Mathematical Modeling of Loop Heat Pipes Part II: Secondary Wick Analysis

Abstract

Thermal requirements of today’s spacecraft and instruments exceed the heat transport capacities of conventional heat pipes. Accordingly, spacecraft engineers have turned to the Loop Heat Pipe (LHP) technology as a high-performance replacement. Like heat pipes, a LHP is also a passive capillary-pumped device having no mechanical moving part to wear out or break down. Hence it is reliable, durable, and more importantly maintenance-free for space applications. But while heat pipes can be treated as linear conductors in most thermal analyses, LHP thermal characteristics are not easy to predict for they are highly dependent upon the loop operational conditions. A few examples are given here: power input, sink and ambient temperatures. Even the past history of the aforementioned parameters can have an effect on the LHP performance. Previous LHP modeling efforts focused primarily on the prediction of the loop temperature and pressure drop for steady state or transient operation. For the most part, the model simulations agreed very well with test data from various LHPs. However the “health” of the secondary wick was conveniently ignored for two reasons: (i) to simplify the numerical method and (ii) the secondary wick design was a “trade secret” of the LHP vendor and was not divulged to the general public. In other words, the secondary wick was assumed to have “infinite” liquid transport capability and would not fail under any operating scenario. In truth, available test data had shown that the secondary wick might be the weakest link of the LHP. In an attempt to model the secondary wick fluid transport (without the knowledge of its actual design), a simplified geometry of the LHP pump core that functions like a conventional heat pipe was imposed in the analysis. In this paper, theoretical aspects of mass/heat transfer and fluid dynamics in the pump core will be given along with the model assumptions/simplifications. Discussion of the numerical method and prediction of the secondary wick transport requirements will also be given.