Performance tests of gravity-assist heat pipes with screen wick structures

The equipment and procedures used in the performance testing of sodium heat pipes operating in the gravity-assist mode are described. These tests showed that superheat limitations are a major factor to be overcome in wick design and wick fabrication methods. Oversize voids adjacent to the heat-pipe wall tend to become the loci of vapor pockets which block heat transfer through the wick and seriously reduce performance, particularly at relatively high working-fluid vapor pressures. In general, wick imperfections which may be inconsequential or even beneficial under gravity-neutral operating conditions, are likely to have a performance-limiting effect for gravity-assist operation. Some of the tests also indicate that the performance of gravity-assist heat pipes may be limited by interactions between vapor and liquid. Tests are now being conducted with helical baffles which should provide protected flow paths of low impedance for the returning liquid.