Oversized diameter helium pulsating heat pipe

Abstract

A cryogenic pulsating heat pipe (PHP) whose capillary tube diameter does not satisfy the classical Bond number (Bo) criterion has been experimentally tested and showcased in this work. Helium is the working fluid employed and the PHP diameter is ∼ 1.75 times its recommended critical diameter of 0.57mm. All the investigations reported till date on helium PHP have adhered to tube diameter of 0.5mm evidently meeting the Bo < 4 criterion. However, this helium PHP corresponds to Bo = 12 at 4.2K and, in fact, is the first cryogenic PHP demonstrating its functionality at such a high Bond number for helium. The motivation for such an investigation is initially presented with a deliberation on different PHP critical diameter criteria. A hypothesis emphasizing on the need to re-evaluate the upper limit of tube diameter for cryogenic PHPs is proposed. In support of this, a closed-loop PHP constructed from a 1mm inner diameter stainless steel tube is utilized. It is composed of 20 parallel channels and has a projected length of 0.4m. The functionality of this large diameter helium PHP is validated by experimentally characterizing its thermal performance and pressure as a function of heat load up to 1.5 W within an exhaustive range of filling ratios (10 to 90 %). This is conducted both in vertical and horizontal orientation. Zones where the PHP quickly surpasses the helium critical temperature and pressure values and still functioning are identified. Finally, its reliability is exhibited by 65 h stability test conducted at constant operational conditions.