| The Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP) are thermal management devices which are high performance derivatives of the conventional heat pipe. Both consist of a discrete evaporator containing a porous solid wick which is coupled via smoothed walled tubing to a condenser. In a CPL, fluid inventory is managed by a reservoir attached to the liquid line which is controlled in temperature. In an LHP, fluid inventory is managed passively by a compensation chamber which is integral to the evaporator.;In this study, a borescope was inserted into the liquid core and compensation chamber of LHP evaporators and used to investigate phenomena in operation as an LHP, and in a CPL/LHP hybrid mode with both a reservoir and compensation chamber. These are the first published images of the core obtained during operation of a typical cylindrical evaporator and have confirmed several behaviors inferred over the previous three decades of study. In both modes, the evaporator was substantially more tolerant of vapor in the core than typically acknowledged.;For the LHP mode, which is widely acknowledged to be tolerant of some vapor in the core, it was found that even low-heat-load start-ups could be performed from an initial state in which the core was devoid of liquid. There was typically less excursion of temperature during such startups than when the core was partially liquid filled, and this allowed reliable operation with a low charge level with a relatively small compensation chamber. Due to lower charge level, temperature hysteresis associated with the core becoming fully liquid filled was suppressed, and maximum conductance was higher. These findings suggest that more rigorous consideration of charge and allowable compensation chamber states would yield LHPs of lower mass and with greater and more predicable performance.;For the CPL hybrid mode, the core generally remained liquid filled, except near the edges of the operating envelope. At low heat loads, often a failure scenario for CPLs, and under high imposed pressure loading, vapor sometimes appeared in the core. From there it migrated to the compensation chamber and condensed. This was without a noticeable effect on operating temperatures, unless vapor was generated faster than it condensed and accumulated to the point of blocking liquid entry to the core, causing the wick to dry. Conversely, for operating conditions approaching maximum heat load, the introduction of vapor into the core was explosive, but occurred well after other signs of impending deprime, and it thus appeared to be a symptom of a breakdown of the capillary menisci at the vapor channels, rather than the cause. |
