements.

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.

An update of a Two-Phase Thermal Management System concept is presented including hardware status for flight prototypic components, modeling techniques, and terrestrial test results for both preprototype and some prototypic hardware. A test plan for a microgravity test on a KC 135 is discussed.

Spacecraft have previously used passive cooling or pumped liquid loops to maintain proper thermal environments. Future large spacecraft, such as the Space Station, will have higher power levels and longer heat transport distances. To better meet these requirements, a two-phase thermal bus concept has evolved which includes a thermal transport loop with controlled temperature and pressure. A two-phase cooling system has the advantages over a single-phase system of lower mass flows, lower pumping power requirements, and operation at nearly isothermal conditions, since heat is transferred by evaporation and condensation. The system employs an electrically-driven pitot pump to supply a controlled flow of nearly saturated liquid to any arrangement of series or parallel evaporators.

The central thermal management system for the proposed NASA Space Station will likely employ a two-phase thermal bus to satisfy the high power and long transport distance requirements. Significant potential weight and power savings accrue from this approach. A pumped two-phase cooling loop is described that can meet the requirements while maintaining constant heat source temperatures with large power and sink temperature turndown capability. Predicted performance of the 25 kW ammonia flight conceptual design is presented along with test results from a Freon 114 test loop which confirms predicted characteristics.