Secondary wick operation principle and performance mapping in LHP and FLHP evaporators

High performance Loop Heat Pipe (LHP) technology has advanced to the point that they are the baseline for thermal management for a lot of spacecraft applications. Temperature control and heat dissipation are performed in an efficient, and reliable way thanks to the LHP capillary pumping system. This article summarizes the objectives and performances of a secondary wick incorporated between evaporator and loop reservoir to get the optimum performances. Evaporator co-located with a reservoir is the baseline of LHP concept. This presentation will detail the reason why the thermal disconnection between the reservoir and the evaporator and their hydraulic coupling via a secondary wick is until today the best way to reach the optimum performances of LHP. Sometimes the secondary wick is combined with a bayonet. The article will compare the design without and with this bayonet in terms of impacts on secondary wick operation and performance limitations. The physical laws driving the secondary wick design are summarized. Their performance mapping and their impacts on the evaporator operation are shown. Finally the FLHP (Free LHP) concept is explained. This concept is intended to satisfy the needs of a freely located multi-evaporators loop. The article explains how secondary wicks are used when using evaporators not capillary linked to loop reservoir.