Development of Flat-Plate Loop Heat Pipes for Spacecraft Thermal Control

This paper describes the development, ground experiment results and on-orbit test plan of stainless steel-ammonia flat-plate loop heat pipes for space applications. The pressure-proof evaporator with reinforced structure was designed to endure the high pressure of the working fluid. The cylindrical compensation chamber and the primary sintered nickel wick were connected by the secondary wick for liquid supply under microgravity. To validate the startup and operation capability in space, the on-orbit tests of two FLHPs (flat-plate loop heat pipes) will be carried on in the reentry capsule of the Chinese next generation spaceship. The FLHPs will be used to transfer the dissipated heat of the laser gyro and fiber optic gyro to the cold capsule wall. Extensive ground experiments were conducted to investigate the startup and operation characteristics and the heat transfer capability. Test results indicate that there are only two startup situations for the FLHP. A superheat degree of above 10 degrees C is required to initiate the nucleate boiling in the evaporator when the vapor channels are flooded by liquid. The auxiliary startup measure by locally heating on the top surface near the evaporator outlet is effective to obtain the required superheat, not affecting the equipment's temperature. Orientation of the evaporator and the CC (compensation chamber) has negligible effect on the startup and operation of the FLHP, and the secondary wick can provide effective liquid supply to the primary wick even at an unfavorable position. The heat transfer capacity of the evaporator is greater than 330 W and the critical heat flux is greater than 20 W/cm(2).