High performance heat pipes. Technical progress report, May 1, 1977-May 31, 1978. [TUNNEL computer code

Abstract

Progress in the development of high performance, high temperature heat pipes is discussed. The basis of the work is an unusual wick structure which seeks to combine the high capillary pressure of sintered porous metal powder with low liquid flow resistance provided by longitudinal passages, or tunnels, cast within the metal powder. The program has four tasks: Heat Pipe Analysis, Experimental Research, Heat Pipe Tests and Applications Study. The work of the first year of the project in which the first two tasks were completed and the third begun is summarized. The analytical work of Task 1 was aimed at developing the mathematical methods for predicting the performance of tunnel wick heat pipes. The analyses show that under some conditions the ultimate performance limit may be due to weeping of liquid from the tunnels into the heat pipe vapor space under conditions of high vapor pressure recovery in the condenser area. A computer program TUNNEL was written which predicts the heat pipe performance, including the onset of seeping. The characteristics of powder metal wicks have not been well determined experimentally. Data on permeability, capillary pore radius and tunnel flow/pressure drop were needed to predict heat pipe performance. These properties weremore » measured experimentally on samples of copper, nickel and molybdenum powder metal wicks. Toward the close of the year's work, the first tunnel wick heat pipes were fabricated and tested. These were water/copper units. Performance was at a high level, but did not meet full expectations. The results were being analyzed at the close of the period. The evidence appeared to indicate poor wetting of the wick so that the tunnels were not filling reliably during horizontal operation, but did fill during gravity-aided operation. The testing with water/copper will continue and will be followed by the sodium/nickel and lithium/molybdenum systems.« less