Hybrid Thermosyphon for Power Electronics Thermal Management in High Gravity Applications

In many aerospace applications there exists a need to reject heat passively to air either in a nearby or far away bay or to the ambient through the aircraft skin. In this case the distance from the heat source to the heat sink may be several meters and the temperature difference between them may be limited by only a few degrees. Cooling technology to satisfy this need must be able to operate in a high gravity environment in order to be qualified for aerospace application. A Loop Heat Pipe (LHP) can satisfy the passive high gravity cooling requirement, but has limited coolant pumping capacity under small source/sink temperature differentials. A loop thermosyphon, using gravity as the coolant driving force, can be used to operate under smaller temperature differentials, but because of the gravity dependence, loop thermosyphons can only work in steady environments. In applications where high gravity is only experienced in a transient nature, it is possible to apply a buffer to the thermosyphon to provide continuous operation when normally the evaporator would experience a dry-out. Using a specially configured wick in the evaporator of a loop thermosyphon, it has been demonstrated that up to 80% of the stored coolant can be used without any change in temperature, tested to as much as 10g's. In an open air evaporator test with 16gm of water stored in the wick, uninterrupted cooling for 195s in a 10.8g environment was provided using a rotary test assembly. Matching performance was also demonstrated in a variable gravity condition, where the g's were repeatedly cycled between 1 and 10 throughout the test. A closed thermosyphon test also demonstrated uninterrupted cooling in an adverse 1g environment with matching stored coolant usage with the high g testing.