Transient behaviors of loop heat pipes for alpha magnetic spectrometer cryocoolers

• Transient behaviors of LHPs for AMS cryocoolers have been investigated in the large space simulator.
• Mechanism of physical process during LHP startup has been analyzed.
• Effect of degree of liquid filling in the compensation chamber on LHP startup characteristics has been studied.
• Conditions for temperature oscillation have been revealed.

Startup and oscillation characteristics of loop heat pipes (LHPs) for alpha magnetic spectrometer (AMS) cryocoolers during thermal vacuum and thermal balance (TVTB) testing were determined. The mechanism of the physical process during LHP startup was analyzed in detail. Factors including liquid/vapor distribution in the evaporator, filling rate of the compensation chamber, and LHP elevation were investigated. Startup proved easier when vapor was present in the vapor grooves, whereas vapor in the evaporator core had an adverse effect. When the vapor grooves are liquid-filled, the startup process can be divided into three stages: heating of LHP components before superheating establishes, nucleate boiling after superheating is satisfied, and successful loop circulation. When the vapor grooves are vapor-filled, there are only two stages in LHPs: nucleate boiling and loop circulation establishment. Examination of the effect of compensation chamber filling rate on startup characteristics showed that reverse flow occurred when vapor was present in the compensation chamber and startup proved difficult. Filling rate of the compensation chamber should be sufficient to provide thorough wetting of the capillary structure, as otherwise wick dry-out may occur and prevent LHP startup. Temperature oscillations normally occurred at high sink temperature and/or heat load. Inclusion of a bypass valve was an effective measure to suppress temperature oscillations, because vapor at the condenser was not saturated when the bypass valve was open.