Performance of nitrogen pulsating heat pipes as passive thermal switches in a redundant cryocooler application

Abstract A novel redundant cryocooler system, featuring two cryocooler cold heads linked via separate nitrogen pulsating heat pipe (PHP) thermal switches to a common cold plate, is developed and performance tested. The setup allows the cold plate to be maintained at the design temperature when one of the two cryocoolers is inactive by leveraging the low thermal conductivity of the static vapor in the associated dry PHP as insulation from the warm inactive cryocooler. The PHP thermal switches operate passively and require no mechanical actuation, inherently offering a highly reliable switching mechanism. PHP heat load, effective thermal conductivity, effective conductance, evaporator temperature, and condenser temperature measurements are presented for a range of fill ratios and applied cold plate heat loads. The tested PHPs can transfer at least 16 W before dryout and have effective thermal conductivities in the ON and OFF switch states ranging from 10,000 W/m-K to 55,000 W/m-K and 8 W/m-K to 50 W/m-K, respectively. Thermal switch performance is characterized by the switching ratio – the ratio of its effective conductance in the ON state to that in the OFF state – and is demonstrated to approach about 2500 for the tested PHPs. The parasitic heat load through the PHP switch in the OFF state (about 0.6 W) is found to be about 4 percent of its maximum tested ON state heat transfer capacity (about 16 W).