Operational Behaviors of Multiple Thermally-Coupled Loop Heat Pipes

Loop Heat Pipe (LHP) is a two-phase capillary-pumped heat transport technology that contains no mechanical moving part to wear out or break down. Hence, they are routinely utilized in the spacecraft thermal control systems (TCS). At first glance, the LHP operation seems straightforward. In actuality, the system inner workings are highly complex involving the convoluted interaction among different processes: fluid flow dynamics, phase change thermodynamics, and heat/mass transfer. As a result, even the performance of a single LHP may appear counterintuitive to a novice user, especially during a transient period. However, with a string of successes on orbit, the spacecraft TCS engineers have been emboldened to employ the LHP technology in ways that were not thoroughly verified either on the ground or in space. One example is the use of multiple LHPs thermally coupled (i) to a shared heat source at the evaporators and/or (ii) to the same heat sink at the condensers. In addition, the temperature control of the attached payload was also attempted by adding heat to some or all LHP reservoirs. Often, the modifications worked out nicely accomplishing the intended design purposes while producing only benign operational side effects. Nevertheless, on a few occasions, the new features induced the system to function in ways that did not quite make sense even to the seasoned LHP practitioners. In this paper, the authors will utilize a steady state LHP thermal-fluid model to demonstrate that although the operational behaviors of multi-LHP systems sometimes appear perplexing and irrational, they are mostly predictable. The uneven heat load sharing among the seemingly “identical” LHPs is chosen for the study to show that it is an inherent characteristic of multi-LHP operations and not a result of some hardware malfunction.