Heat and Mass Transfer in Loop Heat Pipes

Loop Heat Pipes (LHPs) have gained acceptance among spacecraft engineers in recent years as high performance heat transport devices for thermal control systems (TCS). However, the most common criticism from people who use LHPs is that their behavior is difficult to predict. Complex interaction of thermodynamics and fluid flow dynamics inside a LHP poses a challenge for the analytical modeling of its performance. The need for a complete understanding of mechanisms involving the heat and mass transfer in a LHP cannot be overstated. During the initial spacecraft TCS design phase, trade studies are usually carried out to select an appropriate thermal control concept for the design. The inability to accurately predict the LHP response in the actual operating environment often leads to the dismissal of LHPs for lack of certainty. This paper attempts to present a simplistic explanation of LHP operation in terms of heat and mass transfer processes, in hope that it will help the potential end-users to understand the technology better. Most of the observed phenomena described herein are based on available test data of various LHP systems. Nevertheless, a few anomalies especially during operational transients are still not well understood. For that, research ideas will also be proposed.