Cryo-ablation is a breast cancer treatment method that utilizes a small probe in precise locations within the body to freeze and destroy unwanted cancer tissue. Recently, there has been a growing interest in combining cryo-ablation with magnetic resonance imaging (MRI). The challenge of combining these two technologies is that MRI devices require the absence of metals, and traditional heat exchangers used in cryoprobes are typically composed of thermally conductive metals that tend to disrupt the image produced by an MRI, impairing its functionality. Subsequently, it becomes of interest to develop a heat exchanger composed of thermally conductive MRI-compatible materials such as zirconium. This report presents the results of a thermal modelling effort to characterize and design a non-metallic Joule-Thomson cryoprobe for cryo-ablation. The model is comprised of a Joule-Thomson valve, as well as a discretized recuperative heat exchanger that includes the effects of axial conduction, pressure drop, two-phase and single-phase convection correlations, and fluid properties for single-components and mixtures. The device operates at 170 K with a heat load of 10 W, which is a viable temperature and heat load for cryo-ablation.

Pulsating heat pipes, also known as PHPs, are passive two-phase heat transfer devices capable of moving heat at cryogenic temperatures with high effective thermal conductivities around two orders of magnitude higher than copper. A recent Helium Pulsating Heat Pipe experiment demonstrated a surprising phenomenon, where Helium PHPs of different lengths (300 mm and 1000 mm) displayed the same thermal conductance at equal heat loads. The purpose of this research is to experimentally characterize this apparent length-independence and determine the size limits for Helium PHPs. An experimental approach is developed where three additional Helium PHP experiments will be conducted with the same operating parameters as the original experiment except with extended adiabatic lengths – 1.25 m, 1.5 m, and 1.75 m. All PHPs considered in this study are in the vertical orientation and are bottom-heated. This will give five complete sets of data from which the influence of length on helium pulsating heat pipes’ performance may be analysed. This paper serves as a work-in-progress report describing the experimental design and fabrication of these Helium PHP experiments.