The influence of aspect ratio on the thermal performance of a cryogenic pulsating heat pipe

• Published in: Applied thermal engineering 2021-09, Vol.196 (C), p.117322, Article 117322
• Main Authors: Pfotenhauer, J.M., Sun, X., Berryhill, A., Shoemaker, C.B.
• Format: Article
• Language: English
• Subjects: Adiabatic flow; Aspect ratio; Gravity; Heat conductivity; Heat pipes; Heat transfer; Helium; Hydrogen; Low temperature; Neon; Orientation; Pipes; Pulsating heat pipe; Segments; Space applications; Storage tanks; Superconducting magnets; Superconductivity; Telescopes; Thermal energy; Thermal management; Thermal resistance
• ISSN: 1359-4311; 1873-5606
• DOI: 10.1016/j.applthermaleng.2021.117322

•Cryogenic pulsating heat pipes perform best with large vertical to horizontal ratio.•Aspect ratio dependence of thermal resistance is characterized for a helium PHP.•The major thermal resistance for a helium PHP occurs at the wall – fluid interface.•Method provided to reduce heat load and orientation sensitivity for a hydrogen PHP. Pulsating heat pipes (PHPs) are expected to serve as a significant design component for the thermal management of superconducting magnets and various cryogenic space applications, including cryogen storage tanks and low temperature detectors on telescopes. A growing body of data is accumulating regarding the performance of cryogenic PHPs, including operation with helium, hydrogen, neon, and nitrogen, and the notable dependence on their orientation with respect to gravity. However, in view of the expectation that the thermal path between a low temperature heat source and the corresponding cryocooler will involve a convoluted route including vertical, horizontal, and sloping segments, we have begun a systematic study regarding the influence of the vertical-to-horizontal aspect ratio of a PHP on its thermal performance. Although various studies available in the literature characterize the performance of completely vertical or completely horizontal PHPs, this study quantitatively displays the relative impact of the vertical and horizontal segments on the performance of a mixed orientation PHP. This systematic investigation is the first of its type to characterize the influence of the vertical-to-horizontal aspect ratio of a PHP on its thermal performance. Data gathered with both helium and hydrogen filled PHPs including five different aspect ratios confirm the expectation that vertically dominated geometries perform better than horizontally dominated geometries. Minimum thermal resistance values ranging from 1.5 K/W to 20 K/W for the 1-meter-long PHPs, with aspect ratios varying from 4:1 to 1:4, are reported as a function of heat load, fill ratio, and fluid type. Here the aspect ratio is defined by the length of the vertical adiabatic section divided by the length of the horizontal adiabatic section. The results with the hydrogen PHP demonstrate that a fill ratio of 70% significantly reduces the influence of orientation and heat load on the PHP’s thermal resistance. The combined results provide valuable guidance for their application as heat transfer components in superconducting magnets and various cryogenic space applications.