See wikipedia:Heat_pipe. A gravity feed heat pipe is typically a vertically oriented permanantly sealed copper pipe containing a refrigerant at a pressure which will result in partial but not complete evaporation of liquid refrigerant over the expected operating temperature range. It has no wick or special machining of the inner surface of the tube to carry liquid refrigerant upward. Thus, it acts like a heat diode, in that it will efficiently carry heat in one direction (upward) but not in the other (downward).

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