Short communication: Thermal performance of a cryogenic helium pulsating heat pipe with three evaporator sections

This short communication reports surprising results from on-going measurements of a helium pulsating heat pipe (PHP) in which the adiabatic length has been increased from 0.3 m to 1 m. In addition, the PHP has been constructed with a total of three evaporator sections. The PHP contains 42 parallel stainless steel tubes with an outer and inner diameter of 0.8 mm and 0.5 mm, respectively. PHPs with this configuration can serve as thermal spreaders for cryocoolers and can be used to cool multiple hot spots. Results and configuration details of the PHP with a 300 mm long adiabatic section have been previously published (Fonseca et al., 2017, 2018). The same PHP, now modified with a 1-meter long adiabatic section has been tested at the same heat load and fill ratio values as the 0.3 m PHP. The PHP operates between 3 K and 5.2 K using a Sumitomo cryocooler model RDK-408A2 with a cooling capacity of 1 W at 4.2 K. Interestingly, the results show that the temperature difference (T) between the evaporator and condenser sections of the PHP with the 1-m adiabatic section length is almost identical to that measured with the 0.3 m adiabatic section length PHP when the two different configurations are each operated at their respective optimum fill ratio. Temperature differences below 1 K are achieved when the heat load is less than 0.26 W per evaporator section (0.78 W total). Very similar optimal thermal conductance (Q/Delta T) values are obtained for both configurations, where a maximum thermal conductance of 0.383 W/K at a fill ratio of 58% and 0.387 W/K at a fill ratio of 70% are achieved for the 1 m and 0.3 m PHP, respectively. In addition, the optimal thermal performance can be expressed as an effective thermal conductivity that is proportional to the adiabatic length, resulting in a higher value for the 1-m PHP of 150 kW/m-K; while the 0.3 m PHP achieved a maximum thermal conductivity of almost 53 kW/m-K. This paper will present results based on equal heat load distribution throughout the evaporator sections. (C) 2018 Elsevier Ltd. All rights reserved.